Dual emission of 4-(1H-pyrrol-1-yl)benzonitrile clusters with acetonitrile in a supersonic jet and its absence in phenylpyrrole clusters

The fluorescence of clusters of 4-(1H-pyrrol-1-yl)benzonitrile (PBN) and of phenylpyrrole with acetonitrile (AN) was studied in a supersonic jet. Two separate bands are found in the case of PBN/AN clusters, one centered at 305 nm, the other at 450 nmn. The blue- and red-shifted bands are assigned to locally excited (LE) and charge transfer (CT) states, respectively, in analogy with solution spectra of PBN in acetonitrile. The CT emission is observed only when mass spectra obtained upon resonance-enhanced multiphoton ionization (REMPI) of the clusters contain cluster ions in which at least four acetonitrile molecules are attached to a PBN molecule. In contrast, only a single emission band centered at 300 nm (assigned to a LE state) is observed from clusters of 1-phenyl-1H-pyrrole (PP) with acetonitrile prepared under similar conditions, regardless of cluster size. REMPI-induced mass spectra measured in coincidence with the fluorescence reveal a similar distribution of cluster sizes for the two phenylpyrrol derivatives. A possible rationalization for the different emission profiles is discussed in terms of excited-state dynamics.