Second-order nonlinear optical (NLO) materials have been investigated for decades driven by the application requirements of electro-optic modulators for optical communication, optical switches, sensors, terahertz generation and detection. The prominent advantages of organic NLO materials are ultra-fast response and structural versatility, which gives the materials large NLO activity, good processability and other necessary properties for specific applications via suitable molecular engineering. In this review, historic and recent studies of organic dipolar chromophores in acentric alignment affording macroscopic anisotropy are overviewed. In particular, recent marked progress in the development of acentric bulky materials, including isolating chromophores to prevent centrosymmetric alignment, is focused on. We also review fascinating methods for the ordered alignment of chromophores, which include supramolecular interaction, light fields, electric fields and so on. It is hoped that better chromophore molecular structures and film construction techniques can be devised using suitable NLO films to achieve higher performances in optoelectronic devices.

A new design of tethered binary chromophores has been studied for nonlinear optics. In this work, a push–pull tetraene structure with a strong dialkylaminophenyl donor and a CF₃-TCF acceptor is used as the primary chromophore due to its large hyperpolarizability, and a short cyanoacetate dye with a smaller dipole moment and a very different molecular aspect ratio is used as a secondary dipolar structure for dipole engineering. We found that such binary chromophore systems (exemplified as chromophore C1) exhibited significantly improved poling efficiency and thermal stability in poled films of guest–host polymers and monolithic glass. A systematic study of materials' physical properties, including analyses of poling-induced order thermally stimulated depolarization and comparison with simple dipolar polyenes, correlates the improved EO performance of poled films containing C1 well with its tethered binary structure. It provides an effective electrostatic screening mechanism for excellent solution processibility of materials, and a cooperative enhancement for higher polar order of poled thin films under the force of the applied poling field. An ultrahigh r₃₃ value of 273 pm V⁻¹ and a high refractive index of 2.12 at the wavelength of 1300 nm have been achieved for monolithic glass of C1, which represents a record-high n³r₃₃ figure-of-merit of 2601 pm V⁻¹ with good temporal stability. This exceptional result is a great demonstration of the advantages offered by dynamically assisted dipolar polarization enhancement of tethered binary chromophores, for significantly improving the poling efficiency and thermal stability of organic EO materials for efficient optical modulation.