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 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 1CT­(eq)
and 1CT­(ax) dominating. The lower energy 1CT­(eq)
contributes to thermally activated delayed fluorescence, whereas the
higher energy 1CT­(ax) does not. Phenothiazine analogues 2a and 2b also have major fluorescence emissions
assigned as 1CT­(eq) and 1CT­(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.