Efficient π bridges based on five-membered heterocyclic rings for second-order NLO properties of push-pull type molecules

Abstract

Thirty-two molecules were designed as donor–bridge–acceptor (D–π–A) push-pull type nonlinear optical (NLO) chromophores with five-membered heterocyclic (O, S, NH, PH) bridges. An eigenvector following (EF) method was employed to optimize the structures of the molecules and a parametric method number six (PM6) method was used to study the second-order NLO properties by calculating the dipole moment and the first- and second-order polarizabilities. The relationship between second-order NLO properties and the structures is discussed through the calculated results. The analyzed results show that the five-membered heterocyclic ring is a highly efficient electronic transfer bridge. By comparing the second-order NLO properties, it was found that the 4H-1,2,4-diazaphosphole heterocyclic bridge (MZ-PH) and furan heterocyclic bridge (M-O) can enhance the second-order polarizability (β) values of molecules more than other bridges. The results can help the selection of electronic donor, conjugated bridge and electronic acceptor to get large second-order NLO properties.