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 CO2-directed lithiation chemistry is reported. This electron donor was coupled with 2,8-dibromodibenzothiophene-S,S-dioxide with Buch-wald 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 (CT)-C-1(eq) and (CT)-C-1(ax) dominating. The lower energy (CT)-C-1(eq) contributes to thermally activated delayed fluorescence, whereas the higher energy (CT)-C-1(ax) does not. Phenothiazine analogues 2a and 2b also have major fluorescence emissions assigned as (CT)-C-1(eq) and (CT)-C-1(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.