Two-color two-photon excited fluorescence of indole: Determination of wavelength-dependent molecular parameters.

We present a detailed study of two-color two-photon excited fluorescence in indole dissolved in propylene glycol. Femtosecond excitation pulses at effective wavelengths from 268 to 293.33 nm were used to populate the two lowest indole excited states ¹L_a and 1L_b and polarized fluorescence was then detected. All seven molecular parameters and the two-photon polarization ratio O containing information on two-photon absorption dynamics, molecular lifetime t_f, and rotation correlation time trot have been determined from experiment and analyzed as a function of the excitation wavelength. The analysis of the experimental data has shown that ¹L_b-¹L_a inversion occurred under the conditions of our experiment. The two-photon absorption predominantly populated the ¹L_a state at all excitation wavelengths but in the 287-289 nm area which contained an absorption hump of the ¹L_b state 0-0 origin. The components of the two-photon excitation tensor S were analyzed giving important information on the principal tensor axes and absorption symmetry. The results obtained are in a good agreement with the results reported by other groups. The lifetime t_f and the rotation correlation time trot showed no explicit dependence on the effective excitation wavelength. Their calculated weighted average values were found to be t_f = 3.83 ± 0.14 ns and t_rot = 0.74 ± 0.06 ns. [ABSTRACT FROM AUTHOR]