Metal-organic semiconducting charge transfer complexes based on TCNQFx (x= 0 and 4) for electron transfer reactions
Creators: name: HUSSAIN, Zakir
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)

description: Metal-organic semiconducting materials based on the charge transfer complexes of metal-7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated analogue (TCNQFx) have attracted significant attention due to their rich optical, electronic, and chemical properties. Of particular interest is the low bandgap of MTCNQFx materials which allows them to be used for photoactive applications such as electronics, catalysis, sensing and microbial management. 
The work undertaken in this thesis outlines new fabrication route to synthesise a range of metal-organic semiconducting materials based on MTCNQFx. The work presented here is split into three aspects viz. (i) increasing the crystallisation of MTCNQ by changing the solvent in which the material is synthesized; (ii) creating MTCNQ/TCNQF4 hybrids using galvanic replacement reactions and (iii) influence of changing the metal in MTCNQFx from commonly used Cu and Ag to Pb.  While the three aspects have strengths in different aspects of MTCNQFx-based materials, the common theme of fabricating MTCNQFx materials and modulating its properties link the different chapters in this thesis. 

Firstly, the influence of solvent and solvent mixtures on the crystallisation of CuTCNQ on Cu foil was studied. In chapter 2, a range of protic and aprotic organic solvents were used for CuTCNQ synthesis using the wet chemical strategy. This study highlighted the importance of solvent properties such as the dielectric constant of the solvent which had significant effect on the reaction kinetics of CuTCNQ crystallisation. The choice of solvent in combination with temperature of the reaction resulted in CuTCNQ with unique morphology, redox characteristics, and phases. These characteristics had a large influence on the catalytic activity of CuTCNQ. For instance, the solvent used for synthesis had more effect on its catalytic ability than the amount of CuTCNQ catalyst. The work also developed a new way to identify the phase of CuTCNQ using electrochemistry as an analytical tool. The outcome from this work was further investigated by fabricating CuTCNQ in water-DMSO bisolvent mixture, where the addition of water increases the dielectric constant of the solvent-mixture resulting in increased crystallisation of CuTCNQ on the surface of the Cu foil. This increased crystallisation improved the catalytic efficiency of redox catalytic reactions. The use of different solvent and solvent mixtures offers avenues to manipulate the crystallization process of CuTCNQ.  

The second aspect of this thesis (chapter 4) focuses on fabrication of CuTCNQ|TCNQF4 hybrids using galvanic replacement (GR) reactions. This work for the first time demonstrated that GR reactions can be used for exchange of anionic species in contrast to commonly used cations. The GR reaction between TCNQF40 in acetonitrile and TCNQQ– from CuTCNQ to form Cu(foil)/CuTCNQ/CuTCNQx(TCNQF4)y(solid) hybrids where (x + y = 1), proceeds due to the favorable difference in the redox potential of the two anions. Electrochemical analysis also established that the metal cation Cu had minimal effect on the GR process. Rather, the Cu metal only facilitated the recrystallisation of the CuTCNQ and CuTCNQF4. Further, the hybrids produced as a result of the GR reaction show improved catalytic activity for redox catalytic reactions.
Lastly, in an attempt to increase the repertoire of MTCNQFx-based materials, the work presented in chapter 5 demonstrates the spontaneous crystallization of Pb(TCNQ)2 and Pb(TCNQF4)2 on Pb foil using a wet chemical strategy. This is the first ever comprehensive report of Pb based TCNQFx materials considering all previous studies on crystallization of TCNQFx has been with transition metals such as Cu and Ag. In-depth analysis of the Pb(TCNQFx)2 materials suggested that these materials only exist in just one phase in contrast to two discrete phases observed in CuTCNQ and AgTCNQ. Another aspect that was interesting with Pb(TCNQFx)2 materials was that the material had significantly lower dissolution in acetonitrile. The Pb(TCNQFx)2 materials showed rich optical properties and showed good catalytic performance.