Up-converted Luminescence of 9,10 diphenyl-Anthracene Solutions Sensitized by a Doubly-photoexcited Metallorganic Complex: Conciliating Spectroscopic Measurements with Photokinetic Modelling

Giannis Antoniou^a, Panagiotis Keivanidis^a

^aDevice Technology and Chemical Physics Laboratory, Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology,, Kitiou Kyprianou, 45, Limassol, Cyprus

Proceedings of International Conference on Emerging Light Emitting Materials (EMLEM22)

Aspects of Emergent Light Emitters:

Limasol, Cyprus, 2022 October 3rd - 5th

Organizers: Maksym Kovalenko, Maryna Bodnarchuk and Grigorios Itskos

Poster, Giannis Antoniou, 032
Publication date: 15th July 2022

Τriplet-triplet annihilation-induced photon energy up-conversion (TTA-UC) refers to a photochemical process occurring in composite systems of small organic molecular and π- conjugated polymeric emitters whereby delayed luminescence is produced with photon energies higher than the photon energy used for photoexciting the composite. Based on an agreed mechanism [1], the TTA-UC process involves a cascade of photophysical events including the (i) low photon energy light absorption by a sensitizer (S) producing singlet excited states in S, (ii) intersystem crossing (ISC) in S, (iii) triplet energy transfer (TET) from S to an activator (A), and (iv) a triplet-triplet annihilation (TTA) reaction between two A molecules that prepares a singlet-excited A from where high photon energy TTA-UC luminescence is produced. TTA-UC is particularly attractive as a wavelength-shifting tool to enable photoactuation i.e. the sensitization of organic solar cells at long wavelengths of incoming light where the photoactive layer of the OSC device is transparent. Encouraging results have been presented recently on the quantum yield (Φ) of the archetypical TTA-UC system of the 9,10 di-phenyl anthracene (DPA) emitter mixed with the (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) platinum^II(PtOEP) metallorganic sensitizer [2]; a green-to-blue Φ_TTA-UC as high as 8% was reported for solution-processable solid-state DPA:PtOEP films. Nevertheless, the binary nature of the DPA:PtOEP composite film introduces severe implications in terms of device engineering aspects, and the incorporation of the DPA:PtOEP up-converting interlayers in OSC devices with vertically-stacked geometries is hard to achieve. Herein we present an alternative TTA-UC excited state pathway that promises to establish a feasible route towards TTA-UC sensitized photoactive devices of vertically-configured geometry. In particular, a TTA-UC composite system is presented whereby following the ISC step, the TET process from sensitizer to activator is circumvented and a sensitizer in its doubly-excited electronic state is prepared by TTA events. In this scheme, TTA-UC luminescence generation is facilitated by electronic energy transfer from the doubly-photoexcited sensitizer to a ground state emitter. Hitherto this excited-state TTA-UC pathway has been limited to systems that were photoexcited by quasi-cw and pulsed laser sources, and the corresponding Φ_TTA-UC were insignificant i.e. ≪0.001% [3, 4].

At present we focus on the spectroscopic study of the TTA-UC system comprising the DPA emitter mixed with the (3,3,7,8,12,13,17,18-octaethylporphyrin-22,24-diid-2-one) platinum^II (PtOEP-K) metallorganic complex. UV-Vis and photoluminescence (PL) spectroscopic measurements are performed on the DPA:PtOEP-K, for a range of compositions in toluene solutions. Under 532 nm cw-laser photoexcitation a green-to-blue Φ_TTA-UC= 0.59 % is obtained when the excitation fluence is 7 × 10¹⁸ cm^-2 s^-1. For the optimum composition, TTA-UC delayed luminescence is attainable also under the incoherent ultra-low power excitation of 1.65 mW cm^-2. Preliminary laser-induced PL measurements demonstrate the possibility to translate the DPA:PtOEP-K system to the solid state. Altogether, the collected results allow for the development of a generic photokinetic model that is applicable in the unconventional class of TTA-UC systems that operate by doubly photoexcited sensitizers.

References:

[1] 1. Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, F., Upconversion-induced fluorescence in multicomponent systems: Steady-state excitation power threshold. Physical Review B 2008, 78 (19), 195112.

[2] 2. Raišys, S.; Juršėnas, S.; Kazlauskas, K., Boost in Solid-State Photon Upconversion Efficiency through Combined Approach of Melt-Processing and Purification. Solar RRL 2022, 6 (6), 2100873

[3] 3. Keivanidis, P. E.; Baluschev, S.; Miteva, T.; Nelles, G.; Scherf, U.; Yasuda, A.; Wegner, G., Up-Conversion Photoluminescence in Polyfluorene Doped with Metal(II)–Octaethyl Porphyrins. Advanced Materials 2003, 15 (24), 2095-2098

[4] 4. Goudarzi, H.; Limbu, S.; Cabanillas-González, J.; Zenonos, V. M.; Kim, J.-S.; Keivanidis, P. E., Impact of molecular conformation on triplet-fusion induced photon energy up-conversion in the absence of exothermic triplet energy transfer. Journal of Materials Chemistry C 2019, 7 (12), 3634-3643.

Acknowledgements:

This work was co-funded by the European Regional Development Fund and the Republic of Cyprus through project EXCELLENCE/1216/0010 ‘Low Photon-Energy Up-Conversion induced Sensitized Photocurrent Generation in Organic Photodiodes’ of the Research and Innovation Foundation.

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