Simulation of solvent dynamics effects on the fluorescence of 3-methylindole in water

The UV absorption and fluorescence of 3-methylindole in water at 300 K has been simulated over periods of 4 to 30 ps in length using a combination of classical molecular dynamics simulations and a spectroscopically calibrated semiempirical molecular orbital method (INDO/S-SCI). The absorption redshift is predicted to be 545 ±150 cm^-1 for ¹L_a, and 85 ±15 cm^-1 for ¹L_b. The distribution of transition energies due to solvent fluctuations suggests inhomogeneous broadening at lower temperatures of about 2000 cm^-1 for ¹L_a, and 400 cm^-1 for ¹L_b, (fwhm). The large time-dependent shift of the ¹L_a fluorescence was determined both by direct nonequilibrium response to an instantaneous charge redistribution and from the autocorrelation function of the fluctuations of the ¹L_a transition energies from the equilibrated ground state. The two methods gave similar response times, roughly fitting a double exponential with time constants of 17 and 290 fs. The magnitude of the calculated shifts with partial and complete solute polarizability are 3000 and 5000 - 6000 cm^-1, bracketing an experimental estimate of 3800 cm^-1. A connection with perturbation-free energy of solvation (ΔA_s) results is made which identifies ΔA_s as the average of absorption and fluorescence maximum shifts upon solvation, (▽TAν_a + Δν_f)/2.