Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

Donor-pi-acceptor structured thermally activated delayed fluorescence (TADF) molecules are likely to have distributions of dihedral angles in solids. However, the impact of the dihedral angle distribution of TADF molecules on their electronic structures and excited-state dynamics regarding the triplet charge-transfer states ((CT)-C-3) has rarely been studied. Herein, we report unique dual phosphorescence from a series of methyl-substituted TADF molecules in a frozen matrix at 77 K. The origin of dual phosphorescence is the dihedral angle distribution of the TADF molecules rather than the anti-Kasha behavior of the TADF molecules. Based on the time-dependent density functional theory (TD-DFT) calculations and experimental investigations, we show that the dihedral angle distribution of the TADF molecules in solids induces a large energy distribution of (CT)-C-3 states over 0.27 eV leading to a dual phosphorescence from (CT)-C-3 and local triplet states ((LE)-L-3). The ratio of the (CT)-C-3 phosphorescence and the (LE)-L-3 phosphorescence depends on the position of the (LE)-L-3 state within the energy band of the (CT)-C-3 state, which determines the pathway of intramolecular triplet energy transfer (ITET). Our findings contribute to the understanding of the complex excited-state dynamics of triplet states of TADF molecules and shed light on the design of efficient TADF emitters and dual phosphorescence emitters. Moreover, the photophysical model we describe provides fundamental and new insights into the excited-state dynamics of luminescent molecules along with Kasha's rule, one of the most fundamental principles in photochemistry and photophysics.