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 yet, unobserved phenyl−amino stretching vibration will substantiate a definitive explanation of the ICT mechanism in DMABN.
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