EVIDENCE OF PURE L-1(B) FLUORESCENCE FROM REDSHIFTED INDOLE-POLAR SOLVENT COMPLEXES IN A SUPERSONIC JET

Two-photon polarized fluorescence excitation and vibronically resolved one-photon dispersed fluorescence spectra of the long-wavelength abso rbing van der Waals complexes of indole with water, methanol, and form amide were examined for the purpose of firmly assigning the nature of the lowest singlet excited electronic state. The two-photon spectra fo r all of these complexes have circular/linear polarization ratios of a bsorptivity (Omega-values) that show that excitation is to the L-1(b) state. Analysis of the Franck-Condon (FC) patterns of the dispersed fl uorescence for these indole-polar solvent complexes show that emission is also from the L-1(b) State in each case. In the emission spectra, the intensity ratios of the origin and v(26) lines are about 2:1, whic h is the value expected for L-1(b) emission. The v(26) vibration is th e most intense nonorigin vibronic line, as expected for L-1(b) emissio n. Finally, there is little or no intensity from the v(8), y(9), and v (10) vibrations, which would be strongly active with emission from the L-1(a) state. These results show unequivocally that for these indole- polar solvent complexes the L-1(a) state does not shift in energy belo w the L-1(b) state-contrary to the interpretation of several recent pa pers. Further support is given to the assignment of L-1(b) 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. (C ) 1998 American Institute of Physics.