Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States

Abstract

The rates for up-conversion intersystem crossing (UISC) from the T state to the S state are calculated for a series of organic emitters with an emphasis on thermally activated delayed fluorescence (TADF) materials. Both the spin-orbit coupling and the energy difference between the S and T states (ΔE) are evaluated, at the density functional theory (DFT) and time-dependent DFT levels. The calculated UISC rates and ΔE values are found to be in good agreement with available experimental data. Our results underline that small ΔE values and sizable spin-orbit coupling matrix elements have to be simultaneously realized in order to facilitate UISC and ultimately TADF. Importantly, the spatial separation of the highest occupied and lowest unoccupied molecular orbitals of the emitter, a widely accepted strategy for the design of TADF molecules, does not necessarily lead to a sufficient reduction in ΔE; in fact, either a significant charge-transfer (CT) contribution to the T state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small ΔE. Also, having S and T states of a different nature is found to strongly enhance spin-orbit coupling, which is consistent with the El-Sayed rule for ISC rates. Overall, our results indicate that having either similar energies for the local-excitation and charge-transfer triplet states or the right balance between a substantial CT contribution to T and somewhat different natures of the S and T states, paves the way toward UISC enhancement and thus TADF efficiency improvement.

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…Our results are broadly in agreement with recent work on small molecule TADF systems which suggest that having a substantial CT contribution to T1 and that the energetic proximity of a (π-π*) T2 state enhances RISC. 32 Given the success of donor-acceptor motifs to engineer efficient RISC in small molecules, we expect that the complex mechanism described here may be broadly applicable to other materials capable of TADF…”

Thermally Activated Delayed Fluorescence Emitters for Deep Blue Organic Light Emitting Diodes: A Review of Recent Advances

“…The spin-orbit vibronic coupling is also another major important factor in the design of TADF materials. To date, investigations concerning the relationship existing between spin-orbit coupling and RISC are still rare [60][61][62][63]. While aiming at designing deep blue TADF emitters, several parameters should be taken under consideration, such as the π-conjugation length, the redox potential of both the electron-releasing and electron-withdrawing moieties, as well as the degree of conjugation, which is allowed between the two moieties.…”

Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?

“…[11][12][13][14] In addition to these exciting applications of TADF, the fundamental processes in TADF are also attracting considerable interest in both experimental and theoretical fields. [15][16][17][18][19][20][21][22][23][24][25][26] In this case, the goal is to achieve a sufficient understanding of these processes to realize a high RISC rate (k RISC ) by judicious designing and optimizing the most suitable chemical structures, emission environments, and so on. Increasing the k RISC suppresses triplet state quenching and is, therefore, beneficial for reducing roll-off in OLEDs and organic laser diodes.…”

“…18,19,21,25 In addition, some groups have proposed that second-order spin-orbit coupling enables utilization of higher order triplet states, which further facilitates RISC. 18,20,23 Conventional and time-dependent density functional theory calculations based on such orbital selection rules and/or second-order perturbation theory have succeeded in reproducing the experimentally determined k RISC values of some, but not all, of the organic molecules investigated.…”

“…18,20,23 Conventional and time-dependent density functional theory calculations based on such orbital selection rules and/or second-order perturbation theory have succeeded in reproducing the experimentally determined k RISC values of some, but not all, of the organic molecules investigated. 21 Thus, a broad consensus of the TADF mechanism has yet to be reached.…”