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 kr, however, the more freedom of rotation of the D–A bond causes larger knr, which is comparable to the kr, 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 kr, the suppressed rotation of the DA bond significantly reduces knr, resulting in more competitive kr 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 kr, much larger than its knr, 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%, Lmax = 12 000 cd m−2, which is among the best deep-red OLED devices.