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The relaxation pathways of excited trans-N,N′-diacylindigo dyes (trans-1–trans-4) and of the rigid trans (5) and cis (6) indigo analogues have been studied by nanosecond flash photolysis and steady state measurements. In fluid solution the trans to cis photoisomerization of 1–4 occurs via a singlet excited state mechanism on direct excitation, and the triplet state appears to be reached only by sensitized reaction. The lower quantum yield for trans → cis isomerization of 2–4 at room temperature, as compared to 1, is due to a thermally activated internal conversion step leading to the trans ground state. This pathway competes with fluorescence and twisting of the molecule around the central double bond in the excited singlet state. Increasing the viscosity of the medium decreases the twisting of the flexible molecules and, as a consequence, increases the population of the lowest triplet state. A triplet transient, the intensity of which increases with the viscosity of the medium, could be characterized at low temperature under direct excitation. A transient species, which can be assigned to a triplet state on the basis of quenching experiments and on the fact that the same transient could be found under sensitized conditions, also has been observed at room temperature for the rigid 5 and 6 molecules. Quenching measurements of triplet sensitizers by 5, trans-1, and trans-2 enables the localization of the triplet energy at around 40 kcal/mol.

The photochemical behaviour of N,N1-disubstituted indigo dyes has been investigated. The trans to cis and cis to trans photoisomerization quantum yields are found to be of the order of 0.25 or less (down to 0.03). These dyes, which fluoresce only weakly in solution and at room temperature, show a drastic increase in their fluorescence when inserted into a polymer matrix or at 77 K in butyronitrile glass. This has been interpreted in terms of the inhibition of formation of a twisted excited intermediate as well as of the rotation of the substituent on the nitrogen atom. An important electron transfer occurs from the singlet excited state when the indigo dyes are placed in the presence of electron-donating molecules. When irradiating with visible light, the photostationary state is always in favour of the trans isomers. This, and kinetic as well as thermodynamic considerations, excludes these compounds as candidates for solar energy storage.JOSEPH POULIQUEN, VERONIQUE WINTGENS, VICENTE TOSCANO, BRAHIM BEN JAAFAR, SADHANA TRIPATHI, JEAN KOSSANYI et PIERRE VALAT. Can. J. Chem. 62, 2478 (1984.Le comportement des indigos substituks 1-4 a kt6 examink. On trouve que les rendements quantiques de photoisomerisation sont faibles (de l'ordre de, ou infkrieurs 9, 0,20). Ces composks qui fluorescent faiblement en solution et B 20°C, prksentent une augmentation considkrable de leur fluorescence quand ils sont inclus dans un polymkre ou bien lorsque la tempkrature est abaisske B 77 K dans le butyronitrile. Ceci a kt6 interpret6 comme un empschement a la rotation autour de la double liaison centrale, et comme une gene i la rotation du substituant sur l'azote ainsi que de l'inversion de cet atome. L'irradiation en lumi2re blanche est toujours en faveur des isomkres trans. Ceci, ainsi que des considkrations cinktiques et thermodynamiques excluent ces composks pour le stockage de l'tnergie solaire. IntroductionIn the search for new sources of energy, photochemical transformations have been examined as a potential means of obtaining energy-rich stable products. Criteria for efficient photochemical conversion of solar energy are: absorption in the visible or near-ultraviolet region, high quantum efficiencies, large positive ground state enthalpies, and stable photoproducts. In addition, the reactant must be inexpensive and not dangerous.The importance of dyes has been widely recognized in different converting systems related to photobiology (1,2), photoelectrochemistry (3, 4), and photochemistry (5).Photochemical systems mostly involve (Fig. 1) absorption of visible light by a reagent R to produce a high energy content product P which can later release the stored energy while reverting to the starting material. Among the systems reported thus far, one finds intramolecular cycloadditions (the best system investigated seems to be the norbornadiene-quadricyclane one (6) which can store about 26 kcal/mol(7)) and valence isomerization (such as the photochemical transformation of 1-methyl 5-phenyl A-2 pyrazoline into 2-phenylcyclopropylazo...