Evidence for La1 fluorescence from jet-cooled 3-methylindole-polar solvent complexes

The ratio of nonresonant two-photon induced fluorescence excitation spectra using circularly and linearly polarized light for jet-cooled 3-methylindole complexed with a series of increasingly basic hydrogen bond acceptors (water, methanol, ethanol, diethylether, diethylamine and triethylamine) is consistent with an avoided crossing of the two lowest excited singlet states, La1 and Lb1. The dispersed fluorescence of these from this series also reflects the crossing, providing a definitive La1 jet-cooled fluorescence spectrum. The jet-cooled La1 fluorescence spectrum is not broad and redshifted, but has vibronic structure that agrees well with ab initio predictions and is similar to that of La3 phosphorescence.