Article Cite This: Acc. Chem. Res. 2017, 50, 2487-2495
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New Triplet Sensitization Routes for Photon Upconversion: Thermally Activated Delayed Fluorescence Molecules, Inorganic Nanocrystals, and Singlet-to-Triplet Absorption Nobuhiro Yanai*,†,‡ and Nobuo Kimizuka*,† †
Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ PRESTO, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan CONSPECTUS: Photon upconversion based on triplet−triplet annihilation (TTA-UC) has attracted much interest because of its possible applications to renewable energy production and biological fields. In particular, the UC of near-infrared (NIR) light to visible (vis) light is imperative to overcome the Shockley−Queisser limit of single-junction photovoltaic cells, and the efficiency of photocatalytic hydrogen production from water can also be improved with the aid of vis-to-ultraviolet (UV) UC. However, both processes have met limitations in the wavelength range, efficiency, and sensitivity for weak incident light. This Account describes recent breakthroughs that solve these major problems, new triplet sensitization routes to significantly enlarge the range of conversion wavelength by minimizing the energy loss during intersystem crossing (ISC) of triplet sensitizers or bypassing the ISC process. The photochemical processes of TTAUC in general start with the absorption of longer wavelength incident light by triplet sensitizers, which generate the triplet states via ISC. This ISC inevitably accompanies the energy loss of hundreds of millielectronvolts, which significantly limits the TTA-UC with large anti-Stokes shifts. The small S1−T1 gap of molecules showing thermally activated delayed fluorescence (TADF) allows the sensitization of emitters with the highest T1 and S1 energy levels ever employed in TTA-UC, which results in efficient vis-to-UV UC. As alternatives to molecular sensitizers in the NIR region, inorganic nanocrystals with broad NIR absorption bands have recently been shown to work as effective sensitizers for NIR-to-vis TTA-UC. Their small exchange splitting minimizes the energy loss during triplet sensitization. The modification of nanocrystal surfaces with organic acceptors via coordination bonds allows efficient energy transfer between the components and succeeding TTA processes. To remove restrictions on the energy loss during ISC, molecules with direct singlet-to-triplet (S−T) excitation are employed as triplet sensitizers. Although the S−T absorption is spin forbidden, large spin−orbital coupling occurs for appropriately designed metal complexes, which allow S−T absorption in the NIR region with large absorption coefficients. While the triplet lifetime of such S−T absorption sensitizers is often short (less than microsecond), the integration of the molecular sensitizers with emitter assemblies allows facile Dexter energy transfer to the surrounding emitter molecules, leading to efficient NIR-to-vis UC emission through triplet energy migration (TEM) in the condensed state. By judicious modification of the chromophore structures, the first example of NIR-to-blue UC has also been achieved. It is essential to combine these new triplet sensitization routes with an upconverted energy collection (UPCON) approach in molecular assemblies to effectively populate emitter triplets and to overcome remaining issues including back energy transfer. We propose two overall materials designs for the TEM−UPCON strategy, core−shell−shell structures and trilayer structures composed of triplet donor, acceptor, and energy collector. The fusion between triplet science and chemistry of self-assembly would overcome previous difficulties of NIR-to-vis and vis-to-UV TTA-UC toward real-world applications ranging from energy to biology. photovoltaic cells.9 For example, it is expected to enhance the efficiency of recently spotlighted perovskite solar cells, whose absorption range has been limited to the visible range below 800 nm.13,14 Likewise, vis-to-UV UC would improve the efficiency of photocatalytic systems including hydrogen
1. INTRODUCTION Photon upconversion (UC) is a methodology to convert longer-wavelength light (lower-energy photons) to shorterwavelength light (higher-energy photons).1−12 The potential application of UC widely ranges from energy to biology fields, and the UC from near-infrared (NIR) light to visible (vis) light has particularly attracted much attention because of its potential to overcome the Shockley−Queisser limit of single-junction © 2017 American Chemical Society
Received: May 9, 2017 Published: September 20, 2017 2487
DOI: 10.1021/acs.accounts.7b00235 Acc. Chem. Res. 2017, 50, 2487−2495
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Accounts of Chemical Research production and carbon dioxide reduction, which currently work only under the UV light or high-energy vis light (
