Abstract
Thermally activated delayed fluorescence (TADF) has recently become an extensively investigated phenomenon due to its high potential for application in organic optoelectronics. Currently, there is still lack of a model describing correctly basic photophysical parameters of organic TADF emitters. This article presents such a photophysical model describing the rates of intersystem crossing (ISC), reverse ISC (rISC), and radiative deactivation in various media and emphasizing key importance of molecular vibrations on the example of a popular TADF dye 9,10-dihydro-9,9-dimethyl-10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-acridine (DMAC-TRZ). The presented experimental and theoretical investigations prove that ISC and rISC can occur efficiently between the singlet and triplet states of the same charge-transfer nature (1CT and 3CT, respectively). In emitters with the orthogonal donor and acceptor fragments, such spin-forbidden 1CT ↔ 3CT transitions are activated by molecular vibrations. Namely, the change of dihedral angle between the donor and the acceptor affords reasonable spin–orbit coupling, which together with a small energy gap and reorganization energy enable 1CT ↔ 3CT transition rates reaching 1 × 107 s–1. Evidence of direct 1CT ↔ 3CT spin-flip and negligible role of a second triplet state, widely believed as a key parameter in the design of (r)ISC materials, change significantly the current understanding of TADF mechanism. In authors’ opinion, photophysics, and molecular design principles of TADF emitters should be revised considering the importance of vibrationally enhanced 1CT ↔ 3CT transitions.
Highlights
Over the last decade, fast development of all-organic optoelectronics with an increasing number of potential applications provided a plethora of experimental solutions on themolecular level
There has been no general theoretical model which would correctly describe the basic photophysical parameters of thermally activated delayed fluorescence (TADF) materials such as rates of intersystem crossing (ISC), reverse intersystem crossing (rISC), and radiative deactivation in various media. This impedes correct understanding of the mechanism and design principles of efficient TADF materials. We present such a model and on the example of one of the most popular TADF emitters explain how high rISC and ISC rates are achieved under various conditions
Taking into account the 440 nm onset of 3LE phosphorescence measured in frozen methylcyclohexane solution,[16] the ΔE1CT−3LE values in these experiments varied from +0.1 eV in hexane to −0.4 eV in acetone
Summary
Fast development of all-organic optoelectronics with an increasing number of potential applications provided a plethora of experimental solutions on the (sub)molecular level. In heavy-atom free molecular systems, triplet excitons usually cannot ensure fast rates of light emission and should be converted to the singlet ones for high electroluminescence efficiency. Probably the best solution for triplet harvesting is the use of materials with fast reverse intersystem crossing (rISC). This can be realized either by emitters exhibiting thermally activated delayed fluorescence (TADF) dispersed in common OLED hosts[1] or by common fluorescent emitters dispersed in TADF emitters in the role of hosts, the so-called “hyperfluorescence” approach.[2] Numerous photo- and electroluminescent investigations revealed strong dependence of internal and external quantum efficiencies on the rISC rate. The importance of materials with fast rISC for further development of optoelectronics cannot be overestimated
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