scholarly journals Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state

2016 ◽  
Vol 2 (1) ◽  
pp. e1500889 ◽  
Author(s):  
Larissa Bergmann ◽  
Gordon J. Hedley ◽  
Thomas Baumann ◽  
Stefan Bräse ◽  
Ifor D. W. Samuel
Keyword(s):  
Solid State ◽  
Delayed Fluorescence ◽  
Average Lifetime ◽  
Intersystem Crossing ◽  
Time Resolved ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Singlet States ◽  
Time Resolved Photoluminescence

Intersystem crossing in thermally activated delayed fluorescence (TADF) materials is an important process that controls the rate at which singlet states convert to triplets; however, measuring this directly in TADF materials is difficult. TADF is a significant emerging technology that enables the harvesting of triplets as well as singlet excited states for emission in organic light emitting diodes. We have observed the picosecond time-resolved photoluminescence of a highly luminescent, neutral copper(I) complex in the solid state that shows TADF. The time constant of intersystem crossing is measured to be 27 picoseconds. Subsequent overall reverse intersystem crossing is slow, leading to population equilibration and TADF with an average lifetime of 11.5 microseconds. These first measurements of intersystem crossing in the solid state in this class of mononuclear copper(I) complexes give a better understanding of the excited-state processes and mechanisms that ensure efficient TADF.

scholarly journals A Unified Framework for Photophysical Rate Calculations in TADF Molecules

2021 ◽  
Author(s):  
Leonardo Evaristo de Sousa ◽  
Piotr de Silva
Keyword(s):  
Delayed Fluorescence ◽  
Intersystem Crossing ◽  
Unified Framework ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Theoretical Approaches ◽  
Singlet States ◽  
Photophysical Behavior ◽  
Theoretical Standpoint

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.

scholarly journals A Unified Framework for Photophysical Rate Calculations in TADF Molecules

2021 ◽  
Author(s):  
Leonardo Evaristo de Sousa ◽  
Piotr de Silva
Keyword(s):  
Delayed Fluorescence ◽  
Intersystem Crossing ◽  
Unified Framework ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Theoretical Approaches ◽  
Singlet States ◽  
Photophysical Behavior ◽  
Theoretical Standpoint

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.

Over 20% external quantum efficiency in red thermally activated delayed fluorescence organic light-emitting diodes using a reverse intersystem crossing activating host

2018 ◽  
Vol 6 (20) ◽  
pp. 5363-5368 ◽  
Author(s):  
Ji Han Kim ◽  
Dong Ryun Lee ◽  
Si Hyun Han ◽  
Jun Yeob Lee
Keyword(s):  
Quantum Efficiency ◽  
External Quantum Efficiency ◽  
Light Emitting Diodes ◽  
Delayed Fluorescence ◽  
Organic Light Emitting Diodes ◽  
Intersystem Crossing ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Organic Light

Highly efficient red thermally activated delayed fluorescence organic light-emitting diodes were developed using a reverse intersystem crossing activating host derived from phenylcarbazole and pyridofuropyridine.

scholarly journals Hot Exciplexes in U-Shaped TADF Molecules with Emission from Locally Excited States

2021 ◽  
Author(s):  
A. Lennart Schleper ◽  
Kenichi Goushi ◽  
Christoph Bannwarth ◽  
Bastian Haehnle ◽  
Philipp Welscher ◽  
...  
Keyword(s):  
Delayed Fluorescence ◽  
Organic Light Emitting Diodes ◽  
Intersystem Crossing ◽  
Thermally Activated Delayed Fluorescence ◽  
New Approach ◽  
Light Emitting ◽  
Thermally Activated ◽  
Donor And Acceptor ◽  
Low Efficiency ◽  
High Color Purity

Rapid reverse intersystem crossing and high color purity are vital characteristics of emitters with thermally activated delayed fluorescence in opto-electronic devices. We present a new approach, called “hot exciplexes” that enables access to both attributes at the same time. Hot exciplexes are produced by coupling facing donor and acceptor moieties to an anthracene bridge, yielding an exciplex with large T1 to T2 spacing. The hot exciplex model is investigated using optical spec-troscopy and quantum chemical simulations. Reverse intersystem crossing is found to occur preferentially from the T3 to the S1 state within only a few nanoseconds. Application and practi-cality of the model are shown by fabrication of organic light-emitting diodes with up to 32 % hot exciplex contribution and low efficiency roll-off.

Reverse Intersystem Crossing Managing Assistant Dopant for High External Quantum Efficiency Red Organic Light-Emitting Diode

2022 ◽  
Author(s):  
Dong Jin Shin ◽  
Seung Chan Kim ◽  
Jun Yeob Lee
Keyword(s):  
Light Emitting Diode ◽  
Delayed Fluorescence ◽  
Intersystem Crossing ◽  
Energy Transfer Rate ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Organic Light Emitting Diode ◽  
Thermally Activated ◽  
Organic Light ◽  
Donor Moiety

Thermally activated delayed fluorescence (TADF) materials working as an assistant dopant with reduced Dexter energy transfer rate was designed by replacing the donor moiety of 2,3,5,6-tetra(9H-carbazol-9-yl) terephthalonitrile (4CzTPN) with 5H-benzo[4,5]thieno[3,2-c]carbazole...

scholarly journals “Rate-limited effect” of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes

2016 ◽  
Vol 7 (7) ◽  
pp. 4264-4275 ◽  
Author(s):  
Xinyi Cai ◽  
Xianglong Li ◽  
Gaozhan Xie ◽  
Zuozheng He ◽  
Kuo Gao ◽  
...  
Keyword(s):  
Rate Constant ◽  
Fluorescence Lifetime ◽  
Delayed Fluorescence ◽  
Organic Light Emitting Diodes ◽  
Intersystem Crossing ◽  
Limited Effect ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Organic Light

The rate constant of reverse intersystem crossing was found to be the “rate-limited step” in thermally activated delayed fluorescence lifetime governing.

scholarly journals Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

2015 ◽  
Vol 373 (2044) ◽  
pp. 20140447 ◽  
Author(s):  
Fernando B. Dias
Keyword(s):  
Energy Gap ◽  
Delayed Fluorescence ◽  
Threshold Temperature ◽  
Intersystem Crossing ◽  
Triplet Energy ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Kinetics Of ◽  
Triplet Harvesting

The kinetics of thermally activated delayed fluorescence (TADF) is investigated in dilute solutions of organic materials with application in blue light-emitting diodes (OLEDs). A method to accurately determine the energy barrier (Δ E a ) and the rate of reverse intersystem crossing ( k Risc ) in TADF emitters is developed, and applied to investigate the triplet-harvesting mechanism in blue-emitting materials with large singlet–triplet energy gap (Δ E ST ). In these materials, triplet–triplet annihilation (TTA) is the dominant mechanism for triplet harvesting; however, above a threshold temperature TADF is able to compete with TTA and give enhanced delayed fluorescence. Evidence is obtained for the interplay between the TTA and the TADF mechanisms in these materials.

scholarly journals Molecular Design Realizing Very Fast Reverse Intersystem Crossing in Purely Organic Emitter

2019 ◽  
Author(s):  
yoshimasa wada ◽  
Hiromichi Nakagawa ◽  
Soma Matsumoto ◽  
Yasuaki Wakisaka ◽  
Hironori Kaji
Keyword(s):  
Energy Gap ◽  
Molecular Design ◽  
Delayed Fluorescence ◽  
Intersystem Crossing ◽  
Triplet States ◽  
Thermally Activated Delayed Fluorescence ◽  
Light Emitting ◽  
Thermally Activated ◽  
Donor And Acceptor ◽  
Spin Inversion

Reverse intersystem crossing (RISC), originally considered forbidden in purely organic materials, has been recently enabled by minimizing the energy gap between the lowest singlet excited state (S<sub>1</sub>) and lowest triplet state (T<sub>1</sub>) in thermally activated delayed fluorescence (TADF) systems. However, direct spin-inversion between S<sub>1</sub> and T<sub>1</sub> is still inefficient when both states are of the same charge transfer (CT) nature (i.e. <sup>1</sup>CT and <sup>3</sup>CT, respectively). Intervention of locally excited triplet states (<sup>3</sup>LE) between <sup>1</sup>CT and <sup>3</sup>CT is expected to trigger fast spin-flip. Here, we report on the systematic-design of the ideal TADF molecules with near-degenerate <sup>1</sup>CT, <sup>3</sup>CT and <sup>3</sup>LE states by controlling the through-space distance between the donor and acceptor segments in a molecule with tilted intersegment angles. The new system realizes very fast RISC with a rate constant (<i>k</i><sub>RISC</sub>) of 1.2×10<sup>7</sup> s<sup>−1</sup>. The large <i>k</i><sub>RISC</sub> of the emitter resulted in great device performance in the applications to blue TADF assisted fluorescence organic light-emitting diodes (OLEDs) as well as TADF-emitter OLEDs.<br>

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