Intramolecular-rotation driven triplet-to-singlet upconversion and fluctuation induced fluorescence activation in linearly connected donor–acceptor molecules

Abstract

Thermally activated triplet-to-singlet upconversion is attractive from both fundamental science and exciton engineering, but controlling the process from molecular configuration is still unrevealed. In particular, the flexibility of the freedom of molecular geometry is of major importance to understand the kinetics of the phonon-induced upconversion. Here, we focus on two linearly connected donor–acceptor molecules, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) and hexamethylazatriangulene-2,4,6-triphenyl-1,3,5-triazine (HMAT-TRZ), as the model system. While DMAC-TRZ possesses a rotational degree of freedom in the dihedral angle between the donor and acceptor moieties, i.e., C–N bond in tertiary amine, the rotation is structurally restricted in HMAT-TRZ. The rotationally flexible DMAC-TRZ showed significant triplet-to-singlet upconversion caused by thermal activation. On the other hand, the rotation-restricted HMAT-TRZ showed negligible thermal upconversion efficiency. We elaborate on the origin of the photophysical properties from the viewpoint of the geometries in the excited states using time-resolved infrared spectroscopy and quantum chemical calculations. We uncovered that the structural restriction of the intramolecular flexibility significantly affects the optimized geometry and phonon modes coupled to the spin conversion. As a result of the rotation restriction, the spin flipping in HMAT-TRZ was coupled to bending motion instead of the rotation. In contrast, the free rotation fluctuation in the DMAC-TRZ mixes local-excitation and charge-transfer characters, leading to successful activation of the delayed fluorescence as well as the reverse intersystem crossing. Our discovery sheds light on the mechanism of the triplet-to-singlet upconversion, providing a microscopic strategy to control the optoelectronic properties from a molecular viewpoint.

Characterization of Excited States in a Multiple-Resonance-Type Thermally Activated Delayed Fluorescence Molecule Using Time-resolved Infrared Spectroscopy

“…These structural features are entirely different from typical D-A type TADF molecules we already reported 34,35 in which the structural change is small in the excited state. This rigidity must bring about the high oscillator strength and a narrow FWHM with a small Stokes shift of DABNA-1.…”

“…Here, we have investigated the excited-state geometries and molecular orbitals in DABNA-1 using TR-IR in conjugation with density functional theory (DFT) calculations and time-dependent (TD-) DFT calculations. Using the same method, we reported the correlations between structural dynamics and TADF activities 34,35 . Because the results of TD-DFT calculations strongly depend on a choice of functionals 36 , comparison to experimental data is essential.…”

Host‐Dopant Interaction between Organic Thermally Activated Delayed Fluorescence Emitter and Host Material: Insight into the Excited State

“…According to the second-order spin-vibronic mechanism as discussed in section 2.2, the RISC process of TADF involves two triplet excited states vibronically coupled to each other, thus efficient TADF requires a certain degree of intramolecular vibrations, as demonstrated in many studies. [93][94][95][96] For example, Ward et al [94] found that by sterically hindering the intramolecular motion of the D and A groups, the emission of the materials could be switched from TADF to room temperature phosphorescence, which indicates that TADF depends on molecular vibrations. These results have quite beneficial implications for the choice or design of host materials in terms of their rigidity.…”

Aggregation-induced emission active thermally-activated delayed fluorescence materials possessing N-heterocycle and sulfonyl groups

“…Time-resolved IR (TR-IR) spectroscopy, a powerful tool to obtain information about geometries of excited species, 22,[45][46][47] was carried out on 2Cz2SB in a benzene solution and KBr pellet. An FT-IR spectrum of 2Cz2SB in KBr contains multiple bands in the 1100-1400 cm À1 region (Fig.…”