Structural Switching in Tetrathiafulvalene based Metal-Organic Frameworks

Abstract

The ability for [Cd2(bpdc)2(Py2TTF)2] (bpdc = 1,4 biphenyldicarboxylate) to undergo a double [2+2] photo-induced cycloaddition between the TTF cores when exposed to white light inspired further investigation into its analogues. One of these frameworks, [Zn2(TDC)2(Py2TTF)2] (Zn-C) (TDC = 2,5-thiophenedicarboxylate) was observed to undergo a photo-induced structural change, but contained crossed TTF ligands, that were far outside the known Schmidt criteria for photocycloaddition. This photo-induced structural change caused significant crystal cracking, preventing the determination of the resulting structure.

Chapter 3 discusses a series of optimisations of the synthetic conditions forming [Zn2(TDC)2(Py2TTF)2]. A novel framework [Zn2(TDC)2(Py2TTF)] (Zn-Pad) was found to form alongside [Zn2(TDC)2(Py2TTF)2]. These optimisations served to to reduce the formation of Zn-Pad, and to reduce the extreme crystal cracking that had previously prevented structural characterisation of the product of the double [2+2] photo-induced cycloaddition (Zn-Cyc). Zn-Cyc was found to contain disordered Py4C12S8H4 produced by the photocyclisation. Due to the crossed nature of the Py2TTF dimers in Zn-C, one of the ligands within each dimer must undergo significant molecular motion to form Zn-Cyc.

Chapter 4 reports the synthesis of a novel Cd-Analogue of Zn-C, [Cd2(TDC)2(Py2TTF)2] (Cd-C). This framework was found to undergo a reversible photo-induced structural transition. Solid-state UV-Vis spectroscopy, Raman spectroscopy and cyclic voltammetry demonstrated the presence of Py2TTF and the formation of Py4C12S8H4.

A Raman spectroscopy based kinetic study of the photo-induced transitions in Zn-C and Cd-C was undertaken, modelling kinetic results with the JMAK model and the FW autocatalytic and autoinhibitive models. This study develops considerations to modelling kinetics to account for ambient light exposure, through the development of a modified FW kinetic model and a normalisation method.