New Direct Approach for Determining the Reverse Intersystem Crossing Rate in Organic Thermally Activated Delayed Fluorescent (TADF) Emitters

Abstract

We developed a new optical method to determine the rate of reverse intersystem crossing (k rISC) in thermally activated delayed fluorescent (TADF) organic chromophores using time-resolved transient absorption spectroscopy. We successfully correlated the k rISC of the TADF-chromophores with device performance. Specifically, we focused on the external quantum efficiency (ηEQE) and the stability of the device at high brightness levels. It is believed that by obtaining a large k rISC one may reduce the possibility of triplet–triplet annihilation (TTA) and increase the long-term stability of organic light emitting diodes (OLEDs) devices at high brightness levels (ηEQE roll-off). In this contribution, we investigate the photophysical mechanism in a series of TADF-chromophores based on carbazole or acridine derivatives as donor moieties, and triazine or benzonitrile derivatives as the acceptor moieties. We found a relationship between large k rISC values and high ηEQE values at low operating voltages for the TADF-chromophores investigated. In addition, those chromophores with a larger k rISC illustrated a smaller ηEQE roll-off (higher stability) at high operating voltages. These features are beneficial for superior OLEDs performing devices. Contrarily, we found that if a chromophore has a k rISC ≤ 105s–1 its ηEQE is ≤5%. Such a small k rISC suggests that there is no TADF effect operating in these organic systems and the molecule is not efficient in harvesting triplet excitons. Emission lifetime-based methodologies for determining the k rISC were included for comparison but failed to predict the devices performance of the investigated TADF-chromophores to the same extent of our proposed methodology.

Intersystem crossing processes in the 2CzPN emitter: a DFT/MRCI study including vibrational spin–orbit interactions

“…Our calculated RISC rate constant associated with the back population of the S 1 state of 2CzPN in vacuum through the T 1 -S 1 channel (k RISC = 3 Â 10 4 s À1 ) certainly is one order of magnitude larger than the smallest constants (5-6 Â 10 3 s À1 ) reported in solution and in OLEDs, 10,13,76 but significantly smaller than the highest value (3.5 Â 10 6 s À1 ) 77 determined by kinetic modelling of transient triplet absorption spectra in toluene solution. Astonishingly, the experimentally derived RISC rate constants and DE ST values do not appear to be correlated.…”

Exact Solution of Kinetic Analysis for Thermally Activated Delayed Fluorescence Materials

“…Other parameters such as exciton diffusion coefficients and length are also estimated. 10 14 = is triplet lifetime from transient absorption spectra.…”

“…[13] Goodson and co-workers extracted the triplet decay rate using transient absorption (TA) measurement as a proxy for the RISC rate constant, but they did not appear to consider cycling between the singlets and triplets. [14] The constraints and assumptions for these RISC rate constant estimations are summarized in Table S1. Since there are now a large number of different models used to estimate , each with their own set of assumptions, it has become impossible to accurately compare the estimated rate constants across these studies, complicating any meta-analysis.…”

TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix

“…Especially, the SOC of 1 CT and 3 CT states of TADF molecules with orthogonal D-A structure such as acridine donor-based ones are too weak for typical RISC rate (10 4 ~10 6 s -1 ) indicating the presence of another RISC pathway. 12,[25][26][27][28][29][30][31][32] Recent researches emphasize that locally excited triplet ( 3 LE) or hybrid local and charge-transfer state ( 3 HLCT) that has sufficient SOC with the 1 CT state must exist for efficient RISC process. 12,[30][31][32][33][34] In addition, the energy difference between 1 CT, 3 CT, 3 HLCT, and 3 LE should also be minimized for a faster RISC rate.…”