Modulation of the Dihedral Angle in Donor-Functionalized Tris (2, 4, 6-Trichlorophenyl) Methyl Radical-Carbazole Dyads

Abstract

Tris (2, 4, 6-trichlorophenyl) methyl (TTM) radical is the most widely studied luminescent radical, and the carbazole-TTM donor–acceptor (D-A) dyad is recognized for both high photoluminescence quantum yield and robust photostability. However, despite numerous studies on various D-A radical dyads, their structure-property relationship including carbazole-TTM remains elusive. Here, a series of TTM radical acceptor coupled with methyl-modified carbazole donor is synthesized and used as a model system to investigate the influence of the D-A dihedral angle, known as a primal parameter of luminescent closed-shell D-A compounds, on the photophysical properties of D-A radicals. The results reveal that the dihedral angle tunes the hybridization of locally excited transitions and intramolecular charge transfer (CT) transitions. The larger angles lead to an enhancement of the CT character in the excited state, which suppresses radiative decay due to decreased donor–acceptor orbital overlap and increases non-radiative decay according to the energy-gap law, leading to reduced photoluminescence quantum yields. Furthermore, the increased CT character in the excited state bypasses photocyclization of the TTM moiety and improves photostability. These findings indicate that D-A dihedral angle plays a crucial role in balancing the luminescence efficiency and photostability of the organic luminescent radicals.