Dihedral Angle Control of Blue Thermally Activated Delayed Fluorescent Emitters through Donor Substitution Position for Efficient Reverse Intersystem Crossing

This
study shows a molecular design strategy for controlling the
dihedral angle of two carbazole donors linked to a 2,4-diphenyl-1,3,5-triazine
acceptor by a phenyl unit. Using this approach, six thermally activated
delayed fluorescence emitters were synthesized with donors placed
in various positions around a central phenyl core, and the photophysical
relationship between the donor position and its dihedral angle was
investigated. We demonstrate that this angle can affect both the strength
of the charge transfer state and the conjugation across the entire
molecule, effectively changing the singlet–triplet energy gap
of the system. We conclude that materials containing two substituted
−ortho donors or one −ortho and an adjacent −meta
have the smallest energy gaps and the shortest delayed fluorescence
lifetimes. On the other hand, emitters with no −ortho substituted
donors have larger energy gaps and slow-to-negligible delayed fluorescence.
When applying these materials to organic light-emitting diodes, these
blue-emitting devices have a range of electrical properties, the best
producing efficiencies as high as 21.8% together with high resistance
to roll-off that correlate with the reverse intersystem crossing rates
obtained.