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Abstract
Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin-orbit coupling (SOC) between the first singlet and triplet excited states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg-Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally-resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multipleresonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD), with the inclusion of the FranckCondon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the
rates has also been approximately evaluated. We find that the experimentally-measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about ten times larger than HTVC effect, and the latter increases the ISC rates by more than one order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to the inaccurate description of excited states calculated by the time-dependent density functional theory as well as not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and opens design routes that go beyond the FC approximation for
the future development of high-performance OLED devices.
Supplementary materials
Title
Vibronic Coupling Effect on the Vibrationally-Resolved Electronic Spectra and Intersystem Crossing Rates of a TADF Emitter: 7-PhQAD
Description
The supplementary materials include the excitation energies of 7-PhQAD calculated by ADC(2)/CC-pVDZ at the S$_0$ and S$_1$ geometries, the main geometrical parameters for the ground and excited-state geometries of 7-PhQAD, the charge density difference between the corresponding excited states and ground state by (TDA-)CAM-B3LYP*, and the differences between the geometric structures of S$_0$ and S$_1$, of S$_0$ and T$_1$, and between those of S$_{1}$ and T$_{1}$
optimized at TD-CAM-B3LYP*/6-311++G** level in toluene, and the derivation about the HT-related term and the damping-related term in the time-dependent approach.
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