Entrapping of Push-Pull Zwitterionic Chromophores in Hybrid Matrices for Photonic Applications

A new class of heterocycle-based push-pull chromophores showing enhanced nonlinear properties characterized by an aromatic and highly zwitterionic ground state and a quinoid/neutral excited state have been synthesized to be incorporated in sol-gel hybrid systems. This class of compounds shows very large first molecular hyperpolarizabilities ( as high as 27000 × 10^–48 esu) and is a promising candidate for photonic applications where large second order non-linearities are required. In spite of their very large hyperpolarizability and chemical and thermal stability, these chromophores are decomposed in presence of light and oxygen (photobleaching) and are sensitive to acidic environments due to the carbanionic nature of the donor moiety. A hybrid matrix, based on N-(3-trimethoxysilyl)propyl]ethylenediamine and 3-glycidoxypropyltrimethoxysilane, has been specifically designed to allow the incorporation of such zwitterionic compounds assuring at the same time a good temporal stability of the optical properties. Amine functionalisation has been found very effective in reducing the photobleaching by acting on these chromophores via the singlet oxygen. Second harmonic generation has been observed on poled films, and an order parameter, , of 0.17 has been estimated. The nonlinear coefficient d_eff of the samples has been thus estimated at a value two times larger than for d₁₁ of quartz that, from literature data, is about 0.335 pm/V.