Exceptionally fast intermolecular hydrogen abstraction by 4,4′-bipyridine from alcohols:: Picosecond time-resolved Raman analysis

We present a picosecond resonance Raman pump-probe study of the photochemical reactivity of 4,4'-bipyridine in methanol and ethanol. The data show the decay of the S-1(n pi*) State via two parallel routes, intersystem crossing toward the T-1 state and hydrogen abstraction from the solvent, with estimated rate constants of k(ISC) (3.0 +/- 0.7) x 10(10) s(-1) and (1)k(q) = (6.0 +/-. 1.4) x 10(10) s(-1), respectively. This (1)k(q) rate value is 5 orders of magnitude higher than that found for the same abstraction process by the T-1(pi pi*) state. The mechanism of this surprisingly fast process is discussed on the basis of a comparison of these data and those obtained previously from picosecond transient absorption. Although the reaction is nondiffusional and takes place within H-bonded 4,4'-bipyridine/alcohol complexes, it results from the abstraction of an alkyl hydrogen of the alcohol donor and does not involve the H-bond. II is an example of reaction with a very low structural barrier where the rate is governed in part by the dynamics of reorientation of the alcohol molecules in the solvent cage following the photoexcitation.