Recent progress in second-order nonlinear optical polymers and dendrimers

This review provides a survey of nonlinear optical (NLO) chromophores and materials incorporating them which have been demonstrated over the last several years. Conventional polymeric materials, dendrimers, and other material design approaches are reviewed. Macroscopic nonlinear optical properties are introduced mainly in terms of second-harmonic generation (SHG) and the electro-optic (EO) effect. The temporal and thermal stability of nonlinear optical properties are also discussed. The review begins with a brief introduction explaining overall principles relating to the origin of second-order nonlinear optical properties. The structure of NLO materials, methods for their characterization, and structure-property relationships are also introduced. Much current research is aimed at optimizing microscopic nonlinearity in well-defined heterocyclic NLO chromophores which are then embedded in materials such as polymers, dendrimers, etc. Strong electron density donor and acceptor groups are connected through an efficient π-electron conjugative bridge to yield a highly electronically asymmetric and hyperpolarizable NLO chromophore. This review highlights the design and synthesis of recent chromophores that simultaneously exhibit large molecular hyperpolarizability, and low optical absorption at an operating wavelength, in addition to good processability and thermal/photochemical stability. Although NLO chromophores have been designed to exploit both dipolar and octupolar symmetry, the former has exhibited the strongest potential for the development of practical NLO devices. Organic materials based on polymers and dendrimers containing dipolar chromophores are mainly demonstrated for nonlinear optics. Experimental results corresponding to such dipolar compounds are mainly considered in this review. In addition, new strategies to improve thermal stabilities are also discussed herein.