RESUMO
The photophysics of 3-benzoxazol-2-yl-chromen-2-one was studied in different solvents. High molar absorptivities, between 14,800 and 22,900 dm3/mol cm, were observed for the absorption peak related to the S0 --> S1 transition which suggests a pi --> pi* character. This compound presents a limited solvatochromism, attributed to the benzoxazole group, and high fluorescence quantum yields, phi(f). The fluorescence quantum yield is lowered with the increase of solvent polarity, favouring the participation of internal conversion as deactivation path of the S1 state. The Stokes shift shows that the excited state is stabilised with increasing solvent polarity. The dipole moment was estimated by ab initio calculations as being between 5.28 and 5.62 Debye for S1, and 4.75 Debye for S0. Phosphorescence was not observed. A small but not negligible quantum yield of singlet oxygen generation (phidelta = 0.15) was measured in chloroform. The geometric parameters obtained by semi-empirical calculation (PM3) are in good agreement with crystallographic data, showing a r.m.s. deviation of 0.153 A for the superposition of both structures. The predicted structure is all planar, while the crystallographic data reveal a dihedral angle of 6.5 degrees, between the coumarin and benzoxazole rings. The theoretical description of the electronic spectra, obtained from a PM3 CI calculation, shows excellent agreement with the experimental data. Deviations lower than 2% are observed in the predicted absorption maxima, with best results when solvation is considered. For electronic states calculation, ZINDO/S gave a better prediction of excited state energies, with a deviation lower than 7% for the S1 energy. The most probable sequence for the first four excited states is: Ti(n pi*) < T2(pi pi*) < S1(pi pi*) < S2(n pi*).
