Lili Lin

Excited state properties of non-doped thermally activated delayed fluorescence emitters with aggregation-induced emission: a QM/MM study

The excited state properties of dibenzothiophene-benzoyl-9,9-dimethyl-9,10-dihydroacridine (DBT-BZ-DMAC) in the solid phase are theoretically studied through a combined quantum mechanics and molecular mechanics (QM/MM) method. The results indicate that the non-radiative decay rate of the molecule in the solid phase is significantly decreased due to the suppression of the rotation of the DMAC and DBT units in the molecule, while the radiative rate is greatly increased owing to the enhancement of the transition dipole moment. Moreover, the fluorescence efficiency in the solid phase (20.5%) is shown to be much larger than that in the gas phase (0.01‰), which confirms that DBT-BZ-DMAC is a typical aggregation-induced emission (AIE) system. The results further display that both the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes take place between the first singlet excited state (S1) and the lowest degenerate triplet excited states (T1 and T2). In addition, the charge transfer rate is studied using the Marcus theory and the intrinsic charge mobility is calculated by performing the kinetic Monte Carlo method. The results show that the DBT-BZ-DMAC crystal is a p-type semiconductor with a hole mobility of 0.14 cm2 V−1 s−1 at room temperature. Our investigation elucidates the experimental measurements and helps one to understand the AIE mechanisms of the DBT-BZ-DMAC fluorescence emitter, which is beneficial for developing new TADF emitters.

Excited state dynamics of new-type thermally activated delayed fluorescence emitters: theoretical view of light-emitting mechanism

ABSTRACT Excited state dynamics of two new-type thermally activated delayed fluorescence emitters (DABNA-1 and DABNA-2) synthesised based on multiple resonance effect is studied based on first-principles calculation, and their light-emitting mechanism is explored. The excited state dynamics combing with the adiabatic energy structure of several low-lying excited states indicates that three lowest triplet states are involved in the light-emitting process. The analysis of reorganisation energy indicates different performances for two molecules in solvent and in film. Our theoretical work provides rational explanation for experimental results, and also gives clear picture for light-emitting mechanism of these new-type molecules.

Influence of donor and acceptor groups on the S-T energy gap for thermally activated delayed fluorescence emitters

ABSTRACT Molecules composed of different donors and acceptors are theoretically designed as potential thermally activated delayed fluorescence emitters, and their singlet–triplet (S-T) energy gap is studied using the optimal Hartree–Fork method. It is found that the S-T energy gap is in reverse proportional to the electron-donating ability. Stronger electron-donating ability of donors will induce smaller highest occupied molecular orbital–lowest unoccupied molecular orbital overlap and also a smaller S-T energy gap. Based on our calculation results, three molecules are proposed to have great potential to be used as thermally activated delayed fluorescence emitters in organic light-emitting diodes.

Theoretical perspective of the excited state intramolecular proton transfer for a compound with aggregation induced emission in the solid phase

In this study, we have investigated the excited state intramolecular proton transfer (ESIPT) for the diphenylethylene-modified 2-(2-hydroxyphenyl)benzothiazole derivative (HBT-d-Ph) with aggregation induced emission (AIE) in the solid phase through a combined quantum mechanics and molecular mechanics (QM/MM) approach and thermal vibration correlation formalism for non-radiative decay rate. In comparison with the molecule in the solid phase, we find that the ESIPT for HBT-d-Ph is prone to occur in toluene due to the intramolecular hydrogen bonding (H-bond). In addition, the rotation of benzene units involved in the low frequency vibration modes (<500 cm−1) is effectively impeded due to the intermolecular interaction in the solid state, thus the energy consumption pathway through the rotation of benzene units could be blocked. Further, a dual fluorescence with blue and green emission in the solid phase could be obtained. Our study could provide some useful information for designing highly efficient emitting materials with ESIPT and AIE features.

Modulating excited state properties of thermally activated delayed fluorescence molecules by tuning the connecting pattern

Due to the slow radiation rate caused by weak overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in the thermally activated delayed fluorescence molecules, the design of molecules with a hybridized local and charge-transfer (HLCT) excited state is thought as one good solution. In this paper, an efficient way by modifying the connection bridges between donor (D) and acceptor (A) groups is provided based on first-principles study. It is found that the HLCT state can be obtained by reducing the torsion angle or the steric hindrance between D and A. Besides, the D–A distance should be considered, which can influence the proportion of local excitation and charge-transfer compound in the HLCT state. The effect of the aromaticity of the acceptor unit on the photophysical properties is also investigated. It is found that the effect is quite system dependent, and its applications should be carefully ascertained. Our research provides helpful and new insights into the design of high-efficient organic light-emitting molecules.

Efficient modulation of optical and electrical properties of X-shaped thermally activated delayed fluorescence emitters by substitution

A series of X-shaped thermally activated delayed fluorescence (TADF) emitters are systematically studied by first-principles calculations. Effects of the cyano group adding to the acceptor unit and the hydroxyl group adding to the donor part on the optical and electrical properties are analyzed. It is found that both kinds of groups can efficiently increase the emission wavelength to realize full-color emission. Although they play different roles in modulating the energy level of frontier orbitals, the S-T energy gap, the reorganization energy and transfer integral for different molecules, they can efficiently increase the charge transfer rate and reduce the difference of electron transfer rate and hole transfer rate. These results indicate that these designed strategies are efficient to achieve balanced charge transfer rates and modulate emission colors. By analyzing the energy matching between the TADF emitters and three kinds of hosts, the emission spectra of the 3,5-bis(N-carbazolyl)benzene (mcp) and the absorption spectra of most TADF emitters have a large overlap, which provides helpful information in application of these TADF molecules.

Luminescent properties of thermally activated delayed fluorescence molecule with intramolecular π–π interaction between donor and acceptor

Influence of intramolecular π–π interaction on the luminescent properties of thermally activated delayed fluorescence (TADF) molecule (3, 5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-phenyl)(pyridin-4-yl) methanone (DTCBPY) is theoretically studied by using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Four conformations (named as A, B, C, and D) of the DTCBPY can be found by relax scanning, and the configuration C corresponds to the luminescent molecule detected experimentally. Besides, we calculate the proportion of each conformation by Boltzmann distribution, high configuration ratios (44% and 52%) can be found for C and D. Moreover, C and D are found to exist with an intramolecular π–π interaction between one donor and the acceptor; the intramolecular interaction brings a smaller Huang–Rhys factor and reduced reorganization energy. Our work presents a rational explanation for the experimental results and demonstrates the importance of the intramolecular π–π interaction to the photophysical properties of TADF molecules.