A novel acid-controlled second-order nonlinear optical switch based on dimethyldihydropyrene/cyclophanediene photoswitch

Abstract

Nonlinear optical (NLO) switches, especially multi-state NLO switches, have attracted much attention because of their applications in optical memory with multiple storage and nondestructive capacity. However, it is still a challenge to design new second-order NLO switch molecules with high first hyperpolarizability (β) contrasts. In this study, we theoretically investigated the second-order NLO property and second-order NLO switch effect of dimethyldihydropyrene photoswitch derivatives with a push–pull electronic structure during protonation. Remarkably, the quinoxaline acceptor-based systems have a larger β ratio contrast, demonstrating a simple “ON”–“OFF” NLO switch. Interestingly, the system with a pyridine acceptor and aniline donor showed a multi-state “ON”–“OFF”–“ON” NLO switch under light-triggered and pH-triggered transformations. Furthermore, the DFT and TDDFT calculations provided meaningful insights into their structure–property relationship. Specifically, a large second-order NLO response was displayed in the protonation of the acceptor, resulting from the production of a stronger electron push–pull structure, larger π-conjugation, and more obvious CT transition. However, a weak NLO response was observed upon the protonation of the donor because of inhibiting electron-donating ability, reducing the degree of π-conjugation and producing a predominant LE transition. We hope that the present work will provide a theoretical guide for the design of multifunctional organic materials based on the DHP core.