Crystal-field effects on dipole moments and static first hyperpolarizability in noncentrosymmetric ionic organic crystals

Abstract

Understanding how crystal fields modify local nonlinear optical responses is essential for the rational design of acentric ionic organic crystals. Here, we investigate a structurally diverse set of noncentrosymmetric push−pull ionic crystals, including classical stilbazolium benchmarks and 6MNEP-related GUR/GUS crystals identified by their CCDC refcodes, to evaluate how electrostatic embedding affects the dipole moment and total static first hyperpolarizability (βtot) of asymmetric units. A selfconsistent electrostatic-embedding approach combined with density functional theory (DFT) and time-dependent-DFT (TDDFT) calculations was used to compare isolated and in-crystal ionpair responses within a consistent local-descriptor framework. The results show that the crystal field does not act as a uniform amplifier or suppressor of βtot. Instead, its effect depends on the local electrostatic environment, ion-pair organization, and tensor-component balance. Classical stilbazolium salts generally retain large embedded hyperpolarizabilities, with DAPSH, DSTMS, DSCHS, and DSNS-1 among the most responsive systems. In contrast, most 6MNEP-related GUR/GUS crystals show substantial attenuation of βtot upon embedding, while DSCHS, DSNS-1, and the structurally related MBST salt display enhanced embedded responses relative to their isolated counterparts. These results indicate that favorable local crystal-field alignment and contact patterns can reinforce, rather than suppress, the molecular response in selected cases. Overall, explicit crystal embedding provides a useful comparative strategy for analyzing how ion-pair composition and lattice organization modulate local dipolar and hyperpolarizability descriptors in ionic organic Organic nonlinear optical (NLO) crystals