Further study on intramolecular hydrogen-atom transfer originating from the lowest excited triplet state of 2-methylbenzophenone

Sub-picosecond to nanosecond laser photolysis of the title compound (MBP) a t room temperature reveals that its lowest excited triplet state probably w ith the form cis-(MBP)-M-3*, undergoes intramolecular hydrogen atom transfe r from the methyl group to the carbonyl oxygen. The lowest excited triplet cis-(BR)-B-3* and then trans-(BR)-B-3* 1, 4-radicals thus generated convert to the ground-state cis-DE and trans-DE, respectively. At room temperature , both the dienols revert to MB but extremely rapid reversion of trans-DE i n ethanol compared with that in acetonitrile or benzene is tentatively ascr ibed to double-proton transfer through ethanol-bridged two hydrogen bonds ( cf. Scheme 2). At 77 K, steady-state photolysis of MBP yields trans-DE as a stable product but its nanosecond excitation gives rise to the appearance of no emission and transient absorption spectra responsible for the lowest excited singlet or triplet state of trans-DE. This indicates that trans-(BR )-B-3* (generated from cis-3MBP* via cis-(BR)-B-3* --> trans-(BR)-B-3* conv ersion) is different from the lowest excited triplet state of trans-DE. By measurements of the phosphorescence and its excitation spectra after steady -state photolysis of MBP or trans-DE at 77 K, it is proposed that the non-e missive lowest excited singlet or triplet state of trans-DE yields the keto form of dihydroanthrone (DHA); at a higher temperature than 77 K, this ket o form reverts to MBP via DHA and then trans-DE. (C) 1998 Elsevier Science S.A. All rights reserved.