Femtosecond photoselectivity in intramolecular double proton transfer in [2,2'-bipyridyl]-3,3'-diol

Abstract

For photoexcited [2,2'-bipyndyl]-3,3'-diol in liquid solution two alternate double proton transfer processes were found to occur simultaneously: a concerted double proton transfer (one-step process), in less than 100 fs, and the sequential double proton transfer (two-step process), with time constants of less than 100 fs and 10 ps. The experiments show that the yield of the diketo tautomer (formed in the concerted process) is enhanced relative to that for the monoketo intermediate product, when laser pulses with longer wavelengths are used for photoexcitation. From the results, an energy barrier in the dienol-to-monoketo tautomerization reaction of about 600 cm-1 is inferred.