Theoretical Study of the Electronic Structure of [Tetrathiafulvalene]22+ Dimers and Their Long, Intradimer Multicenter Bonding in Solution and the Solid State

The long, multicenter bonding in tetrathiafulvalenium cation radical dimers, [TTF](2)(2+), observed in both the solid state and in solution, were computationally investigated via B3LYP, CASSCF(2,2), and MCQDPT/CASSCF(2,2) calculations. The stability of [TTF](2)(2+) dimers originates from [TTF](center dot+)center dot center dot center dot anion(-) or [TTF](center dot+)center dot center dot center dot solvent electrostatic interactions, whose sum exceeds the [TTF](center dot+)center dot center dot center dot[TTF](center dot+) repulsion, in solution, or the sum of the [TTF](center dot+)center dot center dot center dot[TTF](center dot+) and anion(-)center dot center dot center dot anion(-) repulsions in solid state. MCQDPT/CASSCF(2,2) calculations indicate that the electronic ground state of the [TTF](2)(2+) dimers is a closed-shell single state with non-negligible contribution of the open-shell singlet, as is experimentally observed. This ground state is not well reproduced in B3LYP or CASSCF(2,2) calculations.