Shedding light on excited-state structures by theoretical analysis of femtosecond transient infrared spectra: Intramolecular charge transfer in 4-(dimethylamino)benzonitrile

We performed a detailed theoretical analysis of femtosecond transient infrared spectra to determine excited-state structures involved in photoinduced intramolecular charge transfer (ICT) in 4-(dimethylamino)benzonitrile (DMABN). For comparison, 4-aminobenzonitrile (ABN) is studied. We present the first ab initio CASSCF study with all states under consideration being fully optimized. We derive two different models for the locally excited (LE) states: a planar and a novel pyramidal conformation. Three different mechanisms are treated for the ICT state formation: the twisted ICT (TICT), the pseudo-Jahn-Teller ICT (PICT), and the rehybridized ICT (RICT) models. By use of this combined theoretical and experimental approach, we can evaluate the respective models and thus provide new insight into the ICT process. We assign a pyramidal LE state to ABN, and, in contrast, a planar LE state to DMABN. We can conclusively rule out RICT as the ICT mechanism in DMABN. Although our results favor TICT as the ICT mechanism in DMABN, a final statement cannot be made. We predict, however, that determination of the position of the, as pet, unobserved phenyl-amino stretching vibration will substantiate a definitive explanation of the ICT mechanism in DMABN.