Light driven mesoscale assembly of a coordination polymeric gelator into flowers and stars with distinct properties

Abstract

Control over the self-assembly process of porous organic-inorganic hybrids often leads to unprecedented polymorphism and properties. Herein we demonstrate how light can be a powerful tool to intervene in the kinetically controlled mesoscale self-assembly of a coordination polymeric gelator. Ultraviolet light induced coordination modulation via photoisomerisation of an azobenzene based dicarboxylate linker followed by aggregation mediated crystal growth resulted in two distinct morphological forms (flowers and stars), which show subtle differences in their physical properties.

Fig. 1. (a) Absorption spectra of the ADA solution in DMSO (1.85 × 10^–5 M) before and after irradiation with ultraviolet (UV) and visible (Vis) light. Inset shows zoomed region of 379–527 nm corresponding to the n–π* transition band of cis azobenzene. (b) Reversibility of photoisomerisation, shown for three cycles. (c) Schematic representation of synthesis of CPG1 and CPG2 (corresponding gel images are shown). Trans and cis isomers of ADA (carbon – grey; nitrogen – blue; oxygen – red; hydrogen – white) and probable structure of inorganic octahedra (iron – green; oxygen – red) are also shown.

Fig. 2. SEM analysis of CPG1. (a) Different morphological forms of CPG1 captured in one frame. Platelets (white box), rough sheets (green box), cabbage (blue box), large smooth sheets (yellow box) and flower (red box) like structures. (b–f) Time dependent SEM analysis of CPG1. (b) Nanoscale CPG1 platelets formed immediately, inset shows platelets undergoing fusion. (c and d) Images obtained after 5–6 h. (c) Gradual smoothening of rough sheets; smoothened portions marked with arrows. (d) Formation of cabbage like structure. (e) Matured metal–organic flowers formed after 1 day. (f) Zoomed view of the box marked in (e), showing nanoscale sheets (∼25 nm).

Fig. 3. SEM analysis of CPG2. (a) Different morphological forms of CPG2 captured in one frame. Leaves (white box), thick leaves (yellow box), tentacles (green box), stars in the formation (blue box), and crystalline stars (red box). (b–d) Time dependent SEM analysis of CPG2. (b) Nanoscale leaves of CPG2 after immediate gelation. (c) Aggregation of nano leaves after 1 h. (d) Crystalline metal–organic stars obtained after 1 day.

Fig. 4. (a) TEM image of CPG1 nanosheets. HRTEM image of a nanosheet is shown in the inset. (b) IFFT reconstruction of the HR-TEM image shown as inset in (a). Zoomed view of the marked box region is shown as inset. (c and d) Molecular packing of the coordination polymeric chains observed from the reconstructed HR-TEM image. Crystal structure viewed down axis ‘a’ and reciprocal cell axis ‘b’ of PCN 243, are shown as a guide to the eye.

Fig. 5. Proposed formation pathways of (a) flowers in CPG1 and (b) stars in CPG2. Detailed view of the secondary building unit (SBU) containing the [Fe₃(μ₃-O)(COO)₆] cluster with coordinated solvent molecules (Ow). Each Fe^III center is located in a highly distorted octahedral environment. Fusion of platelets in CPG1 leads to the formation of 2-D sheets and finally into flowers. The anisotropic growth along different facets in CPG2 leads to the formation of stars. Blue, green and red arrows represent maximum, moderate and restricted rates of crystal growth. Blue broken lines indicate directions of polymeric extension. Not all short contacts are shown so as to retain visual clarity. A different crystallographic view of PCN 243, can be found in Fig. S8.

Fig. 6. (a) XRD pattern of CPG1 and CPG2. Simulated XRD pattern of PCN 243 and stick pattern of rhombohedral hematite (α-Fe₂O₃, JCPDS card no. 24-0072) phase also shown for comparison. (b) XRD pattern of CPG1 and CPG2 activated at 150 °C. Rheological behaviour of (c) CPG1 and (d) CPG2. (e) Comparative N₂ adsorption (ads) and desorption (des) isotherms of CPG1 and CPG2 at 77 K. (f) Comparative CO₂ adsorption isotherms of CPG1 and CPG2 at 195 K. Inset shows the zoomed low-pressure region. Red and blue arrows indicate stepwise uptake of CO₂ for CPG1 and CPG2, respectively.