Common mechanism of dual emission in linearly-linked donor-acceptor-type thermally activated delayed fluorescence molecules.

Linearly-linked donor-acceptor-type (D-A) thermally activated delayed fluorescence molecules have been expected to be used as efficient emitters in organic light emitting diodes. Despite their simple molecular structures, some of these molecules exhibit a complex dual emission mechanism due to their two conformers: quasi-coplanar (q-copl.) and perpendicular (perp.) conformers. We have investigated three molecules of this type: phenothiazine-triphenyltriazine, 9,9-dimethyl-9,10-dihydroacridine-triphenyltriazine, and phenoxazine-triphenyltriazine using picosecond time-resolved photoluminescence and femtosecond transient absorption spectroscopy measurements. We have revealed the dual emission mechanism common to the three molecules: after photoexcitation, in the q-copl. conformer, the second singlet excited state with local excitation character emits strong fluorescence, which decays in 3-7 ps as it relaxes to the lowest singlet excited state with charge transfer (CT) character. The CT state exhibits relatively weak fluorescence with a lifetime of tens to hundreds of picoseconds. In the perp. conformer, the excited state shows a pronounced CT character with a weaker oscillator strength reduced by two orders of magnitude, structural relaxation in about 4 ps and a slow decay in >1 ns. The dual emission intensity ratio is determined by the population ratio between the q-copl. and perp. conformers in the ground state. The difference in this intensity ratio between the three molecules is ascribed to the difference in relative energetic stability between the two conformers in the ground state. The emission mechanism common to the linearly-linked D-A molecules deepens the understanding of their photophysical properties and opens new pathways for the development of advanced photofunctional materials.