Incorporation of spiropyran functionality into framework materials

Chong, M.W.S. (2016) Incorporation of spiropyran functionality into framework materials. PhD thesis, University of Nottingham.

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Abstract

This thesis describes developments towards incorporating spiropyran functionality into metal-organic frameworks (MOFs).

Chapter 1 outlines the reported literature concerning the study of photochemistry with respect to MOFs; such materials have demonstrated potential as both alternative environments for the study of photoactivated processes and platforms to integrate photosensitive moieties. Incorporation of photoactive groups into MOFs has enabled these supramolecular materials to be altered chemically and physically via photo-initiated processes. Existing studies into reversible photoswitching groups are largely focussed on azobenzene. The chronological development of azobenzene incorporation into MOFs reflects the evolving strategies of exploiting this functionality to achieve photocontrol over the properties of MOF materials. These advances in accommodating photoswitching azobenzene into MOFs have been applied in reported studies with other photochromic groups and taken into consideration with the work described herein concerning spiropyrans.

Chapter 2 focusses on the preparation of a carboxylic acid functionalised salicylaldehyde 3-formyl-4-hydroxybenzoic acid (H2L1), a key precursor in the synthesis of carboxylic acid functionalised spiropyrans. The serendipitous outcome in solvothermal reaction of H2L1 and copper nitrate in dimethylformamide affords {Cu2L12·(DMF)2(H2O)}n (1-Cu-DMF) which has been crystallographically characterised and is further described in this chapter. Channels run through the direction of the crystallographic a-axis of 1-Cu; its connectivity and porosity is retained upon solvent exchange of the single crystals with ethanol and tetrahydrofuran. Gas sorption experiments show 1-Cu exhibits type I adsorption behaviour with a Brunauer-Emmett-Teller (BET) surface area of 948 ± 1 m2 g−1. Notably, 1-Cu adsorbs negligible quantities of methane compared to carbon dioxide and other C2Hn hydrocarbons; the selectivities are confirmed by analysis via the ideal adsorbed solution theory (IAST) and Henry’s law. Of particular importance, 1-Cu demonstrates exceptional selectivity for acetylene, which has applicability in separation technologies for the isolation of acetylene.

Chapter 3 details the design and synthesis of a series of carboxylic acid functionalised spiropyrans and bisbenzospiropyrans. These compounds serve as ligand precursors for MOFs but also have interesting photophysical properties as organic compounds, which are studied in this chapter. Condensation of prefunctionalised fragments, H2L1 and carboxylic acid functionalised Fischer’s base 5, afforded a novel dicarboxylic acid functionalised spiropyran H2L2. A second synthetic route to extended ligand precursors, via Suzuki-Miyaura cross coupling of ethyl ester functionalised boronic acids to dibrominated photoactive cores and subsequent hydrolysis, is described. Crystallographic characterisation of the ethyl esters indicates flexibility of the core moieties around the spiro carbon. Comparison of the UV-visible absorption spectra shows the properties of related spiropyrans and bisbenzospiropyrans to be influenced by electronic effects arising from both the type and positioning of the functional groups. The fluorescence quantum yields of novel spiropyrans 13, 15 and H2L5 have been determined as 0.025, 0.032 and 0.068 respectively. Cyclic voltammetric experiments show the electrochemical behaviour of spiropyrans to be influenced primarily by electronic effects related to the type of functional group attached, whereas the electrochemical properties of bisbenzospiropyrans is dominated by electronic effects arising from the positioning of the functional groups. Density functional theory (DFT) calculations of the spectroscopic properties are described and are consistent with experimental observations.

Chapter 4 describes investigations to incorporate the carboxylic acid compounds prepared in Chapter 3 into framework materials. To mitigate potential instability problems from using solely photoresponsive and highly flexible components, co-crystallisation with pillaring agents was considered. Reaction of H2L2 with zinc nitrate and a dipyridyl terephthalamide pillaring agent L7 affords coordination network 2 Zn. Two isomers, {Zn(L2)(syn-L7)·(DMF)3(H2O)}n (2-syn-Zn) and {Zn(L2)(anti-L7)·(DMF)3.5(H2O)1.5}n (2-anti-Zn), have been crystallographically characterised; their differences rest upon the conformations adopted by ligand L7. The structure of the 2 anti Zn isomer has higher potential porosity, appearing as rhomboid channels running down the direction of the crystallographic b-axis. The two dimensional sheets of both 2-syn-Zn and 2-anti-Zn are linked in a third dimension through hydrogen bonding interactions between the carboxylate of (L2)2− and amide moiety of L7 in adjacent layers. UV irradiation (325 nm) of single crystals of both forms of 2 Zn initiates a growth in fluorescence of the material observed in situ on a Raman microscope. In situ monitoring of the fluorescence using a 785 nm laser shows a decay over 23 hours to recover the original Raman spectrum of the material. The fluorescence decay can be fitted to a biexponential process; the faster process (13 270 s−1 and 1290 s−1, 2-syn-Zn and 2-anti-Zn respectively) is approximately an order of magnitude greater than the slower process (3980 s−1 and 350 s−1 respectively). DFT calculations suggest the theoretical spectroscopic and electrochemical properties of (L2)2− are not significantly changed by coordination to zinc in 2-Zn. The structures 2-syn-Zn and 2-anti-Zn are the first known examples of spiropyran functionality being incorporated into frameworks.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Schröder, Martin
Easun, Timothy
Champness, Neil
Subjects: Q Science > QD Chemistry > QD450 Physical and theoretical chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 33949
Depositing User: Chong, Magdalene
Date Deposited: 22 Oct 2018 12:53
Last Modified: 08 Feb 2019 10:15
URI: http://eprints.nottingham.ac.uk/id/eprint/33949

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