Photophysics and photochemistry of planar alkylanthraquinones (the 1-methyl and 1,4-dimethyl compounds) studied by subpicosecond and nanosecond laserphotolysis as well as steady-state photolysis

By means of subpicosecond and nanosecond laser photolysis of the title comp ounds (MAQ and DMAQ), it is concluded that both the lowest excited singlet and triplet states undergo intramolecular hydrogen-atom transfer and the co rresponding excited biradicals thus generated yield 9-hydroxy-1,10-anthraqu inone-1-methide from MAQ and 9-hydroxy-di-methyl-1,10-anthraquinone-1-methi de from DMAQ. Although these methides are stable at 77 K, they revert to th e original anthraquinones at a higher temperature than 77 K. In ethanol and EPA (diethyl ether-isopentane-ethanol in 5:5:2 volume ratio) at room tempe rature, furthermore, 313 or 366 nm steady-state photolysis reveals that 1-m ethyl-9,10-dihydroxyanthracene (MAQH(2)) is formed from MAQ irrespective of the excitation wavelength but 1,4-dimethyl-9,10-dihydroxyanthracene (DMAQH (2)) is formed from DMAQ only upon 313 nm excitation. Undoubtedly, the lowe st excited triplet state of MAQ abstracts a hydrogen atom from ethanol gene rating the semiquinone radical followed by formation of MAQH2. In contrast, not the lowest excited triplet state but the third excited singlet or trip ler stare of DMAQ may abstract a hydrogen atom from ethanol yielding DMAQH( 2) finally.