GB2170505A - Lasing with substituted 2, 6-diphenyl-benzo-(1.2d;4.5d')- bisoxaszoles and 2,6-diphenyl-benzo-(1.2d;5.4d')- bisoxazoles - Google Patents

Abstract

New compounds (I) and (II) are photostable laser dyes capable of stimulated emission in the u.v. range below 400 nm in organic solvents when optionally excited. <IMAGE> wherein all R groups are the same and all R' groups are the same, R being CH3 or OCH3; and R' being CH3, OCH3, tertiary butyl, or F. The illustrated laser system comprises 1-power supply; 1a-excimer laser; 1b-beam splitter; 2-dye laser; 3-grating; 4a,4b-lense systems; 6,7-Joulometers; 8-flowing dye cell; 9-dye solution. <IMAGE>

Description

SPECIFICATION Lasing with substituted 2,6-diphenyl-benzo-(1 .2d;4.5d')-bisoxazoles and 2,6-diphenyl-benzo-(1 .2d;5.4d')- bisoxazoles The present invention relates to a method of lasing with laser dyes and a dye laser, and more particularly relates to the use of substituted 2,6-di phenylbenzo(l .2d ;4.5d')-bisoxazoles and 2,6 diphenylbenzo(1 .2d;5.4d')-bisoxazoles as laser dyes.

Organic molecules with fluorescent properties, in suitable solvents are often capable of stimulated emission through optical excitation. The stimulated emission takes place in a so-called optical resonator in which a solution containing a laser dye in a quartz cell is pumped out inside of a closed circuit. The closed unit of the optical system with mirrors, prism, reflection grating, and the dye circuit is called the dye laser.

The optical stimulation of the organic dye is achieved, as a rule, with gas lasers (pumping lasers) such as nitrogen, argon or krypton ion lasers, as well as with excimer lasers. The great advantage of dye lasers over gas lasers is that in dye lasers wavelengths tuned over a larger range of the fluorescence spectrum of the organic dye can be selected for a multitude of applications.

In the ultraviolet (uv) range below 400 nanometer (nm), there are so few dyes which are also photostable.

Since in the uv range the energy radiated by the pumping laser is within the area of the binding energy of organic covalent bonds, the stimulated emission competes with the photochemical degradation of the laser dye. The photostability of the laser dye material is important, particularly for long-term experiments with dye lasers. Compounds in the coumarin class are, therefore, good laser dyes emitting in the blue range, but they lack photostability. See B.H. Winters, H.l. Mandelberg and W.B. Bergmark and W.B. Mohr, Appl. Phys. Letters 25,723(1974); and G. Jones, W.R. Bergmarkand W.R. Jackson, Opt.Commun. 50,320 (1984).

Elaborate efforts with additives are supposed to produce chemical stability of the dissolved laser dyes, but often the results are limited. See, for instance, R.V. von Trebra and T.H. Koch, Appl. Phys. Letters 42 129 (1983)).

Summary of the invention A primary object of the present invention is to solve the problem of finding laser dyes which can be excited by pumping lasers and which in the near ultraviolet range, particularly in the range under 400 nm, are capable of stimulated emission and which possess improved photostability.

The achieve the foregoing objects and in accordance with its purpose the present invention provides a method for lasing in which a laser dye is excited by irradiation comprising providing as the laser dye a substituted 2,6 -diphenyl-benzo-(1 .2d;4.5d')- bisoxazole of formula (I) or a substituted 2,6-diphenyl-benzo (1.2d;5.4d')- bisoxazole of formula (II)

wherein all R groups are the same and all R' groups are the same, and wherein R, independent of R', is CH3 or OCH3, and R', independent of R, is H, CH3,OCH3, tertiary butyl, or F. The dyes employed in the present invention are used as uv laser dyes and exhibit improved properties.

In another aspect of the present invention, there is provided a laser system comprising a means for exciting a laser dye in a dye laser containing the laser dye, wherein the laser dye is a dye of formula (I) or (II) above.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, but are not restrictive of the invention.

Brief description of the drawing The sole figure of the drawing schematically illustrates one embodiment of a laser system employing a laser dye in accordance with the present invention.

Detailed description of the invention It is true that in a Chinese publication Pan, Jiaxing; Wang, Mingzhen; Zhou, Yimin; Kao, Chen-heng: Gaodeng Xuexiao Huaxue Xuebao 1982, Zhuankan), 61-4; CA 98,215514 e (1983), some distributed 2,6-diphenyl-benzobisoxazoles with their comparable fluorescense quantum efficiency were mentioned.

These compounds are, however, with the exception of the CH3 and fluorine substituents, too insoluble for practical laser operations. Furthermore, the disubstituted 2,6-diphenyl-benzo-bisoxazoles of the Chinese prior art exhibit, because of their planar molecular structure, a very small Stokes shift which, in laser operations, leads to a small tuning range.

In the Chinese publication, lasing efficiencies are incorrectforthe listed substances; they are actually relative fluorescence quantum efficiencies. The relative fluorescence intensity of these disubstituted 2,6-diphenyl-benzo-bisoxazoles in relation to the fluorescence intensity of a fluorescence standard (PPO 2,5-diphenyloxazole) was measured in a fluorimeter. This fluorescence standard is known to lase. The lasing efficiency of PPO is 19.4 %. The relative fluorescence intensity of the 2,6-diphenyl-benzobisoxazoles was then incorrectly coverted to the laser efficiency relatively to the standard PPO with 19.4 % by calculation. This ist incorrect because lasing ability of a fluorescent compound can not be calculated.The laser efficiency in %, however, can only be measured in a pumping laserldye system such as the one described hereafter in the present specification. Out of the enormous number of fluorescent compounds only a few are capable of laser emission.

In accordance with the present invention, it has been found that improved laser properties of tetra- or hexa-substituted 2,6-diphenyl-benzo-bisoxazoles are obtained by introducing space filling substituents in the ortho,ortho'- positions of both of the 2,6-phenyl groups. The substitution achieves the following: Methyl or methoxy groups in R increase the Stokes shift and increase the solubility to the required concentration by 103 Molil required for practical laser operations. Despite steric hindrance through R, the fluorescence quantum efficiency and the lasing efficiency is not reduced.In accordance with the present invention, the usable substituted 2,6-diphenyl-benzo-bisoxazoles exhibit high photostability in comparison to the known laser dyes, and are capable of stimulated emission in the uv range below 400 nm in organic solvents when optically excited.

The substituted, 2,6-di phenyl-benzo-(1 .2d;4.6d')-bisoxazoles preferably are synthesized in absolute ethanol through condensation of two mols of the corresponding ortho-ortho'-substituted benzaldehyde with one mol of 2,5-diamino-p-quinone, whereby the condensation product is subsequently oxidized under reflux with lead tetra-acetate in a 30% solution of acetic acid.

The substituted 2,6-diphenyl-benzo-(1 ,2d; 5.4d')-bisoxazoles preferably are synthesized in pyrophosphoric acid at about 1 500C through condensation of two mols of the corresponding ortho-ortho'-substituted benzoic acid with one mol 4,6-diamino-resorcinol-dihydrochloride in pyrophosphoric acid.

2,6-dimesityl-benzo-(1 ,2d ;4,5d')-bisoxazo le: 0,02 Mol of 1,4-dihydroxy-2,5-diaminobenzene dihydrochloride and 0,04 Mol of 2,4,6-trimethylbenzoic acid were added to 100 g of pyrophosphoric acid. The reaction mixture was stirred and heated 10 110"C under nitrogen atmosphere while, HC1 gas evolved from the reaction mixture. The mixture was kept for 4 hours at 150"C. After cooling the reaction mixture was poured on to 400 g of crushed ice in water. The crude product was collected by filtration and washed with 5% Na2CO3 solution. After recrystallisation from cyclohexane pure white needles of m.p. 249-250"C were obtained in 44% yield.

2,6-dimesityl-benzo-(1 ,2d;5,4d')-bisoxazole: 0,02 Mol of 1,3-dihydroxy-4,6-diaminobenzene dihydrochloride and 0,04 Mol of 2,4,6-trimethylbenzoic acid were added to 100 g of pyrophosphoric acid and heated in the same way as described above.

The crude product was purified by recrystallisation from cyclohexane. White needles of m.p. 238"C were obtained in 32% yield.

2,6-di(2,4,6-trimethoxyphenyl)-benzo-(1 ,2d;4,5d')-bisoxazole, m.p. 296"C, was obtained in 49% yield.

The following photophysical and laserspectroscopic properties were measured: -fluorescence quantum efficiency, QF, - fluorescence relaxation time, TF, - Stokes shift, AXst, -tuning range, dx - lasing efficiency, Tl - photostability, End2.

In an apparatus, in accordance with the drawing which shows the construction of the laser system schematically with the high voltage power supply 1, excimer laser 1 a, beam splitter 1 b, and dye laser 2, etc., the tuning range, lasing efficiency, and photostability are measured in the oscillator stage of the dye laser.

One part of the optical resonator is a reflection grating which allows continuous tuning of the dye laser wavelengths. The dye laser beam is widened through a lense system 4a before it meets the grating 3. The second part of the resonator consists of a lense system 4b, which serves as an output coupling mirror. From the ratio between the energy, measured by Joulemeters 6 and 7 (pyroelectric detectors), of the pumping laser and dye laser, the lasing efficiency is obtained. The concentration of the dye material, or respectively its absorption at the wavelength of 308 nm, the pump wavelength of the excimer laser la, is adjusted for all measurements in such a way that 99.9% of the excitation light is absorbed in the flowing dye cell 8, with an absorption path length of 0.15 cm. The pumping energy with a maximum of 20 millijoule (mJ) per pulse, is focused with a maximum energy density of 0.4 J/cm2 in the dye solution 9. The repetition rate is 10 Hz.

The photostability is determined by measuring the energy of the dye laser as a function of the total pumping energy which is absorbed in the dye solution 9, whereby the energy half-value Ego/2 is the absorbed pumping energy at which the dye laser energy has decreased to half of its original value. It is given as a molecule dependent value in photons per molecule while considering the used photon energy and the applicable dye concentration. The photophysical and laser spectroscopic properties of some of the 2,6-diphenylbenzo-(l .2d;4.5d')-bisoxazoles or the 2,6-diphenylbenzo-(1 .2d;5.4d')-bisoxazoles described here are consolidated in the Table shown below.The first compounds under the first and the second formula in the table are given for comparison and are not covered by our claims. The Stokes' shift and the tuning range is smaller compared to the claimed tetra- or hexa-substituted 2,6-diphenyl-benzobisoxazoles of the formula I and 11 (see Table). Data for the commercially available laser dye "Stilbene 3" are also listed in the Table for purpose of comparison.

T A 8 L E Photophysical and Laser Spectroscopic Properties of 2.6-Diphenyl-benzo-(1.2d;4.5d")-bisoxazo in Dioxane and 2,6-Diphenyl-benzo-(1.2d;5.4d')-bisoxazoles in Dioxane (unless otherwise noted)

compound 0F* * Stokes R of Stokes Tuning range lasing effi- Photostability Rft' 0 Shift Sst (nm) ciency Photons/molecule a R (I) nhSt (nm) (%) Photons/molecules) . R(I) (nm) R'= H R = H 0.82 0.ô6 45 383-3g1 -- - OCH3 OCI!3 0.90 ~~ 79 367-GOx 14.5 S230 OCH3 CH3 o.so -- 79 | 367-aOi 14.5 5230 Cllg CH3 G78 -- 86 369-37 R fi 12.3 3800 RPk R' = H R = H 0.81 1.17 42 374-381 -- - .

CH3 CH3 0.78 | 1.12 76 359-383 | 7.8 288 "Stilbene 3" 0.76 | 0.82 -- 416-439 | 10.5 85 * In Ethanol As can be seen from the Table, the methyl or methoxy groups in R in the dyes employed in the present invention serve to increase the so-called Stokes shift. See the Table. This has the consequence that through steric hindrance of the ortho- ortho ' - methyl or methoxy groups of the 2,6-diphenyl groups the overlapping area between the absorption and the fluorescence spectrum is reduced. This results in an increased tuning range. See the Table. The compounds employed in the present invention exhibit an increased photostability by a factor of 3,4 to a factor of 50 when compared to the known uv laser dye illustrated in the Table. The double anellation of two oxazole rings to benzene apparently provides the molecules with a high degree of photostability. The lasing efficiency and the photochemical stability of the 2,6-diphenyl-benzo (1.2d;5.4d') bisoxazoles is g reater than that of the 2,6-di phenyl-benzo (1.2d;5.4d') bisoxazoles.

Claims (12)

1. A method for lasing in which a laser dye is excited by irradiation comprising providing as the laser dye a substituted 2,6.diphenyl-benzo-(1 .2d;4.5d')-bisoxazole having the formula (I) or a substituted 2,6-diphenyl benzo-(1 .2d;5.4d')-bisoxazle having the formula (II)

wherein all R groups are the same and all R' groups are the same, wherein R, independent of R' is CH3 or OCH3; and R', independent of R, is H, CH3, OCH3, tertiary butyl, or F.

2. Method according to claim 1, wherein the laser dye is a dye of formula (I).

3. Method according to claim 2, wherein R is OCH3.

4. Method according to claim 3, wherein R' is OCH3.

5. Method according to claim 2, wherein R is CH3.

6. Method according to claim 2, wherein R' is CH3.

7. Method according to claim 1, wherein the laser dye is a dye of formula (II).

8. Method according to claim 7, wherein R is CH3.

9. Method according to claim 8, wherein R' is CH3.

10. A laser system comprising a means for exciting a laser dye, and a dye laser containing the laser dye wherein the laser dye is a substituted 2,6-diphenyl-benzo-(1.2d;4.5d')-bisoxazole having the formula (I) or a substituted 2,6-diphenyl-benzo-(1 .2d;5.4d')- having the formula (II)

wherein all R groups are the same and all R' groups are the same, wherein R, independent of R' is CH3 or OCH3; and R', independent of R, is H, CH3, OCH3, tertiary butyl, or F.

11. Substituted 2,6-diphenyl-benzo-(f.2d;4.5d')-bisoxazoles having the formula (I)

wherein all R groups are the same and ail R' groups are the same, wherein R, independent of R' is CH3 or OCH3; and R', independent of R, is H, CH, OCH3, tertiary butyl, of F.

12. Substituted 2,6-diphenyl-benzo-(l .2d;5.4d')-bisoxazoles having the formula (11)

wherein all R groups are the same and all R' groups are the same, wherein R, independent of R' is CH3 or OCH3; and R', independent of R, is H, CH3, OCH3, tertiary butyl, or F.