The Exploration of Novel Photochromic Materials

This thesis describes the synthesis and photochromic response of some novel T-type 1,3,3-trisubstituted spiro[indoline-2,2'-phenanthro[9,10-b][1,4]oxazines], 1,3,3-trisubstited spiro[indoline-2,2'-[1,4]-oxazino[2,3-f][1,10]phenanthrolines] and their nickel complexes and additionally some novel spiropyrans (1',3',3'-trisubstituted 6,8-dinitrospiro[chromene-2,2'-indolines]).

An efficient and versatile route to several novel 1-alkyl-3,3-dimethyl-2-methyleneindolines (Fischer’s bases) has been extended. The route commences with a two-stage selective, reductive N-alkylation of aniline using commercially available aliphatic aldehydes with benzotriazole serving as a synthetic heterocyclic auxiliary. The intermediate 1-(1H-benzo[d][1,2,3]triazol-1-yl)alkan-1-ols were cleanly reduced with sodium borohydride in THF to afford a series of N-alkylanilines excellent yield. High yielding N-nitrosation of the foregoing N-alkylanilines was accomplished using the classical method of generation of the nitrosyl chloride upon treatment of a cold solution of sodium nitrite with aqueous HCl. 1-Phenyl-1-(alkyl)hydrazines were obtained by reduction of the N-alkyl-N-phenylnitrous amides using Red-Al [sodium bis(2-methoxyethoxy)aluminium hydride] in toluene with careful temperature control. Treatment of the foregoing hydrazines with 3-methylbutan-2-one afforded the hydrazones which were used directly to form the Fischer’s bases in moderate yield upon heating in glacial acetic acid.

9,9-Di(n-hexyl)-9H-fluoren-2-amine was obtained in three steps in 48 % overall yield by nitration (HNO3, AcOH), C-9 bis-(n-hexylation) (KOH, n-hexyl iodide, DMSO) and reduction (hydrazine, Pd[C], EtOH) of 9H-fluorene. Exposing this highly electron rich dialkyl-9H-fluoren-2-amine to the benzotriazole-mediated N-alkylation protocol resulted in efficient conversion to N-(n-butyl)-9,9-di(n-hexyl)-9H-fluoren-2-amine however subsequent N-nitrosation could not be cleanly achieved.

During the mono-N-(n-butylation) of 9,9-di(n-hexyl)-9H-fluoren-2-amine an interesting by-product containing a 2,3-disubstituted tetrahydroquinoline unit (3-ethyl-10,10-dihexyl-2-propyl-2,3,4,10-tetrahydro-1H-indeno[1,2-g]quinoline) was isolated and fully characterised. Preliminary investigation of this ring-forming protocol revealed that increasing the amount of aldehyde in the benzotriazole-mediated N-alkylation sequence favours the formation of the quinoline unit. The formation of the tetrahydroquinoline ring by the foregoing protocol constitutes a new quinoline ring forming reaction.

Commercially available 2-methoxy-dibenzo[b,d]furan-3-amine was also subjected to the benzotriazole-mediated N-alkylation, N-nitrosation sequence. Interestingly, the N-isobutyl-N-(2-methoxy-dibenzo[b,d]furan-3-yl)nitrous amide was isolated as a mixture of atropisomers with each isomer being readily observed by 1H NMR spectroscopy.

Fischer indolisation of the 1-methyl-1-phenylhydrazones derived from acetylcyclohexane, ethyl 2-methyl-3-oxobutanoate and 3-acetyldihydrofuran-2(3H)-one gave moderate yields of the 3,3-disubstituted Fischer’s bases, 1'-methyl-2'-methylenespiro[cyclohexane-1,3'-indoline], ethyl 1,3-dimethyl-2-methyleneindoline-3-carboxylate and 1'-methyl-2'-methylene-4,5-dihydro-2H-spiro[furan-3,3'-indolin]-2-one, respectively; the latter pair of which are novel. Interestingly, (E)-2-(1-(2-methyl-2-phenylhydrazineylidene)ethyl)cyclopentan-1-one, the hydrazine derived from 1-methyl-1-phenylhydrazone and 2-acetylcyclopentanone was shown by X-ray crystallography to exist as the ene-hydrazine tautomer, ((Z)-2-(1-(2-methyl-2-phenylhydrazineyl)ethylidene)cyclopentan-1-one), with a stabilising intramolecular H-bond and as such failed to undergo a Fischer indolisation.

Condensation of the foregoing Fischer’s bases with 10-nitrosophenanthren-9-ol, derived from the C-nitrosation of 9-phenanthrol, in refluxing ethanol gave novel positive, T-type photochromic 1-alkyl-3,3-trimethylspiro[indoline-2,2'-phenanthro[9,10-b][1,4]oxazines] in good yield which generated blue-green photoisomers (max ca. 585 nm) upon exposure of their toluene solutions to UV irradiation. The persistence (half-life) of the coloured photoisomers was markedly dependent upon the position of the branching point and extent of the branching of the N-alkyl group of the Fischer’s base. Remarkably, the oxazine, (1'-methyl-4,5-dihydro-2H-dispiro[furan-3,3'-indoline-2',2''-phenanthro[9,10-b][1,4]oxazin]-2-one) derived from the lactone containing Fischer’s base, exhibited negative photochromism giving an initial blue-green solution (max ca. 585nm) which could be reversibly bleached with white light; this behaviour constitutes the first report of a negatively photochromic phenanthrol[9,10-b][1,4]oxazine.

6-Nitroso-1,10-phenanthrolin-5-ol was obtained in two steps by the oxidation of 1,10-phenanthroline with potassium bromate in 60% aqueous sulfuric acid at room temperature, and subsequent routine mono-oximation with hydroxylamine hydrochloride. Condensation of 6-nitroso-1,10-phenanthrolin-5-ol with the novel N-alkyl Fischer’s bases provided a new series of T-type positively photochromic 1-alkyl-3,3-trimethylspiro[indoline-2,2'-[1,4]oxazino[2,3-f][1,10]phenanthrolines] which exhibited turquoise coloured solutions upon excitation with UV irradiation (max ca. 585 nm). Complexation of the foregoing oxazinophenanthrolines with Ni2+ salts followed by a counterion exchange from chloride to tetra(phenyl)borate gave fully reversible, blue-green coloured (max 605 nm), negatively photochromic complexes in moderate yield.

Condensation of 3,5-dinitrosalicylaldehyde with the new 1-alkyl-3,3-dimethyl-2-methyleneindolines gave a series of 1',3',3'-trisubstituted 6,8-dinitrospiro[chromene-2,2'-indolines] in good yield and which exhibited excellent negative photochromic properties with the initial red-orange solutions (max ca. 560 nm) being reversibly bleached upon exposure to white light.

Several of the novel Fischer’s bases were condensed with squaric acid in a toluene / n-butanol mixture to afford new, vibrant blue coloured (max ca. 640 nm), squaraine dyes in 30 – 55 % yield, with molar extinction coefficients of ca. 4.4 – 5.2 × 105 mol-1dm3cm-1.