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

Abstract

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. Φ_ST is found to be small, but not negligible. However, Φ_Δ 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*).