G. Buntinx et al., SUBPICOSECOND TRANSIENT ABSORPTION ANALYSIS OF THE PHOTOPHYSICS OF 2,2'-BIPYRIDINE AND 4,4'-BIPYRIDINE IN SOLUTION, Journal of physical chemistry, 100(50), 1996, pp. 19380-19388
The transient absorption spectra of 2,2'-bipyridine (22BPY) and 4,4'-b
ipyridine (44BPY) following femtosecond excitation at 266 nm in a seri
es of organic solvents have been measured in the 300-670-nm wavelength
range with subpicosecond time resolution. The results have been corre
lated with those obtained in the microsecond time domain. The S-1 stat
es are characterized and attributed to n pi levels in both molecules.
The S-1 state of 22BPY decays essentially via intersystem crossing to
the T-1 pi pi state in all solvents (tau s(1) = 50-80 ps). The 44BPY
S-1 lifetime varies from 10 to 70 ps depending on the solvent. The sh
ortest lifetimes are found in alcohols (10-20 ps) and alkanes (approxi
mate to 11 ps) where the fast formation of the N-hydro radical 44BPYH(
.), already evidenced in previous nanosecond experiments, is shown to
occur with high yields from the S-1 state in parallel to ISC. Ln alcoh
ols this process is found to be activated by the protic character of t
he solvent and to take place within H-bonded solute/solvent complexes
pre-existing in the ground state. A mechanism involving a surprisingly
fast H-atom abstraction by the 44BPY S-1 state via homolytic breaking
of the alcohol OH bond is suggested.