Large second-order optical nonlinearities of s-triazine derivatives:: View from micro molecules to macro crystals

Three s-triazine derivatives have been synthesized and their structures been determined. These are Compound 1: 2,4-dimethyl-6-(p-N,N-dimethylaminostyryl)-s-triazine (abbreviated to NMe-1), Compound II: 2,4-dimethyl-6-(p-N,N-diethylaminostyryl)-s-triazine (to NEt-1), and Compound III: 2-methyl-4,6-bis(p-N,N-dimethylaminostyryl)-s-triazine (NMe-2). NMe-1 and NEt-1 molecules belong to the D-pi-A type with a one-dimensional charge-transfer character. NMe-1 crystallizes to centrosymmetric P (1) over bar space group, while NEt-1 to non-centrosymmetric polar Cc space group with all of the molecular charge-transfer axes taking the same orientation. Under the irradiation of 1064 nm fundamental laser, NEt-1 powder crystals radiate a 2-order harmonic light with an intensity 46.8 times that of urea. The typed NMe-2 molecule, which has a two-dimensional charge-transfer character, crystallizes to polar P2(1) space group. Its powder crystals exhibit a 2-order harmonic intensity 46.2 times that of urea. The quantum chemical calculations and tensor analyses have been carried out to reveal the dependence of the macro 2-order nonlinear optical (NLO) efficiency on the micro 2-order NLO coefficients (beta) and on the crystalline packing styles.