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

Abstract

Sub-picosecond to nanosecond laser photolysis of the title compound (MBP) at room temperature reveals that its lowest excited triplet state probably with the form cis- 3 MBP ∗ , undergoes intramolecular hydrogen atom transfer from the methyl group to the carbonyl oxygen. The lowest excited triplet cis- 3 BR ∗ and then trans- 3 BR ∗ 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 in ethanol compared with that in acetonitrile or benzene is tentatively ascribed 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- 3 BR ∗ (generated from cis- 3 MBP ∗ via cis- 3 BR ∗→trans- 3 BR ∗ conversion) 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-emissive lowest excited singlet or triplet state of trans-DE yields the keto form of dihydroanthrone (DHA); at a higher temperature than 77 K, this keto form reverts to MBP via DHA and then trans-DE.