Control of structural flexibility of layered-pillared metal-organic frameworks anchored at surfaces

Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.


Supplementary Methods
Materials. All reagents and solvents were purchased from commercial suppliers (Sigma Aldrich, TCI and Alfar Aesar) and used without further purification.
Cu(NO3)23H2O and Cu(OAc)2H2O were used as metal precursors for powder synthesis and thin-film fabrication of the Cu-based layered-pillared metal-organic frameworks (MOFs), respectively. The alkoxy-functionalised linker, i.e. 2,5-diethoxy-1,4-benzenedicarboxylic acid (H2DE-bdc) and 2,5-bis(2-methoxyethoxy)-1,4benzenedicarboxylic acid (H2BME-bdc), were prepared via Williamson ether synthesis from dimethyl 2,5-dihydroxy-1,4-benzenedicarboxylate according to published procedure. 1  Bulk powder MOF synthesis. Cu2(DE-bdc)2(dabco) powder (1bulk) was prepared under solvothermal reaction condition, which was slightly modified from the previously reported procedure for the Zn 2+ containing analogue. 1 Cu(NO3)23H2O (241.6 mg, 1 mmol), H2DE-bdc (254.2 mg, 1mmol) and dabco (56.0 mg, 0.5 mmol) were suspended in DMF (15 ml) and sonicated until the precursors were fully dissolved. The solution was left at room temperature for 20 min and then a precipitate formed. This precipitate was removed by filtration prior to transferring the clear filtrate into a screw jar (25 ml), which was sealed and subsequently heated to 120 °C for 48 h. After cooling down to room temperature, the mother liquor was firstly decanted and then exchanged by fresh DMF, the mixture was stirred for 30 min and left to settle for 24 h. Afterwards, the DMF was exchanged by CHCl3, the mixture was stirred for 30 min and left to settle for 24 h.
The solvent exchange procedure with CHCl3 was repeated for 2 times prior to collecting 1bulk by filtration with a frit. The collected product was washed 3 more times with 10 ml CHCl3. 1bulk was subsequently transferred to a Schlenk tube and activated overnight at 130 °C. Cu2(BME-bdc)2(dabco) powder (2bulk) was prepared using the same synthetic procedure as for preparing 1bulk, instead of H2DE-bdc, H2BME-bdc (314.3 mg, 1mmol) was used as a linker.
Characterisations of MOF powders. Crystallinity and phase purity of the compounds 1bulk and 2bulk was identified by powder X-ray diffraction (XRD, D8 Bruker-AXS advance instrument, flat mode, Debye-Scherrer geometry, slit width of 0.05°, Cu K radiation, 2 range from 5° to 50°, position sensitive detector, Ni filter and step size of 0.0141°). Infrared (IR) spectra were recorded on a Bruker Alpha-P FT-IR (ATR-Mode, 48 scans) located in a glovebox. Thermogravimetric analysis (TGs) were recorded on a Netzsch STA 409 PC TG-DSC apparatus (heating rate of 5 K min -1 , in a stream of N2 gas with constant flow rate of 20 ml min -1 ). Roughly 10 mg of samples were placed in a pre-weighted, clean aluminium oxide crucible. The TG curves were backgroundcorrected by subtraction of a measurement conducted with an empty crucible under the same conditions. MOF thin-film fabrication. Au-coated quartz crystal microbalance sensors (QCM, commercially available by Q-Sense, AT cut type, Au electrode, diameter 14 mm, thickness 0.3 mm and fundamental frequency ca. 4.95 MHz) were used as the substrates. Prior to the fabrication of MOF thin-films, the substrates were immersed in 20 M solution of 16-mercaptohexadecanoic acid (MHDA) in ethanol mixed with 5% v/v acetic acid for 24 h to generate the COOH terminated substrate. In order to generate the pyridyl terminated surface, 20 M solution of 4-(4pyridyl)phenylmethylthiol (PMBT) in ethanol was used instead of MHDA. These substrates were subsequently used for the fabrication of MOF films. Thin-films of 1 and 2 (coined the terms as 1tfx and 2tfx; x = total deposition cycles, respectively) were fabricated by a stepwise liquid-phase epitaxial process (LPE, Supplementary Figure   2) at controlled temperature of 40 °C using an automated QCM instrument (Q-Sense E4 Auto) operated in the continuous flow mode with a constant flow rate of 100 L min -1 for a total of 40, 60, 80 and 120 deposition cycles (1tf40, 1tf60, 1tf80 and 1tf120, respectively). In each deposition cycle, the functionalized QCM substrate was alternatingly exposed to the precursor solutions as follows: Cu(OAc)2H2O (0.5 mM in ethanol) 10 min, ethanol (rinse) 5 min, the mixed organic linkers (H2DE-bdc + dabco for 1tfx and H2BME-bdc + dabco for 2tfx, 0.2 mM in ethanol) 10 min and finally ethanol 5 min. Note that, the QCM frequency change was monitored in-situ during the fabrication process. In addition, 1tf was fabricated on the -pyridyl terminated QCM substrate by LPE process for 60 cycles (named as 1tf60-Py) in order to study the influence of crystallite orientation on the structural flexibility of the MOF thin-films. For comparison, the prototypic layered-pillared Cu2(bdc)2(dabco) (3) thin-film was prepared by LPE for 60 cycles using commercially available 1,4-benzenedicarboxylic acid (H2bdc) as the carboxylate linker (named as 3tf60). The adsorption amount at each P/P0 (M) on the MOF thin-films (M0) was derived as: Adsorption amount (g/g): M0 : initial weight of the MOF film sample, F : measuring frequency at each relative vapour pressure and Fs : frequency after final activation of the MOF film sample.
In-situ synchrotron X-ray diffraction during methanol adsorption. Crystalline phase and structural flexibility during methanol adsorption of the MOF bulk powders and thin-films were identified by grazing incidence XRD (GIXRD, Beamline BL9 of the DELTA synchrotron radiation source, Dortmund, Germany). 3  -

Supplementary Figures
Supplementary Figure 1 1bulk, lp phase appears as a minor phase in 1bulk activated form, which may occur due to the incomplete activation process of the MOF powder sample. Supplementary Figure 18 Film thickness of (a) 1tf40, (b) 1tf60, (c) 1tf80, and (d) 1tf120 measured from the cross-sectional SEM images. The thin films were fabricated by LPE process on the -COOH functionalised QCM substrates.

Supplementary Figure 19Methanol adsorption isotherms at 25 °C of (a) 1tf40, (b) 1tf60, (c) 1tf80, and (d) 1tf120 measured by environmentally-controlled QCM.
Filled and empty symbols depict adsorption and desorption. 1tf40 exhibits a single-step methanol adsorption, whereas 1tf60, 1tf80 and 1tf120 show two-steps methanol adsorption. A significant change in the slope of the methanol adsorption isotherm at P/P0 of 0.15 indicates the starting point of the framework transformation from np to lp.
In correspondence with the in-situ GIXRD profiles during the methanol adsorption process, these methanol adsorption data reveal the dependency of the structural flexibility of 1tfx on the total number of LPE fabrication cycles or in other word, the MOF crystallite dimension anchored at the substrate surface.
Supplementary Figure 20 Out-of-plane and in-plane GIXRD profiles measured during methanol sorption at 25 °C of 1tf60-Py. 1tf60-Py was fabricated by LPE process on the -pyridyl functionalised QCM substrates for 60 cycles. According to the out-ofplane GIXRD patterns, the as-synthesised (or MeOH-solvated) 1tf60-Py exhibits a preferred growth along the dabco-related orientation of the framework, which can be assigned to the (001)-orientation by referring to the previously-reported Zn-based analogous structure. 1