From Studies of the Electronic Structures of Molecules to Molecular Electronics

Abstract

This Thesis presents work that advances our understanding of the ground and excited state electronic structures, charge transfer properties and electrical conductivity of linearly and cross-conjugated conjugated organic compounds and organometallic complexes containing these species as ligands. The themes, concepts and methods have been further applied to aspects of cyanocarbon chemistry, organometallic cluster chemistry, the chemistry of metal acetylide complexes, and further aspects of molecular materials chemistry. Across these studies, main themes involving the chemistry of mixed-valence complexes and molecular electronics are readily identified. The model compound 1,4-bis(phenylethynyl)benzene (BPEB) has been demonstrated to exhibit conventional fluorescence properties in fluid solution, with slower rates of relaxation of the singlet excited state in viscous media leading to heterogeneous emission. Such conformational distributions in solution have also been identified in other molecular systems, and add considerably to the understanding of the charge transfer processes in hole-transport materials as well as the descriptions of the electronic structures of mixed valence complexes. Synthetic work has given functionalised BPEB derivatives, allowing the development of liquid-crystalline materials from this prototypical rigid molecular rod, including identification of an unusual optical texture arising from a SmBhex phase. Studies of metal acetylide complexes have shown how both the symmetry properties of the ancillary ligands and the radial distribution of the metal valence d-orbitals can be used to tune the electronic structure of both the parent complex and the radical cations derived from one-electron oxidation. Consequently, the redox character of the representative complexes [Mo(C≡CR)(dppe)(η7-C7H7)]+, [Fe(C≡CR)(dppe)Cp´]+ and [Ru(C≡CR)(dppe)Cp´]+ (Cp´ = Cp, Cp*) vary from the strongly metal-localised processes of the Mo examples to substantially ligand based processes in the case of the Ru systems. In the case of Fe, the good symmetry match but poor spatial overlap of the 3d and C≡C π systems gives more metal-based redox character, with greater alkynyl character than in the analogous Mo complexes but appreciably less than in the case of Ru systems. These characteristics have been used to design bimetallic mixed-valence complexes with electronic structures that test and explore the limits of the assumptions of Marcus-Hush theory and the two-state model, and more complex cases where the degree of ligand vs. metal character and localised vs. delocalised character is strongly conformationally dependent. The work carried out here has shown how the concerted application of UV-vis-NIR and IR data and computational sampling of the potential energy hypersurface at an appropriate level of theory can assist in deriving more accurate descriptions of the electronic structures of the resulting conformational ensembles. Particular effort has been directed towards the electronic structures of complexes [{Cp´(PP)M}(μ-C≡CXC≡C){M(PP)Cp´}]+ (X = bond, arylene; M = Fe, Ru), resulting in a re-classification of the well-known iron complex [{Cp*(dppe)Fe}(μ-C≡CC≡C){Fe(dppe)Cp´}]+ as a localised, Class II mixed-valence system. This work has further demonstrated the utility of spectroelectrochemical methods in such studies. The extension of these studies to metal complexes bearing cyanoacetylide, cyanocarbon, cumulated and cross-conjugated carbon ligands has opened new avenues for the exploration of electron transfer through ‘extended’ cyanides and branched (cross-conjugated) ligands. These works have added considerable insight to the understanding of chemical bonding with metal acetylide, polyynyl, polyyndiyl and related species as a function of redox state, and the chemical reactivity of novel quinoidal cumulenes. The enabling work with BPEB systems, metal acetylide complexes and polyynes has allowed extensive explorations of the electrical properties of such systems within single molecule and monolayer film molecular junctions. Metal complexes of general form trans-[M(C≡CR)Ln] and [M(terpy)2] have been studied in single molecule junctions, allowing development of structure-property relationships including HOMO and LUMO conductance channels, and identifying adventitious contacts (short circuits) to aryl-rich ancillary ligands. Solvent and electrolytic gating effects in single-molecule junctions have been explored, leading to the development of ideal gate coupling in ionic-liquids and consequently an exceptionally efficient single-molecule transistor. Monolayer molecular junctions formed from BPEB-based scaffolds have been used in conjunction with single-molecule measurements demonstrate that near-neighbour effects do not significantly effect molecular conductance in the tunnelling regime. By combining strongly coupled mixed-valence dimer-of-dimer Rh2-paddlewheel fragments and efficient hopping mechanisms with layer-by-layer methods of synthesis, highly efficient metal-complex molecular wires capable of long-range transmission of electric currents have been designed. A number of strategies that introduce a ‘top contact’ electrode directly onto the exposed top surfaces of molecular monolayers have been developed. The electrical characterisation of the resulting nascent monolayer-film molecular electronic devices provides new directions for future hybrid molecular electronic devices. Taken as a whole, this body of work expands and advances our understanding of the chemistry, electronic structures, electron transfer and electrical properties of linearly conjugated and cross-conjugated compounds and complexes. The critical role that distributions of molecular conformations play in the observable spectroscopic properties and charge transfer characteristics of such systems, especially mixedvalence derivatives, has been identified, and proposals for the description of the electronic structures of such complex ensembles have been made. Molecular junctions have been used to explore the electrical characteristics of these systems, novel surface contacting groups introduced and methods for device fabrication established.