A Unified Framework for Photophysical Rate Calculations in TADF Molecules

Abstract

One of the challenges in organic light emitting diodes research is finding ways to increase device efficiency by making use of the triplet excitons that are inevitably generated in the process of electroluminescence. One way to do so is by thermally activated delayed fluorescence, a process in which singlet excitons undergo up-conversion to singlet states, allowing them to relax radiatively. The discovery of this phenomenon has ensued a quest for new materials that are able to effectively take advantage of this mechanism. From a theoretical standpoint, this requires the capacity to estimate the rates of the various processes involved in the photophysics of candidate molecules, such as intersystem crossing, reverse intersystem crossing, fluorescence and phosphorescence. Here we present a method that is able to, within a single framework, compute all these rates and predict the photophysics of new molecules. We apply the method to two TADF molecules and show that results compare favorably with other theoretical approaches and experimental results. Finally, we use a kinetic model to show how the calculated rates act in concert to produce different photophysical behavior.

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence

“…For instance, we computed gaps at different optimized reference geometries. As has been previously shown, 5 such calculations may not be representative of actual singlet−triplet gaps, given the role played by vibrations in TADF. A second important point concerns the fact that experimental gaps are reported either in solution or the solid state, conditions in which CT states tend to undergo deeper energetic stabilization that affects the gaps.…”

Triplet-to-singlet exciton transfer in hyperfluorescent OLED materials

“…These are presented in Table 3 and were calculated following the methodology developed in ref. 28 and summarized in the ESI † file. We note that, as expected, fluorescence rates are between 3 and 6 orders of magnitude higher than phosphorescence rates.…”

“…Absorption, fluorescence, and phosphorescence spectrum simulations were performed according to the nuclear ensemble method, 28,39 which is outlined in the ESI † file. One of the shortcomings of the nuclear ensemble approach to spectrum simulations is that it lacks vibrational resolution, with only the envelope of the band being more reliably described.…”

“…Intersystem crossing rates were similarly obtained using the NEMO software 46 with the methodology described in ref. 28 and outlined in the ESI † file. This approach constitutes an extension of the nuclear ensemble method that aims at determining intersystem crossing rates while accounting for vibrational effects, as these have been shown to be important.…”

“…8 It is done by combining a set of theoretical tools that we have previously developed for optical spectra simulations, estimation of exciton diffusion lengths, and calculations of TADF rates. 28,29 This multiscale modeling approach enables us to estimate the rates for the key processes occurring in hyperfluorescent OLED materials, i.e., FRET, ISC, rISC, fluorescence, and phosphorescence. Importantly, FRET rates are calculated not only for transfers from TADF molecules to fluorescent emitters but also for those in the opposite direction and those that take place among homodimers.…”