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

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