Structure–property analysis of julolidine-based nonlinear optical chromophores for the optimization of microscopic and macroscopic nonlinearity

Four NLO chromophores with different acceptors and modified julolidine donors were facilely synthesized to tune their intramolecular charge-transfer energy gaps for the investigation of the structure–property relationships. The photophysical properties of the UV-Vis absorption spectra, fluorescence spectra and solvatochromism were measured and analyzed to understand the effect of electron-donating strength and electron-withdrawing strength on intramolecular charge-transfer. Density functional theory calculations were carried out to investigate the electron density of the frontier orbitals, energy gap, molecular microscopic dipole moment and hyperpolarizability. The change tendency of the microscopic hyperpolarizability in different dielectric environments in association with the solvatochromism in absorption was analyzed. Meanwhile, the macroscopic electro-optic performance in relation to molecular structure variation and microscopic properties was further analyzed, revealing that chromophores with a larger solvatochromism exhibited higher microscopic hyperpolarizability and macroscopic electro-optic coefficients. Our investigation of the structure–property relationship gave a reliable insight into molecular design for high performance nonlinear optical chromophores.