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

Vertical S1–S0 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 S1 state has a slope of 0.59 and intercept of 9100 cm−1. These results support the concept that no geometry twist in the S1 state of PRODAN is necessary to explain the observed solvent effects on fluorescence.