Solvent effects on the photophysics of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one
Tóm tắt
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 S0 → S11(π,π*) transition with a 1(n,π*) 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 S1 and S2 excited states permits vibronic coupling between them, making configurational changes of the S2 excited state possible, leading to the formation of an S2(TICT) state. Analysis of the TCSPC data indicates an equilibrium between the S2(TICT) and S1(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 S2(HICT) state, making internal conversion an important deactivation process. Quantum mechanical calculations for the isolated molecule show that the diethylamino group in the S2(TICT) state is twisted at least 56° from the plane of the coumarin ring, with partial electronic decoupling between—NEt2 and the coumarin ring. This twisting angle must be positively influenced by solute-solvent interactions. ΦST is found to be small, but not negligible. However, ΦΔ can be considered negligible, an indication that T1 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 T1(n,π*) < S2(TICT) < S1(π,π*) ≈ S2(n,π*).
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