In connection with quenching of the lowest excited singlet state of 9,10-dichloroanthracene (DCA) by ground-state 2,5-dimethylhexa-2,4-diene (DMHD), measurements of the fluorescence spectra reveal formation of an exciplex ((DCA-DMHD)*) irrespective of the dielectric constant (ε) of a solvent. By means of subpicosecond and nanosecond laser photolysis as well as steady-state photolysis, however, the following results are obtained: (a) in acetonitrile, methanol or acetone, decomposition of (DCA-DMHD)* generates the DCA radical anion (DCA•-) as an intermediate for the formation of 9-chloroanthracene (CA), but its rate (υ) decreases in the order of υ(acetonitrile, ε=37.5)&gt
υ(acetone, ε=20.7)&gt
υ(methanol, ε=32.7)
(b) in 1-octanol (ε=10.3), diethyl ether (ε=4.34) or heptane (ε=1.92), decomposition of (DCA-DMHD)* generates an open-form transient (OFT, i.e. a ground-state singlet biradical or its zwitterionic form (ZIF)) as an intermediate for formation of the [4+2] adduct
(c) in ethanol (ε=24.6) or 2-propanol (ε=19.9), not only OFT (major) but also DCA•- (minor) are generated from (DCA-DMHD)* and formation of two compounds (the [4+2] adduct and CA) in ethanol (or only the former compound in 2-propanol) can be seen. We thus conclude that not only the dielectric constant of a solvent but also its hydrogen-bonding ability affects the photochemistry of DCA originating from (DCA-DMHD)*. © 2000 Elsevier Science S.A. All rights reserved.