Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Abstract

For a better understanding of the exciton decay process in thermally activated delayed fluorescence (TADF) emitters, the intersystem crossing rate, k ISC , is one of the important physical constants that have to be determined. Herein, a method to calculate the k ISC value from photoluminescence (PL) measurements is reconsidered. The k ISC value can be determined at very low temperatures where delayed fluorescence (DF) is completely suppressed, as well as around room temperature where triplet excitons mainly decay into the ground state by emitting DF. However, there is a temperature range where the k ISC value cannot be determined accurately because the influences of nonradiative decay paths can be neither ignored nor corrected. For such a temperature range, an alternative approach, which utilizes the temperature dependence of an observed PL decay rate, is presented. In this way, k ISC values from 300 to 10 K are determined for thin films of two TADF emitters, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene, which are known as 4CzIPN and 2CzPN, respectively.

Color‐Tunable Dual Persistent Emission Via a Triplet Exciton Reservoir for Temperature Sensing and Anti‐Counterfeiting

“…Compounds At ambient condition; b) The decay curves were fitted to a multi-exponential function: R(t) = ∑A i exp( − t/τ i ) and average lifetime (τ avg ) was calculated according to τ avg = ∑A i τ i ; c) At 77 K. k are the rate constants of radiative transition (r), and nonradiative transition (nr) of singlet (S) and triplet (T) states, respectively. [10,14] As expected, the lifetimes of phosphorescence increase and those of DF decrease at low temperature, respectively, for the crystals of Ph-C8Br and PhBr-C8Br. However, 100-fold increment of the lifetime is observed for the DF emission of PhBr-C8 from 3.99 ms at ambient condition to 347.29 ms at 77 K, which can be attributed to the rigid environment, suppressed nonradiative relaxation, and the greatly reduced nr T k at low temperature.…”

Exact Solution of Kinetic Analysis for Thermally Activated Delayed Fluorescence Materials

“…[18] 𝜏 𝐷𝐹 = 𝜏 𝑃ℎ𝑜𝑠 0 − ( 11). [19] This is a reasonable assumption for efficient organic TADF emitters. .…”

“…In this case, the efficiencies in Eq. S10 19. should be modified by using overall efficiencies (OEs), which are the final distributed exciton ratio between S1 and Tn population via ISC/RISC cycles; those are 𝛷 𝑅𝐼𝑆𝐶 𝑂𝐸 > 𝛷 𝑅𝐼𝑆𝐶 and 𝛷 𝐼𝐶 𝑇 𝑂𝐸 > 𝛷 𝐼𝐶 𝑇 .…”

Halide-containing organic persistent luminescent materials for environmental sensing applications

“…The lifetime and quantum yield of phosphorescence and TADF can be expressed as eqn (4)–(7): 13,25,26,38,100,101 φ P = φ ISC k T r τ P = φ ISC [1 − ( k T nr + k T q ) τ P ]where superscripts S and T refer to the singlet and triplet excited states, respectively; τ TADF and τ P are the lifetimes of TADF and phosphorescence; k ISC and k RISC are the rate constants of ISC and the reverse intersystem crossing (RISC) process; k S r , k S nr , k T r and k T nr are the rate constants of radiative transition (r), nonradiative transition (nr), respectively, and k T q is the quenching rate coefficient of triplet states by the environmental quenchers. Nonradiative transition and the RISC process are endothermic processes, 26,102 which are strongly affected by temperature, leading to significant difference in phosphorescence and TADF emission at distinct temperatures.…”