THE STRUCTURE OF THE LOWEST EXCITED SINGLET (S-1) STATE OF 4,4'-BIPYRIDINE - A PICOSECOND TIME-RESOLVED RAMAN ANALYSIS

Time-resolved Raman spectra of the very short-lived excited S-1 state of 4,4'-bipyridine and of two deuterated isotopomers in Liquid solutio ns have been obtained by using a picosecond pump-probe experiment for different probe excitations in resonance with two S-1 --> S-n electron ic transitions. A complete confident vibrational assignment of these s pectra is established and compared to those found for the ground state and N-hydro radical. This comparison and the analysis of the resonanc e band activity and enhancement effects confirm the n pi nature of S- 1 and reveal a surprising asymmetric structure resulting from the conf inement 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 S-1 state, in agreement with the exceptionall y high reaction rate found for this process. (C) 1998 Elsevier Science B.V. All rights reserved.