Spin–orbit charge transfer intersystem crossing and thermal activation delayed fluorescence (TADF) studies of compact orthogonal anthraquinone phenothiazine derivatives

Abstract

Thermally activated delayed fluorescence (TADF) in compact electron donor–acceptor dyads has attracted great attention due to their potential application in organic light-emitting diodes. However, the lack of understanding of the microscopic mechanisms of this phenomenon hinders the design strategies for effective TADF materials. In this work, we systematically compared the TADF mechanism of three compact dyads with those of electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C–N bond (AQ-PTZ, AQ-PTZ-O, and AQ-PTZ-O₂) by means of quantum chemical calculations in conjunction with electron paramagnetic resonance (EPR) spectroscopy. It turns out that three targeted molecules for TADF are identified depending on the relative energy positions of the charge-transfer excited states, ¹CT/³CT, and the local triplet state, ³LE states. In most TADF-active molecules, AQ-PTZ-O and AQ-PTZ-O₂, the structure deformation of excited states is suppressed by oxidation of the PTZ unit, leading to a small relaxation energy and a reverse intersystem crossing (RISC) energy barrier, as well as ³LE being on-resonance with the CT states, thus resulting in high efficiency RISC. Analyses of the electron paramagnetic resonance (EPR) parameters have confirmed that the electron spin polarized (ESP) patterns of T₁(¹LE) for AQ-PTZ-O and AQ-PTZ-O₂ are AEAEAE (A, absorption; E, emission) with a preferential population of T_x and T_z, which is characteristic of ISC mediated by SOC interactions. Namely, the primary population mechanism of the singlet excited state is spin–orbit charge-transfer RISC due to the presence of a perpendicular π system. Their D values are expected to be positive, and display a more “disk-like” spin density distribution with an oblate shape; the calculated RISC rates also support these views. These results provide new gateways for designing effective TADF materials and studying TADF mechanisms.