Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter

We report the characterization of rotaxanes based on a carbazole–benzophenone thermally activated delayed fluorescence luminophore. We find that the mechanical bond leads to an improvement in key photophysical properties of the emitter, notably an increase in photoluminescence quantum yield and a decrease in the energy difference between singlet and triplet states, as well as fine tuning of the emission wavelength, a feat that is difficult to achieve when using covalently bound substituents. Computational simulations, supported by X-ray crystallography, suggest that this tuning of properties occurs due to weak interactions between the axle and the macrocycle that are enforced by the mechanical bond. This work highlights the benefits of using the mechanical bond to refine existing luminophores, providing a new avenue for emitter optimization that can ultimately increase the performance of these molecules.

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Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet States and Appreciable Fluorescence Rates

10.26434/chemrxiv.13087319.v1 ◽

2020 ◽

Author(s):

Robert Pollice ◽

Pascal Friederich ◽

Cyrille Lavigne ◽

Gabriel dos Passos Gomes ◽

Alan Aspuru-Guzik

Keyword(s):

Excited State ◽

Energy Difference ◽

Delayed Fluorescence ◽

Triplet States ◽

Electron Hole ◽

Thermally Activated Delayed Fluorescence ◽

Light Emitting ◽

Thermally Activated ◽

Singlet And Triplet States

One of the recent proposals for the design of state-of-the-art emissive materials for organic light emitting diodes (OLEDs) is the principle of thermally activated delayed fluorescence (TADF). The underlying idea is to enable facile thermal upconversion of excited state triplets, which are generated upon electron-hole recombination, to excited state singlets by minimizing the corresponding energy difference resulting in devices with up to 100% internal quantum efficiencies (IQEs). Ideal emissive materials potentially surpassing TADF emitters should have both negative singlet-triplet gaps and appreciable fluorescence rates to maximize reverse intersystem crossing (rISC) rates from excited triplets to singlets while minimizing ISC rates and triplet state occupation leading to long-term operational stability. However, molecules with negative singlet-triplet gaps are extremely rare and, to the best of our knowledge, not emissive. In this work, based on computational studies, we describe the first molecules with negative singlet-triplet gaps and considerable fluorescence rates and show that they are more common than hypothesized previously.

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Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet States and Appreciable Fluorescence Rates

10.26434/chemrxiv.13087319 ◽

2020 ◽

Author(s):

Robert Pollice ◽

Pascal Friederich ◽

Cyrille Lavigne ◽

Gabriel dos Passos Gomes ◽

Alan Aspuru-Guzik

Keyword(s):

Excited State ◽

Energy Difference ◽

Delayed Fluorescence ◽

Triplet States ◽

Electron Hole ◽

Thermally Activated Delayed Fluorescence ◽

Light Emitting ◽

Thermally Activated ◽

Singlet And Triplet States

One of the recent proposals for the design of state-of-the-art emissive materials for organic light emitting diodes (OLEDs) is the principle of thermally activated delayed fluorescence (TADF). The underlying idea is to enable facile thermal upconversion of excited state triplets, which are generated upon electron-hole recombination, to excited state singlets by minimizing the corresponding energy difference resulting in devices with up to 100% internal quantum efficiencies (IQEs). Ideal emissive materials potentially surpassing TADF emitters should have both negative singlet-triplet gaps and appreciable fluorescence rates to maximize reverse intersystem crossing (rISC) rates from excited triplets to singlets while minimizing ISC rates and triplet state occupation leading to long-term operational stability. However, molecules with negative singlet-triplet gaps are extremely rare and, to the best of our knowledge, not emissive. In this work, based on computational studies, we describe the first molecules with negative singlet-triplet gaps and considerable fluorescence rates and show that they are more common than hypothesized previously.

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Solvent Dependence of Structural Dynamics and Spin-flip Processes in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)

10.26434/chemrxiv.11798568.v2 ◽

2020 ◽

Author(s):

Masaki Saigo ◽

Kiyoshi Miyata ◽

Hajime Nakanotani ◽

Chihaya Adachi ◽

Ken Onda

Keyword(s):

Dft Calculations ◽

Excited States ◽

Structural Changes ◽

Photophysical Properties ◽

Delayed Fluorescence ◽

Time Resolved ◽

Thermally Activated Delayed Fluorescence ◽

Thermally Activated ◽

Solvent Dependence ◽

Acetonitrile Solutions

We have investigated the solvent-dependence of structural changes along with intersystem crossing of a thermally activated delayed fluorescence (TADF) molecule, 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (o-3CzBN), in toluene, tetrahydrofuran, and acetonitrile solutions using time-resolved infrared (TR-IR) spectroscopy and DFT calculations. We found that the geometries of the S1 and T1 states are very similar in all solvents though the photophysical properties mostly depend on the solvent. In addition, the time-dependent DFT calculations based on these geometries suggested that the thermally activated delayed fluorescence process of o-3CzBN is governed more by the higher-lying excited states than by the structural changes in the excited states.<br>

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A quantitative description of photoluminescence efficiency of a carbazole-based thermally activated delayed fluorescence emitter

New Journal of Chemistry ◽

10.1039/c9nj00101h ◽

2019 ◽

Vol 43 (15) ◽

pp. 6032-6039

Author(s):

Songyan Feng ◽

Xugeng Guo ◽

Jinglai Zhang

Keyword(s):

Quantitative Description ◽

Energy Difference ◽

Delayed Fluorescence ◽

Intersystem Crossing ◽

Small Energy ◽

Thermally Activated Delayed Fluorescence ◽

Thermally Activated ◽

Fluorescence Efficiency ◽

High Fluorescence ◽

Photoluminescence Efficiency

The present results reveal that the dominant charge transfer characteristics in the S1 and T1 states produce a small energy difference between the two states, and consequently an efficient reverse intersystem crossing process and a high fluorescence efficiency.

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Exploiting trifluoromethyl substituents for tuning orbital character of singlet and triplet states to increase the rate of thermally activated delayed fluorescence

Materials Chemistry Frontiers ◽

10.1039/d0qm00429d ◽

2020 ◽

Vol 4 (12) ◽

pp. 3602-3615 ◽

Cited By ~ 2

Author(s):

Jonathan S. Ward ◽

Andrew Danos ◽

Patrycja Stachelek ◽

Mark A. Fox ◽

Andrei S. Batsanov ◽

...

Keyword(s):

Excited States ◽

Organic Molecules ◽

Delayed Fluorescence ◽

Triplet States ◽

Thermally Activated Delayed Fluorescence ◽

Thermally Activated ◽

Conjugated Organic Molecules ◽

Singlet And Triplet States ◽

Triplet Excited States ◽

Orbital Character

This work shows that trifluoromethyl (CF3) substituents can be used to increase the rate of thermally activated delayed fluorescence (TADF) in conjugated organic molecules by tuning the excitonic character of the singlet and triplet excited states.

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