Frequency up-conversion of s-triazine derivatives via two photon absorption and second-harmonic generation

A three-branched octupolar compound, 2,4,6-tris[2-(4-N,N-diethylamino) phenylethenyl]-1,3,5-s-triazine (compound III), as well as its one- and two-branched analogues (compounds I and II respectively) have been synthesized. The structures of I and II have been determined by X-ray diffraction. The linear and nonlinear spectra of these compounds regularly vary: (1) the spectral positions (lambda(max) values) of the linear absorption, the single-photon excited fluorescence (SPEF) and the two-photon excited fluorescence (TPEF) show regular red-shifts when the branch number increases from 1 to 3; (2) the spectral intensities, including the linear absorbance epsilon(max) and the two-photon absorption cross-section sigma, the SPEF and the TPEF intensities are all increased significantly when going from I to III. The values of epsilon(max)/MW and sigma/MW (of compounds I, II and III) vary with the ratios of 1.0:1.3:1.7 and 1.0:1.4:1.3, respectively. All the photophysical data indicate that by incorporating more branches into the triazine ring, the linear and nonlinear optical responses have been significantly enhanced, and that the spectral behaviour of II is closer to that of III, while more different from that of I. The single-branched compound I crystallizes in the noncentrosymmetric polar space group Cc. Its crystalline powder sample shows remarkable macro second-order nonlinear response with a SHG intensity of 46.8 times that of urea under a fundamental picosecond Nd:YAG laser beam (1064 nm). An important factor of such a large SHG intensity is that all the molecular dipoles take the same-oriented packing style that makes the crystal maximally polarized, fulfilling the optimization in the sense of crystal engineering.