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 (TADF), a process in which triplet excitons undergo upconversion 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 of 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.

Effective models for TADF: the role of the medium polarizability

“…S4, ESI†). Since, as explained above, the Marcus factorization does not apply when the Q -dependence of the interaction matrix element is non-negligible, augmenting the Marcus (or MLJ) expression just introducing a Q -dependent interaction 18 is formally incorrect and leads to uncontrolled results.…”

“…[4][5][6][7][8] Tiny spin-orbit coupling (SOC) interactions must be understood in this complex scenario and a wealth of experimental and theoretical work is devoted to unveiling the relevant physics. [9][10][11][12][13][14][15][16][17][18][19] From a theoretical perspective, the detailed understanding of TADF, as needed to actually simulate the time-dependent phenomena, is highly non-trivial. Indeed the non-adiabatic coupling between electronic and vibrational/conformational degrees of freedom must be accounted for in a system characterized by large anharmonicity.…”

A New Entry to Purely Organic Thermally Activated Delayed Fluorescence Emitters Based on Pyrido[2,3‐b]pyrazine‐Dihydrophenazasilines Donor‐Acceptor Dyad

“…Finally, spectrum simulations and all rate estimates were calculated using the nuclear ensemble approach as implemented in the NEMO software. [28] A total of 500 geometries were sampled for each case and the QChem 5.0 software [29] was used throughout. We started our computational analysis by investigating conformational preferences for the three compounds.…”

Triplet-to-singlet exciton transfer in hyperfluorescent OLED materials

“…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.…”

“…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.…”