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, ${}^1{L}_a$ and ${}^1{L}_b.$ The dispersed fluorescence of these from this series also reflects the crossing, providing a definitive ${}^1{L}_a$ jet-cooled fluorescence spectrum. The jet-cooled ${}^1{L}_a$ fluorescence spectrum is not broad and redshifted, but has vibronic structure that agrees well with ab initio predictions and is similar to that of ${}^3{L}_a$ phosphorescence.