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

The synthesis of 1-methylphenoxazine via CO 2 -directed lithiation chemistry is reported. This electron donor was coupled with 2,8-dibromodibenzothiophene- S, S -dioxide with Buchwald–Hartwig chemistry to give a new donor–acceptor–donor 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 nonmethylated 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 1 CT(eq) and 1 CT(ax) dominating. The lower energy 1 CT(eq) contributes to thermally activated delayed fluorescence, whereas the higher energy 1 CT(ax) does not. Phenothiazine analogues 2a and 2b also have major fluorescence emissions assigned as 1 CT(eq) and 1 CT(ax), respectively. 2a and 2b have substantial room temperature phosphorescence (RTP), whereas 1a and 1b do not, highlighting the importance of the sulfur atom in 2a and 2b to obtain RTP emission.