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2. Effect of tert‐Butylation on the Photophysics of Thermally Activated Delayed Fluorescence Emitters.

Author

Thakur, Kalyani, van der Zee, Bas, Sachnik, Oskar, Haese, Constantin, Graf, Robert, Michels, Jasper J., Wetzelaer, Gert‐Jan A. H., Ramanan, Charusheela, and Blom, Paul W. M.

Subjects
DELAYED fluorescence, REDUCED instruction set computers, DECAY constants, QUANTUM efficiency, EXCITON theory
Abstract

Thermally activated delayed fluorescence (TADF) emitters potentially can provide organic light‐emitting diodes with 100% internal quantum efficiency by harvesting triplet excitons. Generally, TADF emitters are small molecules that are not applicable for solution processability. The addition of tert‐butyl groups to the periphery of TADF emitters has proven to improve their solubility in various organic solvents, reduce aggregation‐induced quenching, and enhance the photoluminescence quantum yield (PLQY). This article studies the photophysical influence of the tert‐butyl group attached to an emitter with a carbazole acceptor and a triazine donor. The resulting t3CzTrz‐F is a blue–green TADF emitter, in which the addition of a tert‐butyl group increases the rate of reverse intersystem crossing (rISC), while simultaneously decreasing the nonradiative decay rate substantially. In addition, dilution of t3CzTrz‐F in a host matrix in film results in an enhanced PLQY, which is associated with a decrease in the nonradiative decay constant, while there is no change in the rISC rate. Through a solid‐state NMR study, the change in rISC and nonradiative rate upon tert‐butylation by enlarged intermolecular spacing and reduced vibrational and rotational freedom is rationalized, resulting in improved photophysical performance. [ABSTRACT FROM AUTHOR]

Published
2024
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3. Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst

Author

Backus, Ellen H.G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., Bonn, Mischa, Backus, Ellen H.G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., and Bonn, Mischa

Abstract

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2O into Ti−OH groups and the deprotonation of pre-existing Ti−OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.

Published
2024

4. Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst

Author

Sub Nanophotonics, Nanophotonics, Backus, Ellen H.G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., Bonn, Mischa, Sub Nanophotonics, Nanophotonics, Backus, Ellen H.G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., and Bonn, Mischa

Published
2024

6. Ultraschnelle oberflächenspezifische Spektroskopie von Wasser an einem photoangeregten TiO2‐Modell‐Photokatalysator zur Wasserspaltung

Author

Backus, Ellen H. G., primary, Hosseinpour, Saman, additional, Ramanan, Charusheela, additional, Sun, Shumei, additional, Schlegel, Simon J., additional, Zelenka, Moritz, additional, Jia, Xiaoyu, additional, Gebhard, Maximilian, additional, Devi, Anjana, additional, Wang, Hai I., additional, and Bonn, Mischa, additional

Published
2024
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10. Ultrafast Surface‐Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water‐Splitting Photocatalyst.

Author

Backus, Ellen H. G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., and Bonn, Mischa

Subjects
PHOTON upconversion, SPECTROMETRY, INTERFACIAL reactions, IRRADIATION, WATER temperature, TIME-resolved spectroscopy
Abstract

A critical step in photocatalytic water dissociation is the hole‐mediated oxidation reaction. Molecular‐level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time‐resolved, surface‐specific vibrational sum frequency generation spectroscopy to study the photo‐induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub‐20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2O into Ti−OH groups and the deprotonation of pre‐existing Ti−OH groups. This study provides unique experimental insights into the early steps of the photo‐induced dissociation processes at the photocatalyst‐water interface, relevant to the design of improved photocatalysts. [ABSTRACT FROM AUTHOR]

Published
2024
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11. Ultraschnelle oberflächenspezifische Spektroskopie von Wasser an einem photoangeregten TiO2‐Modell‐Photokatalysator zur Wasserspaltung.

Author

Backus, Ellen H. G., Hosseinpour, Saman, Ramanan, Charusheela, Sun, Shumei, Schlegel, Simon J., Zelenka, Moritz, Jia, Xiaoyu, Gebhard, Maximilian, Devi, Anjana, Wang, Hai I., and Bonn, Mischa

Subjects
TITANIUM dioxide
Abstract

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Published
2024
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16. Grain Engineering for Improved Charge Carrier Transport in Two-Dimensional Lead-Free Perovskite Field-Effect Transistors

Author

Wang, Shuanglong, primary, Marszalek, Tomasz, additional, Zhang, Heng, additional, Yildiz, Okan, additional, Mandal, Mukunda, additional, Ugur, Naz, additional, Jeong, Beomjin, additional, Frisch, Sabine, additional, Andrienko, Denis, additional, Wang, Hai, additional, Bonn, Mischa, additional, Blom, Paul, additional, Kivala, Milan, additional, Pisula, Wojciech, additional, and Ramanan, Charusheela, additional

Published
2022
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18. Pseudo‐Octahedral Iron(II) Complexes with Near‐Degenerate Charge Transfer and Ligand Field States at the Franck‐Condon Geometry

Author

Moll, Johannes, primary, Naumann, Robert, additional, Sorge, Lukas, additional, Förster, Christoph, additional, Gessner, Niklas, additional, Burkhardt, Lukas, additional, Ugur, Naz, additional, Nuernberger, Patrick, additional, Seidel, Wolfram, additional, Ramanan, Charusheela, additional, Bauer, Matthias, additional, and Heinze, Katja, additional

Published
2022
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20. Role of Linker Functionality in Polymers Exhibiting Main‐Chain Thermally Activated Delayed Fluorescence

Author

Philipps, Kai, primary, Ie, Yutaka, additional, van der Zee, Bas, additional, Png, Rui‐Qi, additional, Ho, Peter K. H., additional, Chua, Lay‐Lay, additional, del Pino Rosendo, Esther, additional, Ramanan, Charusheela, additional, Wetzelaer, Gert‐Jan A. H., additional, Blom, Paul W. M., additional, and Michels, Jasper J., additional

Published
2022
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25. Reduced Intrinsic Non‐Radiative Losses Allow Room‐Temperature Triplet Emission from Purely Organic Emitters

Author

Li, Yungui, primary, Jiang, Lihui, additional, Liu, Wenlan, additional, Xu, Shunqi, additional, Li, Tian‐Yi, additional, Fries, Felix, additional, Zeika, Olaf, additional, Zou, Yingping, additional, Ramanan, Charusheela, additional, Lenk, Simone, additional, Scholz, Reinhard, additional, Andrienko, Denis, additional, Feng, Xinliang, additional, Leo, Karl, additional, and Reineke, Sebastian, additional

Published
2021
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26. Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2+

Author

Reichenauer, Florian, primary, Wang, Cui, additional, Förster, Christoph, additional, Boden, Pit, additional, Ugur, Naz, additional, Báez-Cruz, Ricardo, additional, Kalmbach, Jens, additional, Carrella, Luca M., additional, Rentschler, Eva, additional, Ramanan, Charusheela, additional, Niedner-Schatteburg, Gereon, additional, Gerhards, Markus, additional, Seitz, Michael, additional, Resch-Genger, Ute, additional, and Heinze, Katja, additional

Published
2021
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27. Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2+

Author

Reichenauer, Florian, Wang, Cui, Forster, Christoph, Boden, Pit, Ugur, Naz, Baez-Cruz, Ricardo, Kalmbach, Jens, Carrella, Luca M., Rentschler, Eva, and Ramanan, Charusheela

Subjects
Luminescence, Energy, 500 Naturwissenschaften und Mathematik::540 Chemie::540 Chemie und zugeordnete Wissenschaften, Quantum yield
Abstract

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)2]3+ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D2O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)2]3+ and its facile reduction to [Cr(bpmp)2]2+ result in a high excited state redox potential. The ligand���s methylene bridge acts as a Br��nsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(��-diimine)3]2+ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Published
2021
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29. Trap-Assisted Triplet Emission in Ladder-Polymer-Based Light-Emitting Diodes

Author

Khodabakhshi, Elham, Ramanan, Charusheela, Michels, Jasper J., Bonus, Sven, Hertel, Dirk, Meerholz, Klaus, Forster, Michael, Scherf, Ulrich, Blom, Paul W. M., Khodabakhshi, Elham, Ramanan, Charusheela, Michels, Jasper J., Bonus, Sven, Hertel, Dirk, Meerholz, Klaus, Forster, Michael, Scherf, Ulrich, and Blom, Paul W. M.

Abstract

The charge transport and recombination in light-emitting diodes (LEDs) based on a methyl-substituted poly(p-phenylene) ladder polymer (Me-LPPP) are investigated. The transport is characterized by a high room-temperature hole mobility of 2 x 10(-8) m(2) V-1 s(-1) combined with anomalously strong electron trapping. Their electroluminescence (EL) spectrum is characterized by a blue singlet emission, a broad green featureless peak, and a yellow-orange triplet emission. The voltage dependence of the EL spectrum and negative contribution to the capacitance indicate that the triplet-emission is of trap-assisted nature, consistent with the strong electron trapping. Consequently, the color purity of the blue emissive Me-LPPP polymer LEDs can be strongly improved using trap dilution.

Published
2020

36. Quantifying Exciton Annihilation Effects in Thermally Activated Delayed Fluorescence Materials.

Author

Thakur, Kalyani, van der Zee, Bas, Wetzelaer, Gert‐Jan A. H., Ramanan, Charusheela, and Blom, Paul W. M

Subjects
DELAYED fluorescence, PHOTOLUMINESCENCE, LIGHT emitting diodes
Abstract

Important parameters for the design and performance of thermally activated delayed fluorescence (TADF) emitters are the forward and reverse intersystem crossing rates between singlet and triplet states. The magnitude of these rates is determined from the prompt and delayed transient photoluminescence decay. It is demonstrated that this photoluminescence decay strongly depends on the initial photoexcited population density due to exciton–exciton annihilation processes. By kinetic modeling of the power‐dependent time‐resolved photoluminescence of the TADF emitter 9,10‐bis(4‐(9H‐carbazol‐9‐yl)‐2,6‐dimethylphenyl)‐9,10‐diboraanthracene (CzDBA), singlet–triplet annihilation and triplet–triplet annihilation are identified as the main loss processes with rate constants in the order of 10−17 m3 s−1. Neglecting these quenching processes leads to erroneous estimates of the (reverse) intersystem crossing rates. [ABSTRACT FROM AUTHOR]

Published
2022
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38. Asymmetric Covalent Triazine Framework for Enhanced Visible‐Light Photoredox Catalysis via Energy Transfer Cascade

Author

Huang, Wei, primary, Byun, Jeehye, additional, Rörich, Irina, additional, Ramanan, Charusheela, additional, Blom, Paul W. M., additional, Lu, Hao, additional, Wang, Di, additional, Caire da Silva, Lucas, additional, Li, Run, additional, Wang, Lei, additional, Landfester, Katharina, additional, and Zhang, Kai A. I., additional

Published
2018
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39. Ein asymmetrisches kovalentes Triazin‐Netzwerk für effiziente Photoredox‐Katalyse durch Energietransfer‐Kaskaden unter sichtbarem Licht

Author

Huang, Wei, primary, Byun, Jeehye, additional, Rörich, Irina, additional, Ramanan, Charusheela, additional, Blom, Paul W. M., additional, Lu, Hao, additional, Wang, Di, additional, Caire da Silva, Lucas, additional, Li, Run, additional, Wang, Lei, additional, Landfester, Katharina, additional, and Zhang, Kai A. I., additional

Published
2018
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42. Controlled Morphology of ZnO Nanorods for Electron Transport in Squaraine Bulk-Hetero Junction Solar Cells With Thick Active Layers

Author

Peukert, Andreas, Vaillant Roca, Lidice, Scherer, Michael, Lovrincic, Robert, Ramanan, Charusheela, Chanaewa, Alina, von Hauff, Elizabeth, Peukert, Andreas, Vaillant Roca, Lidice, Scherer, Michael, Lovrincic, Robert, Ramanan, Charusheela, Chanaewa, Alina, and von Hauff, Elizabeth

Abstract

The influence of ZnO seed layer thickness in Squaraine (SQ) is investigated: PC71BM bulk heterojunction solar cells that incorporate ZnO nanorods. The thickness of the ZnO seed layer varies between 16–249 nm by changing the concentration of the precursor solution. With atomic force microscopy (AFM), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) studies, it is shown that this approach allows to systematically tune the thickness of the ZnO seed layer without influencing seed layer grain size, or the morphology of the ZnO nanorods that are deposited on top of the seed layer. The proof-of-concept is demonstrated in SQ:PC71BM solar cells. It is found that seed layers with 55 nm thickness yield the highest short circuit current densities, resulting in power conversion efficiencies of 2.5 ± 0.1%. These results are compared to SQ:PC71BM solar cells prepared in planar architectures, and it is observed that both device architectures yield comparable results. The optimized nanostructured ZnO electrode enables the fabrication of BHJ devices with thick active layers without the loss in solar cell performance.

Published
2017
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44. Ultrafast intersystem crossing and spin dynamics of photoexcited perylene-3,4:9,10-bis(dicarboximide) covalently linked to a nitroxide radical at fixed distances

Author

Giacobbe, Emilie M., Qixi Mi, Colvin, Michael T., Cohen, Boiko, Ramanan, Charusheela, Scott, Amy M., Yeganeh, Sina, Marks, Tobin J., Ratner, Mark A., and Wasielewski, Michael R.

Subjects
Electron paramagnetic resonance spectroscopy -- Usage, Nitrogen oxide -- Chemical properties, Nitrogen oxide -- Structure, Chemistry
Abstract

Time-resolved transient optical absorption and EPR (TREPR) spectroscopies are used for detecting the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) (PDI) with a stable tert-butylphenylnitroxide radical ([super 2]BPNO) at specific distances and orientations. The rates of polarization transfer has depended on the molecular connectivity between PDI and [super 2]BPNO and is rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.

Published
2009

50. Electron donor-free photoredox catalysis viaan electron transfer cascade by cooperative organic photocatalystsElectronic supplementary information (ESI) available: Experiment details and characterization, 1H and 13C NMR spectra, fluorescence quenching experiments and control experiments are described. See DOI: 10.1039/c8cy01072b

Author

Wang, Lei, Rörich, Irina, Ramanan, Charusheela, Blom, Paul W. M., Huang, Wei, Li, Run, and Zhang, Kai A. I.

Abstract

Electron-donating sacrificial reagents are highly important for certain photo-redox reactions. However, the use of excessive amounts of sacrificial reagents, mostly amines, often leads to undesired side products and is especially troublesome for product purification. Herein, we take the light-induced electron transfer cascade process of natural photosystems as a role model and assemble organic photocatalysts into cooperative photocatalyst couples. The cooperative photocatalyst couples could undergo intermolecular electron transfer to facilitate the charge separation process and overcome the need for an extra electron donor. Time-resolved photoluminescence spectroscopy was conducted to precisely characterize the photo-excited dynamics within the cooperative photocatalyst couples. As a model photoredox reaction, the carbon–carbon formation reaction between heteroarenes and malonates, which usually requires electron-donating sacrificial reagents such as amines, was conducted to demonstrate the feasibility of the cooperative photocatalyst couples under visible light irradiation. A significant reaction conversion improvement from trace conversion for a single photocatalyst system to over 90% by cooperative photocatalyst couples was achieved.

Published
2018
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