Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked Polymeric Nanoparticles: Toward the Next Generation of Solid-State Upconverting Materials

Monguzzi, Angelo; Mauri, Michele; Frigoli, Michel; Pedrini, Jacopo; Simonutti, Roberto; Larpent, Chantal; Vaccaro, G.; Sassi, Mauro; Meinardi, Francesco

doi:10.1021/acs.jpclett.6b01115

Cited by 40 publications

(41 citation statements)

References 45 publications

(72 reference statements)

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“…The relatively short lifetime of the acceptor triplet limits the triplet diffusion distance and encounter probability. The triplet lifetime of acceptors needs to be extended to improve the upconversion efficiency and may be improved by new molecular design or by introducing rigid framework confinement …”

Section: Resultsmentioning

confidence: 99%

Light‐Harvesting Organic Nanocrystals Capable of Photon Upconversion

Zeng

et al. 2017

ChemSusChem

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Harvesting and converting low energy photons into higher ones through upconversion have great potential in solar energy conversion. A light‐harvesting nanocrystal assembled from 9,10‐distyrylanthracene and palladium(II) meso‐tetraphenyltetrabenzoporphyrin as the acceptor and the sensitizer, respectively effects red‐to‐green upconversion under incoherent excitation of low power density. An upconversion quantum yield of 0.29±0.02 % is obtained upon excitation with 640 nm laser of 120 mW cm−2. The well‐organized packing of acceptor molecules with aggregation‐induced emission in the nanocrystals dramatically reduces the nonradiative decay of the excited acceptor, benefits the triplet–triplet annihilation (TTA) upconversion and guides the consequent upconverted emission. This work provides a straightforward strategy to develop light‐harvesting nanocrystals based on TTA upconversion, which is attractive for energy conversion and photonic applications.

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Section: Resultsmentioning

confidence: 99%

Light‐Harvesting Organic Nanocrystals Capable of Photon Upconversion

Zeng

et al. 2017

ChemSusChem

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“…We note that at early times (t < 3 µs), the apparent lifetime of triplet excitons even at low current densities is in the order of a few µs, significantly shorter than the triplet lifetimes identified in rubrene crystals (about 100 µs) [24] and optical TTA-upconverters with a polymer host (up to about 1 ms). [25] However, due to the bimolecular nature of the triplet fusion process, the decay of triplet density (n T ), described by dn T /dt = −k T n T − k TTA n 2 T , at high population density regime is dominated by the rate of TTA (k TTA ), and is significantly faster than the intrinsic monomolecular decay rate of triplets (k T ) (…”

Section: Doi: 101002/adma201605987mentioning

confidence: 99%

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

et al. 2017

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probability of a triplet pair forming a singlet) in solution is >60%, much higher than the spin-statistical prediction of 25% (one pair of triplets collides to form one of the four states: one singlet S 1 and three triplets T 1 , assuming higher triplet and quintet states are inaccessible). Practical application of solar energy conversion requires the TTA-UC material to be in solid state rather than in solution phase. However, the TTA-UC quantum yield of solidstate systems remains low, typically below 5%, [10] and is moderately high (about 10%) in only one example. [19] To achieve high efficiencies, high excitation intensities (200 mW cm −2 or above) are commonly required. These imply that in solid state, the experimentally observed reaction efficiency of TTA-UC was up to about 20%, below the 25% spin-statistical limit.To achieve highly efficient triplet fusion (TTA-UC) in OLEDs and other TTA upconverters, we consider four criteria for the selection of emitters: (1) high fluorescence quantum yield, (2) short singlet lifetime, (3) long triplet lifetime, and (4) the energy of two triplet excitons, 2E(T 1 ) lies slightly above that of the singlet exciton, E(S 1 ), but below the second triplet state, E(T 2 ) (and also the energies of any spin-quintet states) (E(S 1 ) ≲ 2E(T 1 ) < E(T 2 )). The first and second criteria are prerequisites for efficient fluorescence. The third criterion is essential for the accumulation of a sufficiently high triplet population density required for rapid triplet-triplet collision processes. The fourth criterion ensures that higher-lying triplet or quintet states do not provide loss channels. The spin states of the triplet excitons give in principle nine spin configurations for the interacting triplet exciton pair, five associated with a quintet, three with a triplet, and one with the singlet state. It is generally considered that the quintet is always higher in energy than the initial triplet pair, so is neglected. If there are no energetically accessible higher-lying triplet states at E(T 2 ), we expect only the S 1 and T 1 excitons to form. Triplets produced from this reaction can be recycled and participate in a further fusion reaction. [7] The basic working principle of a triplet fusion LED (FuLED) is illustrated in Figure 1a. The initial stage (Stage I) of the device operation includes charge injection and exciton formation. Exciton formation on the emissive molecules may occur directly or indirectly through an additional exciton transfer step from a host material. If a host material is present, the S 1 and T 1 of the host are required to be higher than that of the emitter to allow efficient host-emitter energy transfer and to ensure long triplet lifetime of the emitter (Criterion 3 discussed above). The 25% singlet population can be converted to light emission (and nonradiative losses) from the singlet channel immediately, resulting in prompt electroluminescence (EL). The 75% triplet excitons remain nonemissive, but the population of triplets is When an organic light-emitting dio...

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“…As discussed earlier, a long triplet diffusion length can arise either from an extended exciton lifetime or from a larger diffusion coefficient, D. Monguzzi and co-workers increased the triplet exciton lifetime by embedding the molecular upconversion materials in a rigid polystyrene (PS) matrix that inhibits intramolecular relaxation. 102 This rigid PS host simultaneously minimizes the decay of the triplets by first-order processes while introducing a barrier to oxygen. The efficiency of TET was 70% of that in the diffusion-limited case in solution.…”

Section: Hierarchical Order In Harvesting Triplet Excitonsmentioning

confidence: 99%

Triplet transport in thin films: fundamentals and applications

Tang

2017

Chem. Commun.

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Triplet excitons are key players in multi-excitonic processes like singlet fission and triplet-triplet annihilation based photon upconversion, which may be useful in next-generation photovoltaic devices, photocatalysis and bioimaging. Here, we present an overview of experimental and theoretical work on triplet energy transfer, with a focus on triplet transport in thin films. We start with the theory describing Dexter-mediated triplet energy transfer and the fundamental parameters controlling this process. Then we summarize current experimental methods used to measure the triplet exciton diffusion length. Finally, the use of hierarchically ordered structures to improve the triplet diffusion length is presented, before concluding with an outlook on the remaining challenges.

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Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked Polymeric Nanoparticles: Toward the Next Generation of Solid-State Upconverting Materials

Cited by 40 publications

References 45 publications

Light‐Harvesting Organic Nanocrystals Capable of Photon Upconversion

Light‐Harvesting Organic Nanocrystals Capable of Photon Upconversion

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

Triplet transport in thin films: fundamentals and applications

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