Toward verdazyl radical-based materials: ab initio inspection of potential organic candidates for spin-crossover phenomenon.

The origin of magnetic interactions in verdazyl-based radical stackings is examined using ab initio wave function and density functional theory (DFT) calculations. Starting from the reported crystal structure of the 1,1'-bis(verdazyl)ferrocene diradical compound, the singlet-triplet energy difference has been evaluated on the basis of multireference difference dedicated configurations interaction calculations and suggested the innocent role of the ferrocene spacer. Using the underlying pi-dimer verdazyl structures, the J variations and potential energy surfaces of parallel and antiparallel face-to-face arrangements have been studied with respect to the verdazyl-verdazyl separation to evaluate the Coulomb repulsion U and bandwidth W. While coupled-cluster CCSD(T) calculations are suggestive of a weak bond formation in both dimer arrangements (approximately 40 kJ mol(-1)), the DFT approach fails to reproduce the bonding character. The intrinsically delocalized character of the magnetic orbitals favors an S = 0 ground state, but importantly, the S = 1 spin state is also bound. A typical 0.4 A increase (i.e., 10%) of the verdazyl-verdazyl equilibrium distance accompanying a 16 kJ mol(-1) adiabatic energy difference is calculated between the S = 0 and S = 1 states. In this distance separation regime, we finally suggest that either a relative 1.2 A slippage or a approximately 42 degrees relative orientation of the verdazyl rings is likely to give rise to a high-spin S = 1 ground state. These features are symptomatic of a bistable system, and an interpretation of the exchange interaction in verdazyl pi-dimer structures in terms of spin transition is proposed.