Radical ions from 3,3′′′,3′′′′-tris(butylsulfanyl)-2,2′:5′,2":5",2′′′,5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene:: An experimental and theoretical study of the p- and n-doped oligomer

The 3,3"',3""' tris(butylsulfanyl)-2,2':5,2":5",2"',5"',2"":5"", 2""'- sexithiophene 1 was investigated through spectroscopic (NMR, EPR, UV/Vis-NIR), electrochemical, spectroelectrochemical and theoretical (DFT) studies. The charged species obtained upon its oxidation and reduction were characterised, showing that 1 can exist in at least five different oxidation states, that is, a neutral species, a radical cation, a dication, a radical anion, and a dianion. The long term stability of the radical cation 1(+circle) was evidenced by the H-1 NMR study in the presence of small quantities of trifluoroacetic acid (TFA). This approach allowed a comparison of the relative broadening of proton signals of 1, induced by the electron exchange process with traces of radical cation 1(+circle), and the hfc (hyperfine coupling) constants obtained from the EPR study and DFT calculations. In the radical cation, all of the heterocyclic sulphur atoms are not significantly involved in the delocalisation of the unpaired electron, whereas the opposite holds for the radical anion. Time-dependent DFT calculations reproduced well the wavelengths of the optical transitions observed in the spectroelectrochemical experiments for all the five oxidation states and support the formation of the dianion 1(2-).