Raman spectroscopy of short-lived terthiophene radical cations generated by photochemical and chemical oxidation

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-copped 2,2':5',2"-terthiophene OT) and 3'-[(E)-2-(4-nitrophenyl)ethenyll-2,2'.5',2"-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5"-dimethyl-3'-[(E)-2-phenylethenyll-2,2'.5',2"-terthiophene QM-pe3T) is stable for up to five minutes as a result of the two a end cops and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bonds in the low-frequency region. These bonds have been assigned, by density functional theory methods, to CS stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the Gr-dimer of NO(2)pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe(3)T)(2)]. This observation is attributed to a smaller change in the C-S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the loser excitation wavelength.