Evidence of pure Lb1 fluorescence from redshifted indole-polar solvent complexes in a supersonic jet

• Published in: The Journal of chemical physics 1998-06, Vol.108 (24), p.10189-10196
• Main Authors: Short, Kurt W., Callis, Patrik R.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.476478

Two-photon polarized fluorescence excitation and vibronically resolved one-photon dispersed fluorescence spectra of the long-wavelength absorbing van der Waals complexes of indole with water, methanol, and formamide were examined for the purpose of firmly assigning the nature of the lowest singlet excited electronic state. The two-photon spectra for all of these complexes have circular/linear polarization ratios of absorptivity (Ω-values) that show that excitation is to the Lb1 state. Analysis of the Franck–Condon (FC) patterns of the dispersed fluorescence for these indole-polar solvent complexes show that emission is also from the Lb1 state in each case. In the emission spectra, the intensity ratios of the origin and ν26 lines are about 2:1, which is the value expected for Lb1 emission. The ν26 vibration is the most intense nonorigin vibronic line, as expected for Lb1 emission. Finally, there is little or no intensity from the ν8, ν9, and ν10 vibrations, which would be strongly active with emission from the La1 state. These results show unequivocally that for these indole-polar solvent complexes the La1 state does not shift in energy below the Lb1 state—contrary to the interpretation of several recent papers. Further support is given to the assignment of Lb1 emission by spectral simulations. The jet-cooled complexes do not exhibit excited state complex (exciplex) characteristics, wherein the solvent is much more strongly bound in the excited state than in the ground state.