The structure of the lowest excited singlet (S1) state of 4,4′-bipyridine: a picosecond time-resolved Raman analysis Original Research Article

Time-resolved Raman spectra of the very short-lived excited S1 state of 4,4′-bipyridine and of two deuterated isotopomers in liquid solutions have been obtained by using a picosecond pump–probe experiment for different probe excitations in resonance with two S1→Sn electronic transitions. A complete confident vibrational assignment of these spectra is established and compared to those found for the ground state and N-hydro radical. This comparison and the analysis of the resonance band activity and enhancement effects confirm the nπ* nature of S1 and reveal a surprising asymmetric structure resulting from the confinement of the excitation in one pyridyl ring. This structure shows a manifest resemblance with that of the N-hydro radical. In this regard a low structural barrier can be predicted for the reaction of hydrogen-atom abstraction by the S1 state, in agreement with the exceptionally high reaction rate found for this process.