Exact Solution of Kinetic Analysis for Thermally Activated Delayed Fluorescence Materials

Abstract

The photophysical analysis of thermally activated delayed fluorescence (TADF) materials has become instrumental for providing insights into their stability and performance, which is not only relevant for organic light-emitting diodes but also for other applications such as sensing, imaging, and photocatalysis. Thus, a deeper understanding of the photophysics underpinning the TADF mechanism is required to push materials design further. Previously reported analyses in the literature of the kinetics of the various processes occurring in a TADF material rely on several a priori assumptions to estimate the rate constants for forward and reverse intersystem crossing. In this report, we demonstrate a method to determine these rate constants using a threestate model together with a steady-state approximation and, importantly, no additional assumptions. Further, we derive the exact rate equations, greatly facilitating a comparison of the TADF properties of structurally diverse emitters and providing a comprehensive understanding of the photophysics of these systems.

Toward rational design of TADF two-coordinate coinage metal complexes: understanding the relationship between natural transition orbital overlap and photophysical properties

“…Organic TADF molecules were chosen as listed in Table S12 (ESI†) and their photophysical properties were collected from literature. 13,49,57–77 Although the theoretical methods used to determine values for the NTO overlap were the same as those applied for the coinage metal complexes, different methods used to obtain experimental values for organic TADF molecules make the comparisons problematic. Nevertheless, the theoretical Δ E ST values can still be evaluated as all compounds were calculated using the same method and basis set and show a clear increase with greater NTO overlap (Fig.…”

“…12 Organic TADF materials generally have slow ISC rates (k ISC = 10 5 -10 8 s À1 ) owing to weak spin-orbital coupling (SOC), which makes fluorescent radiative decay k S 1 r À Á competitive with intersystem crossing. 49 Consequently, the radiative decay rate for TADF (k TADF radiative rate of S 1 k S 1 r À Á…”

“…12 Organic TADF materials generally have slow ISC rates ( k ISC = 10 5 –10 8 s −1 ) owing to weak spin-orbital coupling (SOC), which makes fluorescent radiative decay competitive with intersystem crossing. 49 Consequently, the radiative decay rate for TADF ( k TADF r ) is intimately tied to k ISC (both S 1 → T 1 and T 1 → S 1 ) and the radiative rate of S 1 . In contrast, cMa complexes have ISC rates that are markedly faster ( k ISC = 10 10 –10 11 s −1 ) than owing to high SOC imparted by the central metal ion.…”

Diindolocarbazole – achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units

“…S2, ESI‡), m -CzBNCz 27 and BSBS-N1 , 38 where k RISC reaches 1.08 and 1.90 × 10 6 s −1 , respectively. Even direct comparison between reported RISC rates from different research teams is challenging though, due to the plurality of reported methods for determining its value 39,40 and subtle yet important practical concerns. 41 The MR-TADF emitters with the fastest k RISC are nevertheless two orders of magnitude slower than the best performing D–A TADF emitter (Fig.…”

Diindolocarbazole – Achieving Multiresonant Thermally Activated Delayed Fluorescence Without The Need for Acceptor Units

“…S2, ESI ‡), m-CzBNCz 27 and BSBS-N1, 38 where k RISC reaches 1.08 and 1.90 Â 10 6 s À1 , respectively. Even direct comparison between reported RISC rates from different research teams is challenging though, due to the plurality of reported methods for determining its value 39,40 and subtle yet important practical concerns. 41 The MR-TADF emitters with the fastest k RISC are nevertheless two orders of magnitude slower than the best performing D-A TADF emitter (Fig.…”