Regulation of the Molecular Architectures on Second-Order Nonlinear Optical Response and Thermally Activated Delayed Fluorescence Property: Homoconjugation and Twisted Donor–Acceptor

The nonlinear optical (NLO) and thermally activated delayed fluorescence (TADF) properties of organic push–pull materials consisting of π-conjugated electron-donating (D) and electron-accepting (A) subunits are dominated by the interaction of D and A moieties via intramolecular charge transfer (ICT). Understanding the structure–property relationship, at the microscopic level, is the prerequisite for further performance optimization or improvement. In this work, we theoretically investigated the geometric and electronic structures, CT properties, polarizabilities (α), first hyperpolarizabilities (β tot ), and singlet–triplet energy gap (Δ E ST ) of the homoconjugation (as type I) and the conventional conjugation D–A (as type II) compounds. A noteworthy finding was that the type II molecule was suggested to promote the performance in NLO due to the lower excited energy and larger dipole moment variations for the crucial excited state, as well as the larger separate distributions of first hyperpolarizability density. In addition, the electron transition properties, second-order NLO responses, and Δ E ST values strongly depend on the nature of different electron acceptors (pyrazine → dicyanopyrazine → dicyanoquinoxaline). Further, based on the polarizable continuum model analysis, the increment in the β tot of all studied compounds is preferable for NLO applications. Moreover, the Δ E ST values of the molecules in which the acceptor are replaced by dicyanopyrazine/dicyanoquinoxaline ( 2, 3, and 6 ) in vacuum are reduced by an order of magnitude when embedded in a polarizable environment, indicating they are potentially efficient TADF materials. Overall, we envision that the various architectures and the polarization effect introduced in the present work will offer a route toward the rational design of such kind of D–A system for novel functional second-order NLO and TADF materials.