Reverse
intersystem crossing (RISC) rate of a thermally activated delayed fluorescence
(TADF) molecule is sensitive to the energy alignment of singlet charge-transfer
state (<sup>1</sup>CT), triplet charge-transfer state (<sup>3</sup>CT), and
locally excited triplet state (<sup>3</sup>LE). However, the energy distribution of the charge-transfer states originating
from the conformational distribution of TADF molecules in a solid matrix
inevitably generated during the preparation of a solid sample due to the
rotatable donor-acceptor linkage is rarely considered. Moreover, the
investigation of the energy distribution of the <sup>3</sup>CT state is both
theoretically and experimentally difficult due to the triplet instabilities of
time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence
measurements, respectively. As a result, the relation between conformational
distribution, configurations of excited state transition orbitals, and excited
state energies/dynamics have not been clearly explained. In this work, we
determined the energy distribution of CT states of the TADF emitter TPSA in
frozen toluene at 77 K by the measurement of time-resolved spectra in the full
time range (1 ns ~ 30 s) of emission including prompt fluorescence, TADF, <sup>3</sup>CT
phosphorescence, and <sup>3</sup>LE phosphorescence. We obtained the energy
band of CT states where <sup>1</sup>CT and <sup>3</sup>CT states are
distributed in the range of 2.85-3.00 eV and 2.64-2.96 eV, respectively. We
tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of <sup>3</sup>CT energy of TPSA and found
that only the M11 functional shows consistent results without triplet instability.
We performed TD-DFT with the M11* functional optimized for robust dihedral
angle scan of <sup>3</sup>CT states without triplet instability and reproduced
the energy band structure obtained from the experiment. Through TD-DFT and
experimental investigations, it is estimated that the dihedral angle of
donor-acceptor (θ<sub>D-A</sub>) and acceptor-linker (θ<sub>A</sub>) of TPSA in
frozen toluene lie within the range of 70°≤θ<sub>D-A</sub>≤90° and 0°≤θ<sub>A</sub>≤30° respectively. Our results show that the dihedral
angle distribution must be considered for further investigation of the
photophysics of TADF molecules and the development of stable and efficient TADF
emitters.
Excited‐State Charge Transfer in Covalently Functionalized MoS
2
with a Zinc Phthalocyanine Donor–Acceptor Hybrid
