Comment on “Pseudo-Jahn–Teller and TICT-models: a photophysical comparison of meta-and para-DMABN derivatives” [Chem. Phys. Lett. 305 (1999) 8]: The PICT model for dual fluorescence of aminobenzonitriles

Due to the larger admixture of charge transfer (CT) in the intramolecular charge transfer (ICT) state than in the locally excited (LE) state of dual fluorescent molecules such as 4-(dimethylamino)benzonitrile, the radiative rate constant kf′(ICT) is smaller than kf(LE), irrespective of the ICT molecular structure. The validity of the TICT or PICT model can therefore not be tested with these data. The absence of dual fluorescence for 4-(methylamino)benzonitrile, 3-(dimethylamino)benzonitrile and two dicyano-N,N-dimethylanilines, which cannot be explained by the TICT hypothesis, is caused by their large energy gap ΔE(S1, S2), which prevents the ICT state from becoming lower in energy than LE.