Critical analysis of spectral solvent shifts calculated by the contemporary PCM approaches of a representative series of charge-transfer complexes between tetracyanoethylene and methylated benzenes‡

Applications of contemporary polarisable continuum model (PCM) quantum chemical approaches to account for the solvent shifts of UV-Vis absorption charge transfer (CT) transitions in electron donor–acceptor (EDA) complexes (as well as to account for their stability and other properties in solvents) have been rather rare until now. In this study, we systematically applied different – mainly state-specific – PCM approaches to examine excited state properties, namely, solvatochromic excitation energy shifts in a series of EDA complexes of a tetracyanoethylene (TCNE) acceptor with methyl substituted benzenes with different degrees of methylation N (NMB). For these complexes, representative and reliable experimental data exist both for the gas phase and in solution (dichloromethane). We have found that the linear response (LR) solvent shifts are too small compared to the experimental values, while self-consistent SS approaches give values that are too large. The best agreement with experimental values was obtained by corrected LR (cLR). The transition energies were calculated by means of TD-DFT methodology with PBE0, CAM-B3LYP and M06-2X functionals as well as the wave function CC2 method for the gas phase, and the PCM solvent shifts were added to account for the solvent effects. The best results for transition energies in solvents were obtained using the CC2 method complemented by CAM-B3LYP/cLR for the gas phase transition energy red solvent shift, while all three TD-DFT approaches used gave insufficient values (ca. 50%) of the slope of the dependence of the transition energies on N compared to experimental values.