4-Methyl-8-hydroxy-benzo(6,7)coumarin, 1,4-methyl-6-hydroxy-benzo(5,6)coumarin, II, and 2-methyl-6-hydroxy-benzo(5,6)chromone, III, have shown similar absorption and fluorescence emission spectra. Fluorescence emission quantum yields for I and III are found to be very low, phi(f) = 0.02, but 4-methyl-6-hydroxy-benzo(5,6)coumarin, II, has a eight-fold higher fluorescence quantum yield of the other two specie, in acetonitrile solution, of = 0.16. Quenching of anthracene fluorescence emission by 1, II and III are found to give k(q) values of 1.0 x 10(7)-1.2 x 10(9) M-1 s(-1). Benzo(5,6)coumarin, II, which gives the most intense fluorescence also presents the highest quenching rate, k(q) = 1.2 x 10(9) M-1 s(-1). Experimentally determined k(q) values are seen to correlate well with the free energy of electron transfer (DeltaG(ET)) which are calculated to be in the range of -8.0 to -9.4 kcal/mol, where benzo(5,6)coumarin, II, gives the lowest free energy of electron transfer DeltaG(ET) = -9.4 kcal/mol. These results indicate that I-III behave as electron acceptor moieties toward a condensed aromatic ring, anthracene. The Stokes shift values of 88-105 mm and broad fluorescence emission bands respect to absorption-excitation bands, indicates a molecular structure change in the excited states of I-III. Fluorescence lifetimes of 0.1-0.9 ns in I-III, singlet oxygen quantum yields of 0.15 and 0.40 for I and II, respectively, may be taken as evidence of singlet-triplet intersystem crossings. The photooxidation products of alpha-terpinene, sensitised by II, under direct and concentrated sun light conditions that are mainly p-cymene and ascaridole. In accordance with literature data on coumarin derivatives, benzocoumarins also seem to produce singlet oxygen and beside singlet oxygen, in addition super oxide anion radical production appear to be dominant especially under concentrated sun light. Under direct sun light conditions ascaridole is the major product. Some by-products of alpha-terpinene photooxidation are also determined at GC-MS analysis. Those by-products are assumed to be generated from ascaridole decomposition. (C) 2002 Elsevier Science B.V All rights reserved.
