Photophysics and spectroscopic properties of 3-benzoxazol-2-yl-chromen-2-one.

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*).

Solvent effects on the photophysics of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one.

The photophysics of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one was studied in different solvents and in SDS micelles. This compound presents characteristics which include an S(0)---> S(1) ( pi,pi*) transition with a (1)(n,pi*) perturbative component, due to the electronic coupling between the diethylamino group and the coumarin ring, considerable solvatochromism, dual fluorescence and high fluorescence quantum yields in almost all solvents studied. The electronic structure of the S(1) and S(2) excited states permits vibronic coupling between them, making configurational changes of the S(2) excited state possible, leading to the formation of an S(2)(TICT) state. Analysis of the TCSPC data indicates an equilibrium between the S(2)(TICT) and S(1)(LE) states in favour of the former. In protic solvents, the hydrogen bonding between the solvent and the diethylamino moiety results in the formation of an S(2)(HICT) state, making internal conversion an important deactivation process. Quantum mechanical calculations for the isolated molecule show that the diethylamino group in the S(2)(TICT) state is twisted at least 56 degree from the plane of the coumarin ring, with partial electronic decoupling between -NEt(2) and the coumarin ring. This twisting angle must be positively influenced by solute-solvent interactions. [capital Phi](ST) is found to be small, but not negligible. However, Phi (delta) can be considered negligible, an indication that T(1) is a short-lived state. Based on the experimental data and theoretical calculations, the most probable sequence for the first excited states, including the TICT state, is T(1)(n, pi*) < S(2)(TICT) < S(1)(pi,pi*) approximate S(2) (n,pi*).