Photophysical Behavior of Coumarins as a Function of Substitution and Solvent: Experimental Evidence for the Existence of a Lowest Lying 1(n,?*) State

Article abstract of DOI:10.1021/j100075a002

The nature of the lowest excited state for coumarin and some derivatives was investigated using steady-state and time-resolved fluorescence data at room temperature, as well as fluorescence anisotropy at 77 K in nonpolar and polar solvents in conjuction with theoretical data obtained with different methods: INDO/S-CI, CNDO/S-CI, MNDO-CI, and AM1-CI.The results show that S₁ is actually n,?^* for coumarin in any solvent.Substitution with methoxy and methyl and/or chlorine and/or the increase of solvent polarity reduces the energy gap between the S₁ (n,?^*) and S₂ (n,?^*) states, promoting the mixing of these states and finally inducing inversion to S₁ (?,?^*) with trisubstitution and for the disubstituted case in dioxane:water (1:4).The presence of a lowest lying S₁ (n,?^*) mediates a large degree of radiationless processes in the singlet mannifold.