NB-Type Electronic Asymmetric Compounds as Potential Blue-Color TADF Emitters: Steric Hindrance, Substitution Effect, and Electronic Characteristics.

Abstract

This article is devoted to the theoretical study of the effects of a connection pattern and stereo hindrance of different π-bridges, nitrogen-containing donors, and boron-containing acceptors on the electrooptic properties of NB-type electronic asymmetric compounds in conventional D−π–A frameworks by the density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches. By introducing three different connection groups (−O–, −CH2–, and −CMe2−) and guided by structural rationality, we formed 30 NB-type molecules, which have been classified into four types: D−π–A, D–X1−π–A, D−π–X1–A, and D–X1−π–X2–A (Xn are connection groups). Then, the energy gaps (ΔEST) between the first singlet and triplet excited states were evaluated by TD-LC-ωPBE with the optimal values of ω*, as well as an approximate method, which only considers the interaction between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). It is found that for the compounds with strong vibronic coupling, the calculated ΔEST defined as the difference of vertical excitation energies largely deviates from the experimental result. The consistency between the estimated and experimental values indicates that ΔEST is predominantly determined by the frontier molecular orbitals, which can be tuned by adjusting the modular overlap between HOMO and LUMO or the orientation of the donor and acceptor groups. Accompanied with the other electronic and optical properties, our study suggests that the interaction mode, D−π–X1–A, the modified D−π–A system with a rigidly fixed acceptor and a relatively free donor, can serve as a valuable molecular design pattern for new blue-colored thermally activated delayed fluorescence (TADF) emitters. Specifically, our calculations predict that ARD-BZN-2CMe2-PYN and its relatives might have excellent potential as TADF emitters.