Synthesis and Photo-Bleaching of Epoxy-Based Nonlinear Optical Polymers Containing Triene Chromophores
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ABSTRACT Bis(diglycidyl)ether of bisphenol-A-aniline (BP-AN) based second-order nonlinear optical (NLO) polymers containing triene chromophores have been developed through post-polymer functionalization reactions. One-pot aldol condensation reaction of dicyanomethylidene functionalized verbenone as well as isophorone compounds with aldehyde functionalized BP-AN polymer enabled the introduction of various chromophore moieties with high degree of functionalization. These polymers were studied using various structural (1H NMR, FTIR), linear (UV-Vis) and NLO characterization techniques. A d33 value of ca. 139 pm/V at 1.064 µm was determined for a representative polymer containing verbenone unit. We have observed fast photobleaching in these polymers. Detailed studies on the photophysical properties of this class of materials have been carried out using various analytical techniques, the structureproperties relationship and photo-bleaching mechanisms will be presented. INTRODUCTION Design of suitable organic polymeric materials containing second order nonlinear optical (NLO) chromophores continues to be the major challenge in the development of high performance electro-optic devices for applications such as electro-optic modulation and optical switching [1]. To meet the requirements of photonic applications, the materials should possess large nonresonant second order optical nonlinearity, high thermal and photo-chemical stability, easy processability and low optical loss at the operating wavelength [2]. In general, dipolar chromophores used in NLO applications can be represented as D - π - A, where D is an electron donor group and A is an electron acceptor group providing the requisite ground-state asymmetry. The donor and acceptor groups are connected through π, an electron-conjugated bridge that provides a pathway for the redistribution of the electric charges under the influence of an electric field [1]. The molecular optical response (β) increases with increasing donor and acceptor strength and extending conjugation length between the donor and acceptor groups [3]. It was established that chromorphores with thiophene rings or polyene segments yield much larger nonlinearities than conventional chromophores, which have strongly aromatic ground-state structures. In the case of molecules containing multiple phenyl groups, the aromatic stabilization in the neutral canonical resonance form will hinder charge separation
and consequently lead to large values of bond length alternation and diminished nonlinearities [4]. This nonlinearity – aromatic stabilization tradeoff can be evaded by incorporating configuration-locked triene groups in between the aromatic donor and fairly effective cyanomethylidine acceptor groups [5]. In this light, we have developed a simple and well-defined methodology to incorporate dicyanomethylidene functionalized isophorone or verbenone based chromophores into highly soluble and easily processable aldehyde functional epoxy based polymers by one step aldol condensation [6]. The synthesis, NLO pro
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