Photochromic metal–organic frameworks for inkless and erasable printing

A media for inkless and erasable printing has been developed using photochromic MOFs. Different coloured printing has been achieved by varying the structure of the MOF. The resultant printing has a good resolution and stability, is capable of being read both by human eyes and smart electronic devices and the paper can be reused for several cycles without any significant loss in intensity.


S2
FT-IR spectroscopy of BINDI linker and derived MOFs S14 S6 TG analyses of the materials S18 S7 UV-vis spectroscopic measurements S19 S8 Electron Paramagnetic Resonance (EPR) studies S22 S9 Photochromic property of BINDI S26 S10 SEM micrograph of Mg-NDI coated paper S27 S11 Resolution test of printing with Mg-NDI coated paper S28 S12 Detection of 1D Barcode S29 S13 Stability of the Mg-NDI coated paper towards mechanical deformation S30 S14 Preparation of stencils S31 S15 References S32 S3 S1: Materials and methods All reagents were commercially available and used as received without any further purification. Single Crystal X-Ray Diffraction data were collected on a Super Nova Dual source Xray Diffractometer system (Agilent Technologies) equipped with a CCD area detector. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Smartlab diffractometer for Cu K  radiation ( = 1.5406 Å), with a scan speed of 2° min -1 and a step size of 0.02° in 2.
Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Optics ALPHA-E spectrometer with a universal Zn-Se ATR (attenuated total reflection) accessory in the 600-

S3: Single crystal XRD and crystal structure of Sr-NDI
As synthesized crystal of Sr-NDI was placed inside a glass capillary (Hampton research) and then mounted in the diffractometer. The data collection was done at 200 K. The crystals were mounted on a Super Nova Dual source X-ray Diffractometer system (Agilent Technologies) equipped with a CCD area detector and operated at 250 W power (50 kV, 0.8 mA) to generate Mo Kα radiation (λ = 0.71073 Å) and Cu Kα radiation (λ = 1.54178 Å) at 298(2) K. Initial scans of each specimen were performed to obtain preliminary unit cell parameters and to assess the mosaicity (breadth of spots between frames) of the crystal to select the required frame width for data collection. CrysAlis Pro program software was used suite to carry out overlapping φ and ω scans at detector (2θ) settings (2θ = 28). Following data collection, reflections were sampled from all regions of the Ewald sphere to re-determine unit cell parameters for data integration. In no data collection was evidence for crystal decay encountered.
Following exhaustive review of collected frames the resolution of the data set was judged.
Data were integrated using CrysAlis Pro software with a narrow frame algorithm. Data were subsequently corrected for absorption by the program SCALE3 ABSPACK 2 scaling algorithm.
These structures were solved by direct method and refined using the SHELXTL 97 3 software suite. Atoms were located from iterative examination of difference F-maps following least squares refinements of the earlier models. Final model was refined anisotropically (if the number of data permitted) until full convergence was achieved. Hydrogen atoms were placed in calculated positions (C-H = 0.93 Å) and included as riding atoms with isotropic displacement parameters 1.2-1.5 times Ueq of the attached C atoms. In some cases modeling of electron density within the voids of the frameworks did not lead to identification of recognizable solvent molecules in these structures, probably due to the highly disordered contents of the S6 large pores in the frameworks. Highly porous crystals that contain solvent-filled pores often yield raw data where observed strong (high intensity) scattering becomes limited to ~1.0 Å at best, with higher resolution data present at low intensity. A common strategy for improving X-ray data, increasing the exposure time of the crystal to X-rays, did not improve the quality of the high angle data in this case, as the intensity from low angle data saturated the detector and minimal improvement in the high angle data was achieved. Additionally, diffused scattering from the highly disordered solvent within the void spaces of the framework and from the capillary to mount the crystal contributes to the background and the 'washing out' of the weaker data. Electron density within void spaces has not been assigned to any guest entity but has been modeled as isolated oxygen and/or carbon atoms. The foremost errors in all the models are thought to lie in the assignment of guest electron density. The structure was examined using the ADSYM subroutine of PLATON 4 to assure that no additional symmetry could be applied to the models. The ellipsoids in ORTEP diagrams are displayed at the 50% probability level unless noted otherwise.
It is noteworthy that despite of our several attempts to model the disorder the solvent molecules present inside the MOF pores, we couldn't able to completely assign them as chemical entities. This has lead to the generation of some A and B level errors in the IUCr Checkcif report. This modeling has also caused some checks for C-H bonds which have appeared as A level error and couldn't be removed by refinement. This model has also developed a minute shift in some of the peaks of simulated PXRD pattern; although the experimental PXRD pattern holds good all the cases, proving our hypothesis. Despite of this weak modeling of the non co-ordinated solvent molecules, the provided structure is good enough to describe the structure of the framework.
CCDC 1412539 contains the crystallographic data for Sr-NDI MOF.  Figure S2: ORTEP drawing of Sr-NDI. Thermal ellipsoids set to 50% probability level. Figure S3: Construction of pore walls of Mg-NDI from naphthalenediimide moieties showing their parallel stacking (distance between two parallel oriented NDI moieties is 7.1 Å). S10 Figure S4: Parallel stacking of NDI moieties in BINDI ligand (distance between two parallel oriented NDI moieties is 2.6 Å). S11 Figure S5: Crystal Structure of Sr-NDI showing orthogonal orientation of NDI moieties from interpenetrated form (distance between two orthogonally oriented NDI moieties is 2.4 Å).

S4: PXRD patterns of the materials
As synthesized crystals of the MOF materials were taken out from the mother solution and washed with dry DMF and pure ethanol and then dried in vacuum. The dried materials were then placed in the quartz PXRD plates and kept in the powder diffractometer to record their XRD pattern as a tool of checking the bulk phase purity of the MOFs. Figure S6: PXRD pattern of Ca-NDI from As-synthesized state (Red) and Radiated State (Blue).
Change in background noise has been observed for the case of radiated samples, however the major peaks remained intact in all the cases. S13 Figure S7: PXRD pattern of Sr-NDI from As-synthesized state (Red) and Radiated State (Blue). S14 S5: FT-IR spectroscopy of BINDI linker and derived MOFs Figure S8: FT-IR spectra of BINDI linker and the derived MOFs. S15 Figure S9: FT-IR spectra for Mg-NDI recorded before and after sunlight irradiation. S16 Figure S10: FT-IR spectra for Ca-NDI recorded before and after sunlight irradiation. S17 Figure S11: FT-IR spectra of Sr-NDI recorded before and after sunlight irradiation. S30 S13: Stability of the Mg-NDI coated paper towards mechanical deformation Figure S24: Mechanical Deformation of a Mg-NDI coated paper after printing content on it. It was observed that the contents remained clearly visible even after all kind of deformations applied to it.

S14: Preparation of Stencils
Stencils have been used to control the incidence of light on the MOF coated papers. Thus, any object capable of allowing passage of light through the required path can be used as a stencil for printing.
Such objects include paper cut as per desired outline, suitable photomask, etc. However, for acheiveing a precise impression of the desired content, we have used the filtering of light through a inversely printed object on a transparent sheet. Thus, by printing the inverted object on a polyurethane sheet, we have generated a precise impression with sharp lines, edges that are legible not only to human eyes, but also to smart devices in the form of version-5 QR code. However, for large scale application, we are in process of developing new devices that can spontaneously and continuously control the incidence of light on the photochromic media, without requirement of any stencil.