The Effects of Substituents and Coverage on the Bonding of Thiophene to Cu(111)

Milligan, Peter K (2000) The Effects of Substituents and Coverage on the Bonding of Thiophene to Cu(111). PhD thesis, University of Glasgow.

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Abstract

The purpose of this project was to study the effects of substituent groups and coverage on the bonding of thiophene to Cu(lll). The substituent groups that were studied were chloro-(3-chlorothiophene), methyl-(3-methylthiophene) and methoxy-(3-methoxythiophene). In each case the substitution was at the 3-position of the thiophene ring. This was a two-fold project in that the characterisation of the adsorption was performed in Glasgow by using TPD, AES and LEED and further structural information was obtained by using the synchrotron-based techniques NIXSW and NEXAFS, at the Daresbury Laboratory. The characterisation experiments indicated that the adsorption of thiophene on Cu(l 11) is molecular and reversible. From TPD data, four adsorption states could be identified. These were desorption from multilayers, two monolayer states alpha- and beta- and defect sites. NIXSW and NEXAFS experiments were then performed in order to determine the structure of thiophene at alpha-state and beta-state coverage. It was found that the adsorption site of thiophene at both alpha-state and beta-state coverage was displaced atop but the orientations were significantly different. At alpha-state coverage, the molecules are oriented in an almost flat lying, pi-bonded orientation but at beta-state coverage, the molecules are oriented in a much more inclined geometry. This coverage dependent phase transition is a result of the dual bonding modes of thiophene in that the molecule can bond either in a pi-bonded manner or via the S lone pairs. The object therefore of adding a substituent group at the 3-position of the thiophene ring was to try and influence the bonding mode of the molecule. TPD studies of 3-chlorothiophene, 3-methoxythiophene and 3-methylthiophene all show that there is no beta-like adsorption state. Therefore there is no evidence of any coverage dependent phase transition on Cu(111) for these molecules. As in the case of thiophene, NIXSW studies show that at monolayer coverage, which corresponds to the thiophene alpha-state coverage, the adsorption site is displaced atop and the Cu-S bond length for all four molecules studied is identical within experimental error. NEXAFS studies show that at monolayer coverage, 3-chlorothiophene, 3-methylthiophene and 3-methoxythiophene are all bonded to the surface in roughly flat orientations with the same angle of inclination as thiophene at alpha-state coverage, within experimental error. The adsorption of 3-methylthiophene was found to be molecular and reversible. There is a small amount of dissociation in the adsorption of 3-chlorothiophene and 3-methoxythiophene. This is probably due to the molecules bonding strongly to defect sites. The chloro- and methoxy-groups are found to have a subtle effect on the bonding of thiophene to Cu(111) because they interact, albeit weakly, with the surface via lone pair-surface interactions.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Malcolm Kalodwaia
Keywords: Physical chemistry, Organic chemistry
Date of Award: 2000
Depositing User: Enlighten Team
Unique ID: glathesis:2000-76238
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 09:15
Last Modified: 19 Dec 2019 09:15
URI: http://theses.gla.ac.uk/id/eprint/76238

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