Highly Efficient and Thermally Stable Second-Order Nonlinear Optical Chromophores and Electrooptic Polymers

Abstract

Organic polymeric electro-optic (E-O) materials have attracted significant attention because of their potential use as fast and efficient components of integrated photonic devices (1,2). However, the practical application of these materials in optical devices is somewhat limited by the stringent material requirements imposed by the device design, fabrication processes and operating environments. Among the various material requirements, the most notable ones are large electro-optic coefficients (r 33 ) and high thermal stability (3). The design of poled polymeric materials with high electro-optic activity (r33) involves the optimization of the percent incorporation of efficient (large βµ) second order nonlinear optical (NLO) chromophores into the polymer matrices and the effective creation of poling-induced non-centrosymmetric structures. The factors that affect the material stability are a) the inherent thermal stability of the NLO chromophores, b) the chemical stability of the NLO chromophores during the polymer processing conditions, and c) the long-term dipolar alignment stability at high temperatures. Although considerable progress has been made in achieving these properties (4), organic polymeric materials suitable for practical E-O device applications are yet to be developed. This chapter highlights some of our approaches in the optimization of molecular and material nonlinear optical and thermal properties. Push-Pull Heteroaromatic Stilbenes: Experimental and Theoretical StudiesFor some time, the EniChem group has been studying thiophene incorporated donor-acceptor (D-A) compounds for second order nonlinear optical applications (5-9). Our initial studies focused on the replacement of the benzene rings of the well known N,N-diethylamino-4-nitrostilbene (DANS, 1) with thiophene moieties (5). The rationale was that the lower resonance energy of thiophene relative to benzene should reduce the aromatic derealization and thereby increase the electronic transmission between donor and acceptor substituents resulting in an increase of p. Indeed, the experimental EFISH studies (1.907 u.m, dioxane solvent) revealed that the Pp. value of 2 was significantly higher than that of 1 (Table I). This result in conjunction with the comparable dipole moments of 1 (7.3 debye) and 2 (7.1 debye) demonstrated that the increase in pu, upon thiophene substitution is solely due to the increase in first-order hyperpolarizability, p.

Quantum Mechanical Design of Efficient Second-Order Nonlinear Optical Materials Based on Heteroaromatic Imido-Substituted Hexamolybdates: First Theoretical Framework of POM-Based Heterocyclic Aromatic Rings

“…Furthermore, the relationship between the molecular hyperpolarizability and structure was elucidated based on the theoretical calculation of molecular orbitals (MOs). To produce heteroaromatic NLO materials, the heteroaromatic stilbenes have already been studied experimentally − while monoaromatic and diaromatic rings had already been introduced through imido linkage in hexamolybdates, as shown in Figure . Why are the various heteroaromatic rings chosen in imido-substituted hexamolybdates?…”

“…Polyoxometalates (POMs) are a rich class of inorganic clusters and display incredible chemical and physical properties, which have been applied to a variety of fields, for instance, medicine, catalysis, analytical chemistry, and materials science. − For a few years, there had been increasing interest in the chemistry of organically derivatized POMs . In many organic derivatives of POMs, organoimido derivatives have been drawing much interest in that the strong d−π interaction between the organic delocalized π electrons and the cluster d electrons may result in synergistic effects .…”

“…The barrier due to the aromatic delocalization energy of the benzene ring is supposed to be accountable for the reduced or saturated β values. To overcome the problem of saturation of the molecular nonlinearity, numerous groups have developed systems that contain easily delocalizable five-membered heteroaromatic rings. − …”