Tuning of Second-Order Nonlinear Optical Properties Based on [2.2]Paracyclophanes Isomer: the Relative Configuration and Polarizable Environment

Through-space charge transfer provides more flexibilities and possibilities to regulate the properties of organic optoelectronic materials, biotechnology, photochemistry, and so on, by changing spatially the aromatic packing configuration. In this study, a series of [2.2]­pCp-based cis (pseudogeminal) and trans (pseudopara) molecules are designed to explore the effect of isomers on the activities quantified by the second-order nonlinear optical (NLO) properties from density functional theory calculations. The results show that the first hyperpolarizability (β_tot) of the trans isomer is larger than that corresponding to cis ones, especially for cis/trans-TPA-TRZ systems. Moreover, the designed molecule trans-TPA-TRZ shows remarkably large β_tot value up to 39.7 × 10^–30 esu, which demonstrates that the activity of a material depends not only on the intrinsic property of the acceptor or donor itself but also on their combination, intramolecular conformation, and conjugate packing mode. Interestingly, visualization of the full hyperpolarizability tensor, termed the unit sphere representation, has been used to provide insight and intuition into the relationship between the hyper-Rayleigh scattering response coefficient and molecular structure. Furthermore, the influence of the solid-state environment on the NLO coefficient is substantial, particularly for the trans-TPA-TRZ molecule, whch increases further to 84.2 × 10^–30 esu according to the polarizable continuum model and optimally tuned range-separated hybrid functional. Our work provides rich insight into designing and developing high-performance second-order NLO materials by tuning the relative configuration and polarizable environment of the [2.2]­pCp-based through-space π-extended conjugated systems and might be of potential application in optoelectronics.