A coupled-cluster analysis of the electronic excited states in aminobenzonitriles

The excited states of 4-(N,N-dimethylamino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) are characterized by the similarity transformed equation-of-motion coupled-cluster method with single and double excitations (STEOM-CCSD). The long wavelength band of DMABN is assigned by STEOM-CCSD to an emission of a geometrically relaxed twisted intramolecular charge transfer (TICT) state. In contrast, no lowering in total energy upon twisting is found in ABN. The different behavior of the TICT states in ABN and DMABN can quite clearly be understood from the behavior upon twisting of the correlated ionization potentials and electron affinities from IP/EA-EOM-CCSD results that are obtained as a side: product of STEOM-CCSD calculations. Inclusion of electron correlation is found to be crucial however, as the effect is largely lost at the Hartree-Fock (Koopmans') level of accuracy. The STEOM-CCSD results are similar to results from DFT/SCI calculations. in general they also agree well with results from previous CASPT2 calculations and experimental data, where available. The crucial behavior of the energy of the TICT states upon rotation of the amino group in ABN and DMABN differs markedly however, between STEOM-CCSD and CASPT2. Whereas in STEOM-CCSD upon twisting we find the expected lowering for the TICT state in DMABN and rise in energy for ABN, this is not the case in CASPT2. Previous benchmarks indicate that CASPT2 may have difficulties describing differential dynamical correlation associated with transitions from lone pair orbitals, and we think therefore that the results at the STEOM-CCSD and DFT/SCI levels may be more accurate for this aspect of the problem.