Twist Angle and Rotation Freedom Effects on Luminescent Donor–Acceptor Materials: Crystal Structures, Photophysical Properties, and OLED Application

The twist angle and rotation freedom between the donor (D) and acceptor (A) in D–A materials plays an important role in their photophysical properties. Here, the authors select the asymmetric acceptor pyridal[2,1,3]thiadiazole (PT) to construct two D–A isomers (p‐TPA‐PT and d‐TPA‐PT) with different twist angle and rotation freedom due to the donor triphenylamine (TPA) proximal or distal to N‐atom (pyridyl), as well as a symmetric bis‐triphenylamine‐substituted compound DTPA‐PT for their investigation. On the basis of experimental and theoretical analysis, the authors have explained how the difference in twist‐angle and freedom of rotation affects the photophysical properties of these materials. The p‐TPA‐PT has small twist angles with a relative large k_r, however, the more freedom of rotation of the D–A bond causes larger k_nr, which is comparable to the k_r, and thus a lower photoluminescence (PL) efficiency is obtained in the doped film. Although the d‐TPA‐PT has relative large twist angles with low k_r, the suppressed rotation of the DA bond significantly reduces k_nr, resulting in more competitive k_r when compared with the p‐TPA‐PT, consequently relatively higher PL efficiency is observed. The DTPA‐PT, combination of p‐TPA‐PT and d‐TPA‐PT, shows highest k_r, much larger than its k_nr, therefore shows highest PL efficiency. As a result, the DTPA‐PT based organic light emitting diode (OLED) shows beautiful deep‐red emission, maximum external quantum efficiency of 3.87%, L_max = 12 000 cd m⁻², which is among the best deep‐red OLED devices.