Organogels for Low-Power Light Upconversion

We herein report new organogels that permit efficient optical upconversion (UC) by triplet–triplet annihilation. The materials studied consist of a liquid organic phase, composed of a mixture of N,N-dimethylformamide and dimethyl sulfoxide in which the UC chromophore pair Pd(II) mesoporphyrin IX and 9,10-diphenylanthracene was dissolved, and a three-dimensional polymer network formed by covalently cross-linking poly(vinyl alcohol) with hexamethylene diisocyanate. The new gels are highly transparent, shape-persistent, and display efficient green-to-blue upconversion with UC quantum yields of >0.6 and 14% under ambient and oxygen-free conditions, respectively. The design approach presented here permits the fabrication of a hitherto unexplored class of materials with a unique combination of properties. The framework can easily be extended to other materials based on other solvents, polymer networks, and/or chromophore pairs.

Upconverting toluene solutions were prepared by first dissolving PdOEP (0.64 mg) in toluene (10 mL) to prepare a stock solution with a chromophore concentration of 10 -4 M. In a 10 mL measuring flask, the stock solution (2 mL) was diluted with additional toluene (8 mL) to adjust the chromophore concentration to 2•10 -5 M. DPA (20 mg) was then dissolved in the latter PdOEP solution (6 mL) by stirring the mixture for at least 10 min at 50 °C to prepare a solution with a chromophore concentration of 2•10 -5 M PdOEP and 10 -2 M DPA.
All solutions were prepared under ambient conditions and either directly measured (ambient conditions) or purged with argon for 30 min (air-free conditions, solvent evaporation neglected due to the low volatility of the solvents).

2.2.Standard solutions for quantum yield measurements
A Rhodamine 101 standard solution 1 (10 -6 M) was produced by dissolving rhodamine 101 (4.96 mg) in ethanol (10 mL, 10 -3 M) and subsequently diluting three times 1 mL of this solution with 9 mL of solvent.Absorption and emission spectra were taken under ambient conditions due negligible O 2 -quenching (Fig. S2). 1 An upconverting toluene standard solution 2 was prepared by first dissolving PdOEP (0.64 mg) in toluene (10 mL) to prepare a stock solution with a chromophore concentration of 10 -4 M. The stock solution (0.5 mL) was then diluted with additional toluene (9.5 mL) to adjust the chromophore concentration to 5•10 -6 M. DPA (33 mg) was dissolved in the latter PdOEP solution (10 mL) by stirring the mixture for at least 10 min at 50 °C to prepare a solution with chromophore concentrations of 5•10 -6 M PdOEP and 10 -2 M DPA.This solution was transferred to a custom-made cuvette connected to a round-bottom flask, vacuumdegassed by three freeze-pump-thaw cycles and finally upconversion emission and absorption spectra were measured (Fig. S2).

Reference gels without chromophores prepared under ambient conditions
Solution I was prepared as follows: In a 250 mL round-bottom flask, PVOH (30 g) was dissolved in DMSO (170 g) at 70 °C under constant agitation for 2 h in order to obtain a 15 wt% solution (solution I).This solution was kept over Drierite™ in a desiccator.
Solution II was prepared as follows: In a 25 mL vial, HMDI (57 mg) was dissolved in DMF (4 g) by shaking gently.
To prepare the reference gels, solution I (2 g) was mixed with the freshly prepared solution II (4 g), the viscous mixture was rapidly stirred with a glass rod for 30 s, and filled either into a spectroscopy cuvette (optical glass) for quantitative measurements or a poly(tetrafluoroethylene) mould (25 x 25 x 15 mm) for preparation of self-standing gels.Gel formation was observed after allowing the mixture to stand for ca. 10 min at room temperature.Quantitative measurements were performed at least 1.5 h after gelation.

Chromophore-containing gels prepared under ambient conditions
Solution IIIa was prepared as follows: PdMesoIX (0.67 mg) was dissolved in DMF (10 mL) to prepare a stock solution with a chromophore concentration of 10 -4 M. In a 10 mL measuring flask, the stock solution (2.9 mL) was diluted with additional DMF (7.1 mL) to adjust the chromophore concentration to 2.9•10 -5 M (solution IIIa).
Solution IIIb was prepared as follows: DPA (20 mg) was dissolved in DMF (4 g) by stirring the mixture for at least 10 min at 50 °C to prepare a solution with a chromophore concentration of 1.4•10 -2 M. In order to measure undistorted absorption and direct emission spectra of DPA embedded in gels (Fig. 3), a 500-time diluted solution IIIb was used for the preparation of this specific gel.
Solution IIIc was prepared as follows: DPA (20 mg) was dissolved in a portion of solution IIIa (4 g) by stirring the mixture for at least 10 min at 50 °C.
To prepare the chromophore-containing gels, freshly prepared solutions IIIa-c (4 g) were each used to dissolve HMDI (57 mg) and the resulting solutions were each mixed with a portion of solution I (2 g each), the viscous mixtures were rapidly stirred with a glass rod for 30 s, and filled either into a spectroscopy cuvette (optical glass) for quantitative measurements or a poly(tetrafluoroethylene) mould (25 x 25 x 15 mm) for the preparation of self-standing gels .Gel formation was observed after allowing the mixture to stand for ca. 10 min at room temperature.Quantitative measurements were performed at least 1.5 h after gelation.

Chromophore-containing gels prepared under air-free conditions
Solution IV was prepared as follows: A flame-dried 25 mL Schlenk flask was charged with PVOH (3 g), which was subsequently dissolved in dry DMSO (20 g) at 70°C under constant agitation for 2 h in order to obtain a 15 wt% solution.The resulting solution was then transferred into a nitrogen glove-box for further use.
Solution V was prepared as follows: In a flame-dried 20 mL Schlenk flask, PdMesoIX (1.34 mg) was dissolved in anhydrous DMF (20 mL) to prepare a stock solution with a chromophore concentration of 10 -4 M. A portion of this solution (14.5 mL) was subsequently transferred to a flame-dried 100 mL Schlenk flask using a syringe and the solution was diluted with additional DMF (35.5 mL) to adjust the chromophore concentration to 2.9•10 -5 M. DPA (236 mg) and HMDI (677.32 mg, 0.65 mL, ρ 20°C = 1.05 g cm -3 ) were subsequently added and the mixture was heated to 50 °C for 10 min in order to dissolve the DPA.The resulting solution was then transferred into a nitrogen glove-box for further use,.
In the glove-box, gels were prepared by combining portions of solution IV (2 g) and solution V (4 g), and quickly stirring the mixture with a glass rod for 30 s.The viscous mixture was then filled into a fluorescence cuvette (optical glass), which was capped and additionally sealed with Parafilm M®, and allowed to gel for at least 1.5 h in the glove-box prior to removal from the box and subsequent measurement.
The final concentrations of all the components in air-free gels are given in Table S1, taking into account the mixing of solutions IV and V.

Chromophore-containing gels prepared under air-free conditions with varying polymer and crosslinker-contents
Solution VI was prepared as follows: The procedure for Solution V was followed, omitting the addition of HMDI.
In the glove-box, gels containing 2.5 wt% PVOH with 10 mol% crosslinked hydroxyl-groups were prepared as follows: A portion of solution IV (1 g) was diluted with air-free dry DMSO (1 g).To portion of solution VI (4 g), dry and air-free HMDI (29 mg) was added.Both solutions were then combined and the resulting mixture quickly stirred with a glass rod for 30 s.The viscous mixture was then filled into a fluorescence cuvette (optical glass), which was capped and additionally sealed with Parafilm M®, and allowed to gel for at least 1.5 h in the glove-box prior to removal from the box and subsequent measurement.
In the glove-box, gels containing 5 wt% PVOH with 2.5, 5 or 10 mol% crosslinked hydroxylgroups were prepared as follows: To portion of solution VI (4 g), dry and air-free HMDI (14, 29 or 57 mg) was added.The resulting mixture was then combined with portion of solution IV (2 g) and quickly stirred with a glass rod for 30 s.The viscous mixture was then filled into a fluorescence cuvette (optical glass), which was capped and additionally sealed with Parafilm M®, and allowed to gel for at least 1.5 h in the glove-box prior to removal from the box and subsequent measurement.nm, FWHM = 10 ± 2 nm).Power densities were varied using reflective power density filters (Thorlabs) and measured with an optical power meter (Thorlabs PM100USB with photodiode power sensor S120VC).All samples were measured in 10 x 10 mm quartz (liquid) or optical glass (gels) cells.Vacuum-degassed solutions were prepared and measured in a custom-made quartz glass cuvette bearing a side arm round bottom flask for degassing.Emission spectra were corrected for the spectral response of the detector using the manufacturer's correction files.UV-Vis absorption spectra were recorded on a Shimadzu UV-2401PC using 10 mm path quartz (liquid) or optical glass (gels) cells.
The upconverted quantum yield of the PdMesoIX/DPA solution was first determined using equation (E1) 1  Based on the absorption (Fig. S2a) and emission spectra (Fig. S2b) shown, a quantum yield The PdMesoIX/DPA solution quantum yield Φ PdMesoIX/DPA was also determined by comparison with the standard rhodamine 101 (R101) by using equation (E2) 2 .Here, I represents the integrated upconverted emission intensity from λ = 400-535 nm (I unk ) or the integrated R101 emission intensity from λ = 545-750 nm (I std ); η ethanol = 1.362 was used.

Fig. S1 .
Fig. S1.FTIR spectra monitoring the gelation of a 2:1 w/w solution of DMF/DMSO and using the UC-standard PdOEP/DPA in toluene as reference.Φ, A, I and η represent the quantum yield, the integrated absorbance from λ = 533-543 nm (λ ex = 543 nm), the integrated upconverted emission intensity from λ = 400-535 nm and the refractive index, respectively.The subscripts std and unk refer to the PdOEP/DPA standard and the PdMesoIX/DPA unknown, respectively.The refractive indices used were η toluene = 1.496, η DMF = 1.430 and η DMSO = 1.479 as per the Sigma-Aldrich product page.η DMF/DMSO = 1.446 for 2:1 w/w DMF/DMSO solutions was calculated by linear combination of η DMF and η DMSO .

Table S1 .
Summary of components and concentrations in UC-gels.