Mechanism of the Triplet-to-Singlet Upconversion in the Assistant Dopant ACRXTN

Abstract

In this work, the photophysics of 3-(9,9-dimethylacridin-10­(9H)-yl)-9H-xanthen-9-one (ACRXTN) has been investigated by combined density functional theory and multireference configuration interaction quantum chemical methods. ACRXTN was recently utilized as an assistant dopant in a green emitting organic light-emitting diode (OLED), increasing substantially the external electroluminescence quantum efficiency of the OLED. The efficient triplet-to-singlet upconversion, found experimentally in ARCXTN, cannot be explained solely on the basis of a small singlet–triplet energy gap. We find five interacting electronically excited states in a small energy interval: a charge-transfer excitation (triplet and singlet) from acridine to xanthone, a local xanthone triplet (ππ*) excitation, and a pair of triplet and singlet states originating from an nπ* excitation on xanthone. On the basis of calculated spin–orbit coupling constants and potential energy profiles, we propose here that the triplet (ππ*) state mediates the triplet-to-singlet upconversion in ACRXTN and that the carbonyl bond stretching motion plays an essential role in this process.

Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

“…However, our recent experimental19 and quantum dynamics studies20 suggest that the intersystem crossing (ISC) and reverse intersystem crossing (rISC) between 1 CT singlet and 3 CT triplet states is a more complicated second order process. This second order process is mediated by vibronic coupling of the 3 CT to the local exciton triplet ( 3 LE) states to allow spin–orbit coupling to the 1 CT state182021. These results are also in line with the previous findings of Lim et al 22.…”

“…1a), a structure that promotes the formation of intramolecular charge-transfer (CT) states between the donor and acceptor units. Initially, it was proposed that cyclic intersystem crossing within the CT singlet and triplet manifolds gave rise to TADF12, but with the two uncoupled electrons, on the donor and acceptor, being so far apart and in orthogonal orbitals, the spin–orbit interaction between them must be very small, making this scenario unrealistic18. However, our recent experimental19 and quantum dynamics studies20 suggest that the intersystem crossing (ISC) and reverse intersystem crossing (rISC) between 1 CT singlet and 3 CT triplet states is a more complicated second order process.…”

Using Guest–Host Interactions To Optimize the Efficiency of TADF OLEDs

“…[4][5][6][7][8][9] One way to improve the TADF emission contribution is to minimize the energy splitting (∆E ST ) between singlet and triplet states. Recent experimental studies [10][11][12] along with initial theoretical work [13][14][15] identify that the SOC mechanism in these TADF systems is a complex second order process requiring vibronic coupling between 3 CT and 3 LE to mediate the spin flip back to the 1 CT state, the 3 LE state mediates the SOC between the singlet and triplet CT states 15 . Hence whereas the CT states are very sensitive to the local enviroment 10 the local triplet states are not, therefore the host differential effects the energy gaps which directly changes the rate of reverse intersystem crossing (rISC).…”

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.…”