The effects of conformation change on the nonlinear optical properties of 6,6’-dioxo-3,3’-biverdazyl radical and its derivatives were investigated by using density functional theory (DFT) UB3LYP. In addition, the dependent relationships of the radical polarizability and the second hyperpolarizability with conformation, spin multiplicity were studied at the same level combined with the finite field (FF) approach. The results show that the effective exchange integral values are less than zero under different conformations, so the radicals display the anti-ferromagnetic coupling. No matter whether it is singlet or triplet, polarizability change is very small and the value of the singlet is greater than that of the triplet under different conformations. In the conformation change process, the second hyperpolarizability values of system (a) and (b) for singlet are negative (system (a) of 45° and 135° with the exception), and less than the absolute value of all triplets. The second hyperpolarizability values of system (c) for singlet and triplet states are positive and in molecular close plane configuration time, the value of triplet is greater than that of the singlet. The effects of the different substituent groups R on the system conformation, polarizability and the second hyperpolarizability are also different.

Key words: radical, conformation, effective exchange integral, NLO property