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Feeling blue no more: How TIPS-naphthalene enables efficient visible-to-UV upconversion

2021; Elsevier BV; Volume: 4; Issue: 8 Linguagem: Inglês

10.1016/j.matt.2021.06.024

2590-2393

Zachary A. VanOrman, Lea Nienhaus,

Organic Light-Emitting Diodes Research

Upconversion via triplet-triplet annihilation could efficiently generate ultraviolet light. In a recent work, Harada et al. have doubled the existing visible-to-UV upconversion efficiency and demonstrated a new record using a new triplet annihilator, yielding efficient upconversion using even low-energy LEDs. Upconversion via triplet-triplet annihilation could efficiently generate ultraviolet light. In a recent work, Harada et al. have doubled the existing visible-to-UV upconversion efficiency and demonstrated a new record using a new triplet annihilator, yielding efficient upconversion using even low-energy LEDs. Photon upconversion (UC) is the combination of two or more photons to generate one photon higher in energy. This process has seen a rapid expansion in the past two decades. Some of the most exciting developments have come in the field of triplet-triplet annihilation-based upconversion (TTA-UC), due to the high achievable efficiencies at low incident powers.1Singh-Rachford T.N. Castellano F.N. Photon Upconversion Based on Sensitized Triplet–Triplet Annihilation.Coord. Chem. Rev. 2010; 254: 2560-2573https://doi.org/10.1016/j.ccr.2010.01.003Crossref Scopus (973) Google Scholar The overall process of TTA is shown in the right half of Figure 1A: in molecules capable of TTA, two spin-triplet states can interact, (green arrows) resulting in a singlet excited state, which upon radiative relaxation (purple arrow) emits a photon. TTA-UC commonly occurs in polyaromatic hydrocarbons and is reliant on the efficient population of the spin-triplet state. However, because of selection rules, the triplet state is only weakly directly optically accessible. Therefore, the triplet states in these annihilator molecules are commonly populated via a triplet energy transfer (TET) process from a triplet sensitizer, further detailed in the left half of Figure 1A, where excitation and concomitant absorption populates an excited singlet state (blue arrow), the process of intersystem crossing (ISC) populates the triplet state (red arrow), and a TET process (orange arrows) populates the triplet state of the annihilator, enabling TTA. Thus far, three main classes of triplet sensitizers have emerged: quantum confined semiconductor nanocrystals,2Xu Z. Huang Z. Jin T. Lian T. Tang M.L. Mechanistic Understanding and Rational Design of Quantum Dot/Mediator Interfaces for Efficient Photon Upconversion.Acc. Chem. Res. 2021; 54: 70-80https://doi.org/10.1021/acs.accounts.0c00526Crossref PubMed Scopus (14) Google Scholar,3Nienhaus L. Wu M. Bulović V. Baldo M.A. Bawendi M.G. Using lead chalcogenide nanocrystals as spin mixers: a perspective on near-infrared-to-visible upconversion.Dalton Trans. 2018; 47: 8509-8516https://doi.org/10.1039/C8DT00419FCrossref PubMed Google Scholar bulk semiconductor materials,4VanOrman Z.A. Drozdick H.K. Wieghold S. Nienhaus L. Bulk Halide Perovskites as Triplet Sensitizers: Progress and Prospects in Photon Upconversion.J. Mater. Chem. C. 2021; 9: 2685-2694https://doi.org/10.1039/D1TC00245GCrossref Google Scholar and molecular organometallics.1Singh-Rachford T.N. Castellano F.N. Photon Upconversion Based on Sensitized Triplet–Triplet Annihilation.Coord. Chem. Rev. 2010; 254: 2560-2573https://doi.org/10.1016/j.ccr.2010.01.003Crossref Scopus (973) Google Scholar In the former, quantum-confined semiconductor nanocrystals and bulk metal halide perovskites have been utilized to sensitize annihilator triplet states, either directly through an exciton transfer or by means of transfer of free charge carriers. In molecular systems, commonly composed of organometallics with heavy metal centers, the triplet state is generated through the process of ISC.1Singh-Rachford T.N. Castellano F.N. Photon Upconversion Based on Sensitized Triplet–Triplet Annihilation.Coord. Chem. Rev. 2010; 254: 2560-2573https://doi.org/10.1016/j.ccr.2010.01.003Crossref Scopus (973) Google Scholar Representations of these different classes of triplet sensitizers are shown in Figure 1B, along with their respective pros and cons. Not having to undergo ISC is a positive for nanocrystals, as losses because of exchange energy counteract the energy gains in TTA-UC in molecular systems.5Amemori S. Sasaki Y. Yanai N. Kimizuka N. Near-Infrared-to-Visible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0-T1 Absorption.J. Am. Chem. Soc. 2016; 138: 8702-8705https://doi.org/10.1021/jacs.6b04692Crossref PubMed Scopus (129) Google Scholar However, the required surface-passivating long insulating organic ligands impede efficient energy transfer, necessitating triplet mediator ligands.6Huang Z. Tang M.L. Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules.J. Am. Chem. Soc. 2017; 139: 9412-9418https://doi.org/10.1021/jacs.6b08783Crossref PubMed Scopus (101) Google Scholar Bulk perovskite semiconductors do not require passivating ligands and can enable a direct charge injection process into the triplet state.4VanOrman Z.A. Drozdick H.K. Wieghold S. Nienhaus L. Bulk Halide Perovskites as Triplet Sensitizers: Progress and Prospects in Photon Upconversion.J. Mater. Chem. C. 2021; 9: 2685-2694https://doi.org/10.1039/D1TC00245GCrossref Google Scholar Both nanocrystals and bulk perovskites have been implemented into relatively efficient solid-state UC systems.3Nienhaus L. Wu M. Bulović V. Baldo M.A. Bawendi M.G. Using lead chalcogenide nanocrystals as spin mixers: a perspective on near-infrared-to-visible upconversion.Dalton Trans. 2018; 47: 8509-8516https://doi.org/10.1039/C8DT00419FCrossref PubMed Google Scholar,4VanOrman Z.A. Drozdick H.K. Wieghold S. Nienhaus L. Bulk Halide Perovskites as Triplet Sensitizers: Progress and Prospects in Photon Upconversion.J. Mater. Chem. C. 2021; 9: 2685-2694https://doi.org/10.1039/D1TC00245GCrossref Google Scholar However, both bulk and quantum-confined semiconductor triplet sensitizers are typically broadband UV absorbing materials, resulting in parasitic reabsorption, which will serve as motivation for the work previewed here. Efficient UC has been realized across the visible spectrum for both molecular and semiconductor-based sensitizers, but visible-to-UV UC has lagged behind, necessitating a breakthrough in this area. High-energy UV radiation is used for numerous applications, including photoredox catalysis, photolithography, or as an antimicrobial. However, the efficient generation of UV light, especially while using a low-cost and non-toxic source, has been a challenge.8Du Y. Ai X. Li Z. Sun T. Huang Y. Zeng X. Chen X. Rao F. Wang F. Visible-to-Ultraviolet Light Conversion: Materials and Applications.Adv. Photonics Res. 2021; : 2000213https://doi.org/10.1002/adpr.202000213Crossref Google Scholar Both semiconductor and molecular sensitizers have been paired with 2,5-diphenyloxazole (PPO),9Singh-Rachford T.N. Castellano F.N. Low power visible-to-UV upconversion.J. Phys. Chem. A. 2009; 113: 5912-5917https://doi.org/10.1021/jp9021163Crossref PubMed Scopus (108) Google Scholar culminating in a UC efficiency, ηUC, of 10.2% that was recently realized using perovskite nanocrystals as triplet sensitizers.10He S. Luo X. Liu X. Li Y. Wu K. Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals.J. Phys. Chem. Lett. 2019; 10: 5036-5040https://doi.org/10.1021/acs.jpclett.9b02106Crossref PubMed Scopus (61) Google Scholar While this result was exciting, further optimization is limited by the inherently high UV absorption of semiconductor nanocrystals. Enter Harada et al.,7Harada N. Sasaki Y. Hosoyamada M. Kimizuka N. Yanai N. Discovery of Key TIPS-Naphthalene for Efficient Visible-to-UV Photon Upconversion under Sunlight and Room Light.Angew. Chem. Int. Ed. Engl. 2021; 60: 142-147https://doi.org/10.1002/anie.202012419Crossref PubMed Scopus (20) Google Scholar who used a molecular sensitizer perfectly engineered for visible-to-UV UC, Ir(C6)2(acac), which strongly absorbs visible light while only weakly absorbing UV light, limiting parasitic effects caused by reabsorption. Furthermore, they paired Ir(C6)2(acac) with a novel annihilator, 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Nph), as the energy levels of PPO do not allow for efficient TET when using Ir(C6)2(acac) as the sensitizer. Excitingly, they measured an ηUC exceeding 20%, which was not only the most efficient visible-to-UV UC system ever measured, but it doubled the previous high of 10.2%.10He S. Luo X. Liu X. Li Y. Wu K. Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals.J. Phys. Chem. Lett. 2019; 10: 5036-5040https://doi.org/10.1021/acs.jpclett.9b02106Crossref PubMed Scopus (61) Google Scholar Further, they demonstrate the efficiency qualitatively, where UV light is observed under an air mass 1.5 (AM 1.5) solar simulator, and even with a simple LED light, as shown in Figure 1C. The observable UC under low fluence is another vital characteristic of this system, as exemplified by low Ith values, a figure of merit which yields the power density where TTA-UC becomes an efficient process: the obtained value Ith = 1.1 mW is below the solar flux, meaning that a solar simulator (or simply the sun) can be used to efficiently drive the TTA-UC process for this system. This drastic improvement in both the efficiency and Ith was rationalized through a combination of theory and experiment. First, density functional theory (DFT) calculations showed a lower triplet energy level for TIPS-Nph, allowing for a higher degree of energetic driving force from sensitizer to annihilator. Further, the novel TIPS-Nph proved to be a better triplet annihilator, as it readily converted triplet to singlet states more efficiently than PPO because of reductions in non-radiative decay pathways as a result of the rigid binding of the ethynyl groups found in TIPS-Nph. This insight may show a tangible path toward further breakthroughs in this field, where triplet annihilators can be synthesized one functional group at a time, with record ηUC values in mind. In summary, it can be said that the authors are reaching for the holy grail of UC: driving efficient UC using the abundantly available solar energy. The results shown in this work have immediate and far-reaching impacts, due to the huge leap observed in the ηUC. First, it indicates the vast room still available for molecular discovery for the purpose of both triplet sensitization and annihilation. Further, the eternal collaboration between theory and experiment is highlighted, while a new door is opened. Novel theory and machine learning could screen novel annihilator/sensitizer molecules, which could find pairs capable of even more efficient UC or could push further boundaries in other areas, including molecules capable of efficient UC in water or molecules capable of efficient visible-to-UV UC in the solid state, further bringing this technology closer to device integration. The authors gratefully acknowledge funding by Florida State University . Lea Nienhaus is a member of the Editorial Advisory Board of Matter.