Annabelle Scarpaci

Synthesis and Nonlinear Optical Properties of a Peripherally Functionalized Hyperbranched Polymer by DR1 Chromophores

The first peripheral postfunctionalization of a hyperbranched polyimide by nonlinear optic chromophores (DR1 derivative) was achieved using two different routes. The first one consists in the esterification of the terminal carboxylic acid groups, whereas the second is based on copper-catalyzed Huisgen reaction of the terminal propargylic ester groups. The resulting polymers display good solubility in classical organic solvents and good filmability because thick films can be prepared (up to 2.7 mum). The second-order nonlinear optical properties were measured by SHG at 1064 nm and we show that these hyperbranched polymers exhibit good poling efficiency and good thermal stability since the electro-optic activity remains stable up to 130 degrees C. These results illustrate the potential of hyperbranched polymers to host second-order nonlinear optical chromophores to replace dendrimers or classical linear polymers generally used in this area.

Charge-Transfer State and Large First Hyperpolarizability Constant in a Highly Electronically Coupled Zinc and Gold Porphyrin Dyad

We report the synthesis and the characterizations of a novel dyad composed of a zinc porphyrin (ZnP) linked to a gold porphyrin (AuP) through an ethynyl spacer. The UV/Vis absorption spectrum and the electrochemical properties clearly reveal that this dyad exhibits a strong electronic coupling in the ground state as evidenced by shifted redox potentials and the appearance of an intense charge-transfer band localized at lambda = 739 nm in dichloromethane. A spectroelectrochemical study of the dyad along with the parent homometallic system (i.e., ZnP-ZnP and AuP-AuP) was undertaken to determine the spectra of the reduced and oxidized porphyrin units. Femtosecond transient absorption spectroscopic analysis showed that the photoexcitation of the heterometallic dyad leads to an ultrafast formation of a charge-separated state ((+)ZnP-AuP(*)) that displays a particularly long lifetime (tau = 4 ns in toluene) for such a short separation distance. The molecular orbitals of the dyad were determined by DFT quantum-chemical calculations. This theoretical study confirms that the observed intense band at lambda = 739 nm corresponds to an interporphyrin charge-transfer transition from the HOMO orbital localized on the zinc porphyrin to LUMO orbitals localized on the gold porphyrin. Finally, a Hyper-Rayleigh scattering study shows that the dyad possesses a large first molecular hyperpolarizability coefficient (beta = 2100x10(-30) esu at lambda = 1064 nm), thus highlighting the valuable nonlinear optical properties of this new type of push-pull porphyrin system.

Scope and limitation of the copper free thermal Huisgen cross-linking reaction to stabilize the chromophores orientation in electro-optic polymers

New methacrylate copolymers incorporating two complementary thermally cross-linkable groups (azide or ethynyl) for implementation in electro-optic devices were synthesized and their nonlinear optical properties were investigated. These copolymers were prepared from a monomer containing Disperse Red 1 (DR1) as active NLO chromophore which is end substituted either by an azide or ethynyl group connected via a rigid (phenyl) or flexible spacer (alkyl chain). The second monomer is either a trimethylsilyl-propargyl methacrylate, or an azidopropyl methacrylate or a trimethylsilyl-phenyl methacrylate. The determination of the reactivity ratios showed that the monomer containing the DR1 chromophore is more reactive than trimethylsilyl-propargyl methacrylate. The cross-linking temperatures of these polymers range from 150 °C to 187 °C depending on the rigidity of the spacers connecting the cross-linkable units. These polymers displayed relatively high macroscopic electro-optic stability, enhanced upon cross-linking by more than 40 °C relative to non-cross-linked polymers. The results underscore the importance of the flexibility of the spacers to achieve the stable bulk electro-optic response. While rigidity is favorable to maintain the orientation of the chromophores, the optimal polymer is the one containing a flexible and a rigid spacer, since the mobility of the reactive groups is a key parameter which guarantees a high cross-linking conversion within the polymer. This study demonstrates the versatility of this new cross-linking process because we showed that the reactive groups (azide or trimethylsilylacetylated groups) can be interconverted (on the chromophore or as polymer side chain) with no change on the overall electro-optic activity and its thermal stability. Furthermore, preliminary kinetic study indicates that the Huisgen reaction rate can be controlled by the substituent on the ethynyl group opening the possibility to tune the cross-linking temperature by the careful choice of this substituent.