Organometallic Terthiophene Derivatives: Modulation of the Terthiophene Core of 5,5‘‘-Diphenyl-2,2‘:5‘2‘‘-Terthiophene via [CpRu]+ and [Cp*Ru]+ Substituents

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Abstract

We have utilized electrochemical, UV−vis absorption/emission spectroscopy, and UV−vis spectroelectrochemistry to investigate the electronic structure of 5,5‘‘-diphenyl-2,2‘:5‘2‘‘-terthiophene (Ph2Tth) and the complexes [Cp/Cp*Ru(η6-Ph2Tth)]PF6 and [(Cp/Cp*Ru)2(η6,η6-Ph2Tth)](PF6)2 (Cp = cyclopentadienyl; Cp* = pentamethylcyclopentadienyl). Uncomplexed Ph2Tth behaves as if the 2,2‘:5‘2‘‘-terthiophene (Tth) core has two weakly conjugated, sterically blocking phenyl group substituents. The nature of the phenyl substituents are altered by complexation with “Cp/Cp*Ru+” to create [Cp/Cp*Ru(phenyl)]+ substituents. Electrochemical studies show that Ph2Tth exhibits two reversible oxidation processes that produce the radical cation and dication forms of the Tth core. The complexes of Ph2Tth also exhibit localized Tth core oxidations that have E°s shifted to more positive potentials by about 150 mV per metal center. This shift is attributed to the positive charge of a [Cp/Cp*Ru(phenyl)]+ substituent. The electronic absorption and emission spectra of the complexes are also indicative of the Tth chromophore. The λmax of the π−π* transition for the Tth core is sensitive to the nature of the substituent and is red shifted by metal complexation. The complexes exhibit room temperature emission that is quenched by intramolecular processes involving the metal center and additionally by acetonitrile. For the Cp complexes in acetonitrile, quenching leads to decomplexation of Ph2Tth; for the Cp* complexes, no decomplexation is observed. Spectroelectrochemical studies show that the radical cations of Ph2Tth and the monoruthenated complexes exhibit a small degree of π-dimerization at room temperature; the diruthenated complexes show no evidence of π-dimerization. Studies of the complexes with [Cp/Cp*Ru(phenyl)]+ substituents indicate that the properties of the Tth core can be modulated by the transition metal substituent(s).

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