Primary process for the photochemistry of 9,10-dibromoanthracene yielding 9-bromoanthracene in cyclohexane-amine (triethylamine or N,N-dimethylaniline) studied by picosecond laser photolysis

Abstract Picosecond laser photolysis of 9,10-dibromoanthracene (DBA) in cyclohexane-amine (triethylamine or N , N -dimethylaniline) leads to the formation of a DBA-amine exciplex between the lowest excited singlet state of DBA and ground state amine. However, during the decrease in the exciplex absorption with time, no accompanying appearance of an absorption band attributable to the DBA radical anion (DBA .− ) can be seen. Nanosecond laser photolysis of DBA in cyclohexane-amine also reveals no existence of an absorption band attributable to DBA .− , but its absorption band is clearly seen in acetonitrile-amine (cf. K. Hamanoue, S. Tai, T. Hidaka, T. Nakayama, M. Kimoto and H. Teranishi, J. Phys. Chem. , 88 (1984) 4380, K. Hamanoue, T. Nakayama, K. Ikenaga, K. Ibuki and A. Otani, J. Photochem. Photobiol. A: Chem. , 69 (1993) 305). In accordance with this, the rate of DBA → 9-bromoanthracene debromination on steady state photolysis in cyclohexane-amine is extremely slow compared with that in acetonitrile-amine. On the basis of these results, it can be concluded that the intermediaate in the amine-assisted DBA → 9-bromoanthracene debromination is DBA .− , but the decomposition of the DBA-amine exciplex into DBA .− and the amine radical cation is strongly affected by the dielectric constant of the pure solvent.