Photosensitized generation of singlet molecular oxygen by aryloxazinones

The photophysical properties, the capacity to produce singlet molecular oxygen by sensitization and the photostability of a series of 20 aryloxazinones and one quinoxalinone have been evaluated using a combination of methods including steady-state and time-resolved spectroscopic techniques. The photophysics of aryloxazinone derivatives and consequently its capacity to produce singlet molecular oxygen is very dependent on the structure. Benzoxazinone derivatives substituted in para position to the 2-phenyl group (1)–(3) have low fluorescence quantum yields and produce singlet oxygen in moderate quantum yields. Inclusion of electron donors groups in position 7 of the aromatic fused moiety to the heterocyclic ring (4)–(6), increases significantly the emission from the excited singlet state with a concomitant diminution in the ability to produce singlet oxygen. An exception corresponds to the 7-methoxy-2-phenylbenzoxazinone (4) able to generate larger quantities of excited oxygen and photodecompose in less than a 2% under large energy doses in aerobic irradiation conditions. In general, 2-phenyl and 2-methyl naphthoxazinones derivatives (7)–(17) are more fluorescent than the compounds of the corresponding benzo series, and generate singlet molecular oxygen in low to moderate yields. Exceptions are the 2-methyl (13) and the 9-methoxy-2-methyl (14) derivatives that produce excited oxygen efficiently, however, with appreciable decomposition. The most promising compounds to be employed as singlet oxygen sensitizers are the anthracene like 3-phenyl-2H-naphtho[2,3-b][1,4]-oxazin-2-one (19) and the 1-methyl-3-phenylquinoxalin-2(1H)-one (21). These compounds are photostable in the absence and in the presence of electron donor additives under large doses of irradiation, accomplishing all requisites to be good sensitizer and to produce singlet oxygen in high yields, almost independently on solvent polarity.