Influence of the alkyl chain length on the excited-state properties of 4-dialkyl-benzonitriles. A theoretical DFT/MRCI study

The excited states of various di-alkyl-aminobenzonitriles (ABN) emitting dual fluorescence are investigated theoretically by a combination of density functional theory and multireference configuration interaction (DFT/MRCI). Experimentally, it was found that intramolecular charge transfer (ICT) emission increases on expense of the normal fluorescence with increasing n-alkyl chain length. The long wavelength ICT band becomes the predominant fluorescence band in diisopropylaminobenzonitrile (DIABN) even in an apolar environment. The calculated energy of both the planar and twisted ICT (PICT and TICT) state increases slightly with increasing chain length relative to the energy of the first excited state at optimized planar ground-state geometry. The good agreement of the calculated TICT state dipole moment with the experimental ICT state dipole moment leads us to the conclusion that the dual fluorescence of all linear substituted di-alkyl ABNs originate from a TICT state. The bulky system DIABN is twisted by 33° already in the ground state and complete decoupling of donor and acceptor units takes place without a significant energy barrier. In contrast, planarization toward the PICT state is energetically disfavored. Thus, the red-shifted fluorescence of DIABN is also assigned to emission out of the TICT state. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 149–156, 2001