A surprising asymmetric structure for the short-lived excited S1 state of 4,4′-bipyridine

The combination of ab initio calculation using the CIS method and experimental vibrational data previously obtained from picosecond time-resolved Raman 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 the short-lived excited singlet state S-1 of 4,4'-bipyridine. The n pi* nature of S-1 is confirmed. The excitation is confined in one of the pyridyl rings, breaking the initial ground state symmetry and leading to an asymmetric structure equivalent to an excited pyridine substituted by an unperturbed pyridyl ring. This structure is in complete agreement with the experimental data. (C) 1999 American Institute of Physics. [S0021-9606(99)30413-X].