Electronic Excitation Energies in Dimers between Radical Ions Presenting Long, Multicenter Bonding

The formation of long, multicenter dimers between radical ions is usually monitored through UV–vis spectroscopy given the characteristic low-energy absorption band that they exhibit, not observed for the parent monomers. In this work, the performance of CASPT2, RASPT2, and TD-DFT methods for obtaining excitation energies of the long, multicenter bonded π-[TCNE]₂^2– and π-[TTF]₂²⁺ dimers has been addressed (TCNE = tetracyanoethylene; TTF = tetrathiafulvalene). The impact of the active space on the vertical electronic transitions computed at the RASPT2 and CASPT2 levels has been tested against experimentally observed absorption bands. Analogous tests have been carried out for a wide variety of density functionals within the TD-DFT formalism. Our calculations show that whereas CASPT2 predicts very accurate excitation energies for the π-[TCNE]₂^2–, the mean absolute error for π-[TTF]₂²⁺ is higher for CASPT2 than for TD-DFT calculations, whenever pure density functionals or low % HF exchange hybrid functionals are used. Hybrid functionals with high % HF exchange (and thus RSH functionals) conduct to large errors on the excitation energies in both dimers. Furthermore, vertical electronic transitions are also obtained for 100 configurations extracted from a 45 ps molecular dynamics (CPMD) simulation aimed at providing an accurate description of the thermal fluctuation effects of a π-[TCNE]₂^2– dimer in dichloromethane. These thermal effects explain the shape of the experimental UV–vis spectrum, where the lowest HOMO → LUMO absorption presents a broad band and the following HOMO-1 → LUMO absorption exhibits a narrower band