First hyperpolarizability of dithienylethene-based photoswitches: a structure-property relationship study

Despite their considerable technological potential, structure-property relationships in intramolecular charge-transfer-based photoswitches remain poorly established. Here, we present a quantum-chemical investigation of the first hyperpolarizability (β) of a family of dithienylethene (DTE)-based photoswitches, considering static, electro-optic Pockels, and second-harmonic-generation (SHG) responses. Eight donor (D)/acceptor (A) architectures, with and without alkyne (y) π-extension, are investigated in their open- and closed-ring forms using density functional theory [CAM-B3LYP/6-31+G(d,p)/GD3BJ/(PCM)] in the gas phase and three continuum solvents, with dynamic responses evaluated at 1064 nm. Ring closure generally enhances total first hyperpolarizability (β _tot) by restoring through core conjugation, while donor-acceptor asymmetry emerges as the dominant structural factor governing large NLO responses. Alkyne extension further enhances the response when coupled with a push-pull architecture. Solvation modulates both the magnitude and direction of the response, producing pronounced amplification of the closed/open β _tot ratio in selected systems and a striking reorientation of the SHG response vector for Dy-yA_open. Time-dependent DFT analysis suggests that the reorientation is associated with a redistribution among nearby excited-state contribution. Importantly, β _tot and the directional alignment ratio η are not necessarily correlated. We therefore introduce R _NLO as a combined comparative descriptor that captures both response magnitude and directionality. R _NLO identifies the closed push-pull architectures, particularly Dy-yA_closed, D-yA_closed, and Dy-A_closed, as the most favorable systems among the considered chromophores. Overall, the results show that NLO performance is governed by the coupled effects of donor-acceptor asymmetry, ring closure, π-extension, and dielectric environment, and that optimization of both response magnitude and directionality is essential for establishing meaningful structure-property relationships in DTE photoswitches.