Subpicosecond transient absorption analysis of the photophysics of 2,2'-bipyridine and 4,4'-bipyridine in solution

The transient absorption spectra of 2,2'-bipyridine (22BPY) and 4,4'-bipyridine (44BPY) following femtosecond excitation at 266 nm in a series of organic solvents have been measured in the 300-670-nm wavelength range with subpicosecond time resolution. The results have been correlated with those obtained in the microsecond time domain. The S-1 states 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 shortest lifetimes are found in alcohols (10-20 ps) and alkanes (approximate 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 alcohols this process is found to be activated by the protic character of the 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.