Abstract
The utilization of thermally activated delayed fluorescence (TADF) materials in highly proficient organic light-emitting diodes (OLEDs) has attracted much attention. Based on TADF material TPA-QNX(CN)2, a series of three-dimensional donor-acceptor (D-A) triptycenes have been designed via structural modification of D-fragment. The influences of different D-fragments with various electron-donating strengths on the singlet-triplet energy gap (ΔE(ST)), emission wavelength (λ(em)), and electron/hole reorganization energy (λ(e)/λ(h)) are extensively studied by applying density functional theory (DFT) coupled with time-dependent density functional theory (TD-DFT). The computed results imply that as the electron-donating strength of the D-fragments increases, the ΔE(ST) value decreases and λ(em) is red-shifted for the molecules using the same acceptor units. Analogously, the (1)CT‒(3)CT state splitting (ΔE(ST) (CT)) is also decreased by enlarging the twist angle (β) between the phenyl ring and alternative D-fragment. Therefore, efficient color tuning within a broad emission range (434-610 nm), as well as small ΔE(ST) (CT) values (0.01-0.05 eV), has been accomplished by structural modification of the D-fragments. The greater electron-donating strength, the smaller ΔE(ST), and the smaller λ(h) for PPXZ-QNX(CN)2 make it the best candidate among all the designed molecules.