EP0655064A1

EP0655064A1 - Chromogenic reagents

Info

Publication number: EP0655064A1
Application number: EP93918011A
Authority: EP
Inventors: Ian Oxley The Chase Sutherland; Alastair Foyle 11 Welland Road Sholl; Kaththota Ralalage Arunasiri Saman- Sandanayake
Original assignee: University of Liverpool
Current assignee: University of Liverpool
Priority date: 1992-08-13
Filing date: 1993-08-12
Publication date: 1995-05-31
Also published as: CA2142117A1; JPH09509395A; WO1994004539A1; GB9217149D0

Abstract

Chromogenic reagents, and more particularly compounds which function as selective cation-sensitive dyes are disclosed. The compounds are of formula (II) wherein X is a chromophore; Y and Z are -CH2(CHR-O-CHR)nCH2- in which n is an integer from 1 - 10 (which may be different for Y and Z) and each R independently is hydrogen or a substituent for hydrogen or when n is an integer from 2-10 at least one -CHR-CHR- group may additionally represent a substituted or unsubstituted arylene group; W indicates one or more optional substituents on the benzene ring; and A is a group imparting acidity to the molecule. Preferred compounds are those in which n = 2 for Z and n = 1 or 2 for Y; X is 4-nitrophenylazophenol and R is hydrogen or additionally when n is 2 at least one -CHR-CHR- group may represent a substituted or unsubstituted arylene group.

Description

DESCRIPTION

CHROMOGENIC REAGENTS

The present invention relates to chromogenic reagents and more particularly to compounds which function as selective cation-sensitive dyes (referred to hereafter as chromoionophores) . These chro oionophores are based upon chromophores which are linked to cation-selective ionophores.

Chromoionophores based upon chromophores which are linked to cation-selective ionophores are described in the publications listed below:-

J.F.Alder, D.C.Ashworth, R. Narayanaswamy, R.E.Moss and I.O.Sutherland, Analyst, 1987, 112. 1191; I.P.Danks and I.O.Sutherland, J.Inclusion Phenomena and Mol. Recognition in Chemistry, 1992, 12, 223; A.M.King, C.P.Moore, K.R.A.S.Sandanayake, and I.O.Sutherland, J.Che . Soc, Chem. Commun., 1992, 582.

In the first of these papers an optical fibre probe is reported to respond to potassium ions in aqueous solution with a K⁺/Na selectivity ratio of 6.4 and in the other two papers new chromoionophores are described which show much greater K⁺/Na⁺ selectivity.

A series of modified cryptands has also been prepared as reagents for detecting potassium. These compounds are described in a patent and a recent publication which are, respectively;- R.Klink, D.Bodar, J.M.Lehn, B.Helfert, and R.Bitsch, DE-A- 3,202,779 filed on 4 August 1983 in the name of Merck GmbH (Chem.Abs., 1984, lp). 34574p) and E.Chapoteau, B.P.Czech, C.R.Gebauer, A.Kumar, K.Leong, D.T.Mytych, .Zazulak, D.H.Desai, E.Luboch, J.Krzykawski and R.A.Bartsch, J.Org. Chem. , 1991, 56, 2575.

They have the general formula:

wherein R is 3-phenyl-5-isothiazolylazo, 5-isothiazolylazo, 5-thiazolylazo, 2,4,6-(θ2N)3- 4-0x0-2,5-cyclohexen-l-ylidenamino, or (p- Me₂NC₆H₄)₂COH; m and n are 0 or 1; and

X is N or COH.

All of the documents above however relate to chromoionophores which are, to varying degrees. selective for potassium ions. These chromoionophores show relatively poor selectivity for other cations of physiological importance, such as sodium, magnesium and calcium. There is also poor selectivity for lithium, which has been widely used in the treatment of mental illness for about 40 years. Since the blood-serum concentration of lithium is important in such treatments, it is important to be able to measure this concentration accurately. However, to date the best Li⁺/Na⁺ selectivity that has been reported

(D.Parker and co-workers, J.Chem. Soc, Perkin Trans.

2, 1990, 321) is about 1500 which is still significantly short of the value of about 30000 required for the measurement of serum concentrations of therapeutic Li⁺ (0.5-2.0 mM) in the presence of normal serum concentrations of Na⁺ (140 mM) .

The present invention aims to overcome or at least alleviate the above-mentioned disadvantages.

According to the present invention there is provided a compound of the formula:

(II)

wherein

X is a chromophore;

Y and Z are -CH₂(CHR-0-CHR)_nCH₂- in which n is an integer from 1 to 10 (which may be different for Y and Z) , and each R independently is hydrogen or a substituent for hydrogen or when n is an integer from 2 to 10 at least one -CHR-CHR- group may additionally represent a substituted or unsubstituted arylene group; indicates one or more optional substituents on the benzene ring; and

A is a group imparting acidity to the molecule. A preferably includes an oxygen atom which is attached to the benzene ring. A particular preferred A group is -OH. A may be a cation binding group, for example, methoxy.

The chromophore represented by X helps to impart colour to the molecule. X may be, for example, -N=NAr wherein Ar is a substituted or unsubstituted aryl grou .

The group Ar in X is preferably a substituted or unsubstituted phenyl group. Examples of suitable Ar groups include 4-nitrophenyl,2-nitrophenyl and 2,4- dinitrophenyl.

In Y and/or Z, n is preferably an integer from 1 to 3 more preferably n is 1 or 2 in Y and 2 in Z. The R groups may also be different for Y and Z. The substituent may be, for example, a hydrocarbyl group. The hydrocarbyl group may be, for example, an aryl, al yl or alkenyl group which may itself be substituted. In addition to the case where R is an aryl group Y and Z may also comprise other groups including an aromatic ring. For example, in the case where at least one

-CHR-CHR- group is an arylene group Y and/or z may be:

)

wherein W is as defined above.

Thus, for example, in the case where for Z , n=2 and -CHR-CHR- is a phenylene group Z will be:

In the case where for Z, n=4 and one -CHR-CHR- is a phenylene group, Z could be:

-CH-,-CHR-O-CHR-CHR-0 O-CHR-CHR-O-CHR - Cr. .-.

(V)

may be, for example, an alkyl or alkenyl group which may itself be substituted, or a functional group such as, for example, nitro, amino or substituted a ino, or alkoxy.

Preferred compounds for use as a cryptand for lithium are those in which n represents 1 in Y and 2 in Z and R is hydrogen or alkyl or the -CHR-CHR- group is a substituted or unsubstituted arylene group and the arylene group is preferably phenylene.

Preferred compounds for use as a cryptand for sodium are those in which n represents 2 in Y and Z and R is hydrogen or alkyl or the -CHR-CHR- group is a substituted or unsubstituted arylene group and the arylene group is preferably phenylene.

The cryptands of the present invention are however suitable for complexing cations other than sodium or lithium. Such cations include potassium, calcium and magnesium.

The ion-sensitive dyes represented by the above formula (II) form molecular complexes with certain cations at surprisingly high levels of selectivity. This is accompanied by a change in the absorption spectrum which makes these dyes particularly suitable for use in an optical fibre sensor based upon a probe in which the ion-sensitive compound is immobilised at the tip of the fibre. According to a further aspect of the present invention there is provided an optical fibre sensor comprising a probe in which an ion sensitive compound of the type illustrated in formula II is immobilised at a tip of a fibre of the optical sensor.

Optical fibre sensors are discussed in the Alder et al Analyst 1987 Article referred to above and consist of an optical probe and appropriate instrumentation. Referring to figure 4 of the accompanying drawings an optical fibre tip is sensitised to cations by placing about l g of Amberlite XAD2 resin (4) on to the tip of the optical fibre (1mm diameter core) (1) and encapsulating it in a porous PTFE membrane (such as Millipore FHUP 50μ or the like) (2) . The membrane is held in place by a heat shrinkable piece of tubing (3) . The probe is sensitised by immersion overnight in a methanolic solution of the cryptand reagent followed by washing with distilled water. This probe can be used in an instrument of the type that has been described by J.F.Alder and co-workers (Analyst, 1987, 112. 1191) (see above) .

To be useful in an optical fibre sensor a chromoionophore should be highly selective for a particular cation, should function in the physiological pH range (typical of from pH 6 to pH 8) , and respond to a metal ion in its normal concentration range in biological fluids such as plasma. This invention provides chromoionophores which meet one or more of the above requirements and in particular show surprisingly high selectivity for lithium and sodium cations. In particular the chromoionophores (2a) and (3) show higher selectivity for lithium in extraction experiments, as compared with sodium, than any other chromoionophore that has been reported and the chromoinophores (2b) and (4) show better selectivity for sodium, as compared with lithium and potassium, than a compound which has recently been described, as showing 'potential for the colorimetric determination of sodium using procedures allowing the extraction mode', (in E.Chapoteau,

M.S.Chowdhary, B.P.Czech, A.Kumar, and .Zazulak,

J.Or .Chem.. 1992, 57., 2804-2808).

Indeed, the compounds of the present invention have many applications in optical and electronic devices which are controlled at least to some extent by cations (e.g. in switching by cations).

The compounds of the invention may be prepared by, for example,

(i) the reaction between an aryldiazonium salt of the formula ArN₂ ⁺B~ (where B is an anion) and a phenolic cryptand having the general formula (VI) in

SUBSTITUTESHEET ISA/EP which R=H and in which Y and Z are as defined above, or

(ii) by the prior formation of a quinonoid cryptand by oxidation of the phenolic cryptand shown below in which R=OH followed by reacting the quinone with an arylhydrazine of the formula ArNHNH₂.

Phenolic cryptands of the type

can be prepared as their hydrobromide salts by heating a macrocyclic diamine with a suitable derivative of 2,6-bisbromomethyl-methoxybenzene in acetonitrile at 80°C for 18-24h according to the reaction:-

CH₃Br

They are described in a paper by A.F.Sholl and I.O.Sutherland J.Che .Soc. , Chem. Com un, 1992, 1716- 1718 entitled "selective chromogenic reagents based upon phenolic cryptands".

Compounds of the type illustrated in figure 2 wherein Y and/or Z are illustrated in figures III, IV or V can be prepared as illustrated by the following reaction scheme.

SUBSTITUTE SHEET ISA/EP

In which X and XI are heated in the presence of B2Hg or BH3.S.Me₂ in THF to produce XII which is in turn reacted with 2,6, bis bromomethylanisole and MeCN _to produce XIII. This is then reacted with p- nitrophenyldiazonium choloride to produce XIV. When n=l, XIV is 4-nitropheynlazophenoldiaza 15-crown-5 modified by the addition of a benzene ring fused to the macrocycle (see example 3) and when n=2 XIV is 4-nitrophenylazophenoldiaza 18-crown-6 modified by the addition of a benzene ring fused to the macrocycle (see example 4) .

The present invention also provides a method for sensing cations, comprising co plexing a compound of the present invention with a cation and determining the change in spectroscopic properties brought about by the complexing. Spectroscopic methods for determining the change in properties include measurement of absorbance at the absorption maxima of the chromoionophore and of the appropriate chromoinophore cation complex as illustrated by the spectra shown in Figs. 1, 2 and 3 of the accompanying Drawings.

Fig. 1 shows the absorption spectrum (300-700nm) of cryptand (2a) in CHCl₃ (8.14x10- 5 mol dm-3) after equilibration with an equal volume of aqueous LiCl at pH 7.0

(trishydroxymethyl ethylamine/HCl buffer) at concentrations of

0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.50, and 1.0 mol dm-³.

The absorbance at 406 run corresponds to the free cryptand and at

534 nrα to the lithium complex.

Fig. 2 shows the absorption spectrum (300-800 nm) of cryptand (2a) in CHCl₃ (5.15x10- mol dm- ) after equilibration with equal volumes of aqueous LiCl (1.0 mol dm- ), Nacl (1.0 mol dm-³), and KCl (1.0 mol dm-³ at pH 9.0 (trishydroxymethylmethylamine/HCl buffer). Under these conditions there is no detectable response to either Na⁺ or ⁺.

Fig. 3 shows the absorption spectrum (300-800 nm) of cryptand (2b) in CHCl₃ (2.79x10- mol dm- ) after equilibration with equal volumes of aqueous LiCl (1.0 mol dm- ) , Nacl (0.005 mol dm-³), and KCl (1.0 mol dm-³) at pH 9.0 (trishydroxymethylmethylamine/HCl buffer), under these conditions the relative responses for (a) Na⁺ (b) K⁺, and (c) Li are ca 1 : 2 : 500 based upon absorbance in the 500-600 nm region.

SUBSTITUTE SHEET ISA/EP The present invention will now be described by way of example only, with reference to the preparation and testing of certain compounds.

In the Examples, compounds are identified by reference numerals la), lb), 2a), 2b), 3) and 4). These compounds have the general formula:

in the case of examples la) , lb) , 2a) and 2b) . wherein for la) n=l and X=H for lb) n=2 and X=H for 2a) n=l and X=

SUBSTITUTE SHEET ISA/EP water (7ml) and cooled in ice. Sodium nitrite (0.1029g, 0.15mmol) dissolved in water (3ml) was then added to this solution to form the diazonium salt. An aliquot (2ml) of the solution of the diazonium salt was added to the phenolic cryptand solution which immediately turned a red brown colour. The reaction mixture was stirred for a further 2hours and allowed to warm to room temperature before being basified with NaOH (3M) to pH 11 to produce a deep blue solution.

The blue aqueous solution was extracted with CH2CI2 (3x50ml) and the combined organic extracts were evaporated to give a deep blue residue which was purified by flash column chromatography on neutral alumina (eluent CH₂C1₂ and 2% MeOH/98% CH₂C1₂) to give the azophenol (2a) as a red film (0.0614g, 85%) MS (FAB) m/z 486 (M+H)⁺; ^^ (CHCI3) 406 nm(e 13000); ~U NMR 400 MHZ 6 (CDCI3 10.37 (1H, s br, ArOH) , 8.34- 8.31 (2H, d, J 9.0 Hz, ArH) , 7.92-7.90 (2H, d, J 8.9 Hz, ArH), 7.69 (2H, s, ArH), 4.12 (2H, d, J 12.8Hz, ArCHHN), 3.80-3.76 (2H, , OCH₂) , 3.68 (4H, s, OCH₂CH₂0), 3.60 (2H, d J_AB=12.6Hz, ArCHHN), 3.34 (2H, s br, OCH₂), 3.16-3.12 (6H, , 0CH₂ and NCH₂) , 2.79 (2H, s br, NCH₂), 2.56-2.51 (4H, , NCH₂) ; ¹³C NMR 100 MHz 6 (CDCI3) ¹⁶⁴*⁷- 156.5, 147.6, 144.8, 129.3, substituted aromatic carbons, C-OH, C- O2, C-N=N, C- N=N, C-CH₂, 124.7, 124.6, 122.7, aromatic C-H, 71.2, 70.7, 68.5, 0CH₂, 57.9, 57.7, 54.4, NCH₂. Lithium complex - max (CHC1₃) 534 nm (e 22000); ^~E NMR 400 MHz 6 CDCI3) 8.27 (2H, d, J 9.0 Hz, ArH), 7.83 (2H, d J 9.2 Hz, ArH), 7.69 (2H, s, ArH), 4.30 (2H, d, J 11.4 Hz, ArCHHN), 3.93-3.89 (2H, , OCH₂) , 3.86-3.81 (2H, , OCH₂), 3.69-3.64 (2H, m, OCH₂) , 3.47-3.42 (2H, m, OCH₂), 3.15 (2H, d, J 11.4Hz, ArCHHN), 3.15-3.11 (2H, m, OCH₂), 2.98-2.93 2H, , OCH₂, 2.73-2.64 (6H, m, NCH₂), 2.37-2.31 (2H, , NCH₂) . ¹³C NMR 100 MHz δ (CDCI3) 180.1, 158.1, 145.5, 139.8, 130.8, substituted aromatic carbons, C-0~, C-N0₂, C-N=N, C- N=N, C-CH₂, 124.7, 121.4, aromatic C-H, 68.6, 68.3, 67.5, OCH₂, 59.3, NCH₂Ar, 56.2, 51.1, NCH₂.

The extraction of lithium from aqueous solutions of LiCl by the chromoionophore (2a) was examined by absorption spectroscopy in the range 300-800 nm using a solution of (2a) in CHCI3 and buffered aqueous solutions of LiCl, typical spectra are shown in Figure 1. Even at pH 9.0 there was no detectable extraction of sodium and potassium from aqueous solutions of their chloride salts as indicated by the spectra shown in Figure 2, the sensitivity for lithium is at least 10⁴ times greater than that for sodium or potassium. Details of extraction coefficients for lithium in the pH range 7-9 are given in Table 1. Example 2.

Preparation and testing of 4-nitrophenylazophenol diaza-18-crown-6 (2b)-

Phenolic cryptand (lb) (0.0375g, 0.099 mol) was dissolved in an aqueous solution of sodium hydroxide

(0.0395g, 0.99 mmol in 2ml water) and stirred in an ice bath at 0°C. 4-Nitroaniline (0.1390g, l.Olmmol) was dissolved in hot cone. HC1 (2ml), diluted with water (4.8 ml) and cooled in ice. Sodium nitrite

(0.0704g, 1.02mmol) dissolved in water (2ml) was then added to form the diazonium salt. An aliquot (4ml) of the solution of the diazonium salt was added to the solution of the phenolic cryptand which immediately turned a red brown colour. The reaction mixture was stirred for a further 2hrs and allowed to warm to room temperature before being basified with NaOH (3M) to pH

11 to give a deep blue solution. The product was extracted into CH₂Cl2 (3x50ml) and the combined organic extracts were evaporated to give a deep blue residue which was purified by flash column chromatography on neutral alumina (eluent CH2CI2 followed by 1% MeOH/99% CH₂C1₂) to give the sodium complex of the azophenol 2b) as a deep blue film

(yield 0.0419g, 80%). The sodium complex had MS (FAB) m/z 552 (M⁺); _maχ (CHCI3) 554nm (e 20000); ~ NMR 400

MHz δ (CDCI3) 8.21 (2H, d, J9.0Hz, ArH), 7.77 (2H, d, J 9.0Hz, ArH), 7.70 (2H, s, ArH), 4.67 (2H, t, J

9.75Hz, OCH₂), 4.17 (2H, d, J 11.9Hz, ArCHHN), 3.93-

3.88 (2H, , OCH₂), 3.73-3.66 (4H, , OCH₂), 3.36-3.34

(2H, m, OCH₂), 3.27-3.22 (4H, m, OCH₂) , 3.09-3.06 (2H, , OCH₂), 2.96-2.89 (2H, m, NCH₂), 2.88-2.81 (2H, m,

NCH₂), 2.74 (2H, d, J 12Hz, ArCHHN), 2.47-2.43 (2H, m,

NCH₂), 2.18-2.15 (2H, m, NCH₂) . ¹³C NMR 100 MHz 6

(CDC1₃) 179.4, 158.7, 144.1, 139.2, 129.7, substituted aromatic carbons C-O^", C-N0₂, C-N=N, C-N=N, C-CH₂,

124.8, 121.0, aromatic C-H, 68.9, 67.8, 67.0, 0-CH₂,

56.8,NCH₂Ar, 52.9,N-CH₂.

The free azophenol (2b) was generated by shaking a chloroform solution of the sodium salt with an aqueous solution buffered at pH 7. Evaporation of the dried organic layer gave the azophenol as an orange film. MS (FAB) m/z 530 (M+H)⁺; ^ (CHCI3) 400 nm

(e 15000); ~E NMR 400 MHz 6 (CDCI3) 8.35 (2H, d, J 9.1

Hz, ArH), 7.94 (2H, d, J 9.1 Hz, ArH), 7.71 (2H, s,

ArH), 3.78-3.69 (8H, m) , 3.61-3. (4H, m) , 3.49-3.44

(8H, m) , 2.79 (8H, s br, NCH₂); ¹³C NMR 100 MHz δ

(CDCI3) 162.8, 156.4, 147.8, 145.0, 128.5, substituted aromatic carbons C-OH, C-N0₂, C-N=N, C-N=N, C-CH₂N,

124.7, 122.8, aromatic C-H, 70.6, 68.3 (br) , 0-CH ,

58.2, NCH₂Ar, 56.2, N-CH₂.

The extraction of sodium, potassium, lithium, calcium and magnesium from aqueous solutions of their chloride salts by the chromoionophore (2b) was examined by absorption spectroscopy in the range 300-

800 nm using a solution of (2b) in CHCI3 and buffered aqueous solutions of the salts, typical spectra are shown in Figure 3. At pH 9.0 there was significant extraction of lithium and potassium as indicated by the spectra shown in Figure 3, and also of calcium but the sensitivity for sodium is ca 400 times greater than that for potassium and 800 times greater than that for lithium at this pH. The sensitivity for sodium is ca 16 times greater than that for calcium but in this case the calcium complex has an absorption maximum at 500nm (orange solution) whereas the sodium complex has an absorption maximum at 554 nm. (purple solution) . Details of extraction coefficients for sodium in the pH range 7-9 and for lithium, potassium and calcium at pH 9 are given in the Table 1.

^a For a solution of (2a) or (2b) at ca 10~⁵ to 10~⁴ mol dm^-3 in CHCI3 and solutions of M⁺ at 10~⁴ to 1 mol dm ^J in water using a tris(hydroxymethyl)-methylamine- CH1 buffer. K_e is based upon changes in absorption at 406 and 534 nm for (2a) and Li+, 402 and 554 nm for (2b) and Li⁺, Na⁺ and K⁺, and 402 and 500 nm for (2b) and Ca²⁺

No measurable response for Na⁺, K⁺, Mg , and Ca²⁺ in the pH range 7-9 up to 1 mol dm —^J ~ concentration of the metal salts. ^c No measurable response for Mg^**+ in the pH range 7-9 up to 1 mol dm ^J concentration of the metal salts. ^dK = [H⁺]aq.[M⁺CI^"]org/[M⁺]aq.[CIH]org (where the subscripts aq and org refer to the aqueous and organic phases respectively and CIH refers to the ionisable chromoionophore) .

It will be appreciated from the above that compound (2a) in particular has a remarkable high selectivity for lithium, far greater than anything previously known.

Furthermore, when examples 2a and 2b were compared to two further examples, 3 & 4, in which Z had been modified by the fusion of a benzene ring to the macrocycle as illustrated in formula IV even better results were obtained.

Thus, example 3 was 4 nitrophenyl azophenol diaza 15-crown-5 modified by the addition of a benzene ring fused to the macrocycle and example 4 was 4 nitrophenyl azophenol diaza 18-crown-6 modified by the addition of a benzene ring fused to the macrocycle.

These chromogenic reagents 3 and 4 show higher selectivity in cation extraction experiments than the related compounds 2a and 2b.

The extraction coefficients for the chromogenic reagents 3 and 4 (as compared to 2a and 2b) are listed in Table 2. For lithium extraction the colour change on the formation of the Li⁺ complex formula XVI is from yellow to pink and for the Na⁺ complex formula XVI from yellow to purple. n = 1 or 2 M^ U^ Na'. lC. cr Ca²*

Compound 3 responds to Li⁺ in the pH range 7-9 and to no other cation of physiological importance (K⁺,Na⁺, Mg²⁺, and Ca²⁺) with a selectivity ratio for Li⁺:Na⁺ of >10⁴. Compound 4 responds only to Na⁺ at pH 7 and at higher pH the selectivity for Na⁺:Li⁺ is ca 5000, for Na⁺:K⁺ is ca 6000, and for Na⁺:Ca²⁺ is ca 4000, no response is observed for Mg ~ +. The two compounds 3 and 4 are more lipophilic than 2a and 2b and show no extraction into water in the pH range 7-9 and show selectivity for Li⁺ and Na⁺ that is higher than that reported for any other reagents. They should thus prove ideal for use in optical fibre sensors for Li⁺ and Na⁺ and, in modified form, their exceptionally high selectivity could provide suitable reagents for detecting Li⁺ and Na⁺ in, for example, microscopic examination of biological samples. The response of the reagents is pH dependent so that a pH sensitive reagent must also be used in the optical fibre probe or in the biological sample. If the cation sensing and pH sensing reagents respond in a different wavelength range it should be possible to make both measurements simultaneously.

Table 2 2a,2b,3 and 4 Extraction coefficients ^a'^e and

Selectivities for Chromoionophores log-oKe (±0.2)b

a. For a solution of th^"e²c^3"hromoionophore at ca 10-5

to 10^"4 mol dm in CHCL3 and solutions of M⁺ at 10~⁴ to 1 mol dm - in water using a tris(hydroxy- methyle)methylamine-HCl buffer. b. K_e = [H⁺]aq.[M⁺Cl^""]org/[M⁺]aq.[ClH]org (where the subscripts aq and org refer to aqueous and organic phases respectively and CIH refers to the ionisable chromoionophore. c. _/ - (nm) used for calculating [M⁺Cl~]org, in general M=Li gives -_j^_χ ca 530 nm, M=Na gives _j^x ca 550 nm, _πιaχ ca 560 nm, and M=Ca gives /(_ _maχ ca 500 nm. d. _maχ ca 400 nm used for calculating [CIH] org. e. Compounds 2a and 3 give no measurable response to Na⁺, K⁺, Mg²⁺, and Ca²⁺ in the pH range 7-9 up to 1 mol dm concentrations of the metal salts. Compounds 2b and 4 give no measurable response to Mg^* ~^*+

in the pH range 7-9 up to 1 mol dm —^J ~ concentrations of the metal salts.

Claims

1. A compound of the formula:

wherein

X is a chromophore;

Y and Z are - CH₂ (CHR-0-CHR)_nCH₂ - in which n is an integer from 1 - 10 (which may be different for Y and Z) and each R independently is hydrogen or a substituent for hydrogen or when n is an integer from 2-10 at least one -CHR-CHR- group may additionally represent a substituted or unsubstituted arylene group ;

W indicates one or more optional substituents on the benzene ring; and

A is a group imparting acidity to the molecule.

2. A compound as claimed in claim 1 in which A is a cation binding group.

3. A compound as claimed in claim 1 or 2 in which A indicates an oxygen atom which is attached to the benzene ring.

4. A compound as claimed in claim 1, 2, or 3, in which A is a hydroxy group or methoxy group.

5. A compound as claimed in claim 4, in which A is -OH.

6. A compound as claimed in any of the preceding claims wherein the chromophore X is H.

7. A compound as claimed in any of claims 1 to 5, wherein the chromophore X is -N=NAr, wherein Ar is a substituted or unsubstituted aryl group.

8. A compound as claimed in claim 7 wherein the aryl group is 4-nitrophenyl, 2-nitrophenyl or 2,4 dinitrophenyl.

9. A compound as claimed in claim 8 wherein the aryl group is 4-nitrophenyl.

10. A compound as claimed in claim 6 or 9, in which in Y and/or Z, each R independently is hydrogen or a substituent comprising a hydrocarbyl group.

11. A compound as claimed in claim 10 wherein the hydrocarbyl group is an aryl, alkyl or alkenyl group which may itself be substituted.

12. A compound as claimed in claim 10 in which each R is hydrogen.

13. A compound as claimed in claim 6 or 9 wherein Y and/or Z comprises a group including an aromatic ring.

14. A compound as claimed in claim 13 in which Y and/or Z are of the formula:

wherein W is as defined in claim 1.

15. A compound as claimed in claim 14 wherein there is no substituent W.

16. A compound as claimed in claim 12 or 15 wherein n = 2 for Z.

17. A compound as claimed in claim 16 wherein: n - 1 for Y.

18. A compound as claimed in claim 16 wherein: n = 2 for Y.

19. A cryptand for lithium comprising a compound as claimed in claim 17.

20. A cryptand for sodium comprising a compound as claimed in claim 18.

21. An optical fibre sensor comprising a probe in which an ionsensitive compound as claimed in claims 17 or 18 is immobilized at a tip of a fibre of the optical sensor.

22. A method of sensing cations comprising complexing a compound as claimed in claims 18 or 19 with a cation and determining the change in spectroscopic properties brought about by the complexing.

23. A method as claimed in claim 22 in which the cation is selected from the group comprising sodium, and, lithium.

24. A compound substantially as hereinbefore described with reference to formula XIV, XV and XVI.