Importance of Chromophore Rigidity on the Efficiency of Blue Thermally Activated Delayed Fluorescence Emitters

Four
new symmetrical donor–acceptor–donor (D–A–D)-type
molecules are reported with diphenylamine (DPA) or 10,11-dihydro-5H-dibenz­[b,f]­azepine (Az)
as electron donors and 9,9-dimethylthioxanthene-S,S-dioxide (TXO2) as the electron acceptor. The
donors are attached at different positions on the acceptor core: either para or meta to the sulfone unit. This
series provides new insights into the effects of chromophore rigidity/flexibility
on the efficiency of thermally activated delayed fluorescence (TADF).
The molecules have been characterized by X-ray crystallography, by
in-depth photophysical studies, and by theoretical calculations. The
clear differences observed in the photophysical properties when using
DPA or Az as a donor are shown to originate from different geometries
of the donor unit which, in turn, influence the geometry of the nitrogen
lone pair and the donating strength of the corresponding fragment.
Thus, a para-substituted Az derivative demonstrated
blue TADF in polar media, while the compounds with more flexible DPA
units did not show delayed fluorescence. To obtain deep-blue emitters,
weaker donating units are needed. A more flexible donor unit leads
to increased local excited state (donor) LE emission and reduced TADF.
However, a certain amount of flexibility has to be present to ensure
deep-blue TADF.