Efficient solid-state photoswitching of methoxyazobenzene in a metal–organic framework for thermal energy storage

Efficient photoswitching in the solid-state remains rare, yet is highly desirable for the design of functional solid materials. In particular, for molecular solar thermal energy storage materials high conversion to the metastable isomer is crucial to achieve high energy density. Herein, we report that 4-methoxyazobenzene (MOAB) can be occluded into the pores of a metal–organic framework Zn2(BDC)2(DABCO), where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane. The occluded MOAB guest molecules show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy stored within the metastable Z-MOAB molecules can be retrieved as heat during thermally-driven relaxation to the ground-state E-isomer. The energy density of the composite is 101 J g−1 and the half-life of the Z-isomer is 6 days when stored in the dark at ambient temperature.


Experimental Details
Synthesis of 1. All reagents were obtained from Fluorochem and used without further purification.
The colourless crystals (yield -81%) were collected via vacuum filtration and washed with DMF (3 x 30 mL) before drying under ambient conditions. Phase purity of the compounds was confirmed by Le Bail fitting of the XRPD patterns.
Loading of 1 with MOAB. Samples of 1 were loaded with MOAB using a previously published meltinfiltration procedure. As-prepared samples of 1 were first heated at 120 °C under vacuum for 24 h to remove DMF solvent molecules. 300 mg of the evacuated material was then mixed with a defined mass of E-MOAB and heated at 120 °C for 3 h. Excess MOAB was removed by heating at 120 °C under vacuum for 20 h.

Quantification of MOAB loading level by UV-vis spectroscopy. UV-vis data was collected on a Cary
60 UV/VIS spectrophotometer with a quartz cell (3 mL) within a 200-600 nm range. A calibration curve with known concentrations of E-MOAB was constructed (see Figures S1 and S2).

Quantification of loading level by acid digestion and 1 H NMR spectroscopy. 10 mg of 1⊃MOAB
was suspended in DCl (1.5 mL) and DMSO-d 6 (1.5 mL) and placed in a stainless-steel autoclave (Parr) with a Teflon lining with a 50 mL capacity. The suspension was heated for 12 h at 100 °C to yield a transparent solution. A Bruker Avance III 400 NMR spectrometer with a 5 mm 1 H-X broadband observe probe was used to collected 1 H NMR data. The 1 H NMR spectrum of the solution was taken and the ratio between DABCO, BDC and E-MOAB was determined from the integration of characteristic 1 H resonances.

PSS determination of irradiated 1⊃MOAB composites.
A 25 mg sample of 1⊃MOAB was suspended in benzene-d 6 (0.5 mL) in an Eppendorf and shaken. The Eppendorf was centrifuged to separate the solid from the solution. A Bruker Avance III 400 NMR spectrometer with a 5 mm 1 H-X broadband observe probe was used to collect 1 H NMR data. The population ratio of E-MOAB and Z-MOAB isomers was determined from integration of Z-isomer and E-isomer resonances in the 1 H NMR spectrum based on literature values. The remaining solid was digested in DCl (1.5 mL) and DMSO-d 6 (1.5 mL) and placed in a stainless-steel autoclave (Parr) with a Teflon lining with a 50 mL capacity.
The suspension was heated for 12 h at 100 °C to yield a transparent solution. The 1 H NMR spectrum of the solution was taken, and no residual Z-MOAB or E-MOAB resonances were detected. The process was repeated three separate times and E/Z ratios were consistent. UV light irradiation procedure. Samples were irradiated with an OmniCure LX5 LED Head with a power of 425 mW and a 12 mm focusing lens. 50 mg of finely ground 1⊃MOAB was spread homogenously over a microscope slide. The powder was spread into a circle with a 1 cm radius which was approximately 0.5 mm thick so that irradiation was approximately uniform. The irradiance at central point was 0.3 W cm -2 . The slide was placed under 365 nm light at a distance of 5 cm. The beam was set to 100% intensity and exposed for a fixed duration. The sample was agitated, at increments of 5 minutes, to allow all particulates to be exposed to the beam.
XRPD. X-ray powder diffraction (XRPD) patterns were measured with a Rigaku SmartLab X-ray diffractometer with a 9 kW rotating anode Cu-source equipped with a high-resolution Vertical θ/θ 4-Circle Goniometer and D/teX-ULTRA 250 High-Speed Position-Sensitive Detector System in reflectance mode. The system was configured with parallel-beam optics and a Ge(220) 2 bounce monochromator on the incident side. Powdered solid samples were prepared on glass slides. The measurements were performed as θ/2θ scans with a step size of 0.01 degrees. Variable-temperature measurements were carried out by loading the powder sample into 1 mm diameter thin-walled (0.1mm) borosilicate capillaries, with the capillary loosely sealed with silicone grease to allow for pressure to be released upon heating. The capillary was loaded on a BTS-500 Anton Parr heating stage equipped with a zero-background holder and mounted on a Rigaku SmartLab (9 kW) diffractometer. The heating stage was purged with nitrogen for 30 minutes prior to analysis. The diffractometer was used with parallelbeams optics and a 5 degree soller slit, and a Dtex-250 Ultra 1D detector. The sample run was carried out using a 2-theta scan with a step size of 0.01 degrees 2-theta, and a scanning rate of 4 degrees per minute.
Solid-state NMR. Solid-state NMR experiments were performed on Bruker Avance III HD spectrometer operating at magnetic field strength of 16.4 T, corresponding to 1 H and 13 C Larmor frequencies of 700 and 176 MHz, respectively. Spectra are referenced relative to tetramethylsilane ( 13 C / 1 H) using the CH 3 ( 1 H = 1.1 ppm; 13 C = 20.5 ppm) resonances of L-alanine as a secondary reference. 13 C NMR spectra were recorded at a magic-angle spinning (MAS) rate of 16.0 kHz using cross polarization (CP) to transfer magnetization from 1 H with a contact time of 3 ms. The CP pulse was ramped linearly from 70 -100% power. 1 H heteronuclear decoupling using two-pulse phase modulation (TPPM) 45 with a pulse length of 4.8 µs and a radiofrequency field strength of 100 kHz was applied during acquisition. Spectra are the sum of 512 transients separated by a recycle interval of 10 s. The sample temperature in variable-temperature experiments was calibrated using Pb(NO 3 ) 2 . 46

DFT Calculation Details
First-principles calculations of NMR parameters were carried out under periodic boundary conditons using the CASTEP code 1 employing the gauge-including projector augmented wave (GIPAW) algorithm, 2 which allows the reconstruction of the all-electron wave function in the presence of a magnetic field. The CASTEP calculations employed the generalised gradient approximation Perdew-Burke-Ernzerhof exchange-correlation functional, 3 and core-valence interactions were described by ultrasoft pseudopotentials. 4 Prior to calculation of the NMR parameters, structures were fully geometry optimised using the G06 semiempirical dispersion correction scheme 5 and allowing all atomic positions to vary. For calculations on guest-free frameworks, the input atomic co-ordinates were taken from structures published by Dybtsev et al., with the guest molecule atoms deleted where appropriate. 6 The tetragonal lp structure was based on the empty framework structure, the orthorhombic np structure was based on the benzeneloaded structure, and the distorted tetragonal np structure was based on the DMF-loaded structure.
These structures also required deletion of some of the atoms representing the dynamic disorder of the DABCO group in order to make a chemically sensible input structure. The structures were then optimized while keeping the unit cell parameters fixed to the experimental values.
Single-molecule calculations were carried out in a 20 × 20 × 20 Å cell with fixed cell parameters to ensure molecules remained isolated from periodic replicas.
Geometry optimisations and NMR calculations were carried out using a planewave energy cut-off of   Figure S2. UV-Vis Calibration curve for E-MOAB in MeOH.                   Fast rotational averaging was accounted for by averaging chemical shifts for carbons on opposite sides of the six-membered rings. This simulates the effect of fast rotation of the six-membered rings around the C-N bond. In terms of the predicted chemical shifts, this averaging is also equivalent to fast pedal motion dynamics of the central N=N linkage -indeed rotation of the rings and pedal motion of the N=N linkage cannot be distinguished by this method. Figure S18. Variable-temperature 13 C CPMAS NMR spectra of 1⊃MOAB which are compared to the calculated 13 C chemical shifts of E-MOAB. Static and dynamic models for the E-MOAB molecule are shown. Figure S19. DSC thermogram of 1⊃MOAB at a rate of 20 °C min -1 .       Experimental energy diffence on first heating branch / J g-1 Calculated energy difference from cis-trans thermal relaxation / J g-1 Figure S28. Relationship between the experimental energy difference on the first heating branch and the calculated energy difference due to Z-E thermal relaxation in 1⊃MOAB. Figure S29. Successive DSC thermograms for (top) pristine 1⊃MOAB (middle) irradiated 1⊃MOAB and (bottom) thermally reconverted 1⊃MOAB. Figure S30. XRPD patterns of irradiated 1⊃MOAB with specific times under 365 nm light. Figure S31. Le Bail fit of irradiated 1⊃MOAB. Two phases were identified and the dominant phase's crystal system was found to be tetragonal. The lattice parameters were refined to be a = b = 10.982 (7) Å, c = 9.619(3) Å,  V = 1169.1(2) Å 3 . The space group was found to be P4/mmm. The second crystal system was found to be orthorhombic. The lattice parameters were refined to be a =13.492(2) Å, b = 17.129(4), c = 9.679(8) Å,  V = 2236.7(3) Å 3 . The space group was found to be Cmmm. Which is consistent with the pre-irradiated phase. General formula Zn 8 C 128 H 120 N 16 O 32 . The reliability (R) factor based on the powder profile Rp was 11.19 %. Fast rotational averaging was accounted for by averaging chemical shifts for carbons on opposite sides of the six-membered rings. This simulates the effect of fast rotation of the six-membered rings around the C-N bond. In terms of the predicted chemical shifts, this averaging is also equivalent to fast pedal motion dynamics of the central N=N linkage -indeed rotation of the rings and pedal motion of the N=N linkage cannot be distinguished by this method. Figure S32. Comparison of the DABCO and methoxy regions of 13 C CPMAS NMR spectra of 1⊃MOAB and irradiated 1⊃MOAB. Figure S33. Comparison of the aromatic regions of 13 C CPMAS NMR spectra of irradiated 1⊃MOAB to calculated chemical shifts for Z-MOAB.