Photocontrolled Energy Storage in Azobispyrazoles with Exceptionally Large Light Penetration Depths

Azobispyrazole, 4pzMe-5pzH, derivatives with small terminal substituents (Me, Et, i-Pr, and n-Pr) are reported to undergo facile reversible photoswitching in condensed phases at room temperature, exhibiting unprecedentedly large effective light penetration depths (1400 μm of UV at 365 nm and 1400 μm of visible light at 530 nm). These small photoswitches exhibit crystal-to-liquid phase transitions upon UV irradiation, which increases the overall energy storage density of this material beyond 300 J/g that is similar to the specific energy of commercial Na-ion batteries. The impact of heteroarene design, the presence of ortho methyl substituents, and the terminal functional groups is explored for both condensed-phase switching and energy storage. The design principles elucidated in this work will help to develop a wide variety of molecular solar thermal energy storage materials that operate in condensed phases.


Synthesis
All reactions were performed in the dark. Reagents and solvents were obtained from commercial sources and used as supplied unless otherwise stated. Reactions were monitored by thin-layer chromatography (TLC) using Merck silica gel 60 F254 plates (0.25 mm). TLC plates were visualized using UV light (254 nm) and/or by using the appropriate TLC stain. Column chromatography was performed on Merck Silica Gel 60 (230-400 mesh) with technical grade solvents used as supplied. 1

Quantum Yield Measurements
Ferrioxalate actinometry was used to determine the photon flux from a Nichia NCSU276A 365 nm LED (365 nm, 800 mW @ 100% power) operating at 5% power, fitted with a collimating lens, following a previously reported procedure. 1

Thin Film Preparation for Powder Charging
Thin-film samples of compounds 1-4 were prepared by adding 3 mg of the E isomer on a clean glass slide ( 2.5 cm × 2.5 cm). Photoisomerization of powder film samples of compounds 1-3 and liquid film samples of compound 4 were performed by the following procedure. The films were directly irradiated with 365 nm LED at 1000 mA until films appeared to completely photoliquefy (compounds 1-3), or for 45 minutes (compound 4). NMR samples were prepared in dark and taken to determine the E-Z photoisomerization content.
Discharging of liquid phase films of compounds 1-4 was performed by irradiating the samples with 530 nm LED for 5 to 15 minutes at room temperature until the color changed from dark red-orange to dark yellow. The samples were then taken for NMR spectra using deuterated acetonitrile to determine the percentage of Z-E conversion.

Thin Film Preparation for Penetration Depth Studies [thickness <450 μm]
E isomers of compound 2 were first heated on a hot plate at 100 °C. The melted samples were pipetted dropwise on a clean glass slide (2.5 cm × 2.5 cm). The thickness of the E-rich droplets were then measured using the Zeta-20 Optical Profilometer. Ten measurements were taken over different portions of the samples, which were then averaged and represent film thickness. The samples were then directly irradiated with 365 nm UV light for 24 hours and the percentage of conversion was determined by 1 H NMR using deuterated acetonitrile as solvent.
The discharge of the Z-rich films was conducted by irradiating the samples with 530 nm light until the PSS of Z-E photoisomerization was achieved. The percentage of conversion was also determined by 1 H NMR using deuterated acetonitrile as solvent.

Pan Preparation for Penetration Depth Studies [thickness >450 μm]
Profilometer was first used to determine the thickness of empty aluminum DSC pans. The solid E isomers of compound 2 were broken apart and packed into the empty DSC pans. The pans were then covered with lids and pressed to create a uniform surface. After the lids were taken off, the DSC pans with uniform sample were taken to the profilometer to determine their thickness. Ten measurements were taken over different portions of the samples, which were then averaged and represent film thickness. The thickness of the sample was calculated by subtracting the value of the filled pan from the empty one. The samples were then directly irradiated with 365 nm UV light for 24 hours and the percentage of conversion was determined by 1 H NMR in deuterated acetonitrile.
The discharge of the Z-rich films was conducted by irradiating the samples with 530 nm light until the PSS of Z-E photoisomerization was achieved. The percentage of conversion was determined by 1 H NMR.

Thin Film Preparation for UV-Vis Absorption Measurements in Condensed Phase
The thin film preparation was performed with 5 mg of crystalline compound 1-3 with each being melted on a clean glass slide (2.5 cm x 2.5 cm) on a hot plate at high temperature above their melting points. 5 mg of the liquid compound 4 was obtained by dissolving 2 mg of the compound in 0.2 mL DCM and drop-casting 0.05 mL of the solution on the hot plate until the solvent was totally evaporated. All the thin films were then covered with another clean glass slide for the compounds spread to the entire glass substrate. The samples were then let to cool to room temperature. The sandwiched films were taken to the UV-Vis spectrophotometer to determine the E isomer absorption spectra in condensed phase.
The samples were then directly irradiated with 365 nm UV light for 72 hours before taken to the spectrophotometer to obtain the absorption spectra of solid-state Z isomer.

Bulk Sample E-Z Isomerization
The bulk sample isomerization was conducted with 115 mg of crystalline compound 2 in a UV quartz cuvette with a path length of 10 mm. The cuvette was placed on a piece of black paper and a stir plate (600 rpm) under 365 nm LED at 1000 mA at room temperature for 62 hours. The ratios of Z isomers were determined by 1 H NMR using deuterated acetonitrile as solvent.
The bulk discharge was performed by irradiating the charged sample with 530 nm LED under identical conditions for 430 minutes. The ratios of converted E isomers in the sample were also determined by 1 H NMR using deuterated acetonitrile as solvent. Digital photos were taken by

Photoliquefaction Experiments of Arylazopyrazole Switches 4pzMe, 5pzH and 4pzH
The photoliquefaction experiments were performed with small quantities (~2 mg) of the crystalline  X-ray powder diffraction in the 2θ range 0-40° (step size, 0.014°; time/step, 20 s; 0.04 rad soller; 40 mA × 60 kV) was collected on a PANalytical Empyrean diffractometer equipped with an GaliPIX3D line detector and in Bragg-Brentano geometry, using Mo-Kα radiation (λ=0.7093187 Å) without a monochromator. Around 5 mg samples were loaded into capillary tubes (outer diameter = 0.7 mm) and the measurements were carried out on the capillary spinner.

Supplementary Note 1. Determination of Thermal Half-Life by the Eyring Equation
Thermal half-life of compounds at T = 298 K was extrapolated by use of the following equation:

Supplementary Note 2. Determination of Effective Light Penetration Depth
The effective light penetration depth was determined using the following equation: δ (effective light penetration depth); PSS1 (max. %Z isomer at the photostationary state in a 121 μm thin film, 100% in our studies); PSS2 (%Z isomer at the photostationary state reached in a test film); l (thickness of the test film). The effective light penetration depth was then calculated at each film that was incompletely switched (i.e. PSS2 < PSS1), then an averaged value was obtained.

Supplementary Note 3. Determination of Thickness of Samples Thicker Than 450 μm
Thickness of samples were determined by the following equation: = − (sample thickness); (aluminum pan with compound); (empty aluminum pan) Empty aluminum pans were initially measured by profilometer to determine their thickness prior to adding compound. Each sample was then measured using the profilometer, and ten measurements were taken over different portions of the samples, which were then averaged and represent film thickness. The thickness of the pan was subtracted from the thickness of the sample measured.

Synthetic Route and Procedures
The azobispyrazoles 1 -4 were prepared following two-or three-step syntheses initiated with a diazonium coupling between aminopyrazole 5 and acetylacetone to afford the common diketone Scheme S1. Synthetic route towards azobispyrazoles 1 -4.

(E)-3,5-dimethyl-4-((1-methyl-1H-pyrazol-5-yl)diazenyl)-1-propyl-1H-pyrazole (4)
To a stirred solution of compound        Table S1.  Table S1.    The fitted parameters were used to obtain a Eyring plot that allowed determination of the thermal half-life at 25 °C to 6.0 days by extrapolation.       80 °C and f) Eyring plot. The absorbance at 351 nm at each temperature was fitted to an exponential fit. The fitted parameters were used to obtain a Eyring plot that allowed determination of the thermal half-life at 25 °C to 6.7 days by extrapolation.      Fig. S26. Plots of the static penetration depth of compounds 1, 3, and 4 for their E and Z isomeric forms. The static penetration depth was calculated using the procedure reported by Grossman and co-workers. 7 The UV-vis absorption data used in the calculation was smoothed using the Savitzky-Golay method (points window 10, polynomial order 2).          The O20-and O30-based included water molecules in the structure of 1 were both found to be involved in O-H···N hydrogen bonding, to N6 and N13 respectively. The hydrogen atoms involved in these interactions were located from ΔF maps and refined freely subject to O-H distance constraints of 0.90 Å. Additionally these two water molecules are also linked by O-H···O hydrogen bonds to each other across two independent centers of symmetry to form an extended ···O30···O20···O20···O30···O30···O20··· chain along the crystallographic c axis direction. As a consequence of the two independent centers of symmetry, the positions of the hydrogen atoms along this chain are inherently disordered, and four sites were located from ΔF maps and refined freely at 50% occupancy subject to O-H distance constraints of 0.90 Å.