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[BOND]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.