Exciplex Type Research Articles

Exciplex type emission from anthracene doped aggregates of phenanthrene formed in n-hexane frozen solutions

Fluorescence, absorption and fluorescence excitation spectra of concentrated (3 × 10 −4 M) solutions of phenanthrene in frozen n-hexane in the presence of anthracene have been investigated at 77 and 4 K. Decay time analysis of the anthracene sensitized emission reveals the existence of at least two types of superposed emissions. One emission (τ M ≈ 9 ns) is similar to that from solid solutions of anthracene in phenanthrene crystal while the other is structureless and anthracene-concentration dependent (τ M ≈ 30–65 ns). The latter has been assigned to an exciplex type emission from particular anthracene-phenanthrene configurations which occur in very small size phenanthrene pseudocrystallites, imbedded in an n-alkane matrix.

Photochemical reaction of N-methylnaphthalene-1,8-dicarboximide with alkenes, dienes, and furans. Cyclobutane and oxetane formation

Irradiation of benzene solutions of N-methylnaphthalene-1,8-dicarboximide (1) in the presence of alkenes (2b–k) and dienes (9a–c) give the cyclobutanes (3b–j), (4c,e,f,h,i), and (10a–c). Stereospecificity of cyclobutane formation was observed in the reaction with cis- and trans-but-2-ene (2h,i), and some endo-selectivity was observed in the reaction with styrene (2f). In contrast, irradiation of benzene solutions of (1) in the presence of trans- and cis-stilbene (2j,k), 2,5-dimethylbuta-2,4-diene (9d), and furans (14a,b) gave the fragmentation products (7a,b), (8), (13a,b), (16a,b), and (17a,b) arising via precursor oxetanes. In fluorescence quenching studies of the alkenes (2a–d) a linear relationship was found for log plots of their Stern-Volmer slopes (kqτ) and ionization potentials, and the furans (14a) and (9d) showed new emissions, possibly from exciplexes. A tentative mechanistic scheme for the two types of reactions involving two types of exciplexes generated by the reaction of the singlet excited state of (1) and unsaturated compounds is proposed.

Photocycloaddition of 4-dimethylaminostyrene with 1-vinylpyrene or styrene

Photocycloaddition reactions of 4-dimethylaminostyrene (1) as an electron donor with 1-vinylpyrene (2) or styrene (3) as electron acceptors were studied. In non-polar solvents, photoirradiation of (1) with (2) gave trans- and cis-1-(4-dimethylaminophenyl)-2-pyren-1-ylcyclobutane (4t and 4c), whereas photoreaction of (1) with (3) gave only cis-1-(4-dimethylaminophenyl)-2-phenylcyclobutane (5). The yield of these cycloadducts decreased with increasing solvent polarity, and in polar solvents mainly the copolymer was produced. Transient intermediates were studied by nanosecond ruby laser photolysis of the system (1)+(2). The transient absorption spectrum in the non-polar solvent benzene was ascribed to the exciplex type, while in the polar solvent acetonitrile the spectrum of free cation and anion radicals was observed. These facts imply that photochemical cycloaddition of these systems occurred via an exciplex as the intermediate. The formation of cis- and trans-cycloadducts is discussed in terms of the reactivity of the exciplex state and the conformation of the exciplex.