Nonlinear optical coefficients of the closed and open shell systems of TCNQ connecting different conjugation bridges were computed by using density functional theory (DFT) combining with the finite field (FF) approach. Take the ethylene for an example to discuss the relationship between conjugation length and the NLO properties. The results show that the polarizability and second hyperpolarizability values of open shell systems are larger than those of closed shell systems. Moreover, the polarizabilities and second hyperpolarizabilities become large with the increased conjugation of the conjugation bridge. For the diradical systems, the second hyperpolarizabilities of the singlet increase with increasing the diradical character and spin multiplicity. With the increase of the conjugation bridge length, BLA (Bond Length Alternation, defined as the average of the change of length between adjacent single and double bonds in conjugate molecule) gradually decrease while the diradical character and second hyperpolarizabilities gradually increase.

Key words: diradical character, spin multiplicity, NLO property