High-Resolution Electronic Spectroscopy of the Doorway States to Intramolecular Charge Transfer

Reported here are several of the ground, first, and second excited state structures and dipole moments of three benchmark intramolecular charge transfer (ICT) systems; 4-(1<i>H</i>-pyrrol-1-yl)­benzonitrile (PBN), 4,4′-dimethylaminobenzonitrile (DMABN), and 4-(1-pyrrolidinyl)­benzonitrile (PYRBN), isolated in the gas phase and probed by rotationally resolved spectroscopy in a molecular beam. The related molecules 1-phenylpyrrole (PP) and 4-aminobenzonitrile (ABN) also are discussed. We find that the S₁ electronic state is of B symmetry in all five molecules. In PBN, a second excited state (S₂) of A symmetry is found only ∼400 cm^–1 above the presumed origin of the S₁ state. The change in dipole moment upon excitation to the A state is measured to be Δμ ≈ 3.0 D, significantly smaller than the value predicted by theory and also smaller than that observed for the “anomalous” ICT band of PBN in solution. The B state dipole moments of DMABN and PYRBN are large, ∼10.6 D, slightly larger than those attributed to “normal” LE fluorescence in solution. In addition, we find the unsaturated donor molecules (PP, PBN) to be twisted in their ground states and to become more planar upon excitation, even in the A state, whereas the saturated donor molecules (ABN, DMABN, PYRBN), initially planar, either remain planar or become more twisted in their excited states. It thus appears that the model that is appropriate for describing ICT in these systems depends on the geometry of the ground state