Organic Salts Suppress Aggregation and Enhance the Hyperpolarizability of a π-Twisted Chromophore

Twisted intramolecular charge-transfer (TICT) chromophores exhibit extraordinary hyperpolarizabilities, β, and are therefore promising for electro-optic technologies. Nevertheless, centrosymmetric aggregate formation severely diminishes β in concentrated solutions or in polymer matrices. Herein, the remarkable effects of organic salts on the linear and nonlinear optical response of a high β benzimidazolium-based TICT chromophore, B2TMC-2, are reported. Addition of Bu₄ P⁺ Br^- to B2TMC-2 solution in CHCl₃ induces a halochromic blueshift, primarily reflecting interactions between Bu₄ P⁺ Br^- and the B2TMC-2 cationic portion. DC electric-field induced second-harmonic generation (EFISH) measurements on B2TMC-2+Bu₄ P⁺ Br^- solution reveal a large μβ=-22,160×10^-48  esu, unprecedented for any chromophore with such a broad optical transparency window. Moreover, Bu₄ P⁺ Br^- is shown to suppress B2TMC-2 aggregation, thereby preserving high μβ in concentrated solutions. This phenomenon should be applicable to many other NLO chromophores.