Complementary pairs of oligonucleotides in which the stilbene chromophore is incorporated either in the middle or at the end of a 10 base pair sequence have been prepared and their spectroscopic and photochemical properties investigated. The individual oligonucleotides display fluorescence spectra and photoisomerization similar to that of a model 4,4'-stilbenedicarboxamide. Variations in fluorescence quantum yield and decay times are attributed to interactions with neighboring bases. One-to-one mixtures of complementary oligonucleotides form stable duplexes with melting temperatures of 40 and 46 °C, respectively, for the mid-strand and terminally labeled duplexes. Duplex formation results in hyperchromism of both the nucleotide and stilbene absorption bands and the appearance of broad, long-wavelength fluorescence attributed to an excited stilbene dimer. These duplexes provide a unique opportunity for the investigation of the here-to-for elusive stilbene excimer. The long wavelength absorption bands of both duplexes are efficiently bleached upon irradiation. Bleaching is a consequence of stereospecific stilbene [2+2] photodimerization and results in the formation of cross-linked duplexes of high thermal stability. Excimer and monomer fluorescence are found to provide a highly sensitive probe of duplex formation and dissociation.

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The molecular structure and photophysical behavior of several secondary and tertiary N-(aminoalkyl)phenanthrenecarboxamides have been investigated. Secondary (aminoalkyl)amides exist predominantly in the Z conformation, whereas tertiary amides exist as mixtures of Z and E conformers and semirigid piperazines as mixtures of chair conformers. Rate constants for endergonic intramolecular electron transfer are found to be highly dependent upon molecular structure. The aromatic and amide groups of the tertiary amides are essentially orthogonal, and thus, an E aminoalkyl group can adopt low-energy conformations in which there is spatial overlap between the aromatic and amine groups, whereas such overlap is not possible for either a Z aminoalkyl group or the piperazines. The observation of more rapid intramolecular electron transfer quenching of the phenanthrene singlet by an appended trialkylamine in the E vs Z conformation is attributed to this difference in overlap. An increase in the phenanthrene−amide dihedral angle is also found to result in a decrease in the rate constant for intramolecular electron transfer quenching by a Z aminoalkyl group. In the case of appended tertiary anilines, efficient electron transfer quenching occurs for both Z and E conformers. The Z conformers form fluorescent exciplexes, providing a new example of exciplex-type emission in the absence of direct π−π overlap. Exciplexes formed by the E conformers are nonfluorescent and apparently undergo rapid intersystem crossing. The strong exciplex emission observed at low temperatures both in solution and in frozen glasses is attributed to ground state dimers or aggregates.

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Methylviologen cation radical is produced by photolysis of methyl viologen dichloride entrapped in a sol-gel glass. The sol-gel glass is shown to be microporous, and it is suggested that the microporosity prevents oxygen diffusion thereby helping to extend the lifetime of the cation radical to a period of months. Initial losses of a substantial fraction of the charge separation are not observed. These results bear directly on efficient photochemical energy storage.