Solvent induced shifts of electronic spectra IV. Computational study on PRODAN fluorescence and implications to the excited state structure

Vertical S(1)-S(0) electronic transitions of the highly solvent-sensitive fluorescence label 2-propionyl-6-dimethylamino naphthalene (PRODAN) are modeled by semiempirical CISD AM1 and TD DFT calculations in a large number of solvents of different polarity and hydrogen donating ability. Calculations correctly reproduce the observed solvent induced shifts of the emission maxima. The fluorescence Frank-Condon transition energies in solvent can be predicted quantitatively at the AM1 SM5.42 OPEN(2,2) C.I. = 5 CISD level. For the planar PRODAN emitting state at the latter level we obtain a regression with practically unit slope and zero intercept for aprotic solvents, The respective relationship for the O-twisted S(1) state has a slope of 0.59 and intercept of 9100 cm(-1). These results support the concept that no geometry twist in the S, state of PRODAN is necessary to explain the observed solvent effects on fluorescence. (C) 2008 Elsevier B.V. All rights reserved.