Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked

Letter pubs.acs.org/JPCL

Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked Polymeric Nanoparticles: Toward the Next Generation of Solid-State Upconverting Materials Angelo Monguzzi,*,† Michele Mauri,† Michel Frigoli,‡ Jacopo Pedrini,† Roberto Simonutti,† Chantal Larpent,‡ Gianfranco Vaccaro,† Mauro Sassi,† and Francesco Meinardi*,† †

Dipartimento di Scienza dei Materiali, Università Milano Bicocca, via R. Cozzi 53, 20125 Milano, Italy Institut Lavoisier UMR-CNRS 8180, Université de Versailles Versailles-Saint Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France

‡

S Supporting Information *

ABSTRACT: The technological application of sensitized upconversion based on triplet− triplet annihilation (TTA) requires the transition from systems operating in liquid solutions to solid-state materials. Here, we demonstrate that the high upconversion efficiency reported in hyper-cross-linked nanoparticles does not originate from residual mobility of the embedded dyes as it happens in soft hosts. The hyper-reticulation from one side blocks the dyes in fixed positions, but on the other one, it suppresses the nonradiative spontaneous decay of the triplet excitons, reducing intramolecular relaxations. TTA is thus enabled by an unprecedented extension of the triplet lifetime, which grants long excitons diffusion lengths by hopping among the dye framework and gives rise to high upconversion yield without any molecular displacement. This finding paves the way for the design of a new class of upconverting materials, which in principle can operate at excitation intensities even lower than those requested in liquid or in rubber hosts.

I

as triplet−triplet energy transfer and TTA, which require fast diffusion of the excited molecules to be effective. However, for applications to real-world devices, solid-state systems are highly preferred, also because they allow for the shielding of the dyes from oxygen quenching. Up to now, this goal has been achieved only in few materials, including rubber polymers9−11 and selfassembled molecular systems.12−14 In rubber polymers, as in solution, bimolecular interactions are enhanched by the high residual mobility of embedded dyes in a soft host. On the contrary, in self-assembled materials, the dyes cannot migrate, but a large exciton diffusion length L is obtained by the fast hopping rate of the triplets within a network of close-packed emitters. The sTTA-UC efficiency in rigid hosts is usually rather low because neither the molecules nor the excitons can diffuse.15−17 However, we recently observed an unexpected large sTTA-UC yield in dye-doped rigid polystyrene (PS)-based nanoparticles (NPs), whose origin is still debated.18 Here, we demonstrate that in hyper-cross-linked NPs, the high upconversion yield is due to the extension of triplet exciton lifetimes up to fractions of a second, allowing for a large L even with small molecular/ excitonic diffusivity. This finding explains the reported sTTAUC performances in NPs and suggests a completely new strategy for the fabrication of efficient solid-state systems based

n the last 10 years, the sensitized upconversion based on triplet−triplet annihilation (sTTA-UC) in multicomponent organic systems has proved to be the most effective strategy for obtaining high-energy photons starting from low-energy radiation at excitation intensities comparable to the solar irradiance.1−3 This process was originally developed for improving the performances of photovoltaic cells by blueshifting the solar spectrum in order to exploit the sub-band-gap photons that are otherwise useless for these devices.4 More recently, sTTA-UC was also proposed for other applications, including anti-Stokes fluorescent bioimaging and high-sensitivity oxygen detection.5,6 The ability of sTTA-UC to convert low-power radiation is significantly better than that one of the competitive systems, including nonlinear crystals, two-photon absorbers, quantum dots, and materials based on rare-earth ions, and arises directly from the peculiar nature of the involved electronic states. In particular, sTTA-UC exploits the annihilation of metastable triplets, which are indirectly populated from proper sensitizers via energy transfer (see the Supporting Information, Figure S1) to produce high-energy singlets that generate the upconverted emission. Because triplets are long-living states with typical natural lifetime in the range of milliseconds and over, sTTA-UC does not require the nearly simultaneous absorption of more photons, being effective also at extremely low excitation light intensity.7 The highest sTTA-UC efficiencies have been achieved for sensitizer/emitter pairs dissolved in low-viscosity solvents8 because this process is based on short-range interactions, such © XXXX American Chemical Society

Received: May 23, 2016 Accepted: July 7, 2016

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Figure 1. (a) Normalized absorptions (dashed lines) and upconversion PL spectra (solid lines) of DPA/PtOEP bicomponent sTTA-UC systems in a solution of THF, bulk PS, and an aqueous dispersion of NPs with one (NP-1) and two (NP-2) sensitizers per NP. The dotted spectra are the residual PtOEP phosphorescences. (b) Upconversion quantum yield QYuc as a function of the absorbed photon density for the THF solution (squares), NP-1 (circles), NP-2 (triangles), and PS (diamonds). (c) The top panel shows a sketch of a diffusion-assisted sTTA-UC process in a rigid NP. Upon optical excitation of the sensitizers (red dots), the energy is transferred to the closest emitters (blue dots) and then migrates within the NP by hopping between neighboring dyes. In this way, two triplet excitons generated in different positions inside of the NP can collide and annihilate, yielding upconverted photons. In contrast (lower panel), if excitons cannot migrate, the upconversion process is localized around each sensitizer in a small volume defined by the energy transfer radius. In such a case, TTA occurs only if the same sensitizer simultaneously excites two emitters.

molecule and 50 DPA ones. Sample NP-2 differed from the others by a two times larger sensitizer concentration (4 × 10−5 M), which corresponds to 2 PtOEP and 50 DPA per NP (see Supporting Information, section 1). Previous studies showed that the embedded DPA dyes are homogeneously distributed across the NPs and protected from oxygen.18,19 We started the characterization of the sTTA-UC nanosystem from the measurement of the upconversion quantum yield (QYuc) as a function of the excitation intensity. Usually QYuc rises linearly with the excitation irradiance up to a well-defined value, the excitation power threshold (ITh), and then saturates.3,20 Therefore, ITh is a direct measurement of the excitation power density required by a sTTA-UC system to work properly. It depends on several factors including the sensitizer absorption coefficient, the sensitizer-to-emitter energy transfer efficiency (ΦET), and the ratio between the triplet spontaneous decay rate (kT) squared and the secondorder annihilation rate constant (γTT). The other benchmark parameter for these systems is the largest achievable upconversion yield (QYmax), obtained above ITh when the triplets’ density is so large that they all decay by TTA while the spontaneous recombination becomes a negligible deactivation channel. For well-designed systems, QYmax is typically around 20−25%, and it is limited by the statistical probability to obtain singlet excited states upon TTA,2,21 by the emitter photoluminescence (PL) yield Φem, and by ΦET.8,22 As shown in

on control of the chromophores’ excited-state properties instead of the energy diffusion rate. To shed light on the photophysics controlling the sTTA-UC in rigid NPs, we studied this process using a standard organic bicomponent system based on Pt(II)-octaethylporphyrin (PtOEP) as the sensitizer and 9,10-diphenylanthracene (DPA) as the emitter. This dye pair allows for obtaining a green-to-blue sTTA-UC with an excitation power density as low as 2.5 × 1015 ph cm−2 s−1 (∼1 mW cm−2, Figure 1). We prepared four kinds of samples: (i) a liquid oxygen-free solution in tetrahydrofuran (THF), which is a low-viscosity solvent where the molecules are completely free to migrate, (ii) an oxygen-free, bulk PS sample (MW 105 kDa) in which the molecules are frozen in fixed positions, (iii,iv) two sets of highly cross-linked PS-based NPs dispersed in water (from here on NP-1 and NP-2). The preparation of the THF solution and the synthesis of the PS sample are detailed in the Supporting Information. The polymeric NPs were obtained from a reported procedure and doped by swelling.19 The concentration of NPs in the aqueous suspension was about 2.6 wt %, and the average radius of the NPs, measured by atomic force microscopy and DLS, was ∼7 nm.19 In all of these samples, except NP-2, the dye concentrations were the same (2 × 10−5 and 10−3 M for PtOEP and DPA, respectively). Owing to the overall dye concentrations in the aqueous suspension given above, in sample NP-1, each NP contained on average 1 PtOEP 2780

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The Journal of Physical Chemistry Letters Figure 1b, the reference solution shows a QYmax of ∼23%,23 and a rather low ITh of 2.3 × 1015 ph cm−2 s−1 (∼0.86 mW cm−2). In contrast, the upconversion efficiency of the bulk PS is very small ( 1 ms) annihilation regime, where triplets mainly decay spontaneously. The fit of the experimental data with the decay function that describes the evolution of the sTTA-UC luminescence (solid line; see Supporting Information, section 5)22 gives kT = 460 Hz (τT ≈ 2.2 ms). Considering that in DPA the T1−S0 transition is completely forbidden, this is a relatively large rate, indicating that in THF solution the spontaneous decay is a competitive nonradiative recombination channel for triplets. Here the sTTA-UC is possible thanks to the large annihilation probability granted by the fast molecular diffusion in solution, which prevails on the spontaneous decay probability even for relatively small excited-state densities. The bulk PS sample shows a different behavior. In particular, it is not possible to discriminate different regimes because the PL decay rate remains almost constant over all of the investigated time span. This is consistent with the QYuc versus power measurements, which do not show any excitation power threshold above which TTA becomes the dominant mechanism. The estimated kT is 260 Hz (τT ≈ 3.8 ms), a value significantly smaller than that in THF, indicating that the PS rigid structure reduces the vibrationally assisted recombination of DPA triplets. However, this partial suppression of the nonradiative triplet recombination is not enough to compensate for the lack of excitons mobility, and the sTTA-UC remains inefficient. In the NP-1, the sTTA-UC emission decay is clearly nonexponential as in solution but on a completely different time scale (Figure 3c, triangles). The upconverted PL initially decays with a rate of ∼100 Hz, and after this quick initial decrease, it levels off on a single-exponential decay with an exceptionally low kT of 4 Hz (τT = 250 ms). We ascribe this behavior to the inclusion of dyes inside of a hyper-reticulated NP framework of rigid cages, theoretically smaller than the dye itself, which efficiently hinder all of the intramolecular vibrational motions responsible for the triplet nonradiative internal recombination. It is worth pointing out that the pump/ 2782

DOI: 10.1021/acs.jpclett.6b01115 J. Phys. Chem. Lett. 2016, 7, 2779−2785

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The Journal of Physical Chemistry Letters QY max = 0.5fϕemϕET

probe measurement of the PS upconverted emission shows the same fast decay already observed with the standard setup (Figure 3c, circles), which confirms that the proposed method is fully reliable. The reported findings suggest that in hyper-reticulated NPs the observed high-efficiency sTTA-UC at low power is obtained through a mechanism that is completely different from the one usually exploited in the soft solid-state hosts, in which it is possible to obtain large triplet diffusion lengths, thanks to the large residual dye mobility. In the present NPs, the dyes do not have any possibility to migrate, being completely blocked in their position by the ultrarigid host matrix. However, the intrinsic triplet lifetime becomes so large that even a very small exciton hopping rate between neighboring molecules, possibly due to the overlap of the tails of the molecular orbitals’ wave functions, is sufficient to enable energy diffusion lengths large enough to ensure the high annihilation probability requested for efficient generation of the upconverted emission. To quantitatively support this conclusion, we simulated the stochastic spatial distribution of 50 DPA molecules in a sphere of 7 nm radius. Figure 3d shows the result of the Monte Carlo calculations, obtained imposing a minimum center-to-center intermolecular distance between two NN molecules of 1.0 nm, in accordance with the steric hindrance of the DPA (molecular radius of 0.45 nm). The distribution of NN distances is peaked at 1.1 nm, with more than 70% of dyes having a NN closer than 1.9 nm (blue line). These values are comparable with the distances at which the triplet−triplet interactions, via exchange energy transfer and/or long-range charge transfer, are effective.41−43 Moreover, the distribution of NNN distances, peaked at 1.7 nm, gives intermolecular distances not far from the typical Dexter energy transfer radius. This demonstrates that the excitation energy can migrate within the ensemble of emitters, exploring the entire particle volume to experience TTA. As detailed in the Supporting Information, our data indicate that the proposed energy migration scenario is quite efficient, giving a DPA triplet diffusion length as large as L = 14.5 μm, which is a huge value when compared to the NP dimensions. Remarkably, it should be noted that the strategy to maximize sTTA-UC via a decrease of kT rather than an enhancement of the second-order annihilation constant γTT can be extremely efficient because Ith ∝

(k T)2 γTT

(2)

Because for DPA f is around 0.5 and its ϕem is close to 1 regardless of the environment, eq 2 suggests that QYmax in NPs is mainly limited by the sensitizer/emitter energy transfer yield ϕET. However (Figure S6), from the comparison of the sensitizers’ PL lifetime in the absence and in the presence of the emitter, we observe only moderate differences for ϕET, which ranges from ϕET > 99% in solution to 70% in NPs. This variation does not fully account for the observed QYmax in NPs, which is 1 order of magnitude smaller than that in solution. A more detailed analysis of the PtOEP emission dynamics allows us to clarify this point (Figure S6). In the absence of emitters, the PL decay of sensitizers in solution behaves as a single exponential with a characteristic lifetime of 64.5 μs. The same holds for the PtOEP in PS, with only a moderate lengthening of the lifetime, which goes up to 90.5 μs as a consequence of the increased host rigidity. Conversely, in the NPs, not only is the PtOEP average PL decay the fastest (56.5 μs) despite the extremely large local rigidity but, more importantly, it is clearly multiexponential. This proves that the sensitizer molecules experiment with a distribution of different local environments. In particular, we can suppose that a fraction of sensitizers are on the NP surface, or very close to it, losing the protection from the water in which the NPs are dispersed and especially from the oxygen. This could not only be detrimental for the energy transfer, which is in any case rather efficient as outlined above, but it suggests that probably there are also emitter molecules in the same condition. These latter are expected to show the negligible mobility typical of dyes in solid matrixes but, because of the reduced/suppressed efficacy of the NP shielding effect, may have a significantly shorter triplet natural lifetime, completely ineffective for the sTTA-UC. Therefore, the upconversion yield observed in NPs is lower than that in solution, not as consequence of any photophysical drawback induced by the encapsulation but because not all of the dyes effectively contribute to the process. This is a particularly relevant finding because it implies that there is room to improve the NPs’ performances by enhancing the dyes’ encapsulation protocol. In summary, we elucidated the origin of the high upconversion efficiency observed at low excitation power in dual dye-doped, hyper-cross-linked polymeric NPs. Unlike other solid-state systems in which good conversion outputs are obtained thanks to the residual mobility of the embedded dyes, here the possibility to generate high-energy singlets upon annihilation of triplet metastable states is granted by the extension of their natural lifetime up to a fraction of a second. This is a completely new result, which paves the way for the realization of new classes of TTA-based upconverting solid materials that are no longer characterized by a low viscosity but, on the contrary, are extremely rigid. Moreover, the demonstration of these findings in nanosized systems adds further degrees of freedom to the material design because upconverting particles can be used as is, for instance, in bioimaging applications, or can be included in suitable plastic matrixes for realizing optically active bulk materials. Finally, it is worth noting that, once properly optimized, these systems may be effective at exceptionally low photon fluxes for the very reason that the ultimate excitation power threshold is determined just by the triplet natural lifetime.

(1)

Therefore, the small kT observed in the NPs can easily overcompensate an annihilation process not particularly fast, thus allowing one to obtain Ith comparable to those commonly observed in solution and much smaller than that expected in the bulk PS. In particular, by assuming that γTT is the same in all of the solid amorphous environments where the translation molecular diffusion is negligible, the above equation implies that the threshold in the PS should be at least 3 orders of magnitude larger than that in NPs, undetectable in the irradiance range tested in our experiment. The huge extension of the DPA triplet lifetime in the hyperreticulated NPs explains the origin of the large sTTA-UC efficiency recorded in these systems, but it raises the question of why their ultimate performances are not the same as that of the reference solution. Above Ith, the conversion efficiency is 2783

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(14) Hisamitsu, S.; Yanai, N.; Kimizuka, N. Photon-Upconverting Ionic Liquids: Effective Triplet Energy Migration in Contiguous Ionic Chromophore Arrays. Angew. Chem., Int. Ed. 2015, 54, 11550−11554. (15) Merkel, P. B.; Dinnocenzo, J. P. Low-power green-to-blue and blue-to-UV upconversion in rigid polymer films. J. Lumin. 2009, 129, 303−306. (16) Monguzzi, A.; Tubino, R.; Meinardi, F. Multicomponent Polymeric Film for Red to Green Low Power Sensitized UpConversion. J. Phys. Chem. A 2009, 113, 1171−1174. (17) Islangulov, R. R.; Lott, J.; Weder, C.; Castellano, F. N. Noncoherent Low-Power Upconversion in Solid Polymer Films. J. Am. Chem. Soc. 2007, 129, 12652−12653. (18) Monguzzi, A.; Frigoli, M.; Larpent, C.; Tubino, R.; Meinardi, F. Low-Power-Photon Up-Conversion in Dual-Dye-Loaded Polymer Nanoparticles. Adv. Funct. Mater. 2012, 22, 139−143. (19) Gouanvé, F.; Schuster, T.; Allard, E.; Méallet-Renault, R.; Larpent, C. Fluorescence Quenching upon Binding of Copper Ions in Dye-Doped and Ligand-Capped Polymer Nanoparticles: A Simple Way to Probe the Dye Accessibility in Nano-Sized Templates. Adv. Funct. Mater. 2007, 17, 2746−2756. (20) Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, F. Upconversion-induced fluorescence in multicomponent systems: Steady-state excitation power threshold. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 195112. (21) Khnayzer, R. S.; Blumhoff, J.; Harrington, J. A.; Haefele, A.; Deng, F.; Castellano, F. N. Upconversion-powered photoelectrochemistry. Chem. Commun. 2012, 48, 209−11. (22) Cheng, Y. Y.; Fückel, B.; Khoury, T.; Clady, R. G. C. R.; Tayebjee, M. J. Y.; Ekins-Daukes, N. J.; Crossley, M. J.; Schmidt, T. W. Kinetic Analysis of Photochemical Upconversion by Triplet−Triplet Annihilation: Beyond Any Spin Statistical Limit. J. Phys. Chem. Lett. 2010, 1, 1795−1799. (23) Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Low power, non-coherent sensitized photon upconversion: modelling and perspectives. Phys. Chem. Chem. Phys. 2012, 14, 4322−4332. (24) Monguzzi, A.; Bianchi, F.; Bianchi, A.; Mauri, M.; Simonutti, R.; Ruffo, R.; Tubino, R.; Meinardi, F. High Efficiency Up-Converting Single Phase Elastomers for Photon Managing Applications. Adv. Energy Mater. 2013, 3, 680−686. (25) Keivanidis, P. E.; Baluschev, S.; Lieser, G.; Wegner, G. Inherent Photon Energy Recycling Effects in the Up-Converted Delayed Luminescence Dynamics of Poly(fluorene)−PtIIoctaethyl Porphyrin Blends. ChemPhysChem 2009, 10, 2316−2326. (26) Xu, K.; Zhao, J.; Escudero, D.; Mahmood, Z.; Jacquemin, D. Controlling Triplet−Triplet Annihilation Upconversion by Tuning the PET in Aminomethyleneanthracene Derivatives. J. Phys. Chem. C 2015, 119, 23801−23812. (27) Goldschmidt, J. C.; Fischer, S. Upconversion for Photovoltaics − a Review of Materials, Devices and Concepts for Performance Enhancement. Adv. Opt. Mater. 2015, 3, 510−535. (28) Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband dye-sensitized upconversion of nearinfrared light. Nat. Photonics 2012, 6, 560−564. (29) Maus, A.; Hertlein, C.; Saalwächter, K. A Robust Proton NMR Method to Investigate Hard/Soft Ratios, Crystallinity, and Component Mobility in Polymers. Macromol. Chem. Phys. 2006, 207, 1150− 1158. (30) Sturniolo, S.; Pieruccini, M.; Corti, M.; Rigamonti, A. Probing alpha-relaxation with Nuclear Magnetic Resonance echo decay and relaxation: A study on nitrile butadiene rubber. Solid State Nucl. Magn. Reson. 2013, 51−52, 16−24. (31) Bonetti, S.; Farina, M.; Mauri, M.; Koynov, K.; Butt, H. J.; Kappl, M.; Simonutti, R. Core@shell Poly(n-butylacrylate)@polystyrene Nanoparticles: Baroplastic Force-Responsiveness in Presence of Strong Phase Separation. Macromol. Rapid Commun. 2016, 37, 584−9. (32) Simpson, J. H.; Wen, W. Y.; Jones, A. A.; Inglefield, P. T.; Bendler, J. T. Diffusion coefficients of xenon in polystyrene

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01115. Experimental details, structural analysis details and comments, and time-resolved PL modeling details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.M.). *E-mail: [email protected] (F.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M. acknowledges support from Università degli Studi MilanoBicocca (Grant No. 2016-ATESP-0052).

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REFERENCES

(1) Yanai, N.; Kimizuka, N. Recent emergence of photon upconversion based on triplet energy migration in molecular assemblies. Chem. Commun. 2016, 52, 5354−5370. (2) Aulin, Y. V.; van Sebille, M.; Moes, M.; Grozema, F. C. Photochemical upconversion in metal-based octaethyl porphyrindiphenylanthracene systems. RSC Adv. 2015, 5, 107896−107903. (3) Schulze, T. F.; Schmidt, T. W. Photochemical upconversion: present status and prospects for its application to solar energy conversion. Energy Environ. Sci. 2015, 8, 103−125. (4) Baluschev, S.; Miteva, T.; Yakutkin, V.; Nelles, G.; Yasuda, A.; Wegner, G. Up-Conversion Fluorescence: Noncoherent Excitation by Sunlight. Phys. Rev. Lett. 2006, 97, 143903. (5) Kwon, O. S.; Song, H. S.; Conde, J.; Kim, H.-i.; Artzi, N.; Kim, J.H. Dual-Color Emissive Upconversion Nanocapsules for Differential Cancer Bioimaging In Vivo. ACS Nano 2016, 10, 1512−1521. (6) Borisov, S. M.; Larndorfer, C.; Klimant, I. Triplet−Triplet Annihilation-Based Anti-Stokes Oxygen Sensing Materials with a Very Broad Dynamic Range. Adv. Funct. Mater. 2012, 22, 4360−4368. (7) Monguzzi, A.; Borisov, S. M.; Pedrini, J.; Klimant, I.; Salvalaggio, M.; Biagini, P.; Melchiorre, F.; Lelii, C.; Meinardi, F. Efficient Broadband Triplet−Triplet Annihilation-Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic Systems. Adv. Funct. Mater. 2015, 25, 5617−5624. (8) Hoseinkhani, S.; Tubino, R.; Meinardi, F.; Monguzzi, A. Achieving the photon up-conversion thermodynamic yield upper limit by sensitized triplet-triplet annihilation. Phys. Chem. Chem. Phys. 2015, 17, 4020−4024. (9) Simon, Y. C.; Bai, S.; Sing, M. K.; Dietsch, H.; Achermann, M.; Weder, C. Low-Power Upconversion in Dye-Doped Polymer Nanoparticles. Macromol. Rapid Commun. 2012, 33, 498−502. (10) Kim, J.-H.; Deng, F.; Castellano, F. N.; Kim, J.-H. High Efficiency Low-Power Upconverting Soft Materials. Chem. Mater. 2012, 24, 2250−2252. (11) Monguzzi, A.; Mauri, M.; Bianchi, A.; Dibbanti, M. K.; Simonutti, R.; Meinardi, F. Solid-State Sensitized Upconversion in Polyacrylate Elastomers. J. Phys. Chem. C 2016, 120, 2609−2614. (12) Duan, P.; Yanai, N.; Nagatomi, H.; Kimizuka, N. Photon Upconversion in Supramolecular Gel Matrixes: Spontaneous Accumulation of Light-Harvesting Donor−Acceptor Arrays in Nanofibers and Acquired Air Stability. J. Am. Chem. Soc. 2015, 137, 1887−1894. (13) Duan, P.; Yanai, N.; Kurashige, Y.; Kimizuka, N. AggregationInduced Photon Upconversion through Control of the Triplet Energy Landscapes of the Solution and Solid States. Angew. Chem., Int. Ed. 2015, 54, 7544−7549. 2784

DOI: 10.1021/acs.jpclett.6b01115 J. Phys. Chem. Lett. 2016, 7, 2779−2785

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The Journal of Physical Chemistry Letters determined by xenon-129 NMR spectroscopy. Macromolecules 1996, 29, 2138−2142. (33) Simonutti, R.; Bracco, S.; Comotti, A.; Mauri, M.; Sozzani, P. Continuous Flow Hyperpolarized 129Xe NMR for Studying Porous Polymers and Blends. Chem. Mater. 2006, 18, 4651−4657. (34) Bracco, S.; Valsesia, P.; Ferretti, L.; Sozzani, P.; Mauri, M.; Comotti, A. Spectroscopic observations of hybrid interfaces and gas storage in organo-clays. Microporous Mesoporous Mater. 2008, 107, 102−107. (35) Kaliva, M.; Armatas, G. S.; Vamvakaki, M. Microporous Polystyrene Particles for Selective Carbon Dioxide Capture. Langmuir 2012, 28, 2690−2695. (36) Vaccaro, G.; Bianchi, A.; Mauri, M.; Bonetti, S.; Meinardi, F.; Sanguineti, A.; Simonutti, R.; Beverina, L. Direct monitoring of selfassembly of copolymeric micelles by a luminescent molecular rotor. Chem. Commun. 2013, 49, 8474−6. (37) Loutfy, R. O. Fluorescence Probes For Polymer Free-volume. Pure Appl. Chem. 1986, 58, 1239−1248. (38) Howell, S.; Dakanali, M.; Theodorakis, E. A.; Haidekker, M. A. Intrinsic and Extrinsic Temperature-Dependency of Viscosity-Sensitive Fluorescent Molecular Rotors. J. Fluoresc. 2012, 22, 457−465. (39) Mikhnenko, O. V.; Blom, P. W. M.; Nguyen, T.-Q. Exciton diffusion in organic semiconductors. Energy Environ. Sci. 2015, 8, 1867−1888. (40) Pope, M.; Swenberg, C. E.; Pope, M. Electronic processes in organic crystals and polymers; Oxford University Press: New York, 1999. (41) Scholes, G. D. Long-range resonance energy transfer in molecular systems. Annu. Rev. Phys. Chem. 2003, 54, 57−87. (42) Strambini, G. B.; Galley, W. C. Detection of slow rotational motions of proteins by steady-state phosphorescence anisotropy. Nature 1976, 260, 554−6. (43) Miller, J. R.; Hartman, K. W.; Abrash, S. Long-distance (25.ANG.) electron transfer by triplet excited states in rigid media. J. Am. Chem. Soc. 1982, 104, 4296−4298.

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