Computational and experimental studies of redox-active 1,4-dithiafulvene derivatives

Mehrani, Sepideh (2017) Computational and experimental studies of redox-active 1,4-dithiafulvene derivatives. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

1,4-Dithiafulvene (DTF) is a π-conjugated organic molecule with excellent electrondonating properties and redox activity, which can be widely used in the application of molecular electronics and optoelectronics. The work described in this dissertation investigates on the fundamental properties of a series of novel DTF-based molecules and macromolecules. The first part of this thesis is focused on the studies of a bis(DTF)- end-capped oligo(phenylene ethynylene). Both experimental characterizations (e.g., cyclic voltammetry, UV-Vis absorption, and Raman spectroscopy) and theoretical density functional theory (DFT) modeling have been conducted to gain insight into the structural, electronic and redox properties of this oligomer in the neutral and oxidized states. Further, the self-assembling behavior of the bis(DTF)-oligomer on the surfaces of mica and highly oriented pyrolytic graphite (HOPG) was examined by atomic force microscopy (AFM). It has been noted that the microscopic morphologies of the self-assemblies of bis(DTF)-endcapped oligo(phenylene ethynylene) on surfaces show redox state dependency. In the second part of this thesis, the adsorption behavior of a carboxyl-substituted DTF molecule on the surface of gold was studied. DFT calculations were performed with the Vienna Ab Initio Simulation Package (VASP) under periodic boundary conditions with Perdew-Burke-Ernzerhof (PBE) correlation and the generalized gradient approximation (GGA) exchange functional. The effect of van der Waals interactions was taken into account by the vdW-DFT (optB88-vdW) method. The DTF molecule–gold surface interactions were investigated to understand the process of monolayer formation and associated charge transfer properties. Moreover, calculations with and without dispersion correction were performed to examine the role van der Waals interactions play at organic-metal interfaces. Raman spectroscopy and Kelvin probe force microscopy (KPFM) were employed to validate the accuracy of computed work functions. The results demonstrate that the work function of Au (111) can be tuned by depositing a thin film of redox-active DTF molecules with differing molecular dipole moments. Finally, the mechanisms for the reduction of HAuCl4 with DTF derivatives were investigated by time-dependent UV-Vis analysis. The experimental results show that stepwise temperature control is a key factor dictating the formation of gold nanoparticles (AuNPs). Overall, this finding points to a promising one-phase synthetic approach for AuNPs in aqueous media.

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