Jet cooled spectra of pyrrolobenzene and of pyrrolobenzonitrile: the nature of the excited states

The jet cooled fluorescence and resonance enhanced multiphoton ionization (REMPI) excitation spectra of pyrrolobenzene (PB) and pyrrolobenzonitrile (PBN) were recorded in the UV range. Fluorescence lifetimes and emission spectra were obtained upon excitation up to 2200 cm−1 above the 0–0 band. The data indicate that the emission is from the forbidden Lb-type state, while the absorption spectrum in solution is dominated by the La-type state. The energy gap between S1 and S2, in these molecules is very small (around 500 cm−1); at room temperature, hot bands of the more allowed La-type transition are expected to dominate the absorption spectrum.