EP4735461A2 - Alkylated bodipy compound and methods of using the alkylated bodipy compound - Google Patents

Abstract

A compound having a structure according to Formula (I) or Formula (II) is disclosed, wherein R¹, R², R³, R⁴, Ar, X¹, X², and Z are defined herein. The compound can be particularly useful as a Type I photoinitiator. Curable compositions and methods of using the compounds are also described.

Description

ALKYLATED BODIPY COMPOUND AND METHODS OF USING THE ALKYLATED

BODIPY COMPOUND

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application No. 63/523,426, filed on June 27, 2023, the contents of which is hereby incorporated by reference in its entirety.

FEDERAL RESEARCH STATEMENT

This invention was made with government support under Grant no. CHE2107877 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

[0001 ] Visible light has emerged as a promising stimulus to drive polymerizations for a variety of applications, particularly in the biomedical and advanced manufacturing (e.g., 3D printing) arenas. This arises in part from the spatiotemporal control, high penetration depth, low energy, and discrete absorption it offers to enable benign and wavelength-selective fabrication of multifunctional soft materials. A major hurdle limiting the implementation of visible light driven polymerizations is its low efficiency relative to ultraviolet-light driven processes, arising from a difference in mechanism to generate initiating species for polymerization, such as radicals, which has been predominantly restricted to bimolecular processes for long wavelength light (>500 nm) (i.e., Type II, photoredox), and unimolecular processes for short wavelength light (<500 nm) (i.e., Type I, photolysis). In turn, the requirement for cocatalysts such as tertiary amines, iodonium salts, and/or borate salts in Type II processes are intrinsically diffusion limited, slowing overall photocuring rates, increasing complexity and cost, and potentially decreasing biocompatibility of the concomitant resin formulations. Therefore, long wavelength (>500 nm) Type I photoinitiators offer a compelling alternative yet remain elusive.

[0002] It would therefore be advantageous to provide long wavelength (e.g., >500 nanometers (nm)) Type I photoinitiators. Such photoinitiators would be particularly useful for visible light driven polymerizations.

SUMMARY

[0003] An aspect of the present disclosure is a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur; provided that when each occurrence of X² is I, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl, R² is not a -CH2OH group, a -CH2OCH2PI1 group, or a group having the structure when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl, R² is not a -C FBr group, a -CH2CI group, a phenyl group, a p-tolyl

*— Q group, a -CH2OCH2PI1 group, or a group having the structure ; and when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴is hydrogen, R² is not a phenyl group or a 2,4,6-trimethyl phenyl group.

[0004] Another aspect is a curable composition comprising: a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur; an ethylenically unsaturated compound; and optionally a crosslinker.

[0005] Another aspect is a cured composition derived from the curable composition.

[0006] Another aspect is a process for converting a reactant to a reaction product, the method comprising exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide the reaction product, wherein the reaction mixture comprises: the reactant; and a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur.

[0007] Another aspect is a process for photopolymerization, the process comprising: exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide a polymer comprising repeating units derived from an ethyl enically unsaturated compound, wherein the reaction mixture comprises: the ethylenically unsaturated compound; and a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted Ce-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted 0,-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce 20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur.

[0008] The above described and other features are exemplified by the following figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following figures represent exemplary embodiments.

[0010] FIG. 1 shows a schematic illustration of a BODIPY photoinitiator (PI) synthesis according to an aspect of the present disclosure. Reaction conditions: (i) 1 molar (M) aqueous sodium hydroxide (NaOH(aq)), dichloromethane/methanol (CH2C12/MeOH (1:2)), 3 hours (hr); (ii) methyl magnesium bromide (CHsMgBr), dichloromethane (DCM), 30 minutes (min); (iii) N-bromosuccinimide (NBS), CH2Q2, 1 hr.

[0011] FIG. 2 shows UV-vis absorption spectra of BODIPY Pls (top) and commercial Type I Pls (bottom) in CH3CN (5 micromolar (pM)). Relevant LED emission traces underlaid. Insets: Images of corresponding Pls in CH3CN (2 millimolar (mM)).

[0012] FIG. 3 shows an illustration of the in-situ monitoring of monomer to polymer conversion with attenuated total reflectance-Fourier transform infrared spectroscopy (ATR- FTIR).

[0013] FIG. 4 shows (top) a plot of monomer to polymer conversion versus time for BODIPY Pls upon exposure to a green LED centered at 530 nm ( 'or an orange LED centered at 590 nm) in comparison to commercial type I Pls upon exposure to a green (530 nm) or blue (470 nm) LED (bottom). All PI concentrations were 0.3 mole percent (mol%) (*or 0.1 mol% as noted).

[0014] FIG. 5 shows an electron paramagnetic resonance (EPR) spectrum for a greenlight irradiated sample that contained BODIPY-Me-Br and M-tert-butyl-a-phenylnitrone (PBN) (black trace), and simulated result of PBN-Me (grey trace).

[0015] FIG. 6 shows bond dissociation energy (BDE) values determined computationally using density functional theory. Inset: 3D structures with B-F and B-C bond lengths indicated.

[0016] FIG. 7 shows excited state lifetime characterization by transient absorption spectroscopy. Inset provides fluorescence quantum yield (c?f) values obtained using fluorescence spectroscopy and excited state lifetime (r). Data for BODIPY-F-H was obtained using time- correlated single photon counting fluorescence spectroscopy due to lack of a triplet signal in transient absorption.

[0017] FIG. 8 shows visible light transmittance (T_vis) of a polymer film prepared using BODIPY-Me-Br as the photoinitiator right after preparation and after an extended irradiation with a green LED centered at 530 nm.

[0018] FIG. 9 shows a chemical scheme of the synthesis of exemplary BODIPY derivatives according to an aspect of the present disclosure. Reaction conditions: (i) N,N- Diisopropylethylamine (DIPEA), boron trifluoride etherate (BF3-Et2O); (ii) 1 M NaOH(_aq); (iii) A-bromosuccinimide (NBS); (iv) Methylmagnesium bromide.

DETAILED DESCRIPTION [0019] A classic Type I photoinitiator that absorbs visible light (e.g., 405 nanometers (nm)) is bisacylphosphine oxide (BAPO), which has been employed industrially in lithography and 3D printing owing to its high radical generation efficiency. In analogy to traditional UV- activated aryl ketones, acylphosphine oxides undergo homolytic a-scission (C-P bond) after photoexcitation, directly producing a high concentration of radicals capable of inducing rapid acrylate polymerization (i.e., curing). However, aryl ketone absorption occurs via a spin- forbidden n-n* electronic transition, resulting in low molar absorptivity in the visible region (-405 nm). Efforts to red-shift the absorption have been made by replacing the carbonyl oxygen atom with sulfur or selenium to manipulate the n-bonding energy, or by replacing the a-carbon with germanium (commercially available as Ivocerin). However, absorption peak maxima in these derivatives remain confined to wavelengths <450 nm, and their challenging syntheses and potential toxicity have hindered widespread use as single-component photoinitiators in photocurables. Additional examples of visible light induced polymerizations include those employing titanocene derivatives (commercial product Irgacure™ 784), palladium diimine catalysts, phenyl iodonium n-conjugated boron dipyrromethene (BODIPY) salts, and thiocarbonylthio chain transfer agents used in controlled radical (photoiniferter) polymerization. Still, absorption peak maxima are <500 nm and/or have low molar absorptivity above 500 nm (<1,000 M 'cm ¹), necessitating the use of high light intensities (>50 milliwatt per square centimeter (mW/cm²)) and/or photoinitiator concentrations (>2 weight percent (wt%)) to achieve photopolymerizations on timescales relevant to lithography and 3D printing (e.g., seconds).

[0020] The present inventors have advantageously discovered that alkyl-functionalized variants of BODIPY fluorophores could provide a novel Type I photoinitiator platform for rapid and efficient free radical reactions (e.g., polymerization). It was further discovered that alkylated BODIPY acts as a highly efficient Type I photoinitiator upon exposure to low intensity green light (e.g., 495 to 570 nm, for example 530 nm, <5 mW/cm²) to induce rapid polymerizations (e.g., less than 20 seconds to maximum conversion for an acrylate monomer). In a further advantageous feature, low photoinitiator concentrations could be employed (e.g., less than 5 weight percent, or less than 2 weight percent, or less than 1 weight percent, or less than or equal to 0.6 weight percent, each based on the total weight of a curable composition). A significant improvement in visible light-activated photocatalysts is therefore provided by the present disclosure.

[0021] Accordingly, an aspect of the present disclosure is a compound having a structure according to Formula I or Formula II wherein in the foregoing Formulas, X¹ is boron (B) or gallium (Ga); X² is independently at each occurrence hydrogen, chlorine (Cl), bromine (Br), or iodine (I); Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ (i.e., on a single X¹ moiety) can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur. In an aspect, each occurrence of R³ is the same. In an aspect, each occurrence of R⁴ is the same. In an aspect, each occurrence of R³ and R⁴ are the same. In an aspect, R¹ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group, provided that neither occurrence of R¹ is a substituted or unsubstituted C1-6 alkoxy group. In an aspect, R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group. In an aspect, at least one occurrence of R¹ is a substituted or unsubstituted Ci- 6 alkyl group substituted with one or more in-chain or pendent sulfur atoms, preferably wherein each occurrence of R¹ is a Ci-6 alkyl thioether group (i.e., -S-(Ci-6 alkyl)). In an aspect, R¹ can be a substituted Ci-6 alkyl group, for example a Ci-6 alkyl group substituted with a cyano group (-CN) or a Ci-6 alkyl ester (-(C=O)-O-(Ci-6 alkyl)).

[0022] In an aspect, X¹ is boron (B). In an aspect Z is a carbon atom. In an aspect, X² is preferably Br, Cl, or hydrogen, more preferably Br or hydrogen. In an aspect X² is Br. In an aspect, each occurrence of R¹ can be a substituted or unsubstituted Ci-6 alkyl group. In a specific aspect, each occurrence of R¹ can be an unsubstituted Ci-6 alkyl group, preferably a methyl group. In an aspect, R² can be a substituted or unsubstituted Ci-6 alkyl group. For example, in an aspect, R² can be a Ci-6 alkyl group optionally substituted with a halogen or a hydroxyl group. In a specific aspect, R² is a hydroxyl-substituted Ci-6 alkyl group, for example a hydroxymethylene group (i.e., -CH2OH). In an aspect each occurrence of R³ and R⁴ is independently an unsubstituted C1-6 alkyl group, preferably a methyl group. When the compound is according to Formula (II) and the Ar group is present, Ar can be a substituted or unsubstituted fused aryl group substituted with one or more in-chain heteroatoms selected from the group consisting of oxygen and sulfur. For example, Ar can be a fused furan group, a fused thiophene group, or a fused bithiophene group.

[0023] In an aspect, the compound has a structure according to Formula (I). For example, the compound can have a structure according to Formula (I) wherein X¹ is boron (B), Z is carbon, X² is preferably Br, Cl, or hydrogen, more preferably Br or hydrogen, R¹ can be an unsubstituted C1-6 alkyl group, preferably a methyl group, R² is a substituted or unsubstituted Ci- 6 alkyl group, preferably a hydroxyl-substituted C1-6 alkyl group, and R³ and R⁴ is independently an unsubstituted C1-6 alkyl group, preferably a methyl group.

[0024] When each occurrence of X² is I, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl in Formula (I), R² is not a -CH2OH group, a -CH2OCH2PI1 group, or a group having the structure

[0025] When each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl in Formula (I), R² is not a -CFfeBr group, a -CH2CI group, a phenyl group, a p-tolyl group, a -CFEOCFFPh group, or a group having the structure

[0026] When each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is hydrogen in Formula (I), R² is not a phenyl group or a 2,4,6- trimethyl phenyl group.

[0027] Exemplary compounds according to Formula (I) can include, but are not limited to, compounds of the following structures:

[0028] The corresponding gallium (Ga) analogs of the foregoing compounds are also specifically contemplated by the present disclosure.

[0029] In a specific aspect, the compound can be according to Formula (I) and can have the structure

[0030] The compounds having the structure according to Formula (I) and Formula (II) have been found by the present inventors to be particularly useful as Type I photoinitiators for various chemical reactions, for example photopolymerizations. [0031 ] Accordingly, a curable composition represents another aspect of the present disclosure. The curable composition comprises a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is boron (B) or gallium (Ga); X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted 0,-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur. All variations and combinations of R¹, R², R³, R⁴, Ar, X¹, X², and Z of Formula (I) and (II) discussed above are possible in the curable composition of the present disclosure.

[0032] In addition to the compound of Formula (I) or Formula (II), the curable composition further comprises an ethylenically unsaturated compound. Suitable ethylenically unsaturated compounds comprise free radically active unsaturated groups. Examples of useful ethylenically unsaturated compounds include (meth)acrylates (e.g., Ci-22(meth)acrylic acid esters, Ci-22(meth)acrylamides, hydroxy-functional Ci-22(meth)acrylic acid esters, and the like, for example methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl(meth) acrylate, isobutyl acrylate, sec-butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth) acrylate, 2- ethylhexyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, isobomyl (meth)acrylate, benzyl (meth)acrylate, phenyl (meth)acrylate, and the like), vinyl monomers (e.g., vinyl ester monomers such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl n-sodium caproate, vinyl isocaproate, vinyl caprylate, vinyl caprate, vinyl octoate, vinyl laurate, vinyl myristate, palmitic acid, vinyl stearate, vinyl pivalate, trimethyl vinyl acetate, vinyl chloroacetate, vinyl trichloroacetate, vinyl trifluoroacetate, and vinyl benzoate), styrenic monomers (e.g., styrene, a- methyl styrene, p-methyl styrene, m-methyl styrene, o-methyl styrene, 2,4-dimethyl styrene, 2,5-dimethyl styrene, p-ethyl styrene, p-isopropyl-styrene), and the like, or combinations of any of the foregoing compounds. In an aspect, the ethylenically unsaturated compound can comprise a (meth) aery late or a (meth)acrylamide. For example, the ethylenically unsaturated compound can comprise an acrylate, for example a C 1-22 acrylate ester, or a C1-6 acrylate ester. In an aspect, the ethylenically unsaturated compound can comprise a (meth)acrylamide, for example a C1-22 (meth)acrylamide, or a C1-6 (meth)acrylamide.

[0033] In addition to the compound of Formula (I) or Formula (II) and the ethylenically unsaturated compound, the curable composition can optionally further comprise a crosslinker. Suitable crosslinkers can comprise two or more ethylenically unsaturated groups capable of participating in the polymerization with the ethylenically unsaturated compound. Exemplary crosslinkers can include, but are not limited to, acrylic acid esters of polyols, methacrylic acid esters of polyols and allyl ethers of polyols each of which has hydroxy] groups substituted with at least two substituents. Examples of the above polyols include ethylene glycol, propylene glycol, polyoxyethylene glycol, polyoxypropylene glycol, glycerin, polyglycerin, trimethylolpropane, pentaerythritol, sucrose, sorbitol and the like. Additional examples of crosslinkers can include divinylbenzene, N,N'-methylenebis(acrylamide), N,N'- ethylenebis(acrylamide), N,N'-propylenebis(acrylamide), N,N'-butamethylenebis(acrylamide), N,N'-diallylacrylamide, N,N'-hexamethylenebisacrylamide, trially lisocyanurate, 1,4- diacryloylpiperazine-l,l,l-trimethylolpropane diallyl ether, triethylene glycol divinyl ether, diallyl maleate, bis(acryloylamido)methane, ethyleneglycoldimethacrylate, diethyleneglycoldimethacrylate, 3-(acryloyloxy)-2-hydroxypropyl methacrylate, 1 ,6-hexanediol diacrylate, tripropylene glycol diacrylate, diallyl phthalate, triallyl phosphate, allyl methacrylate, tetraallyloxyethane, triallyl cyanurate, di vinyl adipate, vinyl crotonate, 1,5-hexadiene, allyl glycidyl ether, and pentaerythritol tetraallyl ether.

[0034] The curable composition can optionally further comprise a suitable solvent. When present, the solvent is selected to sufficiently dissolve or disperse the components of the curable composition. Suitable solvents can be selected based on the selected components and guided by the present disclosure. In an aspect, the curable composition does not include a solvent.

[0035] Advantageously, the photoinitiating compounds according to Formulas (I) or (II) can enable efficient photoinitiation at low concentration of photoinitiator. For example, in an aspect, the curable composition can comprise less than 5 weight percent, or less than 2 weight percent, or less than 1 weight percent, or less than or equal to 0.6 weight percent, or greater than 0 to less than 5 weight percent, or 0.01 to 2 weight percent, or 0.01 to 1 weight percent, or 0.01 to 0.6 weight percent, or 0.1 to 5 weight percent, or 0.1 to 2 weight percent, or 0.1 to 1 weight percent, or 0.1 to 0.6 weight percent of the compound according to Formula (I) or (II), each based on the total weight of a curable composition (e.g., the total weight of the photoinitiator and the ethylenically unsaturated compound).

[0036] In another advantageous feature, the present inventors have discovered that the compounds according to Formula (I) or Formula (II) can act as Type I photoinitiators, meaning that no additional components are needed to initiate the chemical reaction (e.g., a co-catalyst). Accordingly, the curable composition can exclude a cocatalyst (also referred to as a co-initiator or a photosensitizer). Exemplary co-catalysts that can be excluded from the composition can include a tertiary amine, an iodonium salt, a borate salt, or a combination thereof.

[0037] The curable composition can optionally further exclude other radical-generating initiators. For example, the curable composition can exclude an initiator compound such as aromatic carbonyl compounds (e.g., benzophenone, benzoin methyl ether, benzoin isopropyl ether, benzyl, xanthone, thioxanthone and anthraquinone); acetophenones (e.g., acetophenone, propiophenone, a-hydroxy isobutylphenone, a, a’ -dichloro-4-phenoxy acetophenone, 1 -hydroxy - 1 -cyclohexylacetophenone and diacetylacetophenone); organic peroxides (e.g., benzoyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butyl hydroperoxide, di-t- butyldiperoxybutyldiperoxyisophthalate and 3,3',4,4-tetra(t- butylperoxycarbonyl)benzophenone); diphenylhalonium salts (e.g., diphenyl iodobromide and diphenyl iodochloride) organic halides (e.g., carbon tetrabromide, chloroform and iodoform); heterocyclic and polycyclic compounds (e.g., 3-phenyl-5-isooxazorone and 2,4,6- tris(trichloromethyl)-l,3,5-triazinebenzanthrone); azo compounds (e.g., 2,2'-azo(2,4- dimethylvaleronitrile), 2,2-azobisisobutyro nitrile, l,l'-azobis(cyclohexane-l-carbonitrile) and 2,2’-azobis(2-methylbutyronitrile)); iron arene complexes; titanocene compounds; bisimidazole compounds; N-arylglycidyl compounds, acridine compounds, combinations of aromatic ketones and aromatic amines, and peroxyketals.

[0038] The curable composition can be used to provide a corresponding cured composition, for example by photoinitiating a chemical reaction of the ethylenically unsaturated compound and optionally the crosslinker in the presence of a suitable wavelength of light. The wavelength of light can be selected based on the chemical structure of the compound according to Formula (I) or (II), guided by the present disclosure. [0039] Accordingly, another aspect of the present disclosure is a process for converting a reactant to a reaction product. In an aspect, the reactant can comprise the aforementioned curable composition (i.e., comprising the ethylenically unsaturated compound, the compound having a structure according to Formula (I) or Formula (II) as a photoinitiator, and optionally a crosslinker). In an aspect, when the reactant comprises the curable composition, the reaction product comprises a polymer comprising repeating units derived from the ethylenically unsaturated compound. When a crosslinker is present, it will be understood that the product will be a crosslinked polymer product.

[0040] The method comprises exposing the reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide the reaction product. The intensity of the irradiated light is not particularly limited and can be, for example, 0.5 to 500 mW/cm². The present inventors have discovered that used of the compound according to Formula (I) or Formula (II) as a photoinitiator can enable the use of lower light intensities, which can be advantageous for certain applications. For example, the reaction mixture can be exposed to the light at an intensity of 50 mW/cm² or less. For example, within this range, the light intensity can be 1 to 50 mW/cm², or 1 to 40 mW/cm², or 1 to 30 mW/cm², or 1 to 20 mW/cm².

[0041] The temperature of the reaction is not particularly limited as long as the reaction proceeds. For example, the method can be conducted at a temperature of 0 to 100 °C, for example 10 to 60 °C, or 20 to 40 °C.

[0042] The irradiation time can also depend of the light source, the intensity, and the chemical identities of the photoinitiator and the ethylenically unsaturated compound. In an advantageous feature, the irradiation time can be short, for example 5 minutes or less, or 1 minute or less, or 30 seconds or less, or 1 to 60 seconds, or 1 to 30 seconds, or 1 to 15 seconds. Such timescales (e.g., 15 seconds or less) can be particularly useful for applications of the disclosed method to lithography or 3D printing (additive manufacturing).

[0043] Advantageously, the compound according to Formula (I) or Formula (II) can act as a Type I photoinitiator, wherein no other compounds are needed to initiate the chemical reaction upon exposure to light of the desired wavelength. Accordingly, in an aspect, the reaction mixtures defined herein can exclude any additional initiating compounds such as a cocatalyst or a photosensitizer. For example, the reaction mixture can exclude a co-catalyst comprising a tertiary amine, an iodonium salt, a borate salt, or a combination thereof. Other initiators or co-catalyst that are generally known in the art may also be excluded from the reaction mixtures and curable compositions of the present disclosure. [0044] This disclosure is further illustrated by the following examples, which are nonlimiting.

EXAMPLES

[0045] A series of BODIPY photoinitiators (PI) were designed to increase the excited- state lifetime (halogenation) and potentially result in bond homolysis (boron methylation), while maintaining ease of synthetic accessibility. A synthetic scheme is provided in FIG. 1. To start, meyo-methylacetate BODIPY was synthesized from 2,4-dimethylpyrrole with 2-chloro-2- oxoethyl-acetate, followed by reaction with boron trifluoride etherate. Subsequent hydrolysis of acetate with sodium hydroxide gave BODIPY -F-H. Alternatively, boron methylation was accomplished by treating /7/c.vo-methyl acetate BODIPY with methyl magnesium bromide, which simultaneously substituted the fluorine atoms and cleaved the ester to provide an alcohol, yielding BODIPY-Me-H. Bromination of the BODIPY-F-H and -Me-H derivatives was accomplished in an analogous manner using A-bromosuccinimide (NBS) to provide BODIPY-F- Br and -Me-Br, respectively. Overall, the synthetic procedure was modular and only required three facile steps, making it more accessible relative to those for alternative visible light Pls, such as acylgermanes (e.g., Ivocerin).

[0046] Upon isolation, it was immediately evident that each compound had a distinct absorption profile, as they were visually different colors in solution (FIG. 2, inset). To ultimately assess the reactivity of each BODIPY derivative, it was deemed necessary to characterize molar absorptivity (e), which was accomplished using UV-vis absorption spectroscopy (FIG. 2). Values were comparable in both dilute solution (5 pM in CH3CN, 1 cm pathlength, FIG. 2) and in a representative resin formulation described later (0.1 mol% in isobomyl acrylate, 25 pm pathlength). From this analysis, the 71-71* absorption bands for all derivatives were found to peak between 500 and 550 nm, with s values in excess of 50,000 M ¹ cm ¹. Notably, methylation resulted in a —5-10 nm hypsochromic (blue-)shift, while bromination resulted in a -20-25 nm bathochromic (red-)shift. Absorption and emission data for BODIPY Pls is shown in Table 1. Table 1

2abs = peak absorption wavelength; A _lll = peak emission wavelength; s= molar absorptivity (extinction coefficient);

<Pt = fluorescence quantum yield; if = fluorescence excited state lifetime; triplet excited state lifetime [0047] Subsequently, the spectral emission profile for blue (470 nm), green (530 nm), and orange (590 nm) LEDs used herein were characterized to determine the extent of overlap with the different BODIPY absorption profiles. Quantification of the relative number of LED photons absorbed ( ABS) was accomplished by normalizing to the derivative with the greatest amount of overlap, BODIPY-F-Br. This provided \BS values of 0.73, 0.66, 0.87, and, 1.0 for BODIPY-Me-H, -F-H, -Me-Br, and -F-Br, respectively.

[0048] To place these derivatives into context they were directly compared with state-of- the-art industrial Type I Pls. Specifically, UV-vis absorption spectra were collected for BAPO, Ivocerin, and Irgacure™ 784 and overlaid with the same blue and green LEDs (FIG. 2). Owing to the “forbidden” n-7i* transition that results in radical formation among these Pls, their So to Si values in the visible region were >40x lower compared to the BODIPYs. Considering the blue LED with only the commercial Pls, BAPO, Ivocerin, and Irgacure™ 784 had ABS values of 0.05, 0.1, and 1.0, respectively. However, in looking at the green LED, BAPO had negligible absorption, and relative to BODIPY-F-Br, Ivocerin and Irgacure™ 784 had <#ABS values of <0.001 and 0.013, respectively. Thus, BODIPY-F-Br absorbed >1000x and 77x more green LED photons relative to Ivocerin and Irgacure™ 784, respectively. Therefore, based solely on absorption (i.e., holding reaction quantum yields equal) one would expect significantly greater radical formation and concomitantly faster rates of polymerization for a Type I BODIPY PI. The total photon absorption by each PI under the irradiation of blue or green LEDs (4 mW/cm²) is shown in Table 2.

Table 2

[0049] The utility of different BODIPY derivatives as Type I Pls to induce free -radical polymerizations was examined using real-time Fourier transform infrared (RT-FTIR) spectroscopy in an attenuated total reflection (ATR) mode, as shown in FIG. 3. Isobornyl acrylate (IBOA) was selected as a model system, owing to its biorenewable sourcing and low volatility. Monomer to polymer conversion was determined by monitoring the peak at -770 cm'¹ (C=C vinylic stretch) during LED irradiation. Initial experiments were performed with 0.3 mol% BODIPY and a green LED intensity of 4 mW/cnr, which are conditions relevant to contemporary projection-based 3D printing (e.g., digital light processing and liquid crystal display). Fully formed resins were purged with an inert gas (e.g., argon) to remove oxygen as a potential radical and triplet excited state scavenger. Under these conditions, the methylated BODIPY derivatives resulted in the fastest polymerization rates (r_p), with BODIPY-Me-Br being ~16x faster than BODIPY-Me-H, as shown in FIG. 4. Specifically, r_p values were 0.007 M/s (0.1 % C=C conversion/s), 0.017 M/s (0.3 %/s), and 0.33 M/s (6.5 %/s) for BODIPY-F-Br, -Me-H, and -Me-Br, respectively (no polymerization was observed for BODIPY-F-H). Versatility in monomer scope was demonstrated by polymerizing isobornyl methacrylate, 2- hydroxyethyl acrylate, and carbitol acrylate. All polymerizable resins showed an excellent temporal response and good stability, as no measurable conversion occurred in the dark, corresponding to the first 15 seconds of RT-FTIR spectroscopy measurements. After turning the LED on, the resin containing BODIPY-Me-Br as the PI reached maximum monomer conversion in 10 seconds (e.g., 80% for neat IBOA due to vitrification).

[0050] Examining BODIPY-Me-Br PI further revealed that rapid green light- induced polymerizations to maximum monomer conversion were possible using a concentration of 0. 1 mol% (3x lower than previous). Under this condition, an r_p of 0.18 M/s (3.6 %/s) and a time to 50% conversion (tso) of 11 s (relative to 7 s for 0.3 mol%) were observed. Note that tso values are provided as a simple benchmark for comparison. Additionally, increasing the light intensity from 4 mW/cnr to 15 mW/cm² resulted in an r_p of 0.58 M/s (12 %/s) and a tso of 4 s, making it nearly 2x faster. Finally, an orange LED centered at 590 nm was employed at 15 mW/cm², which resulted in an r_p of 0.018 M/s (0.4 %/s), comparable to that measured for BODIPY-Me-H with the green LED at 4 mW/cm². Notably, under equimolar conditions of BODIPY-Me-Br, the r_p was ~30x lower in going from green to orange light irradiation (both at 15 mW/cm²), yet the relative number of orange photons absorbed was nearly lOOOOx lower. This suggests that the photopolymerization efficiency is considerably higher under the orange light conditions, which, without wishing to be bound by theory, could arise from competitive side-reactions that occur at higher excited-state and/or radical densities.

[0051] The BODIPY Pls were benchmarked against commercially available Pls, Ivocerin and Irgacure™ 784, which have been reported to operate under blue and green light irradiation. Using equimolar quantities of PI (0.3 mol%) in IBOA, samples were irradiated with 4 or 40 mW/cm² blue and green LEDs, again monitoring with RT-FTIR spectroscopy. Under green light irradiation, only the sample containing Ivocerin exposed at an intensity of 40 mW/cm² displayed an appreciable r_p of 0.05 M/s (1.0 %/s) and a tso of 45 s, despite having a </ ABS of 0.07 relative to Irgacure™ 784 (i.e., absorbing 14x fewer green LED photons). Irradiating instead with a blue LED revealed again only polymerizations occurring with Ivocerin and not Irgacure under the conditions and timeframe (60 s) examined. With a blue LED intensity of 4 mW/cm², the samples containing Ivocerin had an r_p of 0. 11 M/s (2.2 %/s) and tso of 22 s. Increasing the intensity to 40 mW/cm² resulted in an r_p of 0.53 M/s (11 %/s) and tso of 4 s, which was comparable to that observed for BODIPY-Me-Br irradiated with 15 mW/cm² using a green LED. Overall, at equivalent concentrations the BODIPY Pls significantly outperformed current visible light Pls using lower energy and intensity LEDs.

[0052] It was hypothesized that free radical polymerization was caused by light-induced homolytic P-scission at the boron-carbon bond (B-CH3) for the methylated BODIPYs. Additionally, the small extent of polymerization observed for resins with BODIPY-F-Br was rationalized by inefficient homolytic P-scission at the meso-methyl C-0 bond. A series of spectroscopic and computational characterization techniques were employed to determine the mechanism. First, electron paramagnetic resonance (EPR) spectroscopy was used with spintrapping experiments to confirm the radical character and associated chemical composition of the fragmented products. The spin-trapping agent TV-tert-butyl-a-phenylnitrone (PBN, 10 mM) was dissolved together with BODIPY-Me-Br (5 mM) in CH2CI2, and the mixture was irradiated with the green LED for 5 minutes. Characterization of the resulting mixture by EPR spectroscopy revealed one predominant PBN-adduct with hyperfine coupling constants characteristic of methylation (aN = 15.25, an = 3.72), and in agreement with simulations, as shown in FIG. 5. This suggested that methyl radicals were being formed. Additionally, size exclusion chromatography on poly(IBOA) samples prepared using BODIPY-Me-Br had a clear UV absorption (350 nm) signal, indicating boryl-radical initiation given the lack of other 350 nm absorbing components in the initial mixture. Further evidence of P-scission at the B-C bond was obtained using nuclear magnetic resonance (NMR) spectroscopy and liquid chromatographymass spectrometry (LC-MS) characterization, which revealed formation of B-OCH3 and B-OH adducts when irradiating the BODIPY PI in methanol and water, respectively.

[0053] In order for the purported B-C P-scission to occur, the bond dissociation enthalpy (BDE) would need to be lower than the input energy, which for 530 nm photons is ~2.4 eV. Notably, this matches both the spectral (shown in FIG. 2 (bottom)) and electrochemical energy gaps for the BODIPY Pls, shown in Table 3. The electrochemical properties of BODIPY Pls shown in Table 3 were determined using cyclic voltammetry (CV). Table 3 [0054] Using density functional theory, BDE values for the different BODIPY derivatives and Type I PI benchmarks were estimated. While the classic BODIPY structures bearing B-F bonds showed a high BDE of ~5 eV, replacing the fluorine atoms with methyl groups resulted in a dramatic decrease in BDE to ~ 1.7 eV, irrespective of the presence or absence of bromine, as shown in FIG. 6. This significant decrease in BDE was also supported by the larger bond length for B-C (1.63 A) relative to B-F (1.39 A) (FIG. 6, inset). Additionally, the BDE values for the methylated BODIPY Pls were found to be lower in comparison to the benchmarks, Ivocerin and Irgacure™ 784, which were 2.5 and 2.1 eV, respectively. Despite this low value, the BODIPY Pls were qualitatively easy to handle on the bench without.

[0055] The role of bromine in the observed photopolymerization rate enhancement was explored. This was accomplished by characterizing excited state dynamics using a combination of fluorescence and transient absorption spectroscopies (FIG. 7). Initially, fluorescence quantum yield ( i) values were determined using Rhodamine 6G as a standard, finding that BODIPY-F- H was considerably more fluorescent = 0.95) relative to BODIPY-Me-H, -F-Br, and -Me-Br; </>< = 0.39 ± 0.06, 0.21 ± 0.05, and 0.09 ± 0.02, respectively (FIG. 7, inset). Transient absorption spectroscopy was then employed to examine the non-radiative excited-state dynamics, which revealed long-lived populations that were attributed to spin-triplet states, as shown in FIG. 7. Specifically, the excited state lifetime (r) values for BODIPY-Me-H, -F-Br, and -Me-Br were found to be 4.3, 7.0, and 8.1 ps (FIG. 7, inset). Notably, T is ~1000x longer for the triplet excited states relative to the corresponding singlet excited states (~l-6 ns), as characterized using time- correlated single photon counting fluorescence spectroscopy. Based on this result, the competitive non-radiative decay pathway was attributed to intersystem crossing, which would indicate that triplet yield increases in going from BODIPY-Me-H, -F-Br, and -Me-Br.

[0056] As a final proof-of-concept relevant to photocurable technologies (e.g., 3D printing), BODIPY -Me-Br was employed as a PI to rapidly prepare a polymer network. A resin comprising 50 wt% IBOA (monomer), 50 wt% trimethylolpropane triacrylate (crosslinker), and the BODIPY PI (0.3 mol%) was placed into a 100 pm gap between glass slides, followed by irradiation with the green LED under ambient conditions (~5 min.). Separating the glass slides provided a pink plastic film with a visible light transmittance (T_vis) of 66%. As shown in FIG. 8, the color from the film was almost completely removed (bleached) upon extended irradiation with green light (T_ViS = 80%), providing an avenue to access colorless and transparent plastics using low energy green light.

[0057] In summary, BODIPY-Me-X represents a novel, efficient visible light photoinitiator platform to induce rapid radical polymerizations relevant to the photochemistry community, with particularly pertinent applications in photocurables (e.g., coatings, adhesives, (stereo)lithography, etc.). Based on results presented herein, it is postulated that B-C P-scission in alkylated-BODIPY derivatives is a general phenomenon, and thus has exciting potential for improvement owing to the modularity of this chromophore. For example, improvements in radical generation efficiency may be possible through further tailoring the composition of the “X” and “Z” groups presented, while 71-extended BODIPYs have the potential to enable longer wavelength, red light (~1.9 eV) photoinitiation via a Type I mechanism. As a result, it is anticipated that these derivatives will enable applications in the biomedical and advanced manufacturing arenas where selectively absorbed, benign visible light is paramount, such as in the development of next generation cell-laden tissue scaffolds for disease modeling and multimaterial 3D printing to create soft actuators and stretchable electronics. A significant improvement in visible light photoinitiator platforms is therefore provided by the present disclosure.

[0058] Experimental details follow.

[0059] All reagents were used as received unless otherwise noted. Pyridine (99.0%) was purchased from Acros Organics. 4-(Dimethylamino)pyridine (99%) was purchased from Apollo Scientific. 2,4-Dimethyl- 1 W-pyrrole (98%) was purchased from Ark Pharm. N- Bromosuccinimide (>99%) was purchased from Chem-Impex. 4-Nitrophenyl chloroformate (97%) was purchased from Combi-Blocks. Sodium hydroxide (98%) and methylmagnesium bromide 3 M in diethyl ether were purchased from Fischer Scientific. Triethylamine (99.5%) was purchased from Oakwood Chemical. Boron trifluoride diethyl etherate (46%), and isobomyl methacrylate (85%) were purchased from Sigma- Aldrich. 2-Chloro-2-oxoethylacetate (>97%), isobomyl acrylate (90%), carbitol acrylate (>98%), 2-hydroxyethyl acrylate (>95%) and N,N- diisopropylethylamine (>99%) were purchased from TCI Chemicals.

[0060] Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectra were recorded on an Agilent MR 400 MHz spectrometer utilizing CDCh and DMSO-F6 as solvents. ^]H NMR were carried out coupled and referenced to the CDCh chemical shift at 7.26 ppm and DMSO-r/6 at 2.05 ppm. ¹³C NMR were carried out decoupled and referenced to the CDCI3 chemical shift at 77.16 ppm and DMSO-<76 at 39.52 ppm.

[0061] High Resolution Mass Spectrometry (HRMS): HRMS was performed on an Agilent Technologies 6530 Accurate-Mass Q-TOF LC/MS using ESI and the data was subsequently analyzed using Agilent MassHunter Qualitative Analysis Software.

[0062] UV-Vis Spectroscopy: UV-vis spectra were obtained using a system from Ocean Insight. Specifically, the system utilized a balanced deuterium- tungsten halogen light source (DH- 2000-BAL) with a typical output of 194 pW (deuterium bulb) and 615 pW (tungsten bulb) through an SMA 905 connector, covering a range from 230 nm - 2.5 pm. Multimode fiber-optic cables with SMA connectors on both ends and a 600 pm core diameter (QP600-025-SR) connected the light source to the sample holder. For dilute solution measurements a qpod cuvette holder (QNW qpod 2e™) capable of magnetic stirring and Peltier-driven temperature control from -30 °C to 105 °C was used. The sample holder was coupled through another multimode fiber to the spectrometer (QEPRO-ABS) having an entrance slit of 5 pm (INTSMA-005 Interchangeable Slit). The spectrometer measured in the range from 200-950 nm, at an optical resolution of 1.7 nm, using a back-thinned, TE cooled, 1024 x 58 element CCD array.

[0063] Fluorescence Spectroscopy: Photoluminescence data was recorded using a Fluorolog3 Fluorimeter, which was equipped with a 450 W ozone-free Xenon arc lamp as the light source and a photomultiplier tube with spectral resolution of 0.1 nm and detection range from 250 to 1050 nm. Time correlated single photon counting (TCSPC) experiments were also conducted with the Fluorolog3 Fluorimeter. The Xenon lamp was replaced with NanoLEDs (482 nm) with time resolution of 300 ps.

[0064] Nanosecond Transient Absorption Spectroscopy: The nanosecond transient absorption measurements were conducted using the enVISion™ system from MAGNITUDE INSTRUMENTS™. The spectrometer system can measure transient absorption spectra and photoluminescence spectra. The instrument can detect wavelengths from UV to visible to near-IR with a time resolution ~5 ns. The population decay was fitted using the equation shown as follows, M(t) = N_oe ^t/T where No is the initial population, and T is the lifetime.

[0065] Real-Time Fourier-Transform Infrared (RT-FTIR) Spectroscopy: The RT-FTIR measurements were conducted using an INVENIO-R FT-IR spectrophotometer manufactured by Bruker and were controlled by the OPUS Spectroscopy Software. The detection of spectra was performed using a liquid nitrogen cooled (LN-MCT Mid) detector. To investigate the chemical composition and monitor the photocuring of liquid resins upon exposure to light, a modified GladiATR Illuminate ATR accessory provided by PIKE Technologies was utilized. The visible light exposure was performed using a collimated LED light sources (530 nm-P/N LCS-0530-15- 22) from Mightex Systems, together with Lightguide Adapters. The LED Controller M/N SLC- MA02-U was employed, and a 3 mm liquid light guide (LLG-3- 4H) was used in the experiments.

[0066] Cyclic Voltammetry (CV): The cyclic voltammetry (CV) measurements were carried out in an Ni-filled glovebox using the CHI 660D Electrochemistry Workstation. The electrochemical cell utilized was a single-compartment three-electrode configuration, consisting of glassy carbon as the working electrode, a platinum wire as the counter electrode, and an Ag/AgNOs (0.01 M) nonaqueous reference electrode, which was calibrated against Fc/Fc⁺ in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPFe) acetonitrile solutions (E!/2(Fc/Fc⁺) = 0.1 V vs Ag/Ag⁺), with a scan rate of 0. 1 V/s. The oxidation onsets were used to calculate the highest occupied molecular orbital (HOMO) energy levels as -(4.8 eV - EOXFC + E'ox), while the reduction onsets were employed to compute the lowest unoccupied molecular orbital (LUMO) energy levels as -(4.8 eV - E_OXFC + E d).

[0067] Theoretical Calculations: The computational studies were done following our previously reported procedure. All calculations were carried out using Spartan ‘20, Wavefunction, Inc. All molecular geometries were optimized in gas phase with 6-311 + G (2DF, 2P) basis set and B3LYP method. The energies were calculated with DEF2-TZVP basis set and CAM-B3LYP method.

[0068] Electron Paramagnetic Resonance (EPR) Spectroscopy: The X-band EPR spectra was recorded using Broker EMX + CW EPR Spectrometer. The instrument includes standard, high sensitivity, and dual mode EPR spectrometers, as well as a CW-ENDOR EPR spectrometer and an optical transmission EPR spectrometer. In addition, the instrument includes temperature control systems that enable low and high temperature studies, and a range of accessories such as a programmable 1-axis goniometer, an AquaX aqueous mixing solution cell, and a marker assembly. This instrument is designed to be versatile and flexible, accommodating a wide range of samples and applications in the field of EPR spectroscopy.

[0069] Light Emitting Diodes (LEDs): The LED emission profiles were collected using a spectrophotometer (QEPro, Ocean Insight), connected to an optical fiber (QP600-025-SR). Neutral density filters were used to reduce the emission intensity from the LEDs so as to avoid saturating the detector.

[0070] A chemical scheme illustrating the syntheses described below is shown in FIG. 9.

[0071] Synthesis of 5,5-difluoro-l,3,7,9-tetramethyl-5H-4 2. ^,5 A 4-dipyrrolo[l,2-c:2',l'- f][l,3,2]diazaborinin-10-yl)methyl acetate (“Compound 1”). Compound 1 was synthesized following a modified protocol from literature. To a two-neck roundbottom flask equipped with a magnetic stir bar, inlet adapter, septum, and condenser was added 2, 4-Di methyl- 1 //-pyrrole (4 grams (g), 40 millimole (mmol), 2 equivalents (eq.)) and 2-chloro-2-oxoethyl acetate (3 g, 20 mmol, 1 eq.) under a nitrogen atmosphere. The mixture was then dissolved in dry dichloromethane (100 mL) and heated to reflux (40 °C) and stirred for 2 hours. After refluxing for 2 hours, the solution was cooled in an ice bath to ~0 °C followed by adding N,N- diisopropylethylamine (DIPEA, 10 g, 80 mmol, 20 mL, 4 eq) and boron trifluoride etherate (30 g, 200 mmol, 10 eq.) sequentially. The solution was then stirred for 30 minutes at 25 °C. The solution was then washed with water (3x) and brine (lx). The organic phase was dried over MgSCL. Dichloromethane was then removed under reduced pressure and the crude product was purified by column chromatography with a mixture of hexanes and diethyl ether (gradient, v:v = 0:1 to 2: 1 , /?i = 0.5 with hexanes and diethyl ether v:v = 1 :1), providing the desired product as an orange solid. Yield: 65%. ^:H NMR: (400 MHz, CDCh) 5 6.07 (s, 2H), 5.07 (d, J= 1.1 Hz, 2H), 2.51 (s, 6H), 2.34 (s, 6H), 2.12 (d, 7= 1.1 Hz, 3H). ¹³C NMR: (126 MHz, CDC1₃) 6 170.62, 156.65, 141.48, 133.28, 132.66, 122.34, 77.29, 77.04, 76.78, 57.89, 29.73, 20.63, 15.66, 14.74, 14.72, 14.70, 1.05. HRMS (ESI): exact mass calculated for C16H19BF2N2O2 [M+Na]⁺ is 343.14, found [M+Na]⁺ = 343.14.

[0072] Synthesis of BODIPY-F-H (5,5-difluoro-l,3,7,9-tetramethyl-5H-4 ⁴,5A⁴- dipyrrolo[l,2-c:2',l'-f][l,3,2]diazaborinin-10-yl)methanol: To a one-neck roundbottom flask equipped with a magnetic stir bar, inlet adapter and septum was added 100 mL of methanol, followed by compound 1 (1 g, 3 mmol, 1 eq.) dissolved in 50 mL of dichloromethane. Next, 5 mL of IM NaOH(aq) was added to the solution at room temperature and the mixture was stirred for 3 hours in air at which point compound 1 appeared fully consumed by thin layer chromatography (Ri ~ 0.5 with hexanes and diethyl ether = 1:1). The solvent was then removed under reduced pressure, and the crude product was dissolved in dichloromethane and placed into a separatory funnel. The organic phase was washed with brine (3x), dried over MgSCL. filtered, and dried under reduced pressure. The crude product was then purified by silica gel column chromatography with a gradient of hexanes and diethyl ether (v:v = 6:1 to 1:1), followed by concentrating the desired band (Rf = 0.3, dichloromethane on TLC) under reduced pressure to provide the product as an orange solid. Yield: 50%. ¹ H NMR: (400 MHz, CDCI3) 5 6.07 (s, 2H), 4.89 (d, 7 = 5.2 Hz, 2H), 2.50 (d, 7 = 7.4 Hz, 12H). ¹³C NMR: (126 MHz, CDCI3) 5 156.28, 122.12, 77.29, 77.25, 77.04, 76.78, 55.92, 15.63, 14.70. HRMS (ESI): exact mass calculated for C14H17BF2N2O [M+Na]⁺ is 301.13, found [M+Na]⁺ = 301.13.

[0073] Synthesis of BODIPY-F-Br (2,8-dibromo-5,5-difluoro-l,3,7,9-tetramethyl-5H- 414,514-dipyrrolo[l,2-c:2',l'-f][l,3,2]diazaborinin-10-yl)methanol: To a one-neck roundbottom flask equipped with a magnetic stir bar, inlet adapter and septum was added BODIPY-F-H (200 mg, 0.46 mmol, 1 eq.) followed by dichloromethane (50 mL). While stirring at room temperature in air, N-bromosuccinimide (200 mg, 1.15 mmol, 2.5 eq.) pre-dissolved in 20 mL of dichloromethane was added dropwise to the flask. The reaction mixture was then stirred for 30 minutes at room temperature, noting complete consumption of starting material by thin layer chromatography (Rf = 0.1 with dichlorome thane). The dichloromethane was removed under reduced pressure, and the crude product was purified with silica gel column chromatography using a gradient of hexanes: dichloromethane (v:v = 1:0 to 0:1, Rf = 0.2 with dichloromethane), followed by dichloromethane: methanol (0.95:0.05). Concentrating the desired band provided the desired product as a brown solid. Yield: 45%. ^JH NMR: (400 MHz, CDCI3) 5 4.91 (s, 2H), 2.56 (s, 6H), 2.51 (s, 6H). ¹³C NMR: (126 MHz, DMSO-d6) 8 153.23, 142.40, 140.10, 131.39, 111.84, 54.88, 40.46, 40.29, 40.12, 39.96, 39.79, 39.62, 39.45, 30.06, 29.99, 14.82, 14.03, 14.00. HRMS (ESI): exact mass calculated for CMHISBB^FZNZO [M+Na]⁺ is 456.94, found [M+Na]⁺ = 456.95.

[0074] Synthesis of BODIPY-Me-H (l,3,5,5,7,9-hexamethyl-5H-4 A ⁴,5 A ⁴- dipyrrolo[l,2-c:2',l'-f][l,3,2]diazaborinin-10-yl)methanol: To a one-neck roundbottom flask equipped with a magnetic stir bar, inlet adapter and septum was added compound 1 (1 g, 3.6 mmol, 1 eq.) under a nitrogen atmosphere followed by anhydrous dichloromethane (100 mL). The mixture was placed in an ice bath to cool to 0 °C, followed by adding methylmagnesium bromide (3 M in diethyl ether, 12 mL, 72 mmol, 10 eq.) dropwise by syringe. After addition, the unreacted methylmagnesium bromide was quenched with dropwise addition of IM NH4Cl(_aq) until bubble formation ceased. The organic and aqueous phases were separated. The aqueous phase was washed with dichloromethane (3x). The combined organic phase was dried with MgSO4, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography using a gradient of hexanes: diethyl ether (v;v = 6: 1 to 3: 1). The desired band was collected (Rf = 0.6, dichloromethane) and concentrated under reduced pressure to provide the product as an orange solid. Yield: 60%. ¹ H NMR (400 MHz, CDCh) 8 6.07 (s, 2H), 4.89 (s, 2H), 2.52 - 2.46 (d, 12H), 0.05 (s, 6H). ¹ C NMR: (126 MHz, CDCh) 8 152.86, 138.23, 136.99, 130.58, 122.53, 56.42, 20.63, 16.59, 15.95. HRMS (ESI): exact mass calculated for C16H23BN2O [M+H]⁺ is 271.19, found [M+H]⁺ =271.20.

[0075] Synthesis of BODIPY-Me-Br (2,8-dibromo-l,3,5,5,7,9-hexamethyl-5H-414,514- dipyrrolo[l,2-c:2',T-f][l,3,2]diazaborinin-10-yl)methanol: To a one-neck roundbottom flask equipped with a magnetic stir bar, inlet adapter and septum was added BODIPY-Me-H (200 mg, 0.46 mmol, 1 eq.), followed by anhydrous dichloromethane (50 mL). While stirring at room temperature in air, A-bromosuccinimide (200 mg, 1.15 mmol, 2.5 eq.) pre-dissolved in 20 mL dichloromethane and was added to the flask dropwise. The reaction mixture was then stirred for 30 minutes at room temperature, noting complete consumption of starting material by thin layer chromatography (Rf = 0.6, dichloromethane). The dichloromethane was removed under reduced pressure, and the crude product was purified with silica gel column chromatography using hexanes:diethyl ether (2:1, Rf = 0.5) to provide the desired product as a deep red solid after concentration. Yield: 56%. ^!H NMR: (400 MHz, DMSO-c/6) 8 5.62 (s, 1H), 4.75 (s, 2H), 2.50 (s, 6H), 2.44 (s, 6H), 0.12 (s, 6H). ¹³C NMR: (126 MHz, MeOD-J4) 5 149.91, 139.77, 135.07, 129.90, 111.38, 55.23, 14.67, 13.56. HRMS (ESI): exact mass calculated for Ci₆H₂iBBr₂N₂O [M+H]⁺ is 427.01, found [M+H]⁺ =429.02.

[0076] This disclosure further encompasses the following aspects.

[0077] Aspect 1: A compound having a structure according to Formula (I) or Formula

(II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted Ce-₂o aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce-₂o aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce-₂o aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur; provided that when each occurrence of X² is I, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl, R² is not a -CH₂OH group, a -CH₂OCH₂Ph group, or a group having the structure when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl, R² is not a -CH₂Br group, a -CH₂C1 group, a phenyl group, a p-tolyl

*— Q group, a -CH₂OCH₂Ph group, or a group having the structure • _anc| _when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴is hydrogen, R² is not a phenyl group or a 2,4,6-trimethyl phenyl group. [0078] Aspect 2: The compound of aspect 1, wherein X¹ is B.

[0079] Aspect 3: The compound of aspect 1 or 2, wherein Z is carbon.

[0080] Aspect 4: The compound of any of aspects 1 to 3, having a structure according to Formula I, wherein each occurrence of X² is Br, Cl, or hydrogen, preferably Br or hydrogen, more preferably Br.

[0081] Aspect 5: The compound of any of aspects 1 to 4, wherein each occurrence of R¹ is a substituted or unsubstituted Ci-6 alkyl group.

[0082] Aspect 6: The compound of any of aspects 1 to 4, wherein at least one occurrence of R¹ is a substituted or unsubstituted Ci-6 alkyl group substituted with one or more in-chain or pendent sulfur atoms, preferably wherein each occurrence of R¹ is a Ci-6 alkyl thioether group.

[0083] Aspect 7: The compound of any of aspects 1 to 6, wherein R² is a substituted or unsubstituted Ci-6 alkyl group, preferably a halogenated Ci-6 alkyl group or a hydroxylsubstituted Ci-6 alkyl group.

[0084] Aspect 8: The compound of any of aspects 1 to 7, having a structure according to Formula I, wherein each occurrence of R³ and R⁴ is an unsubstituted Ci-6 alkyl group, preferably a methyl group.

[0085] Aspect 9: The compound of any of aspects 1 to 7, having a structure according to Formula II, wherein Ar is a substituted or unsubstituted fused aryl group substituted with one or more in-chain heteroatoms selected from the group consisting of oxygen and sulfur.

[0086] Aspect 10: The compound of any of aspects 1 or 3 to 9, wherein X¹ is Ga.

[0087] Aspect 11: The compound of aspect 10, having a structure according to Formula II, wherein R¹, R², R³, and R⁴ are substituted or unsubstituted Ci-6 alkyl groups, and each occurrence of X² is Br.

[0088] Aspect 12: The compound of aspect 1, wherein the compound is:

[0089] Aspect 13: The compound of aspect 1, wherein the compound is:

[0090] Aspect 14: The compound of aspect 1 , having the structure according to Formula I, wherein X¹ is B; X² is independently at each occurrence hydrogen or Br; Z is carbon; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted Ce-i2 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group.

[0091] Aspect 15: A curable composition comprising: a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur; an ethylenically unsaturated compound; and optionally a crosslinker.

[0092] Aspect 16: A cured composition derived from the curable composition of aspect 15.

[0093] Aspect 17: A process for converting a reactant to a reaction product, the method comprising exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide the reaction product, wherein the reaction mixture comprises: the reactant; and a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur.

[0094] Aspect 18: A process for photopolymerization, the process comprising: exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide a polymer comprising repeating units derived from an ethylenically unsaturated compound, wherein the reaction mixture comprises: the ethylenically unsaturated compound; and a photoinitiator comprising a compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas, X¹ is B or Ga; X² is independently at each occurrence hydrogen, Cl, Br, or I; Z is carbon or nitrogen; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen and sulfur, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group; R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-20 aryl group; R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur.

[0095] Aspect 19: The process of aspect 17 or 18, wherein the photoinitiator comprises a compound having the structure:

[0096] Aspect 20: The process of aspect 18 or 19, wherein the reaction mixture further comprises a crosslinker.

[0097] Aspect 21: The process of any of aspects 17 to 20, wherein the reaction mixture excludes a cocatalyst, preferably wherein the reaction mixture excludes a cocatalyst comprising a tertiary amine, an iodonium salt, a borate salt, or a combination thereof.

[0098] The compositions, methods, and articles can alternatively comprise, consist of, or consist essentially of, any appropriate materials, steps, or components herein disclosed. The compositions, methods, and articles can additionally, or alternatively, be formulated so as to be devoid, or substantially free, of any materials (or species), steps, or components, that are otherwise not necessary to the achievement of the function or objectives of the compositions, methods, and articles.

[0099] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. “Combinations” is inclusive of blends, mixtures, alloys, reaction products, and the like. The terms “first,” “second,” and the like, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” and “the” do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or” unless clearly stated otherwise. Reference throughout the specification to “an aspect” means that a particular element described in connection with the aspect is included in at least one aspect described herein, and may or may not be present in other aspects. The term “combination thereof’ as used herein includes one or more of the listed elements, and is open, allowing the presence of one or more like elements not named. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various aspects. [0100] Unless specified to the contrary herein, all test standards are the most recent standard in effect as of the filing date of this application, or, if priority is claimed, the filing date of the earliest priority application in which the test standard appears.

[0101] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this application belongs. All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.

[0102] Compounds are described using standard nomenclature. For example, any position not substituted by any indicated group is understood to have its valency filled by a bond as indicated, or a hydrogen atom. A dash that is not between two letters or symbols is used to indicate a point of attachment for a substituent. For example, -CHO is attached through carbon of the carbonyl group.

[0103] As used herein, the term “hydrocarbyl”, whether used by itself, or as a prefix, suffix, or fragment of another term, refers to a residue that contains only carbon and hydrogen. The residue can be aliphatic or aromatic, straight-chain, cyclic, bicyclic, branched, saturated, or unsaturated. It can also contain combinations of aliphatic, aromatic, straight chain, cyclic, bicyclic, branched, saturated, and unsaturated hydrocarbon moieties. However, when the hydrocarbyl residue is described as substituted, it may, optionally, contain heteroatoms over and above the carbon and hydrogen members of the substituent residue. Thus, when specifically described as substituted, the hydrocarbyl residue can also contain one or more carbonyl groups, amino groups, hydroxyl groups, or the like, or it can contain heteroatoms within the backbone of the hydrocarbyl residue. The term "alkyl" means a branched or straight chain, saturated aliphatic hydrocarbon group, e.g., methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, t-butyl, n- pentyl, s-pentyl, and n- and s-hexyl. “Alkenyl” means a straight or branched chain, monovalent hydrocarbon group having at least one carbon-carbon double bond (e.g., ethenyl (-HC=CH2)). “Alkoxy” means an alkyl group that is linked via an oxygen (i.e., alkyl-O-), for example methoxy, ethoxy, and sec -butyloxy groups. "Alkylene" means a straight or branched chain, saturated, divalent aliphatic hydrocarbon group (e.g., methylene (-CH2-) or, propylene (-(CTh)?- )). “Cycloalkylene” means a divalent cyclic alkylene group, -CrJbn-x, wherein x is the number of hydrogens replaced by cyclization(s). “Cycloalkenyl” means a monovalent group having one or more rings and one or more carbon-carbon double bonds in the ring, wherein all ring members are carbon (e.g., cyclopentyl and cyclohexyl). "Aryl" means an aromatic hydrocarbon group containing the specified number of carbon atoms, such as phenyl, tropone, indanyl, or naphthyl. “Arylene” means a divalent aryl group. “Alkylarylene” means an arylene group substituted with an alkyl group. “Arylalkylene” means an alkylene group substituted with an aryl group (e.g., benzyl). The prefix "halo" means a group or compound including one more of a fluoro, chloro, bromo, or iodo substituent. A combination of different halo atoms (e.g., bromo and fluoro), or only chloro atoms can be present. The prefix “hetero” means that the compound or group includes at least one ring member that is a heteroatom (e.g., 1, 2, or 3 heteroatom(s)), wherein the heteroatom(s) is each independently N, O, S, Si, or P. “Substituted” means that the compound or group is substituted with at least one (e.g., 1, 2, 3, or 4) substituents that can each independently be a C1-9 alkoxy, a C1-9 haloalkoxy, a nitro (-NO2), a cyano (-CN), a Ci-6 alkyl sulfonyl (-S(=O)2-alkyl), a C1-6 alkyl ester (-C(=O)-O-alkyl), a Ce-12 aryl sulfonyl (-S(=O)2-aryl), a thiol (-SH), a thiocyano (-SCN), a tosyl (CH3C6H4SO2-), a C3-12 cycloalkyl, a C2-12 alkenyl, a C5-12 cycloalkenyl, a C6-12 aryl, a C7-13 arylalkylene, a C4-12 heterocycloalkyl, and a C3-12 heteroaryl instead of hydrogen, provided that the substituted atom’s normal valence is not exceeded. The number of carbon atoms indicated in a group is exclusive of any substituents. For example -CH2CH2CN is a C2 alkyl group substituted with a nitrile.

[0104] While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.

Claims

1. A compound having a structure according to Formula (I) or Formula (II) wherein in the foregoing Formulas,

X¹ is B or Ga;

X² is independently at each occurrence hydrogen, Cl, Br, or I;

Z is carbon or nitrogen;

R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group;

R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-20 aryl group;

R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group;

R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-20 aryl group; and

Ar is a substituted or unsubstituted fused aryl group optionally substituted with one or more in-chain or pendent heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur; provided that when each occurrence of X² is I, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴ is methyl, R² is not a -CH2OH group, a -CH2OCH2PI1 group, or a group having the structure when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴is methyl, R² is not a -CFFBr group, a -CH2CI group, a phenyl group, a p-tolyl group, a -CH2OCH2PI1 group, or a group having the structure when each occurrence of X² is hydrogen, each occurrence of R¹ is methyl, and each occurrence of R³ and R⁴is hydrogen, R² is not a phenyl group or a 2,4,6-trimethyl phenyl group.

2. The compound of claim 1, wherein X¹ is B.

3. The compound of claim 1, wherein Z is carbon.

4. The compound of claim 1, having a structure according to Formula (I), wherein each occurrence of X² is Br, Cl, or hydrogen, preferably Br or hydrogen, more preferably Br.

5. The compound of claim 1, wherein each occurrence of R¹ is a substituted or unsubstituted C1-6 alkyl group.

6. The compound of claim 1, wherein at least one occurrence of R¹ is a substituted or unsubstituted C1-6 alkyl group substituted with one or more in-chain or pendent sulfur atoms, preferably wherein each occurrence of R¹ is a C1-6 alkyl thioether group.

7. The compound of claim 1, wherein R² is a substituted or unsubstituted C1-6 alkyl group, preferably a halogenated C1-6 alkyl group or a hydroxyl-substituted C1-6 alkyl group.

8. The compound of claim 1, having a structure according to Formula (I), wherein each occurrence of R³ and R⁴ is an unsubstituted C1-6 alkyl group, preferably a methyl group.

9. The compound of claim 1, having a structure according to Formula (II), wherein Ar is a substituted or unsubstituted fused aryl group substituted with one or more in-chain heteroatoms selected from the group consisting of oxygen and sulfur.

10. The compound of claim 1, wherein X¹ is Ga.

11. The compound of claim 10, having a structure according to Formula (II), wherein R¹, R², R³, and R⁴ are substituted or unsubstituted C1-6 alkyl groups, and each occurrence of X² is Br.

12. The compound of claim 1, wherein the compound is:

13. The compound of claim 1, wherein the compound is:

14. The compound of claim 1, having the structure according to Formula (I), wherein X¹ is B;

X² is independently at each occurrence hydrogen or Br;

Z is carbon; R¹ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, optionally substituted with one or more in-chain or pendent sulfur atoms, optionally wherein each occurrence of R¹ can combine to form a cycloalkyl group;

R² is a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-12 aryl group;

R³ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group;

R⁴ is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group.

15. A curable composition comprising: a photoinitiator comprising the compound of claim 1 ; an ethylenically unsaturated compound; and optionally a crosslinker.

16. A cured composition derived from the curable composition of claim 15.

17. A process for converting a reactant to a reaction product, the method comprising exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide the reaction product, wherein the reaction mixture comprises: the reactant; and a photoinitiator comprising the compound of claim 1.

18. A process for photopolymerization, the process comprising: exposing a reaction mixture to light having a wavelength of 400 to 1000 nm, preferably 450 to 750 nm to provide a polymer comprising repeating units derived from an ethylenically unsaturated compound, wherein the reaction mixture comprises: the ethylenically unsaturated compound; and a photoinitiator comprising the compound of claim 1.

19. The process of claim 18, wherein the photoinitiator comprises a compound having the structure:

20. The process of claims 18, wherein the reaction mixture further comprises a crosslinker.

21. The process of claim 17, wherein the reaction mixture excludes a cocatalyst, preferably wherein the reaction mixture excludes a cocatalyst comprising a tertiary amine, an iodonium salt, a borate salt, or a combination thereof.