www.acsami.org Research Article Water-Stable Upconverting Coordination Polymer Nanoparticles for Transparent Films and Anticounterfeiting Patterns with Air-Stable Upconversion Junda Zhang, Daniel Ruiz-Molina, Fernando Novio,* and Claudio Roscini* Cite This: ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Photon upconversion (UC) based on triplet−triplet annihilation is a very promising phenomenon with potential application in several areas, though, due to the intrinsic mechanism, the achievement of diffusion-limited solid materials with air-stable UC is still a challenge. Herein, we report UC coordination polymer nanoparticles (CPNs) combining sensitizer and emitter molecules especially designed with alkyl spacers that promote the amorphous character. Beyond the characteristic constraints of crystalline MOFs, amorphous CPNs facilitate high dye density and flexible ratio tunability. To show the universality of the approach, two types of UC-CPNs are reported, exhibiting highly photostable UC in two different visible spectral regions. Given their nanoscale, narrow size distribution, and good chemical/colloidal stability in water, the CPNs were also successfully printed as anticounterfeiting patterns and used to make highly transparent and photostable films for luminescent solar concentrators, both showing air-stable UC. KEYWORDS: triplet−triplet annihilation, upconversion, coordination polymer nanoparticles, metal−organic framework, luminescent solar concentrators 1. INTRODUCTION in their triplet state. Once enough excited molecules of the Photon energy upconversion (UC) is a process in which a acceptor are produced, the triplet−triplet annihilation (TTA) high-energy emission is generated through the absorption of process occurs forming both ground-state and highly excited two or more lower-energy photons.1−3 So, materials exhibiting emitter molecules, which release their energy by emitting this feature have raised much attention for their potential photons from the singlet excited state (S1), i.e., of higher applicability in several fields such as bioimaging,4,5 biotherapy,6 energy than that used to excite the photosensitizer (see solar cells and energy storage,7,8 photocatalysis (to remove air Scheme S1 in the Supporting Information).1 and/or water contaminants upon solar light irradiation),9 While inorganic UC systems are normally obtained as solid photopolymerization,10 optoelectronic devices (e.g., lumines- bulk materials or nanoparticles,18,19 a still ongoing research cent solar concentrators, OLEDs),11−13 and anticounterfeit- study is challenging the achievement of air-stable molecular- ing.14 To date, most of the reported UC materials have been based UC solid materials for practical applications.20,21 The represented by rare-earth nanoparticles and sensitizer−emitter main challenges derive from (i) quenching of the involved molecular organic dye pairs undergoing UC through triplet− excited triplet states by the oxygen in the air atmosphere, triplet annihilation (TTA-UC).15 The former group relates to which reduces the UC efficiency22−24 and (ii) the need for dye inorganic bulk and nanostructured materials,1,16,17 mainly pairs to diffuse and approach within 1 nm for the bimolecular made by lanthanide ions embedded in an inorganic crystalline processes involved in UC (TTET and TTA) to efficiently take host lattice, though typical drawbacks are the very narrow ion place via the Dexter energy transfer mechanism.25,26 Moreover, absorption bands, low extinction coefficients, high radiation it is often desired to have these materials as colloidal and power densities required (∼W cm−2), and relatively low UC chemically stable nanoparticles in aqueous media, for their quantum yields, besides low biocompatibility and toxicity. In biological and environmentally friendly applications, for contrast, organic-based TTA-UC benefits from the broad absorption of the sensitizers and the high fluorescence quantum yield of emitter organic molecules. Generally, Received: September 11, 2022 sensitizer molecules are at first excited upon absorption of a Accepted: January 12, 2023 photon forming the singlet excited state. After generating a Published: February 1, 2023 triplet excited state (via intersystem-crossing), the sensitizer molecules undergo triplet−triplet energy transfer (TTET) to the energy acceptor (or emitter), yielding emitter units excited © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acsami.2c16354 8377 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 1. (a) Scheme of the synthesis of UC-CPNs using an emitter and a sensitizer chemically modified with a spacer as monomers, and Zr2+ as a cross-linker. (b, c) Chemical structures of the sensitizers (Pd-S-COOH and Os-S-COOH) and emitters (DPA-S-COOH and CAEBD-S-COOH) functionalized with COOH-terminated spacers and used to prepare VVUC1-CPNs and VVUC2-CPNs. instance as bioimaging markers or water-based ink formula- sensitizer-to-emitter ratio and/or the sensitizer/emitter dis- tions, and/or redispersed in polymeric matrices as transparent tance, especially when these particles are dispersed in other emitting devices (OLEDs, luminescent solar concentra- media. The only exception is given by recent work by I. Roy et tors).12,13,27,28 al.,43 which reports for the first time a MOF with both the During the years, several strategies have been developed to sensitizer and the emitter being part of the chemical structure accomplish such TTA-UC bulk and nanostructured solids: of the MOFs. However, due to the inherent structural direct dye dissolution in rubber-like low-Tg polymers (e.g., properties of the MOFs, the local emitter/sensitizer ratio is polyurethane),29,30 functionalization of the glassy polymer fixed and depends on the coordination properties of the metal backbone (e.g., polymethyl methacrylate) or epoxy resins with ion. Efficient TTET is normally achieved with a large excess of the dye pairs,31,32 gels (hydro- and organogels),33−35 ionic the emitter; therefore, the local emitter/sensitizer ratio is an liquid-containing or highly dye-concentrated epoxy resins,36 important parameter that cannot be controlled above a certain polymer or silica liquid-core micro- and nanocapsules,4,22,37,38 limit with MOF structures. On top of that, most of the and liquid nanodroplets confined in polymer materials. More reported materials are based on microstructured MOFs, recently, TTA-UC was extended to highly limited diffusion unsuitable for certain applications where colloidal stability materials, with efficient triplet self-exchange or triplet energy and optical transparency are required. migration between highly concentrated and/or aligned Herein, we hypothesize that these challenges can be chromophores. In this respect, we reported the TTA-UC in successfully faced with the use of amorphous coordination solid-state paraffin waxes (with nearly quantitative TTET), polymer nanoparticles (CPNs). These nanostructures, in where the aromatic dye molecules (sensitizers and emitters) addition to hosting both sensitizer and emitter molecules as were forced to coprecipitate and aggregate in the solid-state constitutive building blocks of the polymeric network, aliphatic paraffin, due to their limited solubility.3 A similar guarantee dye density and flexible ratio tunability, i.e., a approach relies on the formation of binary crystals, where broad control of the number of emitter molecules surrounding dense dye packing guarantees efficient triplet migration and the sensitizer, normally required to achieve UC (Figure 1a). good UC efficiency.14,39 However, even if several approaches Moreover, these nanoparticles have already been shown to have been developed, the synthesis of solid TTA-UC materials exhibit good chemical and colloidal stability in water,45,46 still represents a challenge. which opens new venues for the application of these With this aim, recently, coordination chemistry has emerged nanostructures in biology and other environmentally friendly as a promising alternative. In addition to their rich chemistry applications. and a broad range of useful ligands, metal coordination minimizes molecular migration and phase segregation normally 2. RESULTS AND DISCUSSION observed in dye-loaded nanoparticles or polymer matrices. In 2.1. Upconverting Coordination Polymer Nanopar- this context, metal−organic frameworks (UC-MOFs) emerge ticles. To demonstrate the viability of our approach, we as excellent candidates promoting high density and suitable designed two pairs of dyes, as shown in Figure 1b,c: the alignment of the emitter molecules within the framework, with introduction of the alkyl chain contributes the necessary efficient triplet migration and TTA properties.5,40−44 These flexibility to have amorphous systems without significantly materials take advantage of their open-framework structure to diluting the UC active dye weight mass in the nanoparticles. physically trap the sensitizer within the pores, or sometimes This is very relevant to ensure the high dye density required for even on the surface, to promote TTA-UC, though this an efficient energy transfer and triplet migration to take place, configuration might verify leaching issues if the interaction even when the particles are embedded in other solid media with the metal center is not too strong, preventing control over (e.g., polymers). The chemical design of the ligands leads to two important features for the TTET efficiency, which are the obtaining monodisperse and water-stable (chemically and 8378 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 2. (a) Emission spectra of DMF solutions of DPA-S-COOH/Pd-S-COOH at different molar ratios (λexc = 532 nm, N2 atmosphere). (b) SEM image, (c) emission spectrum, and (d) photograph of the emission of the aqueous suspension of VVUC1-CPNs obtained from DPA-S- COOH/Pd-S-COOH = 1281 (Table S1, entry 3; λexc = 532 nm, N2 atmosphere). A magenta subtractive dichroic color filter was used to cut the 532 nm excitation beam. (e) Integrated UC intensities of VVUC1-CPNs (1.0 mg/mL) upon prolonged irradiation (up to 30 min) with a CW 532 nm laser (1.2 W/cm2). (f) SEM image of VVUC1-CPNs after 30 irradiation with a CW 532 nm laser (1.2 W/cm2). colloidal) nanoparticles that exhibit photostable UC in (λmax = 435 nm, 2.85 eV) when irradiated with a 532 nm (2.33 different visible spectral regions, upon irradiation at λexc = eV) laser (i.e., anti-Stokes shift, ΔEUC = 0.52 eV), indicating 532 nm (VVUC1-CPNs) and λexc = 650 nm (VVUC2-CPNs), that the pair is suitable for UC (Figure 2a). From the 0−0 respectively. vibronic band of the absorption spectrum of the DPA-S- First, we synthesized chemically functionalized palladium(II) COOH acceptor (λ0−0 = 396 nm), the energy of the first meso-tetraphenylporphyrin (Pd-S-COOH) and 9,10-dipheny- singlet excited state was estimated as high as 3.13 eV, quite lanthracene (DPA-S-COOH) ligands, whose parent molecules similar to the reported energy of the parent DPA, determined are already well-known dye pairs (sensitizer and emitter) that by TD-DFT and spectral properties.49,50 From the thermody- manifest visible-to-visible UC (see the SI). They were namic driving force of TTA (2 × E(T1)DPA‑S‑COOH > synthesized with 6 (1.8% overall yield) and 4 (41.2%) steps, E(S 1 ) DPA‑S‑COOH ) and the sensitizer-to-emitter TTET respectively, from commercial starting materials. The dyes core (E(T1DPA‑S‑COOH) < E(T1Pd‑S‑COOH)), the triplet state energy units were functionalized with two 5-carbon carboxylic acid- of the emitter was also determined in the range of 1.56 eV < terminated chains, the non-rigid spacers (S), through the T1DPA‑S‑COOH < 1.77 eV, which lays around the value calculated formation of an amide bond (Figure 1b). The detailed for the lowest triplet energy of the parent DPA (1.72 eV).50 synthetic routes and chemical characterization of the These results showed that the chemical functionalization did compounds are described in the SI (Sections S2 and S3). not affect significantly the lowest excited-state energies of the The absorption and emission spectra of DPA-S-COOH and emitter. Pd-S-COOH DMF solutions resembled those reported for the Visible-to-visible VVUC1-CPNs were thus straightforwardly parent DPA (λmaxabs = 375 nm; λmaxem = 433 nm, Figure S1a,b obtained by mixing Pd-S-COOH and DPA-S-COOH, with in Section 6 in the SI) and PdTPP,47 demonstrating that the different relative initial concentrations, in anhydrous DMF for functionalization did not significantly affect the optical 5 min at room temperature (Table S1). Afterward, zirconium properties and the electronic state energies of the dyes. The oxychloride was added leaving the mixture at 90 °C for an emission quantum yield (QY) of DPA-S-COOH was 98.9%, additional 15 h. After this period, a precipitate was obtained, similar to what were reported for the parent dye in different centrifuged, and isolated by freeze-drying. Zr2+ was chosen as a solvents.48 The DMF solution of Pd-S-COOH showed Q- cross-linking metal ion, as it provides very high robustness to band absorption and phosphorescence (under a N2 atmos- the systems against water51,52 but does not provide novel phere) maxima at λmax = 523 and 700 nm, respectively, and a absorption bands, which might affect the optical properties of phosphorescence lifetime of 503 μs (Figure S1c,d). The the CPNs. In all cases, scanning electron microscopy (SEM) maximum phosphorescence at 700 nm recorded at room corroborated the formation of spherical nanoparticles with an temperature allowed us to estimate a triplet state energy of Pd- average size of around 50 nm (Figures 2b and S2). High- S-COOH of 1.77 eV. Noticeably, degassed N,N-dimethylfor- performance liquid chromatography (HPLC) was used to mamide (DMF) solutions of these novel dyes dissolved in determine the sensitizer/emitter ratio as well as to quantify the different molar ratios did show successful anti-Stokes emission amount of emitter molecules per CPN (see Section 1 in the SI 8379 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article for more details). Inductively coupled plasma-mass spectrom- and linear�at higher intensities�trends, see Figure S8). It etry (ICP-MS) was used to quantify the concentration of must be mentioned, though, that the accurate definition of this sensitizers (through the Pd center) and Zr2+ ions. The value is not straightforward in these systems, as differently sensitizer/emitter ratio before and after the reaction for each from homogeneous solutions, data might be affected by the batch is given in Table S1. Final sensitizer/emitter ratios for scattering provided by the suspended nanoparticles, which the different reactions follow the trend initially used for the prevents the exact evaluation of Ith. Several measurements reaction, though the quantitative values are twice as high, carried out with different equipment and using CPN suggesting the non-equal coordination abilities of the two dyes. concentrations that minimized light scattering and self- UC emission was investigated in degassed aqueous absorption, while providing enough UC emission to be suspensions, under irradiation with a continuous wave (CW) detected, allowed us to estimate such a value of Ith that is in 532 nm laser. Noticeably, an intense UC emission with λmax = the range of those reported for other TTA-UC solid 445 nm was measured for all suspensions, with also a residual materials,20,21,54 comparable to the light intensity of 1 sun, phosphorescence contribution at λmax = 653 nm (Figure S3). and much lower than the power density required for The integrated UC/phosphorescence ratio changed signifi- lanthanide-based bulk materials and UCNPs.55 At this power cantly among all samples, confirming the sensitizer/emitter density and considering a maximum ΦUC of 50% due to the ratio influence (Table S1 and Figure S3). Increasing the bimolecular character of the UC process, a final relative ΦUC of amount of the emitter in the CPNs yielded a higher UC/Ph 0.65% (against Ru(bpy)3Cl2, ΦF = 0.04 in H2O)56 was ratio, possibly due to a more favored TTA process. However, measured.25,26,57 Noticeably, UC emission was observed in above a certain DPA-S-COOH amount, the UC/Ph ratio these CPNs despite the sensitizer and emitter molecules being decreased, which we ascribed to an excessive aggregation of the in lower molar amounts than the Zr ion. This result evidences emitter molecules, causing the emission quenching (vide the good approach of amorphous CPNs. The flexibility of the infra). Among all of them, VVUC1-CPNs obtained from an polymeric chain allows the intra/interchain π−π interactions initial DPA-S-COOH/Pd-S-COOH = 1281 ratio (Table S1, between the aromatic moieties of the chromophores and thus entry 3) yielded reproducibly the highest UC-to-phosphor- the proximity to favor the TTET and TTA through triplet escence integrated intensity ratio (6.54) and little residual migration (compensating the lack of dye diffusion in the phosphorescence, suggesting a quite highly efficient sensitizer- CPNs), which finally yield the UC emission. to-emitter TTET (Figure 2c). In fact, nearly pure blue-color It is worth mentioning that this value most likely is fluorescence of the VVUC1-CPNs suspension could be modulated downward by light scattering and DPA aggregation detected by the naked eye upon irradiation with a CW 532 (and self-quenching) due to the high dye density, which lowers nm laser (Figure 2d). Noticeably, even after prolonged the overall value of the UC. To bring more light to this fact, irradiation time (30 min) at a relatively high power density further spectroscopic data were obtained for these CPNs. The (1.2 W/cm2), the UC intensity did not suffer any decrease, absorption spectrum of these VVUC1-CPNs was measured in suggesting very good photostability of the CPNs (Figures 2e water, at different concentrations, from 3.0 to 0.01 mg/mL and S4), which also preserved their spherical nanostructuration (Figure S9). The most concentrated suspensions (1.0−3.0 mg/ (Figure 2f). mL) reveal an absorption band at 524 nm, associated with Pd- Due to the good UC properties, these nanoparticles were S-COOH, despite its low concentration in the CPNs, as further characterized. First, relevant information gathered indicated by the final polymer molecular formula (DPA-S- about this batch of nanoparticles is that they can be easily COOH)2900(Pd-S-COOH)1Zr5703. The progressive increase of resuspended in Milli-Q water with good colloidal stability, the absorption signal at higher energies, particularly in the without forming aggregates at least for 24 h, as confirmed by most concentrated suspensions, was due to the light scattering dynamic light scattering (DLS) analysis (60.0 ± 23.0 nm) over provoked by the nanoparticles, which becomes stronger at this period of time (Figure S5). Fourier transform infrared higher concentrations and shorter wavelengths (Figure S9a,b). spectroscopy (FT-IR) showed a band at 1650 cm−1 and The lowest concentrated suspensions showed the characteristic another medium intensity band at 1371 cm−1; the intensity vibronic progression of the absorption band of anthracene- and frequency of the bands indicate that they correspond to based emitter molecules (DPA-S-COOH). Similar results (i.e., νCOasym and νCOsym vibrations, respectively. The former is scattering effects and spectral shape of DPA-S-COOH, Figure shifted from 1705 cm−1 and the latter from 1440 cm−1, S9c,d) were observed for CPNs made by only the emitter observed in DPA-S-COOH, or from 1688 and 1441 cm−1 of (DPA-S-CPNs, 45.5 ± 5.1 nm, by SEM image, Figure S10). Pd-S-COOH free ligands, indicating the coordination of the The normalized absorption spectrum of DPA-S-COOH in carboxyl groups of the ligands to the metal center (Figure S6). VVUC1-CPNs (1.0 mg/mL) was compared to the absorption X-ray powder diffraction (XRD) showed the absence of spectra of the same dye in DMF solution and DPA-S-CPNs diffraction peaks, which confirms the amorphous nature of the (1.0 mg/mL). Despite all of them showing the characteristic solid nanoparticles. This corroborates that the 5-carbon alkyl vibronic structure of DPA-S-COOH, the bands of VVUC1- chain is a sufficient length to break the crystalline order (Figure CPNs and DPA-S-CPNs were red-shifted (λ0−0 = 400 nm), S7). broadened, and less defined (the peak-to-valley ratio The efficiency of the TTA-UC process is marked by the decreased) than the spectrum of the molecular dye in solution excitation threshold intensity (Ith) parameter, which indicates (Figure S11a), indicating the formation of ground-state the excitation power density at which TTA becomes the main aggregates of the emitter molecules in both types of CPNs. decay channel and the quantum yield of UC (ΦUC) is As expected, the full overlap of the spectra of VVUC1-CPNs maximized.26,53 In our case, an Ith value of 90 mW·cm−2 was and DPA-S-CPNs suggests that the small amount of Pd-S- found from the logarithm plot of the integrated UC intensity COOH molecules in the former does not affect the against the incident light power density (intersection point aggregation of the emitter compound. Worth to mention is between the fitting lines of the quadratic�at low intensities� the lack of variation of the normalized absorption spectra of 8380 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article VVUC1-CPNs and DPA-S-CPNs suspensions, measured at alkyl chain via an amide bond (CAEBD-S-COOH) (Figure different concentrations, which shows that the spectral shape is 1c). The synthetic strategy and the chemical characterization independent of the dilution and that the aggregation is of the final products and the intermediates are shown in the SI produced within the CPNs (Figure S11b,c). The same (Sections S4 and S5). Overall, the sensitizer and the emitter conclusions were extrapolated from the excitation spectra were obtained in 2 (12.6% yield) and 7 (23.4%) steps from the (recorded at λemi = 450 nm, Figure S12a) of VVUC1-CPNs initial commercial compounds, respectively. In DMF, Os-S- and DPA-S-CPNs, broader, less defined (smaller peak-to- COOH showed absorption bands at λmax = 490 and 680 nm valley ratio), and more red-shifted than the spectrum of the (with the tail reaching 730 nm) and phosphorescence at λmax = molecular solution of the ligand. Finally, dye aggregation was 773 nm (1.60 eV), while CAEBD-S-COOH absorption and confirmed from the slightly red-shifted emission (recorded at fluorescence bands were observed at λmax = 410 nm (with the λexc = 375 nm, Figure S11b) of both types of CPN suspensions λ0−0 band = 436 nm, 2.84 eV) and 460 nm, respectively (λemimax = 440 nm), as well as from the reduction of the (Figure S17). Last but not least, irradiation of a degassed DMF absolute DPA-S-COOH emission quantum yield (Figure S13) solution of the sensitizer (5 × 10−4 M) and the emitter (10−3 in VVUC1-CPNs (18.0−34.2%) and DPA-S-CPNs (18.5− M) with a CW 650 nm laser (1.91 eV) yielded the 37.0%), with respect to the free ligand solution (84.2%). These upconverted emission at λmax = 475 nm (2.61 eV) and an spectral analyses, especially those related to the absorption and anti-Stokes shift of ΔEUC = 0.67 eV (Figure S18). From the excitation spectra, corroborate the presence of ground-state driving force of the TTA and TTET processes, we estimated aggregates of the emitter molecules in the nanoparticles, the triplet energy of the emitter in the range of 1.42−1.60 eV, though the presence of excimer species could not be discarded matching with the values reported in the literature for CAEBD at this stage, as the emission spectra variation could be related derivatives (1.43−1.49 eV), determined experimentally or by to both ground- and excited-state aggregates. TD-DFT calculations.61,62 We also analyzed the excitation and emission spectral Considering these promising results, VVUC2-CPNs were features of Pd-S-COOH in VVUC1-CPNs (the absorption synthesized through the reaction of Os-S-COOH and spectrum of Pd-S-COOH in the nanoparticles was too affected CAEBD-S-COOH, using different initial ratios, with zirconium by the scattering) and compared them to those of the free oxychloride in DMF, employing the same experimental ligand in solution and in CPNs made by only the sensitizer Pd- conditions as for VVUC1-CPNs. The results are summarized S-CPNs, synthesized as reference (64.6 ± 10.0 by SEM, Figure in Table S2. Irradiation of Milli-Q water suspensions of S14). The spectral shifts of the excitation and emission bands different VVUC2-CPNs with a CW 650 nm laser, under a N2 of Pd-S-COOH (λexcmax = 533 nm) in both CPNs, compared atmosphere, resulted in all cases of UC emission with λmax = to the spectrum of the free ligand, indicated that the Pd-S- 511 nm, with different intensity ratios compared to the Os COOH porphyrin molecules also undergo homo/heteromo- phosphorescence (Figures 3a and S19 and Table S2). After lecular aggregation within the nanoparticles (Figure S15). Finally, by nanoseconds laser spectroscopy, we investigated the lifetime of the UC emission. Interestingly, the pulsed laser irradiation at 532 nm produced a UC signal, with practically no rise detectable in the monitored timescale, possibly due to a very fast TTET from the donor to the acceptor, which later decays very rapidly, mostly within the first 50 ns (Figure S16). These results indicate that the nanostructuration of coordination polymers made of the sensitizer and emitter monomers allows for the TTET process in the diffusion- limited solid state, which is required for the UC to occur. In the future, in-depth studies will be carried out to accomplish more details on the types of aggregates formed within the Figure 3. (a) Emission spectrum, SEM image (inset), and (b) CPNs and the phenomena regulating the efficiency of TTA photograph of VVUC2-CPNs aqueous suspension (λexc = 650 nm) and TTET. These studies will allow the design and proper obtained from CAEBD-S-COOH/Os-S-COOH = 120 (Table S2, entry 2). A short-pass filter (λcut‑off = 650 nm) was used. modification of the structure of the emitter molecules to prevent their self-quenching, which should yield, in the last instance, higher UC quantum yield values. 2.2. Generality of the Strategy. To demonstrate the detailed analysis, VVUC2-CPNs, of size 89.2 ± 12.8 nm by universality of our approach, we also designed and synthesized SEM (Figure 3a, inset and S20), obtained from a CAEBD-S- nanoparticles upconverting light in a different spectral range COOH/Os-S-COOH = 120 ratio (Table S2, entry 2), and and absorbing at lower energies (>600 nm), of interest for with molecular formula (CAEBD-S-COOH) 89 (Os-S- different applications (VVUC2-CPNs). For this, we aimed to COOH)1Zr235, showed the highest integrated emission use sensitizers forming their triplets upon direct singlet-to- intensity ratio. The UC emission (detectable by the naked triplet (S−T) absorption, without passing through the ISC that eye upon irradiation with a CW 650 nm laser, Figure 3b) was competes unfavorably with vibrational relaxation modes, also stable over 30 min of strong irradiation with a CW 650 nm especially for low-energy singlet excited states (S1).58−61 The laser (6.6 W/cm2, Figure S21), after which the CPNs monomers of choice for this were the terpyridine osmium preserved their spherical shape and size (Figure S22). complex functionalized with succinic acid attached via an ester Also in this case, the amorphous structure of the polymer bond (Os-S-COOH) (sensitizer) and the 4-((10-(4-carbox- promotes π−π interactions between the aromatic portions of yphenyl)-anthracene-9-yl)-ethynyl)-benzoic derivative (emit- the chromophores located in the same or different polymeric ter), chemically modified with a carboxylic acid-terminated chains. These interactions ultimately allow TTET and TTA 8381 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 4. Photographs of transparent (a) VVUC1-CPNs@PVA and (b) VVUC2-CPNs@PVA films under ambient light; photographs of (c) VVUC1-CPNs@PVA and (d) VVUC2-CPNs@PVA films showing UC emission under 532 and 650 nm CW laser irradiation, respectively; photographs of VVUC1-CPNs and VVUC2-CPNs spray-coated patterns under (e) ambient light and (f) UV irradiation (λexc = 365 nm); (g) scheme of the preparation of the material with a UC-based encrypted message; screenshots of the videos showing the UC emission of the spray- coated (h) VVUC1-CPNs- and (i) VVUC2-CPNs-based patterns while scanning with 532 nm (pulsed Nd:YAG) and 650 nm (CW) laser irradiation, respectively. UC emission only appears when the laser beam scans over the “UC” pattern. The beam spot of the 532 nm pulsed laser is large enough (1 cm diameter) to cover a surface area with and without VVUC1-CPNs, developing UC emission selectively in the area patterned with the UC nanoparticles (e.g., the bottom part of the “U” of the “UC” pattern as shown in Figure 4h). processes (via triplet migration) to occur in the solid CPNs, their use for practical applications. Their low dimensions originating the final UC emission. (<100 nm) and narrow size distribution prevent light Again, the UC emission of CAEBD-S-COOH in the CPNs scattering, allowing us to explore first the fabrication of is slightly red-shifted compared to the dye solution nanoparticles-containing highly transparent polymeric films, as fluorescence, possibly due to the dye aggregation. A detailed demonstrated for other low-size oil photochromic nano- spectroscopic study was also carried out for these CPNs. The droplets63 and fluorescent solid lipid nanoparticles.64 These absorption spectra of Os-S-COOH in VVUC2-CPNs could UC transparent polymeric films could be of interest to OLEDs, not be detected even at high CPN concentrations, due to the solar cells, and luminescent solar concentrators.12,13,27,28 As a combination of light scattering, a low amount of the Os proof of concept for this, we explored the water-soluble complex, and its lower extinction coefficient (2.12 × 104 M−1 poly(vinyl alcohol) (PVA) as a matrix because (i) it is an cm−1) due to the less favored S−T absorption (Figure S23). excellent film-forming polymer63−67 and (ii) it has been shown The absorption spectra of VVUC2-CPNs and the reference as one of the most efficient oxygen barrier films (which CPNs made by only the emitter (CAEBD-S-CPNs, 82.8 ± prevents the triplet quenching).59,68 The colloidal stability of 11.3 nm, by SEM, Figure S24) were significantly broader and our CPNs in water allowed us to integrate molecular UC less defined than the spectral band of the monomeric free materials in such a performing polymer, otherwise impossible ligand in solution (Figure S25a). The presence of such for hardly water-soluble UC organic dyes. aggregates in the CPNs was also manifested by the large PVA films were then easily prepared by drop-casting in a bathochromic shift, compared to the free ligand, of the mold VVUC1-CPNs or VVUC2-CPNs suspensions contain- CAEBD-S-COOH emission and the absolute fluorescence ing PVA and letting the water evaporate at room temperature. emission quantum yield of the CPNs (ΦF = 3.0%), significantly Flexible, free-standing, and highly transparent films could be lower than the free ligand (ΦF = 57.3%). The emission red pealed out from the container (Figures 4a,b and S26). Thanks shift in VVUC2-CPNs (λemimax = 510 nm) was observed upon to not only their small dimensions but also their homogeneous both direct excitation and through delayed UC (Figure S25b), distribution within the polymer, the resulting films showed indicating that the emission derives from the same emitter high transparency (%T555 nm = 84 and 75%T against air, species. The detected little spectral difference was ascribed to respectively, Figure S27). Most importantly, VVUC1-CPNs@ self-absorption of the higher energy photons, observed in the PVA and VVUC2-CPNs@PVA films showed blue and green UC experiments. emissions, respectively, upon irradiation with 532 and 650 nm 2.3. Air-Stable UC Printed Patterns and Transparent CW lasers, under an air atmosphere, confirming the generation Films Based on UC-CPNs. Once the synthesis of UC-CPNs of air-stable UC photons (Figure 4c,d). From here, three facts for different spectral ranges was achieved, we aimed to explore deserve to be emphasized. First, the UC/phosphorescence 8382 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article ratio of VVUC1-CPNs was even higher than the correspond- the PVA/VVUC1-CPNs or PVA/VVUC2-CPNs aqueous ing colloidal suspensions (Figure S28), possibly due to better suspensions were spray-coated on the respective prepared protection from oxygen quenching by PVA, which prevents the substrate, interposing the prepatterned “UC” mask between need for additional protective coatings, e.g., nanocellulose, or the nozzle and the substrate (Figure 4g). Irradiation with UV antioxidants, normally used to inhibit oxygen diffusion and/or light (λexc = 365 nm) over the samples showed the emission of quenching of the triplet states.22−24,69 Second, UC was DPA-S-COOH or CAEBD-S-COOH, present in both the observable using a relatively low amount of CPNs (0.67 wt background and the patterned material (Video S1). In this % with respect to PVA), without needing rubber-type polymers way, the “UC” label remains encrypted. On the other hand, or high dye concentrations (as normally happens in UC when 532 and 650 nm CW lasers were used, the corresponding polymeric matrices). Indeed, the structure of the CPNs UC emissions were activated only in the regions where the minimizes dye migration and phase separation and assures CPNs were deposited (Figures 4h,i, S33, and 34). Videos S2− that the high local concentration of the dyes within a particle is S4 show the scanning of the substrate surface with 532 and 650 maintained once embedded in the final film. Third, the nm CW lasers and a pulsed 532 nm beam. UC emission only photostability study of the films in aerated conditions showed appears when the beam scans over the “UC” pattern, that the UC emission not only could be preserved during long decrypting the hidden message. irradiation times (30−40 min) at strong power densities (1.2 mW/cm2, 532 nm and 6.6 mW/cm2, 650 nm respectively) but 3. CONCLUSIONS also increased up to a photostationary state (Figure S29). Such In summary, the introduction of an alkyl chain within emitter enhancement was ascribed to the consumption of O2 present and sensitizer ligands allowed us to synthesize photostable UC in the films, which was trapped during their fabrication or amorphous nanoscale coordination polymers with high dye slowly diffused inside over time. At the beginning of densities and easy dye ratio tunability. This approach prevents irradiation, residual oxygen quenches the photoinduced triplet dye migration and phase segregation of the dyes in the polymer states of the dye molecules and thus the overall UC, forming material (matrix) and allows the optimization of the dye 1 O2 or other reactive oxygen species. With time, these species interactions and the consequent energy transfer processes irreversibly react with the dyes of the CPNs, which consume involved in TTA-UC. We proved that the approach can be the residual quenching oxygen source. When all oxygen is generalized to upconvert photons from/to different spectral consumed, the quenching process is inhibited and UC regions and through distinct mechanisms (intersystem-crossing intensity quickly increases. The UC enhancement over time or direct singlet-to-triplet absorption). Moreover, given their indicates that (i) the amount of dye molecules irreversibly chemical, colloidal, and photostability in water, these CPNs degraded by the reactive oxygen species is negligible compared could be considered the organic counterpart of the lanthanide- to the dyes remaining available for the UC process, and (ii) the based inorganic UCNPs, but with lower excitation intensity oxygen consumption is far faster than the potential diffusion of thresholds. The system might be further improved through the further atmospheric air, due to the extremely good PVA addition of a bulky group in the DPA-S-COOH structure to oxygen barrier effect. Such a nanocomposite structure could reduce aggregation while preserving the high density in the also be extended to other natural polymers, e.g., cellulose, CPNs to favor TTET and TTA. Finally, the combination with which was recently used to obtain stable and recyclable UC the water-soluble and oxygen barrier PVA polymer allowed us films in the air atmosphere, due to similar oxygen barrier to obtain UC waterborne inks and transparent films where UC properties.70 is preserved even at low CPN concentrations. The interest in Finally, as a proof-of-concept application, we explored the these platforms was exemplified by the development of use of these films as UC luminescent solar concentrators, as anticounterfeiting patterns and luminescent solar concentra- UC is desirable to avoid the typical dye reabsorption of re- tors. emitted light during its traveling to the edges.13,27,28 Successfully, localized irradiation with a CW 532 nm laser at 4. EXPERIMENTAL SECTION the center of the VVUC1-CPNs@PVA film yielded intense 4.1. Synthesis of UC-CPNs. Appropriate amounts of dyes (see UC emission at the edges of the film (Figures 4c and S30). Table S1 in Section 6, for the exact amounts and relative ratio of the These results demonstrate that CPNs are small enough to sensitizer and the emitter) were dissolved in 2 mL of anhydrous DMF minimize the scattering of light, which concentrates at the with final addition of ZrOCl2 (6 mg). The resulting mixture was edges through total internal reflection. stirred and heated at 90 °C for 15 h. The solid nanoparticles were The good colloidal dispersion of our nanoparticles in isolated by centrifugation (14 600 rpm, 15 min). The centrifuged material was washed three times with DMF and two times with water aqueous suspensions was also successfully used to prepare to remove unreacted compounds. water-based security inks. Both VVUC1-CPNs and VVUC2- 4.2. Preparation of UC films. A 0.1 mL Milli-Q water suspension CPNs could be easily patterned, in the presence of the PVA (10 mg/mL) of UC-CPNs obtained after the synthesis and binder, onto the glass substrate through spray-coating and purification via centrifugation was mixed with 1.5 mL of a 10 wt % using a prepatterned mask (with “UC” shape, Figure 4e), PVA 4−88 solution while stirring to improve the dispersion. The preserving their fluorescent properties under UV irradiation mixture was poured into a mold (1.7 × 1.7 × 0.5 cm3), and the water (λexc = 365 nm, Figure 4f) and UC emissions under 532 and was allowed to evaporate. Upon precipitation of the polymer, the film 650 nm light (Figure S31). To prepare an encrypted label, a was formed. glass substrate was first spray-coated all over the surface with a 4.3. Spray Coating of the CPNs onto a Glass Substrate. In total, 4 mg of UC-CPNs were suspended in 0.2 mL of Milli-Q water. suspension made of DPA-S-CPNs or CAEBD-S-CPNs The resulting suspension was then mixed with 2 mL of a 10 wt % PVA (containing only DPA-S-COOH or CAEBD-S-COOH) and 4−88 solution while stirring to improve the dispersion. The mixture PVA, obtaining a uniform printed UV active substrate was sprayed through an adapted spray dryer (Buchi 191 spray dryer) (background) with rough surfaces due to the PVA micro- onto a glass slide kept at a distance of 16 cm from the nozzle and capsules formed upon spraying (Figure S32). On top of this, using a prepatterned mask. 8383 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article ■ * ASSOCIATED CONTENT sı Supporting Information ■ ACKNOWLEDGMENTS This work was supported by grant PID2021-127983OB-C21 The Supporting Information is available free of charge at funded by MCIN/AEI/10.13039/501100011033 and by https://pubs.acs.org/doi/10.1021/acsami.2c16354. ERDF A way of making Europe. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is General characterization methods; synthesis and char- supported by the Severo Ochoa Centres of Excellence acterization of the dyes for the CPNs and intermediates programme, grant SEV-2017-0706 funded by MCIN/AEI/ to obtain them; general procedures for the preparation 10.13039/501100011033. J.Z. thanks the BIST PhD Fellow- of CPNs; films containing the CPNs and the printing of ship Programme. This project has received funding from the the CPNs; and supplementary schemes, tables, and European Union’s Horizon 2020 research and innovation figures (PDF) programme under the Marie Skłodowska-Curie grant agree- Scanning with a UV beam over the VVUC1-CPNs- ment No. 754558. The authors thank Jose Bolanos Cardet for based spray-coated pattern deposited onto the DPA-S- the design of Figure 1. CPNs background. The emission does not change all over the scanning as DPA-S-COOH emission is always activated (MOV) ■ ABBREVIATIONS CPNs, coordination polymer nanoparticles Scanning with the 532 nm CW laser over the VVUC1- UC, upconversion CPNs-based spray-coated pattern deposited onto the UC-CPNs, upconverting CPNs DPA-S-CPNs background. The UC emission only DPA-S-COOH, Pd-S-COOH, CAEBD-S-COOH, and Os- appears when the beam crosses the “UC” pattern S-COOH, 9,10-diphenylanthracene, palladium(II) meso- (MP4) tetraphenylporphyrin complex, [4-((10-(4-carboxyphenyl)- anthracene-9-yl)-ethynyl)-benzoic derivative, and terpyri- Scanning with the 650 nm CW laser over the VVUC2- dine Os complex, respectively, all functionalized with CPNs-based spray-coated pattern deposited onto the carboxylic acid-terminated spacers (S-COOH) CAEBD-S-CPNs background. The UC emission only VVUC1-CPNs and VVUC2-CPNs, CPNs upconverting appears when the beam crosses the “UC” pattern (MP4) from different energies (532 and 650 nm, respectively) of Scanning with the 532 nm Nd:YAG nanosecond pulsed the visible spectral region laser over the VVUC1-CPNs-based spray-coated pattern DPA-S-CPNs, Pd-S-CPNs, and CAEBD-S-CPNs, CPNs deposited onto DPA-S-CPNs. The UC emission only made of DPA-S-COOH, Pd-S-COOH, and CAEBD-S- appears from the “UC” pattern (MP4) COOH, respectively PVA, poly(vinyl alcohol) ■ AUTHOR INFORMATION Corresponding Authors VVUC1-CPNs@PVA and VVUC2-CPNs@PVA, PVA films containing VVUC1-CPNs, VVUC2-CPNs, respec- tively Fernando Novio − Catalan Institute of Nanoscience and CW, continuous wave Nanotechnology (ICN2), Bellaterra 08193 Barcelona, Spain; Departament de Química, Universitat Autònoma de Barcelona (UAB), 08193 Cerdanyola del Vallès, Spain; ■ REFERENCES (1) Bharmoria, P.; Bildirir, H.; Moth-Poulsen, K. Triplet−Triplet orcid.org/0000-0002-1517-3612; Annihilation Based near Infrared to Visible Molecular Photon Email: fernando.novio@uab.cat Upconversion. Chem. Soc. Rev. 2020, 49, 6529−6554. Claudio Roscini − Catalan Institute of Nanoscience and (2) Yanai, N.; Kimizuka, N. Stimuli-Responsive Molecular Photon Nanotechnology (ICN2), Bellaterra 08193 Barcelona, Upconversion. Angew. Chem., Int. Ed. 2020, 59, 10252−10264. Spain; orcid.org/0000-0002-0157-8934; (3) Massaro, G.; Hernando, J.; Ruiz-Molina, D.; Roscini, C.; Latterini, L. Thermally Switchable Molecular Upconversion Emission. Email: claudio.roscini@icn2.cat Chem. Mater. 2016, 28, 738−745. Authors (4) Xu, M.; Zou, X.; Su, Q.; Yuan, W.; Cao, C.; Wang, Q.; Zhu, X.; Feng, W.; Li, F. Ratiometric Nanothermometer in Vivo Based on Junda Zhang − Departament de Química, Universitat Triplet Sensitized Upconversion. Nat. Commun. 2018, 9, No. 2698. Autònoma de Barcelona (UAB), 08193 Cerdanyola del (5) Park, J.; Xu, M.; Li, F.; Zhou, H.-C. 3d Long-Range Triplet Vallès, Spain; Catalan Institute of Nanoscience and Migration in a Water-Stable Metal−Organic Framework for Nanotechnology (ICN2), Bellaterra 08193 Barcelona, Spain Upconversion-Based Ultralow-Power in Vivo Imaging. J. Am. Chem. Daniel Ruiz-Molina − Catalan Institute of Nanoscience and Soc. 2018, 140, 5493−5499. Nanotechnology (ICN2), Bellaterra 08193 Barcelona, (6) Huang, L.; Zhao, Y.; Zhang, H.; Huang, K.; Yang, J.; Han, G. Spain; orcid.org/0000-0002-6844-8421 Expanding Anti-Stokes Shifting in Triplet-Triplet Annihilation Upconversion for in Vivo Anticancer Prodrug Activation. Angew. Complete contact information is available at: Chem., Int. Ed. 2017, 56, 14400−14404. https://pubs.acs.org/10.1021/acsami.2c16354 (7) Frazer, L.; Gallaher, J. K.; Schmidt, T. Optimizing the Efficiency of Solar Photon Upconversion. ACS Energy Lett. 2017, 2, 1346−1354. Author Contributions (8) Richards, B. S.; Hudry, D.; Busko, D.; Turshatov, A.; Howard, I. The manuscript was written through contributions of all A. Photon Upconversion for Photovoltaics and Photocatalysis: A authors. All authors have given approval to the final version of Critical Review: Focus Review. Chem. Rev. 2021, 121, 9165−9195. the manuscript. (9) Fang, J.; Zhou, C.; Chen, Y.; Fang, L.; Wang, W.; Zhu, C.; Ni, Y.; Lu, C. Efficient Photocatalysis of Composite Films Based on Plasmon- Notes Enhanced Triplet−Triplet Annihilation. ACS Appl. Mater. Interfaces The authors declare no competing financial interest. 2020, 12, 717−726. 8384 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article (10) Limberg, D. K.; Kang, J.-H.; Hayward, R. C. Triplet−Triplet (28) Nam, S. K.; Kim, K.; Kang, J.-H.; Moon, J. H. Dual-Sensitized Annihilation Photopolymerization for High-Resolution 3d Printing. J. Upconversion-Assisted, Triple-Band Absorbing Luminescent Solar Am. Chem. Soc. 2022, 144, 5226−5232. Concentrators. Nanoscale 2020, 12, 17265−17271. (11) Gao, C.; Wong, W. W.; Qin, Z.; Lo, S. C.; Namdas, E. B.; (29) Monguzzi, A.; Mauri, M.; Bianchi, A.; Dibbanti, M. K.; Dong, H.; Hu, W. Application of Triplet−Triplet Annihilation Simonutti, R.; Meinardi, F. Solid-State Sensitized Upconversion in Upconversion in Organic Optoelectronic Devices: Advances and Polyacrylate Elastomers. J. Phys. Chem. C 2016, 120, 2609−2614. Perspectives. Adv. Mater. 2021, 33, No. 2100704. (30) Singh-Rachford, T. N.; Lott, J.; Weder, C.; Castellano, F. N. (12) Di, D.; Yang, L.; Richter, J. M.; Meraldi, L.; Altamimi, R. M.; Influence of Temperature on Low-Power Upconversion in Rubbery Alyamani, A. Y.; Credgington, D.; Musselman, K. P.; MacManus- Polymer Blends. J. Am. Chem. Soc. 2009, 131, 12007−12014. Driscoll, J. L.; Friend, R. H. Efficient Triplet Exciton Fusion in (31) Dzebo, D.; Borjesson, K.; Gray, V.; Moth-Poulsen, K.; Molecularly Doped Polymer Light-Emitting Diodes. Adv. Mater. Albinsson, B. Intramolecular Triplet−Triplet Annihilation Upconver- 2017, 29, No. 1605987. sion in 9, 10-Diphenylanthracene Oligomers and Dendrimers. J. Phys. (13) Kim, K.; Nam, S. K.; Cho, J.; Moon, J. H. Photon Chem. C 2016, 120, 23397−23406. Upconversion-Assisted Dual-Band Luminescence Solar Concentrators (32) Lee, S. H.; Ayer, M. A.; Vadrucci, R.; Weder, C.; Simon, Y. C. Light Upconversion by Triplet−Triplet Annihilation in Diphenylan- Coupled with Perovskite Solar Cells for Highly Efficient Semi- thracene-Based Copolymers. Polym. Chem. 2014, 5, 6898−6904. Transparent Photovoltaic Systems. Nanoscale 2020, 12, 12426− (33) Bharmoria, P.; Hisamitsu, S.; Nagatomi, H.; Ogawa, T.; 12431. Morikawa, M.-a.; Yanai, N.; Kimizuka, N. Simple and Versatile (14) Yin, W.; Yu, T.; Chen, J.; Hu, R.; Yang, G.; Zeng, Y.; Li, Y. Platform for Air-Tolerant Photon Upconverting Hydrogels by Thermally Activated Upconversion with Metal-Free Sensitizers Biopolymer−Surfactant−Chromophore Co-Assembly. J. Am. Chem. Enabling Exceptional Anti-Stokes Shift and Anti-Counterfeiting Soc. 2018, 140, 10848−10855. Application. ACS Appl. Mater. Interfaces 2021, 13, 57481−57488. (34) Sasaki, Y.; Oshikawa, M.; Bharmoria, P.; Kouno, H.; Hayashi- (15) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Takagi, A.; Sato, M.; Ajioka, I.; Yanai, N.; Kimizuka, N. Near-Infrared Luminescent Materials: Advances and Applications. Chem. Rev. 2015, Optogenetic Genome Engineering Based on Photon-Upconversion 115, 395−465. Hydrogels. Angew. Chem., Int. Ed. 2019, 58, 17827−17833. (16) Liu, Q.; Wu, B.; Li, M.; Huang, Y.; Li, L. Heterostructures (35) Duan, P.; Yanai, N.; Nagatomi, H.; Kimizuka, N. Photon Made of Upconversion Nanoparticles and Metal−Organic Frame- Upconversion in Supramolecular Gel Matrixes: Spontaneous Accu- works for Biomedical Applications. Adv. Sci. 2022, 9, No. 2103911. mulation of Light-Harvesting Donor−Acceptor Arrays in Nanofibers (17) Liu, C.; Zheng, X.; Dai, T.; Wang, H.; Chen, X.; Chen, B.; Sun, and Acquired Air Stability. J. Am. Chem. Soc. 2015, 137, 1887−1894. T.; Wang, F.; Chu, S.; Rao, J. Reversibly Photoswitching (36) Kashino, T.; Hosoyamada, M.; Haruki, R.; Harada, N.; Yanai, Upconversion Nanoparticles for Super-Sensitive Photoacoustic N.; Kimizuka, N. Bulk Transparent Photon Upconverting Films by Molecular Imaging. Angew. Chem., Int. Ed. 2022, 61, No. e202116802. Dispersing High-Concentration Ionic Emitters in Epoxy Resins. ACS (18) Ansari, A. A.; Parchur, A. K.; Chen, G. Surface Modified Appl. Mater. Interfaces 2021, 13, 13676−13683. Lanthanide Upconversion Nanoparticles for Drug Delivery, Cellular (37) Lee, H.; Lee, M.-S.; Uji, M.; Harada, N.; Park, J.-M.; Lee, J.; Uptake Mechanism, and Current Challenges in Nir-Driven Therapies. Seo, S. E.; Park, C. S.; Kim, J.; Park, S. J.; et al. Nanoencapsulated Coord. Chem. Rev. 2022, 457, No. 214423. Phase-Change Materials: Versatile and Air-Tolerant Platforms for (19) Qin, X.; Xu, J.; Wu, Y.; Liu, X. Energy-Transfer Editing in Triplet−Triplet Annihilation Upconversion. ACS Appl. Mater. Lanthanide-Activated Upconversion Nanocrystals: A Toolbox for Interfaces 2022, 14, 4132−4143. Emerging Applications. ACS Cent. Sci. 2019, 5, 29−42. (38) Kwon, O. S.; Song, H. S.; Conde, Jo.; Kim, H.-i.; Artzi, N.; Kim, (20) Ahmad, W.; Wang, J.; Li, H.; Ouyang, Q.; Wu, W.; Chen, Q. J.-H. Dual-Color Emissive Upconversion Nanocapsules for Differ- Strategies for Combining Triplet−Triplet Annihilation Upconversion ential Cancer Bioimaging in Vivo. ACS Nano 2016, 10, 1512−1521. Sensitizers and Acceptors in a Host Matrix. Coord. Chem. Rev. 2021, (39) Oldenburg, M.; Turshatov, A.; Busko, D.; Wollgarten, S.; 439, No. 213944. Adams, M.; Baroni, N.; Welle, A.; Redel, E.; Wöll, C.; Richards, B. S.; (21) Gray, V.; Moth-Poulsen, K.; Albinsson, B.; Abrahamsson, M. Howard, I. A. Photon Upconversion at Crystalline Organic−Organic Towards Efficient Solid-State Triplet−Triplet Annihilation Based Heterojunctions. Adv. Mater. 2016, 28, 8477−8482. Photon Upconversion: Supramolecular, Macromolecular and Self- (40) Ahmad, S.; Liu, J.; Gong, C.; Zhao, J.; Sun, L. Photon up- Assembled Systems. Coord. Chem. Rev. 2018, 362, 54−71. Conversion Via Epitaxial Surface-Supported Metal−Organic Frame- (22) Baluschev, S.; Katta, K.; Avlasevich, Y.; Landfester, K. work Thin Films with Enhanced Photocurrent. ACS Appl. Energy Annihilation Upconversion in Nanoconfinement: Solving the Oxygen Mater. 2018, 1, 249−253. (41) Hosoyamada, M.; Yanai, N.; Okumura, K.; Uchihashi, T.; Quenching Problem. Mater. Horiz. 2016, 3, 478−486. Kimizuka, N. Translating Mof Chemistry into Supramolecular (23) Dzebo, D.; Moth-Poulsen, K.; Albinsson, B. Robust Triplet− Chemistry: Soluble Coordination Nanofibers Showing Efficient Triplet Annihilation Photon Upconversion by Efficient Oxygen Photon Upconversion. Chem. Commun. 2018, 54, 6828−6831. Scavenging. Photochem. Photobiol. Sci. 2017, 16, 1327−1334. (42) Meinardi, F.; Ballabio, M.; Yanai, N.; Kimizuka, N.; Bianchi, A.; (24) Filatov, M. A.; Baluschev, S.; Landfester, K. Protection of Mauri, M.; Simonutti, R.; Ronchi, A.; Campione, M.; Monguzzi, A. Densely Populated Excited Triplet State Ensembles against Quasi-Thresholdless Photon Upconversion in Metal−Organic Frame- Deactivation by Molecular Oxygen. Chem. Soc. Rev. 2016, 45, work Nanocrystals. Nano Lett. 2019, 19, 2169−2177. 4668−4689. (43) Roy, I.; Goswami, S.; Young, R. M.; Schlesinger, I.; Mian, M. (25) Murakami, Y.; Kamada, K. Kinetics of Photon Upconversion by R.; Enciso, A. E.; Zhang, X.; Hornick, J. E.; Farha, O. K.; Wasielewski, Triplet−Triplet Annihilation: A Comprehensive Tutorial. Phys. Chem. M. R.; et al. Photon Upconversion in a Glowing Metal−Organic Chem. Phys. 2021, 23, 18268−18282. Framework. J. Am. Chem. Soc. 2021, 143, 5053−5059. (26) Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, (44) Medishetty, R.; Zaręba, J. K.; Mayer, D.; Samoć, M.; Fischer, R. F. Upconversion-Induced Fluorescence in Multicomponent Systems: A. Nonlinear Optical Properties, Upconversion and Lasing in Metal− Steady-State Excitation Power Threshold. Phys. Rev. B 2008, 78, Organic Frameworks. Chem. Soc. Rev. 2017, 46, 4976−5004. No. 195112. (45) Suárez-García, S.; Solorzano, R.; Novio, F.; Alibes, R.; Busque, (27) Ha, S.-J.; Kang, J.-H.; Choi, D. H.; Nam, S. K.; Reichmanis, E.; F.; Ruiz-Molina, D. Coordination Polymers Nanoparticles for Moon, J. H. Upconversion-Assisted Dual-Band Luminescent Solar Bioimaging. Coord. Chem. Rev. 2021, 432, No. 213716. Concentrator Coupled for High Power Conversion Efficiency (46) Zhang, J.; Ramu, V.; Zhou, X.-Q.; Frias, C.; Ruiz-Molina, D.; Photovoltaic Systems. ACS Photonics 2018, 5, 3621−3627. Bonnet, S.; Roscini, C.; Novio, F. Photoactivable Ruthenium-Based 8385 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386 ACS Applied Materials & Interfaces www.acsami.org Research Article Coordination Polymer Nanoparticles for Light-Induced Chemo- (64) Otaegui, J. R.; Ruiz-Molina, D.; Latterini, L.; Hernando, J.; therapy. Nanomaterials 2021, 11, 3089. Roscini, C. Thermoresponsive Multicolor-Emissive Materials Based (47) Ye, C.; Ma, J.; Chen, S.; Ge, J.; Yang, W.; Zheng, Q.; Wang, X.; on Solid Lipid Nanoparticles. Mater. Horiz. 2021, 8, 3043−3054. Liang, Z.; Zhou, Y. Eco-Friendly Solid-State Upconversion Hydrogel (65) Julià-López, A.; Hernando, J.; Ruiz-Molina, D.; González- with Thermoresponsive Feature as the Temperature Indicator. J. Phys. Monje, P.; Sedó, J.; Roscini, C. Temperature-Controlled Switchable Chem. C 2017, 121, 20158−20164. Photochromism in Solid Materials. Angew. Chem., Int. Ed. 2016, 55, (48) Morris, J. V.; Mahaney, M. A.; Huber, J. R. Fluorescence 15044−15048. Quantum Yield Determinations. 9, 10-Diphenylanthracene as a (66) Julià-López, A.; Ruiz-Molina, D.; Hernando, J.; Roscini, C. Reference Standard in Different Solvents. J. Phys. Chem. A 1976, Solid Materials with Tunable Reverse Photochromism. ACS Appl. 80, 969−974. Mater. Interfaces 2019, 11, 11884−11892. (49) Edhborg, F.; Olesund, A.; Albinsson, B. Best Practice in (67) Zhang, F.; Zhao, T.; Ruiz-Molina, D.; Liu, Y.; Roscini, C.; Leng, Determining Key Photophysical Parameters in Triplet−Triplet J.; Smoukov, S. K. Shape Memory Polyurethane Microcapsules with Active Deformation. ACS Appl. Mater. Interfaces 2020, 12, 47059− Annihilation Photon Upconversion. Photochem. Photobiol. Sci. 2022, 47064. 21, 1143−1158. (68) Lim, M.; Kwon, H.; Kim, D.; Seo, J.; Han, H.; Khan, S. B. (50) Olesund, A.; Gray, V.; Mårtensson, J.; Albinsson, B. Highly-Enhanced Water Resistant and Oxygen Barrier Properties of Diphenylanthracene Dimers for Triplet−Triplet Annihilation Photon Cross-Linked Poly(Vinyl Alcohol) Hybrid Films for Packaging Upconversion: Mechanistic Insights for Intramolecular Pathways and Applications. Prog. Org. Coat. 2015, 85, 68−75. the Importance of Molecular Geometry. J. Am. Chem. Soc. 2021, 143, (69) Askes, S. H. C.; Bonnet, S. Solving the Oxygen Sensitivity of 5745−5754. Sensitized Photon Upconversion in Life Science Applications. Nat. (51) Liang, C.; Ren, J.; El Hankari, S.; Huo, J. Aqueous Synthesis of Rev. Chem. 2018, 2, 437−452. a Mesoporous Zr-Based Coordination Polymer for Removal of (70) Bharmoria, P.; Edhborg, F.; Bildirir, H.; Sasaki, Y.; Ghasemi, S.; Organic Dyes. ACS Omega 2020, 5, 603−609. Mårtensson, A.; Yanai, N.; Kimizuka, N.; Albinsson, B.; Börjesson, K.; (52) Piscopo, C.; Polyzoidis, A.; Schwarzer, M.; Loebbecke, S. Moth-Poulsen, K. Recyclable Optical Bioplastics Platform for Solid Stability of Uio-66 under Acidic Treatment: Opportunities and State Red Light Harvesting Via Triplet−Triplet Annihilation Photon Limitations for Post-Synthetic Modifications. Microporous Mesoporous Upconversion. J. Mater. Chem. A 2022, 10, 21279−21290. Mater. 2015, 208, 30−35. (53) Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Low Power, Non-Coherent Sensitized Photon up- Conversion: Modelling and Perspectives. Phys. Chem. Chem. Phys. 2012, 14, 4322−4332. (54) Huang, L.; Kakadiaris, E.; Vaneckova, T.; Huang, K.; Vaculovicova, M.; Han, G. Designing Next Generation of Photon Upconversion: Recent Advances in Organic Triplet-Triplet Annihi- lation Upconversion Nanoparticles. Biomaterials 2019, 201, 77−86. (55) Wilhelm, S. Perspectives for Upconverting Nanoparticles. ACS Nano 2017, 11, 10644−10653. (56) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara, T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Reevaluation of Absolute Luminescence Quantum Yields of Standard Solutions Using a Spectrometer with an Integrating Sphere and a Back-Thinned Ccd Detector. Phys. Chem. Chem. Phys. 2009, 11, 9850−9860. (57) Ye, C.; Zhou, L.; Wang, X.; Liang, Z. Photon Upconversion: From Two-Photon Absorption (Tpa) to Triplet−Triplet Annihilation (Tta). Phys. Chem. Chem. Phys. 2016, 18, 10818−10835. (58) Mase, K.; Sasaki, Y.; Sagara, Y.; Tamaoki, N.; Weder, C.; Yanai, N.; Kimizuka, N. Stimuli-Responsive Dual-Color Photon Upconver- sion: A Singlet-to-Triplet Absorption Sensitizer in a Soft Luminescent Cyclophane. Angew. Chem., Int. Ed. 2018, 57, 2806−2810. (59) Amemori, 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, 8702−8705. (60) Yanai, N.; Kimizuka, N. New Triplet Sensitization Routes for Photon Upconversion: Thermally Activated Delayed Fluorescence Molecules, Inorganic Nanocrystals, and Singlet-to-Triplet Absorption. Acc. Chem. Res. 2017, 50, 2487−2495. (61) Joarder, B.; Mallick, A.; Sasaki, Y.; Kinoshita, M.; Haruki, R.; Kawashima, Y.; Yanai, N.; Kimizuka, N. Near-Infrared-to-Visible Photon Upconversion by Introducing an S−T Absorption Sensitizer into a Metal-Organic Framework. ChemNanoMat 2020, 6, 916−919. (62) Gray, V.; Dreos, A.; Erhart, P.; Albinsson, B.; Moth-Poulsen, K.; Abrahamsson, M. Loss Channels in Triplet−Triplet Annihilation Photon Upconversion: Importance of Annihilator Singlet and Triplet Surface Shapes. Phys. Chem. Chem. Phys. 2017, 19, 10931−10939. (63) Torres-Pierna, H.; Ruiz-Molina, D.; Roscini, C. Highly Transparent Photochromic Films with a Tunable and Fast Solution- Like Response. Mater. Horiz. 2020, 7, 2749−2759. 8386 https://doi.org/10.1021/acsami.2c16354 ACS Appl. Mater. Interfaces 2023, 15, 8377−8386