TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix

Abstract

Reverse intersystem crossing (RISC) rate of a thermally activated delayed fluorescence (TADF) molecule is sensitive to the energy alignment of singlet charge-transfer state (1 CT), triplet charge-transfer state (3 CT), and locally excited triplet state (3 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 3 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, 3 CT phosphorescence, and 3 LE phosphorescence. We obtained the energy band of CT states where 1 CT and 3 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 3 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 3 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 (θD-A) and acceptor-linker (θA) of TPSA in frozen toluene lie within the range of 70°≤θD-A≤90° and 0°≤θA≤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.

Spiro donor–acceptor TADF emitters: naked TADF free from inhomogeneity caused by donor acceptor bridge bond disorder. Fast rISC and invariant photophysics in solid state hosts

“…6 This conformational disorder, originating from the rotation around the C-N bond has also been investigated in other studies. [7][8][9][10] Furthermore, a molecular design which restricts this torsional motion was reported as an approach to avoid this disorder effect. [11][12][13] However, by using a spiro TADF emitter, the rigidity enforced by the tetrahedral spiro carbon atom gives potential immunity to external conformational distortion of the molecule caused by host interactions.…”

Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

“…The phosphorescence spectra of TPSA can be deconvoluted with the 3 LE SAF and 1 CT (300 K) spectra and the remaining spectra that resemble the hybrid local-CT triplet state ( 3 HLCT) emission spectra that can be observed from TPSA in hexane and 2CzIPN in toluene (Figures S8 and S9). ,,, 3 HLCT state is formed in donor–acceptor-type molecules with a dihedral angle smaller than 90°. Although the optimized D–A angles of the TADF molecules incorporating bulky acridine donors are 90°, a substantial proportion of the real TADF molecules in solids are in the less-tilted conformations.…”

“…As mentioned above, puckering of the acridine moiety is prevented by the rigid spiro linkage between the acridine and fluorene moiety. In addition, the rotation of the two peripheral phenyls linked to the triazine has a minor effect on the CT energy of the TADF molecules . Hence, we considered the rotation of the dihedral angle between the acceptor (triazine) and the linker (θ A ) and the dihedral angle between the donor (acridine) and the linker (θ D ) as the major origin of the energy distribution of the CT states (Figure a).…”

“…All geometry optimizations were performed with Gaussian 09 . Single-point calculations for CT energy scan along the dihedral angles were performed with Schrödinger’s Jaguar, using the modified M11 functional (range separation parameter, ω = 0.10), , which is optimized for the robust calculation of the triplet CT states to avoid triplet instability. − The detailed procedure of the TD-DFT calculations is shown in our previous report …”

“…As mentioned above, such a small energy difference of 1 CT, 3 CT, and 3 LE states has a great impact on the RISC process. However, the energy distribution of 1 CT and 3 CT states stemming from the conformational distribution of TADF molecules in immobile organic solid films or frozen solutions are not considered in many reports that cover the RISC process of TADF molecules. , Especially, the energy distribution of 3 CT states is rarely considered, while the energy distribution of 1 CT states is often considered by time-resolved spectra and transient photoluminescence (PL) decays. − In addition, 3 CT and 1 CT states are often considered degenerate states, assuming the orthogonal D–A structure of the TADF molecules. , Moreover, most of the reports determine the energy and configuration of T 1 states from a phosphorescence spectrum measured at a fixed temperature, delay time, and solvent. ,,, …”