Photochromism is a well-known physical phenomenon and has been detailed in"Photochromism: Molecules and Systems"Studies in Organic Chemistry, 40, Eds. H. Dürr and H. Bouas-Laurent, Elsevier, 1990. Similarly, the phenomenon of pH sensitive dyes/indicators and stains is well established; (see, for example,'Colour Chemistry: Synthesis, Properties and Applications of Organic Dyes and Pigments' ; H. Zollinger, VCH (Germany) 1991).

The 3H-naphtho [2,1-b] pyran and 2H-naphtho [1,2-b] pyran systems are known to be capable of exerting a photochromic effect (see, for example, Y. <BR> <BR> <BR> <P>Hirshberg and E. Ficher, J. Chem. Soc., 1954,3129 and R. Livingstone et al. , J.

Chem Soc., 1958,2422).

The basic 3H-naphtho [2,1-bJpyran and 2H-naphtho [1,2-b] pyran structures are illustrated below: 3H-naphtho [2,1-b] pyran 2H-naphtho [1,2-b] pyran The photochromic properties of both the 3H-naphtho [2, 1-b] pyran and 2H- naphtho [1,2-b] pyran systems have been intensively studied. For examples of 3H- naphtho [2, I-blpyrans, see US patent 4,826,977 (1989), US patent 5,066,818 (1991), PCT WO 91/00861 (1991), PCT WO 92/01959 (1992), PCT WO 92/09593 (1992), PCT WO 94/22850 (1994), PCT WO 95/00866 (1995), US patent 5,532,361 (1996), US patent 5,520,853 (1996), US patent 5,552,090 (1996) and PCT WO 97/06455 (1997); and for examples of 2H-naphtho [1,2-b] pyrans, see EP patent 0,250,193 (1987), US patent 4,818,096 (1989), US patent 5,066,818 (1991), Research Disclosures Pilkington PLC (1992/3), US patent 5,458,814 (1995) and US patent 5,514,817 (1996).

We have now found that changing the pH of a solution, matrix or host material containing certain photochromic dyes can affect the spectroscopic properties, namely those of colour (R x), induced optical density and colourability of the incorporated photochromic dye. Significant shifts in the colour (R together with enhanced induced optical density and improved colourability can be observe without any apparent change in the rate of colouration, though the rate of fade (bleaching) may be altered. This effect is fully reversible and hence provides a means of switching the spectroscopic properties of a photochromic dye by adjusting the pH of its environment. Reversion to the original form of the photochromic dye by adjustment of the pH of its environment results in the return of its associated photochromic properties.

We have further found that, for this new effect to operate, certain structural features of the photochromic molecule are essential, in particular the aromatic moiety of the photochromic dye must have directly bonded to it at least one pH sensitive functional group. Such functional group (s) must contain either (i) one or more'lone pairs'of electrons that may be reversibly protonated or (ici) one or more acidic protons that may be reversibly removed by the action of a base.

The photochromic dyes of the present invention are naphthopyrans of formula I or II: wherein R'and R2, which may be the same or different, are each H, an alkyl group, a substituted alkyl group, an alkenyl group, a substituted alkenyl group, an alkynic group, a substituted alkynic group, a cycloalkyl group, a substituted cycloalkyl group, a heterocycloalkyl group, a substituted heterocycloalkyl group, a cycloalkenyl group, a substituted cycloalkenyl group, an aryl group, a naphthyl group, or a heteroaryl group and their substituted derivatives ; Rl and R2 may be conjoined to form a ring, for exemple but not exclusively, cyclopentane, indane, indene, dibenzosuberane, dibenzosuberene, fluorene, xanthene, thioxanthene, acridine and their substituted derivatives particularly alkoxy and amino derivarives as defined below for R3; the or each R3 which may be the same or different is <BR> <BR> amino, C 1-C20 linear or branche alkylamino, C 1-C20 linear or branche<BR> <BR> dialkylamino, C3-C20 cycloalkylamino, C3-C20 substituted cycloalkylamino, C3-<BR> <BR> C20 cycloalkyl CI-C20 linear or branche alkylamino, C3-C20 substituted<BR> <BR> cycloalkyl C 1-C20 linear or branche alkylamino, C3-C20 dicycloalkylamino, C3- C20 substituted dicycloalkylamino, C3-C20 cycloalkyl arylamino, C3-C20 substituted cycloalkyl arylamino, C 1-C20 linear or branche alkyl arylamino, arylamino, diarylamino, cyclic amino for example but not exclusively aziridino, azetidino, pyrrolidino, piperidino, homopiperidino, perhydroazocino, piperazino, N-alkylpiperazino, N-arylpiperazino, morpholino, thiomorpholino, their substituted derivatives and their mono and di benzologues; aminoaryl in which the amino fonction is defined as above for R3, bridgehead aminoaryl units such as julolidine and lilolidine; hydroxy, hydroxyaryl, thiol, mercaptoaryl, carboxylic acid, thiocarboxylic acid, sulfur and phosphorus based acids. In the above definition, the terms cycloalkyl and substituted cycloalkyl include bi and tri cycloalkyl amino and substituted derivatives; the or each R4 which may be the same or different is C 1-C20 linear or branche alkoxy, C 1-C20 linear or branche alkylthio, alkylsulfinyl, alkylsulfonyl, arylsulfmyl, aiylsulfonyl, halogen, nitro, nitrile, formyl, acyl, aroyl, acetamido, C2-C 10 N-alkylamido, alkoxycarbonyl, aryloxy, arylthio, or is selected from those atoms and groups specified above for R1, R2 and R3; and each'n'is 0 or an integer from 1 to 6 provided that in any one compound the total of all'n's is not more than 6.

The effect of a change in pH may be conveniently illustrated by the scheme below: 'protonated''deprotonated''neutral' photochromic photochromic photochromic molecule molecule molecule -m2 +H+m"3 low-low-low absorbance +OH-absorbance +OH-absorbance hv hv hv thv X. i Xm. 2 xr [=3 high high high absorbance absorbance absorbance The photochromic and pH colour switching properties exhibited by the pyran compound of the present invention render these compound particularly useful as photochromic/pH sensitive indicators, inks, paints, varnishes and stains for'priniing'onto paper and fabrics and other surfaces e. g. glass, plastics and metals. This latter application may be particularly useful for the preparation of security markers (labels) on a broad range of objects e. g. cheques, bonds, bankers drafts, credit cards, charge cards and identity documents and cards and discrete windows. Such inks and other like formulations may also be used for printing documents and greetings cards. The security/identity uses of these pH sensitive photochromic compound and formulations containing them may also extend to include the marking of fuels e. g. petrol and diesel and other oils.

Furthermore, the materials may be used in sensors, opto-chemical transducers, optical data recording systems e. g. compact discs, and read/write optical data storage discs, as waveguides and laser dyes.

Alternatively, these compound may be incorporated into polymeric or sol-gel or colloidal type host materials so as to impart photochromic and pH colour switching properties to the said host materials.

Examples of applications of the polymeric host materials of the present invention include the manufacture of lenses for sunglasses and ophthalmic lenses, protective visors, screens, films,'plastic'sheeting, containers (e. g. bottles and other packaging vesses), mirrors, windows and screens for vehicles such as cars (including sunroofs), motorcycles, aircraft and ships, architectural uses e. g. glazing, and artistic'stained glass'windows and for use in novelty items.

Additionally the materials may be used in vehicle body panels including fairings and spoilers, and related external surfaces and other embodiments where it may be deemed attractive to have said objects change colour in the presence of sunlight.

The photochromic pyrans of the present invention may be incorporated into the'plastic'host material by well established protocols for example as described in European Patent No. 0254020 or U. S. Patent No.

5,066,818.

Typical host materials may include optically clear polymer materials, such as polymers of polyol (allyl carbonate)-monomers, polyacrylates such as polymethylmethacrylates, cellulose acetate, cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate, poly (vinyl acetate), poly (vinyl alcool), polyuretanes, polycarbonate, polyethylene terephthalate, polystyrene, poly (triethyleneglycol dimethylacrylate), poly (diethyleneglycol bis (allyl carbonate)) and various copolymer mixes.

The pH colour switching ability is particularly useful in that a single manufactured photochromic dye may be used to impart different colours to a solution, matrix or host material depending upon the pH of the solution, matrix or host material.

The high induced optical density and enhanced colourability of these photochromic compound of the present invention enables the amount of the photochromic material required so as to impart a useful degree of photochromism to a polymeric host material or to a solution to be greatly reduced, thereby enabling a considerable saving of synthetic effort and cost. Furthermore, the use of reduced quantifies of the photochromic materials of the present invention has the bonus that there is a consequent reduction in any undesirable colour that the photochromic materials may impart in the bleached state either by way of inherent colour of the material itself or by the formation of coloured fatigue/degradation products through use of the photochromic material.

The naphthopyrans of the present invention maybe prepared by a general method which is based on the following rection scheme: ,.'1, OH solvent OH Arl Ar2' \ O 0 i i i This general synthetic methodology has been described, for example by L. Merlini in Advances in Heterocyclic Chemistry,'1975, vol. 18, page 159, and by R. Guglielmetti in "Photochromism : Molecules and Systems,"Studies in Organic Chemistry 40, chap. 8, Eds. H Durer and H. Bouas-Laurent, Elsevier, 1990, and also in several patent documents, for example WO 94/22850 and U. S. Patent No. 5,520,853 (1996). The synthesis of the propargyl alcools shown in the scheme above are obtained in a known manner, for example, T. F. Rutledge in 'Acetylenic Compounds,'Reinhold, New York, 1968.

The subsrituted benzophenones required for the synthesis of the propargyl alcools are either commercially available or obtained by documente procedures described in the literature e. g. B. M. Khadilkar et al. Tetrahedron Letters 1997,38 (9) 1641; J. P. Wolfe et al. Journal of Organic Chemistry, 1997, 62,1264.

The 1-rand 2-naphthols and related hydroxy compound are either commercially available or obtained by known synthetic methods, or derived from such methods; see for example WO 94/22850, W. S. Johnson et al. Organic Rections 1951, vol. 6; D. W. Cameron et al. Australian Journal of Chemistry, 1980,33,2531.

The catalyst may be selected, for example, from alumina, acetic acid, trifluoroacetic acid, aryl or alkyl sulfonic acids, silica, clays (e. g. montmorillionite, tonsil) or acidic exchange resins. Any suitable organic solvent can be used. Those frequently employed for the rection include benzene, toluene, xylene and relatively high boiling alkanes, for example.

In the definition of the naphthopyrans of the invention given above, the term aIkyl group means any linear or branche C 1-C20 alkyl group and inclues haloalkyl and perhaloalkyl groups. The term substituted alkyl group means any linear or branche C 1-C20 alkyl group which is substituted in any position or positions with a functional group which contains the heteroatom nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms. Additionally, the substituted alkyl group may be taken to mean any linear or branche C 1-C20 alkyl group which is substituted in any position or positions with a functional group which contains one or more carbon atoms bonded to one or more of the heteroatoms nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms.

The term alkenyl group means any isomeric linear or branche C2- C20 alkenyl group and inclues haloalkenyl and perhaloalkenyl groups and may contain one or more alkene bonds. The term substituted alkenyl group means any isomeric linear or branche C2-C20 alkenyl group which is substituted in any position or positions with a functional group which contains the heteroatom nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms. Additionally, the substituted alkenyl group may be taken to mean any isomeric linear or branche C2-C20 alkenyl group which is substituted in any position or positions with a functional group which contains one or more carbon atoms bonded to one or more of the heteroatoms nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms.

The term alkynic group means any liner or branche C2-C20 alkynic group and may contain one or more alkynic bonds.

The term cycloalkyl group, a substituted cycloalkyl group, a cycloalkenyl group, and a substituted cycloalkenyl group include mono-, di-, tri- and tetracyclic C3-C20 containing systems and are defined as for their respective non-cyclic analogues.

The terms an aryl group and a naphthyl group refer to phenyl and 1- and 2-naphthyl groups, which are either unsubstituted or substituted with one or more of the same or different of the following substituents; halogen, C l-C6 linear or branche alkyl, C2-C6 linear or branche alkenyl, C2-C6 linear or branche alkynyl, phenyl, aryl, heteroaryl, C 1-C6 linear or branche hydroxyl, C 1-C6 linear or branche alkoxy, C 1-C6 linear or branche alkylthio, alkylsulfmyl, alkylsulfonyl, amino, C1-C6substitutedalkylamino,C1-C6alkylamino, dialkylamino, Cl-C6 alkylarylamino, diarylamino, cyclic amino for example but not exclusively pyrrolidino, piperidino, homopiperidino, perhydroazocino, piperazino, morpholinothiomorpholino,indolino;nitro,piperazino, carboxyalkyl, Cl-C6 alkylcarbonyl, benzol, aroyl, heteroaroyl, formyl, nitrile, carboxyamido, or crown and aza crown systems.

The term a heteroaryl group means for example but not exclusively, any of the following heterocyclic systems and their mono-and di-benzologues and their mono-and di-naphthologues and their substituted derivatives bonded through any carbon or heteroatom possible: thiophene, furan, pyrrole, pyrazole, imidazole, oxazole, isoxazole, thiazole, isothiazole, dithiole, triazole, tetrazole, pyran, thiopyran, pyridine, pyrimidine, pyridazine, pyrazine, oxazine and dithiin.

As used herein, the term alkoxy group means any linear or branche C l-C20 alkoxy group and inclues haloalkyloxy and perhaloalkyloxy groups, and the term substituted alkoxy group means any linear or branche C 1-C20 alkoxy group which is substituted in any position or positions with a functional group which contains the heteroatom nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms. Additionally, the substituted alkoxy group may be any linear or branche C 1-C20 alkoxy group which is substituted in any position or positions with a functional group which contains one or more carbon atoms bonded to one or more of the heteroatoms nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms.

As used herein, the term alkylthio group means any linear or branche C 1-C20 alkylthio group and inclues (as the alkyl part) haloalkyl and perhaloalkyl groups, and the term substituted alkylthio group means any linear or branche C 1-C20 alkylthio group which is substituted in any position or positions with a functional group which contais the heteroatom nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms.

Additionally, the substituted alkyl group may be any linear or branche C l-C20 alkylthio group which is substituted in any position or positions with a functional group which contains one or more carbon atoms bonded to one or more of the heteroatoms nitrogen or oxygen or sulfur or phosphorus or silicon, or any combination of two or more of the aforementioned heteroatoms, irrespective of the substituents directly bonded to said heteroatoms.

In order that the invention may be more fully understood, the following examples are given by way of illustration only: Exemple 1 (1) 6-Morpholino-3 (4-piperidinophenyl)-3-phenyl-3H-naphtho [2,1-b] pyran A solution of 4-morpholino-2-naphthol (6.5 mmol) and 1- (4- piperidinophenyl)-1-phenylprop-2-yn-1-ol (6.5 mmol) in toluene (65 cm3) containing acidic alumina (Brockmann 1) (4.0g) was refluxed for 60 minutes. The cooled solution was filtered and the alumina was washed well with EtOAc (200 cm'). Removal of the solvent from the filtrate gave an oil which solidifie on standing at room temperature. Recrystallisation from EtOAc/hexane gave 6- morpholino-3 (4-piperidinophenyl)-3-phenyl-3H-naphtho [2,1-bpyran (73%), m. p.

= 170.5-172°C.

Examples 2 to 18 Following an identical protocol, but using the appropriate naphtol and prop-2-yn-1-ol, the following naphthopyrans were obtained : (2) 3 (4-Methoxyphenyl)-6-morpholino-3 (4-piperidinophenyl)-3H-naphtho- 4-morpholino-2-naphtholand1-(4-methoxyphenyl)-1-(4-[2,1-b]py ranfrom piperidinophenyl) prop-2-yn-l-ol (75%) after recrystallisation from EtOAc, hexane and ethanol, m. p. = 247-249°C.

(3) 3,3-Di (4-methoxyphenyl)-6-morpholino-3H-naphtho [2,1-b] pyran from 4- morpholino-2-naphthol and 1,1-di (4-methoxyphenyl) prop-2-yn-1-ol (68 %) after recrystallisation from EtOAc, hexane and a trace of ethanol, m. p. = 211-213 °C.

(4) 6-Morpholino-3, 3-di (4-pyrrolidinophenyl)-3H-naphtho [2, 1-b]pyran from 4-morpholino-2-naphthol and 1,1-di (4-pyrrolidinopheny1) prop-2-yn-1-ol (56 %) after recrystallisation. from EtOAc and, m. p. = 243-245 OC.

(5) 6-Morpholino-3-(4-methoxyphenyl)-3-phenyl-3H-naphtho[2,1-b]p yran from 4-morpholino-2-naphthol and 1- (4-methoxyphenyl)-1-phenylprop- 2-yn-1-ol (71 %) after recrystallisation from hexane and a trace of EtOAc, m. p. = 164.5-165.0 °C).

(6) 6-Indolino-3,3-di (4-methoxyphenylf3H-naphtho [2,1-b] pyran from 4- indolino-2-naphthol and 1,1-di (4-methoxyphenyl) prop-2-yn-1-ol (57 %) after recrystallisation from EtOAc and hexane, m. p. = 171-172°C.

9-morpholino-2,2-di(4-methoxyphenyl)-2H-naphtho[1,2-b]pyr an-(7)Methyl 5-carboxylate from methyl 4-hydroxy-6-morpholinonaphthalene-2- carboxylate and 1,1-di (4-methoxyphenyl) prop-2-yn-1-ol (42 %) after recrystallisation from EtOAc and hexane, m. p. =153. 5-155 °C.

(8) Methyl 9-morpholino-2- (4-morpholinophenyl)-2-phenyl-2H-naphtho [1, 2-b]pyran-5-carboxylate from methyl 4-hydroxy-6- morpholinonaphthalene-2-carboxylate and 1- {4-morpholinophenyl)-1- phenylprop-2-yn-1-ol (74 %) after recrystallisation from EtOAc and hexane, m. p. = 248-250 OC.

(9) 5-Hydroxy-3,3-di (4-methoxyphenyl)-3H-naphtho [2,1-b] pyran from 2,3- dihydroxynaphthalene and 1,1-di (4-methoxyphenyl) prop-2-yn-1-ol (53 %) after recrystallisation from EtOAc, hexane and a trace of ethanol, m. p. = 150.5-152 OC.

(10) 6(4-N,N-Diethylaminophenyl)-2, 2-di (4-methoxyphenyl)-2H-naphtho [1,2-b] pyran from 4- (4-N, N-diethylanilino)-1-naphthol and 1,1-di (4- methoxyphenyl) prop-2-yn-1-ol (61 %) after recrystallisation from EtOAc and hexane, m. p. =147. 5-149.5 °C.

(11) 6-Morpholino-3,3-diphenyl-3H-naphtho [2,1-b] pyran from 4-morpholino- 2-naphtol and 1, 1-diphenylprop-2-yn-1-ol (55 %) after recrystallisation from toluene and MeOH, m. p. = 187-188 °C.

(12) 3- (2, 4-Dimethoxyphenyl)-3- (4-methoxyphenyl)-6-morpholino-3H- naphtho [2,1-blpyran from 4-morpholino-2-naphthol and 1- (2,4- dimethoxyphenyl)-l- (4-methoxyphenyl) prop-2-yn-l-ol (68 %) after recrystallisation from hexane and a trace of EtOAc, m. p. = 163-165 °C.

(13) 3,3-Di- (4-methoxyphenyl)-3H-naphtho [2,1-b] pyran from 2-naphtol and 1, 1-di-(4-methoxyphenyl)prop-2-yn-1-ol (48 %) after recrystallisation from hexane and a trace of EtOAc, m. p. =176-177°C.

(14) 3,3-Di (4-methoxyphenyl)-6-piperidino-3H-naphtho [2, 1-b]pyran from 4- piperidino-2-naphthol and 1,1-di (4-methoxyphenyl) prop-2-yn-1-ol (73%) after recrystallisation from EtOAc, hexane and a trace of ethanol, m. p. = 114-119 °C. (15) 6-Morpholino-3 (4-morpholinophenyl)-3-phenyl-3H-naphtho [2,1-blpyran from 4-morpholino-2-naphthol and 1 (4-morpholinophenyl)-1- phenylprop-2-yn-l-ol (73%) after recrystallisation from EtOAc/hexane, m.p. =187-188°C.

(16) 6-Morpholino-3-phenyl-3 (4-pyrrolidinophenylf3H-naphtho [2,1-b] pyran from 4-morpholino-2-naphthol and 1-phenyl-1 (4-pyrrolidinophenyl)- prop-2-yn-1-ol (66%) after recrystallisation from EtOAc, hexane and a trace of ethanol, m. p. = 220-222 OC.

(17)3,3-Di(4-N,N-dimethylaminophenyl)-6-morpholino-3H-nap htho[2,1-b]- pyran from 4-morpholino-2-naphthol and 1.1-di (4-NN-dimethylamino- phenyl) prop-2-yn-1-ol (69%) after recrystallisation from EtOAc, hexane and a trace of ethanol, m. p. = 258.5-260.5 °C.

(18) 3,3-Di (4-N, Ndiethylaminophenyl)-6-morpholino-3H-naphtho [2,1-b]- pyran from 4-morpholino-2-naphthol and 1,1-di (4-N, N diethylamino- afterrecrystallisationfromEtOAc/phenyl)prop-2-yn-1-ol(78%) hexane, m. p. = 235-238 °C.

The photochromic properties of each of the naphthopyrans made in Examples 1 to 14 were measured and the results are set out in the following Table. Compound Neutral Modifie reference species species abs 0.10 abs 0.15 ##max#max#max 466nm482nm abs* 1.59 abs* 2.51 Compound Neutral Modifie reference species species abs 0.19 abs 0.25 ##max#max#max 494nm10484nm abs* 1.63 abs* 2.34 Compound Neutral Modifie reference species species abs 0.08 abs 0.42 ##max#max#max 534nm96438nm abs* 0.95 abs* >3.5 Compound Neutral Modifie reference species species abs 0.31 abs 0.28 ##max#max#max 684nm146538nm abs* 0.58 abs* >3 Compound Neutral Modifie reference species species abs 0.04 abs 0.21 ##max#max#max 426426nm 506 80 abs* 2.48 abs* >3 Compound Neutral Modifie reference species species abs 0.01 abs 0.02 ##ma#max#max 464 nm 604 140 abs* 0.33 abs* 0.64 Compound Neutral Modifie reference species species abs 0.01 abs 0.01 ##max#max#max 442/534 516 nm 74/18 nm abs* abs* 0.10 0.50/0.43 (cold) Compound Neutral Modifie reference species species abs 0.01 abs 0.01 ##max#max#max nm14/84478/548464 nm abs* abs* 0.61 0.72/0.83 Compound Neutral Modifie reference species species abs 0.06 abs 0.16 ##max#max#max 99474 nm 454 nm 20 abs* 0.52 abs* 1. 51 Compound Neutral Modifie reference species species abs 0.06 abs 0.10 ##max#max#max nm498nm4610544 abs* 1.63 abs* 2.63 Compound Neutral Modifie reference species species abs 0.03 abs 0.08 ##max#max#max 1111410 nm 474 64 abs* >2 abs* >2.5 Compound Neutral Modifie reference species species abs 0.16 abs 0.23 ##max#max#max nm530nm9012440 abs* 2.52 abs* >3 Compound Neutral Modifie reference species species abs 0.01 abs 0.02 ##max#max#max 1313468 nm 468 0 abs* 0.24 abs* 0.25 Compound Neutral Modifie reference species species abs 0.01 abs 0.10 ##max#max#max nm526nm8414442 abs* 2.70 abs* 2.90 NOTES 1. abs = absorbance of the photochromic dye in spectroscopic grade acetone prior to activation by a light source.

2. abs* = absorbance of the photochromic dye in spectroscopic grade acetone subsequent to activation by a light source.

3. All solutions are of a similar concentration ca. 1mmoldm-3.

4. The irradiation sequence was identical for all solutions.

5. The tenn'neutral species'refers to the photochromic dye prior to modification with acid or base.

6. The term'modified species'refers to the photochromic dye subsequent to treatment by acid or base.