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By means of subpicosecond and nanosecond laser photolysis of the title compounds (MAQ and DMAQ), it is concluded that both the lowest excited singlet and triplet states undergo intramolecular hydrogen-atom transfer and the corresponding excited biradicals thus generated yield 9-hydroxy-1,10-anthraquinone-1-methide from MAQ and 9-hydroxy-4-methyl-1,10-anthraquinone-1-methide from DMAQ. Although these methides are stable at 77 K, they revert to the 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 temperature, furthermore, 313 or 366 nm steady-state photolysis reveals that 1-methyl-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 lowest excited triplet state of MAQ abstracts a hydrogen atom from ethanol generating the semiquinone radical followed by formation of MAQH 2 . In contrast, not the lowest excited triplet state but the third excited singlet or triplet state of DMAQ may abstract a hydrogen atom from ethanol yielding DMAQH 2 finally.

The lowest excited triplet state of chromone with mixed nπ*-ππ* character abstracts a hydrogen atom from the solvent. Although the ketyl-type radical thus generated is indistinguishable from the β-enol-type radical, this radical (or its keto form, i.e., the β-keto-type radical) gives rise to formation of the β-keto-type solvent adduct(s).

The lowest excited singlet and triplet states of PAQ give rise to formation of the N-ylide compound at 77 K and the reduced compound at room temperature, respectively. In the latter case, however, indirect population of the lowest excited triplet state from a higher excited singlet state (through a higher excited triplet state) is very important.