Bond Rotations and Heteroatom Effects in Donor-Acceptor-Donor Molecules: Implications for Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence

Abstract

The synthesis of 1-methylphenoxazine via CO2-directed lithiation chemistry is reported. This electron donor was coupled with 2,8-dibenzothiophene-S,S-dioxide with Buchwald–Hartwig chemistry to give a new D–A–D charge-transfer fluorescent molecule 1b. X-ray crystal structures and calculations show that the phenoxazinyl groups are coplanar and equatorial (eq) to the acceptor plane in non-methylated 1a, but are pyramidal and axial (ax) in 1b. The bond rotation energy barriers between donor and acceptor groups for 1a and 1b are only 0.13 and 0.19 eV respectively from hybrid-DFT computations at the CAM-B3LYP/6-31G(d) level. Many possible conformers are present in solutions and in zeonex. In zeonex, the methyl groups in 1b shift the emission band 0.13 eV higher in energy compared to 1a. Excited state eq-eq and ax-ax geometries were identified with DFT calculations, with charge transfer (CT) emission assigned as 1CT(eq) and 1CT(ax) dominating. The lower energy 1CT(eq) contributes to thermally activated delayed fluorescence (TADF), whereas the higher energy 1CT(ax) does not. Phenothiazine analogs 2a and 2b, also have major fluorescence emissions assigned as 1CT(eq) and 1CT(ax) respectively. 2a-b have substantial room temperature phosphorescence (RTP) whereas 1a-b do not, highlighting the importance of the sulfur atom in 2a-b to obtain RTP emission.