Effective Impact of Dielectric Constant on Thermally Activated Delayed Fluorescence and Nonlinear Optical Properties: Through-Bond/-Space Charge Transfer Architectures

Recently, three 9,9-dimethylxanthene-based donor (D)/acceptor (A) U-shaped space-through architectures, containing pi-pi intramolecular interactions between the D and A, exhibit unique advantage (i.e., a small singlet (S-1) - triplet (T-1) energy splitting (Delta E-ST)) in thermally activated delayed fluorescence (TADF). To explore the TADF and second-order nonlinear optical (NLO) properties of U-shape compounds with through-space charge transfer (TSCT) between aligned D and A units compared with that of conventional conjugated D-A (L shape) ones, we theoretically investigated the geometric and electronic structures, through space D-A pi-pi interactions, CT properties, Delta E-ST, and first hyper-polarizabilities (beta(tot)) of compounds 1-L similar to 5-U. The calculated Delta E-ST values of the U-shaped molecules are relatively smaller than that of L-shaped compounds in gas phase, indicating that the U-shaped derivatives are excellent thermally activated delayed fluorescent candidates. Furthermore, a noteworthy finding was that the conjugated D-A unit of L-shaped compounds was suggested to promote the performance in NLO due to the lower excited energy and stronger oscillator strength for the crucial excited state. Especially, for compound 2-L, the beta(tot) value is 8 times larger than that of 2-U in gas phase. In addition, we have quantitatively studied Delta E-ST and beta(tot) values in the solid-state polarization for all studied molecules using the polarizable continuum model. Importantly, the results of polarization effects (epsilon from 1.0 to 3.0) show that the marked reduction in the Delta E-ST values of U-shaped derivatives are due to the simultaneous presence of dominant (TSCT)-T-1 and (TSCT)-T-3 excited states in the solid-state polarization, which are favorable for TADF materials. In addition, the increment in the beta(tot), values of L-shaped compounds are preferable for NLO applications. We hope this work may provide a theoretical understanding on the influence of the heteroatom and the pi-pi conjugation between D and A units and polarization effects on the Delta E-ST and beta(tot) and novel design mentality of the efficiency-enhancing TADF and NLO materials.