Volume 4, Issue 5, Pages 1059-1079 (May 2018) Photochromic 2D Metal-Organic Framework Nanosheets (MONs): Design, Synthesis, and Functional MON-Ormosil Composite  Arindam Mukhopadhyay, Vijay Kumar Maka, Govardhan Savitha, Jarugu Narasimha Moorthy  Chem  Volume 4, Issue 5, Pages 1059-1079 (May 2018) DOI: 10.1016/j.chempr.2018.03.013 Copyright © 2018 Elsevier Inc. Terms and Conditions

Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 1 Design Strategy and Target Structures (A) Structures of the target pyridyl and carboxylic acid linkers that feature photochromic 2,2-diphenylbenzopyran as the backbone. (B) T-type photochromism of 2,2-diphenylbenzopyran. (C) Cartoon representation of our strategy for developing 2D layered MOFs by using linkers with a bulged core, which may hinder self-assembly in the orthogonal direction. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 2 Synthetic Routes to the Target Pyridyl and Carboxylic Acid Linkers Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 3 Analysis of X-Ray-Determined Crystal Structures of Zn-DA and Cd-TP (A and F) Crystal packing diagrams of Zn-DA (A) and Cd-TP (F). Hydrogen atoms have been omitted for clarity. (B and G) Horizontal views of the close-packed 2D MONs of Zn-DA (B) and Cd-TP (G). Notice that the diphenyl rings interject into neighboring layers. (C and H) Representation of a single layer of the 2D MONs of Zn-DA (C) and Cd-TP (H) in the space-filling model. Notice the free volumes around the diphenylbenzopyran moieties. The anions and hydrogen atoms have been omitted for clarity. (D) Coordination environments of the metal ion nodes in Zn-DA (left) and Cd-TP (right). (E) Orthogonal orientations of C2 phenyl rings with respect to the benzopyran ring in DA. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 4 T-type Photochromism of the Ethanolic Suspensions of the MOFs (A) UV-vis absorption spectra of an ethanolic suspension (1 mg/mL, sonicated for 30 min) of Cd-DP before and after steady-state photoirradiation (λ = 350 nm) at 288 K for 3–5 min. (B) The T-type photochromism observed for the ethanolic suspension of Cd-DP. (C) Plot of the absorbance of the photogenerated colored species of Cd-DP at 288 K versus ultrasonication time. (D) Biexponential thermal bleaching kinetics of the photogenerated colored suspension of Cd-DP at 288 K in the dark. (E) Spectrokinetic data for the photochromic ethanolic suspensions of all the MOFs (1 mg/mL, sonicated for 30 min). The amplitude percentages (A1 and A2) corresponding to fast (k1) and slow (k2) decaying colored species are given in parentheses; the thermal decay rate constants (k) were calculated by fitting the bleaching curves to a biexponential decay equation: A = A1 exp(−k1t) + A2 exp(−k2t); the goodness-of-fit parameter, i.e., χ2, was found to be close to unity in all cases. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 5 SEM Images of Pristine MOF Crystals SEM images of the pristine crystals of Cd-DP (A) and Zn-DA (B) show the presence of stacked 2D sheets. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 6 DLS Analysis, Demonstration of the Tyndall Effect, and SEM Characterization of the 2D MONs (A and B) DLS analysis (A) of an ethanolic suspension of Cd-DP shows the variation of hydrodynamic diameter of the 2D nanosheets with ultrasonication time. The plot in (B) shows progressive decrease in the mean size of the 2D nanosheets with increasing duration of ultrasonication (see Supplemental Information for details). The inset in (B) demonstrates the Tyndall light-scattering behavior of the ethanolic suspension of Cd-DP obtained after ultrasonication for ca. 30 min. (C and D) SEM images of the 2D nanosheets of Cd-DP (C) and Zn-DA (D) obtained for the ethanolic suspensions of the MOFs subjected to 30 min of ultrasonication. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 7 Characterization of the 2D MONs by AFM (A–D) AFM topographic images (top) and height profile diagrams (bottom) for 2D nanosheets of Cd-DP (A), Zn-DA (B), Cd-TP (C), and Cd-TA (D). The images were obtained for ethanolic suspensions of MOFs subjected to 30 min of sonication. (E) Decrease in the thickness of few-layer 2D nanosheets of Cd-DP with increasing duration of ultrasonication (see Supplemental Information for details). The numbers in parentheses correspond to the number of layers present in the stacks by considering the thickness of a single layer to be ca. 1.5 nm as deduced from X-ray crystal structure determination (Supplemental Information). Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 8 Characterization of the 2D MONs by TEM (A–D) HRTEM images (300 kV) of the MONs at different resolutions. Scale bars: 200 nm (A), 100 nm (B), and 10 nm (C). Notice the presence of well-resolved 0.25 nm periodic lattice fringes in (D). (E and F) SAED patterns of the nanosheets obtained for the TEM images at 300 kV (E) and 120 kV (F). Notice that the SAED pattern in (E) exhibits diffused rings, whereas that in (F) exhibits distinct electron diffraction spots. Scale bars: 10 1/nm. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 9 PXRD Profiles and XPS Characterization Data (A and B) PXRD profiles of Cd-DP (A) and Zn-DA (B). Blue, simulated from single-crystal X-ray structure determination; green, pristine crystals; red, solid material obtained after slow evaporation of ethanol at room temperature from the colloidal suspension of exfoliated 2D MONs; and violet, solid material obtained after slow evaporation of ethanol at room temperature from the colloidal suspension of exfoliated 2D MONs, which was subjected to five photochromic cycles involving steady-state photoirradiation (for ca. 5–10 min) and subsequent thermal decoloration in the dark. (C–E) XPS profiles for the 2D MONs of Cd-DP (C and D) and Zn-DA (E) show the presence of peaks corresponding to the binding energies of Cd 3d, N 1s, Cl 2p, and Zn 2p. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 10 Photoresponsive MON-Ormosil Composite Film (A) SEM image showing the thickness of the film of 2D MONs of Cd-DP entrapped in an ormosil matrix, which is coated on a glass slide. (B) Photographs portraying the photochromic color change of the MON-ormosil composite film. The transparency of the film coated on a glass slide is evident after the latter is placed over white paper printed with “IIT KANPUR” (the authors’ institution). (C) UV-vis absorption spectra of the thin film were recorded before and after photoirradiation with 350 nm; the inset shows biexponential thermal decay kinetics of the colored film at 298 K in the dark. (D) Photochromic cycles showing the fatigue resistance of the film. Chem 2018 4, 1059-1079DOI: (10.1016/j.chempr.2018.03.013) Copyright © 2018 Elsevier Inc. Terms and Conditions