Design And Synthesis Of Multichromophore Dendrimers And Nonlinear Optical Properties Of Films

Recent years, organic nonlinear optical (NLO) materials have drawn great attentions due to their potential applications in electro-optic (EO) modulation, optical switching and optical information processing. In this thesis, the recent progress of organic second-order nonlinear optical materials is reviewed, with specific focus on the design, synthesis and applications of dendrimers in organic NLO materials. A series of multichromophore dendrimers are designed and employed in organic NLO materials, and the tether group effects on the macroscopic optical nonlinearities of the dendrimers are studied. A new type of multichromophore dendrimers with quite large macroscopic optical nonlinearities is explored. Modulating the dipole moments of chromophores to reduce the strong inter-chromophore dipole-dipole interactions that would decrease the macroscopic optical nonlinearities is researched in detail. We also report the nonlinear optical properties of FTC type chromophore incorporated organic-inorganic hybrid NLO materials.Three bichromophores were designed and synthesized by using different tether groups. The influences of tether groups on the linear and nonlinear optical properties were systemically studied via 1H NMR, UV-visible absorption spectra and cyclic voltammetry analysis. It reveals that isophthalic ester used as a tether group could reduce the electron density of the electron donor site of FTC chromophore due to the electron-withdrawing inductive effect of ester group. Yet,3-phenoxypropane-1,2-diyl succinic diester tether group could minimize the detrimental effect on the electron-donating ability of the amine donor portion. The polymer films were fabricated by doping bichromophores and FTC1 into Poly(4-vinylphenol) (PVPh), respectively. The second harmonic generation (SHG) coefficients (d33) of the films are measured by in situ second harmonic generation measurement, and showed that a 1.3-fold enhancement in second harmonic coefficients (d33) is realized in bichromophore B3 by using 3-phenoxypropane-1,2-diyl succinic diester as the tether group in comparison with FTC1 doped polymer films. The long and flexible tether group makes the chromophores to rotate freely in the electrical field, enhancing the poling efficiency. The d33 values of polymer films containing bichromophores B1 and B2 are decreased due to the inflexible and short isophthalic ester tether group. The results indicate that 3-phenoxypropane-1,2-diyl succinic diester tether group can improve the optical nonlinearity and have potential applications in the design and synthesis of multichromophore dendrimers with quite large EO coefficients.A new six-branched chromophoric dendrimer GGQ1 based on 3-phenoxypropane-1,2-diyl succinic diester tether group was designed and synthesized by the incorporation of FTC-type chromophore (FTCl) with a new glycol dendrimer core GC1. The dendrimer shows a glass transition temperature of 133℃and a molecular weight of 3834 Dalton. The dendrimer GGQ1 can form thin films without polymer matrices, with a quite high active chromophore loading density of 73 wt.%. The d33 value of the dendrimer GGQ1 films is 95 pm/V, imparting almost 3-fold enhancement of d33 values in comparison with FTC1 doped polymer films. The results indicate that the dendrimer GGQ1 could efficiently improve the chromophore loading density, thus enhancing the macroscopic optical nonlinearity. The glycol dendrimer core GC1 shows potential applications in the design and synthesis of new type multichromophore dendrimers with large macroscopic optical nonlinearity.A new hetero-trichromophore (HT-1) was designed and synthesized by linking neutral-ground-state (NGS) chromophore NGS1 with zwitterionic (ZWI) chromophore ZWI1. Theoretical calculations show that the dipole moment of HT-1 in gas phase is 13.84 Debye, displaying a decrease of molecular dipole moments when chromophores NGS1 and ZWI1 are linked together. The polymer films were fabricated by doping trichromophore HT-1, chromophores NGS1 and ZWI1 into PVPh. The d33 values of the films and the thermal stability of optical nonlinearities are investigated. The d33 value of the films containing HT-1 is 25.6 pm/V, and is still stable when heating to 110℃. Almost a 5-fold enhancement in second harmonic coefficients is realized as the combination of NGS chromophore and ZWI chromophore. The results indicate that NGS and ZWI chromophore combined hetero-trichromophores can reduce the dipole moments of molecules, and efficiently improve the macroscopic optical nonlinearity.The thiophene-vinyl conjugated FTC-type chromophore FTC1 with quite largeβvalue was introduced into inorganic networks. FTC1 was reacted with 3-isocyannatopropyltriethoxysilane (ICTES) to form the functionalized alkoxysilane dye Al. The hybrid NLO films were successfully prepared, following a sol-gel process of A1 and tetraalkoxysilane. The d33 values of the hybrid films increase with the boost of chromophore loading density, and up to 43 pm/V when the loading density is 20 mol%. Almost a 1.3-fold enhancement in second harmonic coefficients is realized in comparison with FTC1 doped polymer films. The hybrid films exhibit excellent thermal stability of dipole alignment at elevated temperate, and the half-decay temperature of the d33 value for hybrid films at 20 mol% loading density is 165℃, displaying an improvement of 42℃compared with FTC1 doped polymer films. Organic-inorganic hybrid materials show special advantages in the improvement of macroscopic optical nonlinearity and thermal stability of dipole alignment.