Theoretical analysis of charge-transfer electronic spectra of methylated benzenes-TCNE complexes including solvent effects: approaching experiment

The paper brings new accurate theoretical description of charge-transfer (CT) electronic spectra of a complete series of methylated benzenes-tetracyanoethylene (NMB-TCNE) complexes and detail comparison with complete experimental data both in the gas phase and in polar media. It is shown that the energies of the first two (CT) absorption transition in these intermolecular EDA (electron donor-acceptor) complexes are described well by the CC2/aug-cc-pVTZ method. In agreement with experimental data, it reproduces well both the bathochromic shift of the two pi(NMB) -> pi*(TCNE) transitions (ranging from 3.41 to 2.23 eV) with the increasing number of methyl groups N as well as the value of splitting between them. Nevertheless, the CC2 transitions are systematically smaller, that is, red-shifted, with respect to experimental quantities in the gas phase by ca. 0.15-0.2 eV, which is an inaccuracy of the CC2 approach. The TD-LC-BLYP method better describes studied CT transitions than PBE0 or B3LYP functionals; however, the transition energies are too sensitive to the fitting range separation factor mu. The PCM solvation model combined with the CIS or LC-BLYP methods predicts red solvent shifts for all the studied CT transitions in NMB-TCNE complexes due to a larger stabilization of the excited states compared to their ground states in the solvent. The stabilization increases with solvent polarity and decreases with increasing N. The CIS/PCM solvent shifts are smaller than experimental values (taken as the difference for the gas phase and the polar CH2Cl2 solvent) by 0.1-0.15 eV, that is, by 30-40 %, however, being more consistent than those obtained by TD-DFT functionals used. Experimentally interesting (hexamethylbenzene)(2)-TCNE complex (2:1) was also studied by the LC-BLYP approach. The exciton splitting together with the bathochromic effect on absorption in comparison with 1:1 complex was found.