A surprising asymmetric structure for the short-lived excited S-1 state of4,4 '-bipyridine

The combination of ab initio calculation using the CIS method and experimen tal vibrational data previously obtained from picosecond time-resolved Rama n measurements [C. Didierjean, V. De Waele, G. Buntinx, and O. Poizat, Chem . Phys. 237, 169 (1998)] leads to a confident picture of the structure of t he short-lived excited singlet state S-1 of 4,4'-bipyridine. The n pi* natu re of S-1 is confirmed. The excitation is confined in one of the pyridyl ri ngs, breaking the initial ground state symmetry and leading to an asymmetri c structure equivalent to an excited pyridine substituted by an unperturbed pyridyl ring. This structure is in complete agreement with the experimenta l data. (C) 1999 American Institute of Physics. [S0021-9606(99)30413-X].