Second Order Nonlinear Optical Performances of Polymers Containing Imidazole and Benzimidazole Chromophores

Abstract

Summary: Condensation polymers based on two different NLO‐active chromophores, namely 2‐[4‐[(4‐N,N‐dihydroxyethylamino)phenylazo]phenyl]‐5(6)‐nitrobenzimidazole (BZI) and 2‐[4‐(4‐N,N‐dihydroxyethylamino)phenylazo]‐4,5‐dicyanoimidazole (IMI), have been synthesized and their basic properties measured. The second order NLO properties of poled polymers were investigated by SHG procedures at different temperatures (35, 65 and 85 °C) and exposure times. Resonance free d33 coefficients (λ = 1 368 nm) took values in the range 1.8–4.0 pm · V−1, but the time stability of their SHG signal was fairly acceptable, particularly for the high Tg (e.g. 188 °C) polymer for which a relaxation time (at 25 °C) of 2.8 years has been determined.

Polymethacrylate Copolymers Containing 4,5‐Dicyanoimidazole‐Based Chromophores and their Nonlinear Optical Behavior

“…respectively. [3] That NLO activity is lower than the maximum (3.3 pm Á V À1 , X ¼ 15) we have obtained for XA copolymers, which are based on the same chromophore but less tightly bounded to the polymer chain. In fact, in polymers of Scheme 1, any movement of chromophore units, as a whole, can only be accomplished through conformational changes in the polymer chain, while this is not the case in XA copolymers.…”

“…By comparative analysis of results from the present and a preceding paper, [3] we have shown that for the given chromophore unit, 2-[4-(N-alkyl-N-alkylamino)phenylazo]-4,5-dicyanoimidazole, side chain polymethacrylates having the chromophore attached to the chain by an ethylene bridge (Scheme 2) reach higher d 33 values than polycondensates (terephthalate polyesters, Scheme 1) in which the chromophore is directly attached to the chain. Within polymethacrylate copolymers, second order NLO response can be further increased by increasing the lateral bulkiness of the chromophore.…”

“…[1,2] In the case of polymers containing the chromophore units covalently attached to the chain (NLO polymers), the density number of chromophores, the mobility of chromophore units and the glass transition temperature (T g ) of the polymer are relevant features affecting the degree of poling and, therefore, NLO performances. [1,2] We have reported in a recent paper [3] the synthesis and NLO characterization of two terephthalate homopolyesters containing a new 4,5-dicyanoimidazole based chromophore, having the formula given in Scheme 1.…”

NLO Behavior of Polymers Containing Y‐Shaped Chromophores

“…The curves are compatible with a biexponential behavior with two characteristic relaxation times. The initial fast decrease in SHG signal, actually reported in many previous works [20][21][22][23] has been attributed to the discharge of trapped charge [21] carriers or to the rotational mobility of the chromophores [20,23] , whereas the slower relaxation time is related to processes involving longer segments of the chains. It is worth noticing that at 80 8C, after the fast initial reduction, the signal remains unchanged.…”

Effects of the Nature and Length of the π‐Conjugated Bridge on the Second‐Order Nonlinear Optical Responses of Push–Pull Molecules Including 4,5‐Dicyanoimidazole and Their Protonated Forms

“…Since that time, this moiety has been widely used in different branches of material sciences due to its accessibility, solubility, and acceptor character. For instance, 4,5-dicyanoimidazole is an efficient organic electronics component, [14] and recently, photoinduced absorption and birefringence of 4,5-dicyanoimidazole-derived chromophores embedded into poly(methylmethacrylate) films were studied. [15] In addition, 2-bromo-1-methylimidazole-4,5-dicarbonitrile precursors can easily be substituted by donors with systematically extended p-conjugated systems, [16] which are key elements to increase the first hyperpolarizability.…”