Twisted Thiophene-Based Chromophores with Enhanced Intramolecular Charge Transfer for Cooperative Amplification of Third-Order Optical Nonlinearity

Exploiting synergistic cooperation between multiple sources of optical nonlinearity, we report the design, synthesis, and nonlinear optical properties of a series of electron-rich thiophene-containing donor–acceptor chromophores with condensed π-systems and sterically regulated inter-aryl twist angles. These structures couple two key mechanisms underlying optical nonlinearity, namely, (i) intramolecular charge transfer, greatly enhanced by increased electron density and reduced aromaticity at chromophore thiophene rings and (ii) a twisted chromophore geometry, producing a manifold of close-lying excited states and dipole moment changes between ground and excited states that are nearly twice that of untwisted systems. Spectroscopic, electrochemical, and nonlinear Z-scan measurements, combined with quantum chemical calculations, illuminate relationships between molecular structure and mechanisms of enhancement of the nonlinear refractive index. Experiment and calculations together reveal ground-state struct...