Acceleration of Reverse Intersystem Crossing using Different Types of Charge Transfer States

Abstract

There is a need to boost the rate constant of reverse intersystem crossing (k RISC ) in thermally activated delayed fluorescence (TADF) materials for applications to organic light-emitting diodes. Recently, energy level matching of the locally excited state (LE) and charge transfer state (CT) has been reported to enhance k RISC . In this study, we conceptually demonstrate that k RISC can be improved even between CT states without LE states, through the use of different types of CT states. On the basis of this concept, we design a new compound, named DMAC-bPmT, where two phenyl groups of a well-known TADF material DMAC-TRZ are substituted by pyrimidine groups. Theoretical calculations indicated that the energy levels of the different CT states of DMAC-bPmT are very close and enhanced spin orbit coupling may be expected between them. As predicted, DMAC-bPmT experimentally exhibited a k RISC three times as high as that of DMAC-TRZ.

Diazine-based thermally activated delayed fluorescence chromophores

“…Kaji et al designed chromophore 38, starting from its phenyl analogue and substituting two phenyl groups with pyrimidine fragments (Chart 15) so as to modulate the nature of the S 1 and T 1 states. 81 Based on TD-DFT calculations they predict that the S 1 , S 2 , T 1 and T 2 states are nearly degenerated and that all have an important but non-identical CT character. This ensures not only small DE ST but also non-zero SOC values, according to the El-Sayed rule 82 .…”

Unexpected quasi-axial conformer in thermally activated delayed fluorescence DMAC-TRZ. Demonstrating how to turn green OLEDs into blue.

“…DMAC-TRZ has been studied extensively before, but there are many aspects of its photophysics and device performance that have not been revealed until now. [18,22,23,30,34,46,47] The presence of a second conformer, the QA, has proven to be stronger than expected and its contribution to the photophysical performance in solid-state, especially at low concentrations, has proven to be dominant. At higher concentrations (above 1 wt%), the FRET efficiency to the QE conformer increases greatly and its contribution cannot be easily observed, this could be why it was not observed until now.…”

Molecular geometry and the photophysics of thermally activated delayed fluorescence: the strange case of DMAC-py-TRZ

“…Conversely, replacement of the distal phenyl rings in TRZ by electronwithdrawing pyrimidines (DMAC-bPmT) results in a red-shifted emission ( PL of 520 nm vs 500 nm for DMAC-TRZ in toluene). 11 The delayed emission lifetime of DMAC-bPmT is 3.3 s in toluene, which is shorter than that of DMAC-TRZ at 8.8 s in the same medium. The RISC rate constant, k RISC , of DMAC-bPmT is 8.8×10 5 s -1 , is three times faster than that of DMAC-TRZ (2.9×10 5 s -1 ).…”