Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host–Guest Complexes

A series of arylazopyrazole-loaded metal–organic frameworks were synthesized with the general formula Zn2(BDC)2(DABCO)(AAP)x (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo-[2.2.2]octane; AAP = arylazopyrazole guest). The empty framework adopts a large pore tetragonal structure. Upon occlusion of the E-AAP guests, the frameworks contract to form narrow pore tetragonal structures. The extent of framework contraction is dependent on guest shapes and pendant groups and ranges between 1.5 and 5.8%. When irradiated with 365 nm light, the framework expands due to the photoisomerization of E-AAP to Z-AAP. The proportion of Z-isomer at the photostationary state varies between 19 and 57% for the AAP guests studied and appears to be limited by the framework which inhibits further isomerization once fully expanded. Interestingly, confinement within the framework significantly extends the thermal half-life of the Z-AAP isomers to a maximum of approximately 56 years. This finding provides scope for the design of photoresponsive host–guest complexes with high stability of the metastable isomer for long-duration information or energy storage applications.


DFT Calculation Details
First-principles calculations of NMR parameters were carried out under periodic boundary conditions 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. 4Single-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 60 Ry, and for crystal structures, a k-point spacing of 0.05 2π Å −1 was used.For single-molecule calculations, a single k-point at the fractional coordinate (0.25, 0.25, 0.25) was used.The calculations generate the absolute shielding tensor (σ) in the crystal frame.Diagonalisation of the symmetric part of σ yields the three principal components, σ XX , σ YY , and σ ZZ .The isotropic shielding, σ iso , is given by (1/3) Tr [σ].The isotropic chemical shift, δ iso , is given by σ ref − σ iso , where σ ref is a reference shielding.Reference shieldings were determined by comparison of experimental chemical shifts for L-alanine with shieldings obtained from a calculation on a fully optimised crystal structure 7 (Cambridge Structural Database code LALNIN22).For 1 H and 13 C, reference shieldings were determined from the y intercept of a linear fit to the experimental shifts versus calculated shielding, with the gradient of the fit fixed to −1.Calculated shieldings for the three methyl protons were averaged to account for rapid rotation of the methyl group.Respective reference shieldings of 30.2 and 168.4 ppm were obtained for 1 H and 13 C. Calculated chemical shifts for the individual carbons in DABCO groups were averaged to account for the fast rotational dynamics of this group.Single molecule calculations for each guest molecule isomer, confined in a 20 x 20 x 20 Angstrom box, were calculated to determine the expected resonance.These are shown below and labelled 'δ iso and ''δ iso .To simulate the effects of fast rotational dynamics of the rings around the C-N bonds, chemical shifts were averaged for each carbon site across the two conformations, δ iso av .
'δiso ''δiso 'δiso ''δiso  where N Z (0) is the number of guest molecules in the Z isomeric state at the beginning of the measurement and  is a decay constant.This rearranges to the following form whereby plotting ln(N Z (t)/N Z (0)) vs t allows determination of  from the gradient of the straight line of best fit.

Figure S2 .
Figure S2.Optical microscopy photographs of A) 1⊃AP, B) 1⊃F-AP, C) 1⊃MOAP, and D) 1⊃F-MOAP; the black scale bar near the centres of the images represents 1 mm with individual graduations at 0.01 mm.

Figure S3 .
Figure S3.XRPD profiles for synthesised 1⊃AP, 1⊃F-AP, 1⊃MOAP, and 1⊃F-MOAP.Through the occlusion of guest molecules, the framework structure remains stable in air and no changes are seen in the XRPD pattern over a period of 1 week.

Figure S4a )
FigureS4a) Le Bail fit of guest-free 1. Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.98 Å, c = 9.654 Å,   ˚, V = 1162.6Å 3 .The space group was found to be P4/mmm.General formula Zn 2 C 18 H 16 N 4 O 8 .The reliability (R) factor based on the powder profile Rp was 7.51%.y axis (counts), x axis (2theta).

Figure S4f .
Figure S4f.XRPD profiles for synthesised 1⊃AP, 1⊃F-AP, 1⊃MOAP, and 1⊃F-MOAP.Through the occlusion of guest molecules, the framework structure remains stable in air and no changes are seen in the XRPD pattern over a period of 1 week.

Figure S4g )
Figure S4g) Le Bail fit of 1⊃AP at 180 o C. Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.96Å, c = 9.69 Å,   ˚, V = 1166.9Å 3 .The space group was found to be P4/mmm.General formula Zn 8 C 122 H 114 N 36 O 32 .The reliability (R) factor based on the powder profile Rp was 8.45%.y axis (counts), x axis (2theta).

Figure S6 .
Figure S6. 13C CPMAS NMR spectra of 1⊃AAPs at 210 K. Framework resonances are indicated by blue dots.Guest resonances which show a shifting or broadening of their resonances from 310 K to 210 K are highlighted in orange.

Figure S7. 1 H
Figure S7. 1 H NMR spectra of guest molecules in deuterated benzene.E-isomers (left) and Z-isomer (right).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.

Figure S8a )
Figure S8a) Le Bail fit of irradiated 1⊃AP.Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.94Å, c = 9.66 Å,   ˚, V = 1157.1 Å 3 .The space group was found to be P4/mmm.General formula Zn 8 C 122 H 114 N 36 O 32 .The reliability (R) factor based on the powder profile Rp was 5.39%.y axis (counts), x axis (2theta).

Figure S8b )
Figure S8b) Le Bail fit of irradiated 1⊃F-AP.Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.95Å, c = 9.66 Å,   ˚, V = 1157.6Å 3 .The space group was found to be P4/mmm.General formula Zn 8 C 122 H 104 N 36 O 32 F 10 .The reliability (R) factor based on the powder profile Rp was 3.60%.y axis (counts), x axis (2theta).

Figure S8c )
Figure S8c) Le Bail fit of irradiated 1⊃MOAP.Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.96Å, c = 9.67 Å,   ˚, V = 1161.2Å 3 .The space group was found to be P4/mmm.General formula Zn 8 C 116 H 112 N 32 O 36 .The reliability (R) factor based on the powder profile Rp was 4.12%.y axis (counts), x axis (2theta).

Figure S8d )
Figure S8d) Le Bail fit of irradiated 1⊃F-MOAP.Indexing was carried out by N-TREOR09 on EXPO2014.The crystal system was found to be tetragonal.The lattice parameters were refined to be a = b = 10.95Å, c = 9.67 Å,   ˚, V = 1156.9Å 3 .The space group was

Table S2a .
DFT calculated chemical shifts for E-AAPs.

Table S2c .
DFT calculated chemical shifts for E-F-AP.

Table S3 .
The space group was found to be P4/mmm.General formula Zn 8 C 116 H 104 N 32 O 36 F 8 .. The reliability (R) factor based on the powder profile Rp was 6.69%.y axis (counts), x axis (2theta).Crystallographic Details for irradiated 1⊃AAPs.

Table S4d .
DFT calculated energy values for geometry optimised single molecules of F-MOAP and calculated energy difference between E and

Table S5 .
Predicted energy differences on the heating branch of irradiated samples due to Z -E thermal relaxation v calculated values.

Table S6 .
BET surface areas for samples studied by N 2 gas sorption

Calculation of 13 C chemical shifts for guest molecules
Single molecule calculations for each guest molecule isomer, confined in a 20 x 20 x 20 Angstrom box, were calculated to determine the expected resonance.These are shown below and labelled 'δ iso and ''δ iso .The equivalent number carbons were averaged to determine the chemical shift for a dynamic guest molecule undergoing ring flipping within the MOF pores, δ iso .

Table S7a .
DFT calculated chemical shifts for Z-AAPs.

Table S7c .
DFT calculated chemical shifts for Z-F-AP.

Table S8a .
Change in the Z-isomer population in irradiated 1⊃AP over 70 days.

Table S8b .
Change in the Z-isomer population in irradiated 1⊃F-AP over 70 days.

Table S8c .
Change in the Z-isomer population in irradiated 1⊃MOAP over 70 days.

Table S8d .
Change in the Z-isomer population in irradiated 1⊃F-MOAP over 70 days.Assuming the metastable cis isomer undergoes thermal reconversion to the ground-state E isomer according to first-order kinetics, the number of guest molecules in the Z isomeric state as a function of time (N cis (t)) can be expressed by   () =   (0) -