Excited-state dynamics for interpretation of the solvent-dependent photochemistry of 9,10-dichloroanthracene in the presence of 2,5-dimethylhexa-2,4-diene

Abstract 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 ( e ) 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 ( v ) decreases in the order of v (acetonitrile, e =37.5)> v (acetone, e =20.7)> v (methanol, e =32.7); (b) in 1-octanol ( e =10.3), diethyl ether ( e =4.34) or heptane ( e =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 ( e =24.6) or 2-propanol ( e =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) * .