D‐Carbazole‐A Compounds: Modulation of Second‐Order Nonlinear Optical Properties by Substituent Types and Positional Isomerism

Abstract

ABSTRACT Three‐dimensional materials exhibiting through‐space charge transfer (TSCT) have garnered extensive research interest in the field of nonlinear optics (NLO). In this work, density functional theory (DFT) was employed to systematically investigate a series of donor (D)‐carbazole‐acceptor (A) type compounds ( 1–6 ) and traditional D–A type compounds 7 and 8 . Their geometric structures, absorption spectra, charge transfer characteristics, and second‐order NLO properties were analyzed, with emphasis on clarifying the influences of D/A types and substitution positions on the first hyperpolarizability ( β tot ) and charge transfer (CT) processes. The results reveal that replacing the acceptor unit from phthalimide (compound 6 ) to dibenzo[a,c]phenazine (compound 3 ) results in a 3.86‐fold enhancement in the β tot value, whereas variations in the donor moiety or substitution position exert relatively weaker effects. This demonstrates that the D‐carbazole‐A skeleton is highly sensitive to acceptor modifications and plays a dominant role in modulating the second‐order NLO properties. Notably, the β tot value of compound 6 (2.2 × 10 −30 esu) is significantly lower, which can be attributed to its unique orbital contributions (HOMO → LUMO + 1), distinct TSCT character, and smaller oscillator strength of the main excited state. In summary, the types and positions of substituents and the CT mode can effectively modulate the second‐order NLO response, providing new strategies for the design of novel NLO materials.