Femtosecond intramolecular double proton transfer in solution

A review is given of our recent femtosecond fluorescence upconversion studi es of photoexcited [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)), and a few deriva tive compounds thereof. For BP(OH)(2) in liquid solution, concerted and con secutive intramolecular double proton transfers are found to occur concurre ntly. The concerted (one-step) process takes place in less than 100 fs. The consecutive two-step process (with a first step in less than 100 fs and a second step of a few picoseconds) is solvent and excitation wavelength depe ndent. From the results we infer that the excited-state double proton trans fer in BP(OH)(2) is promoted by low-frequency vibrational stretching and be nding modes. Proton transfer dynamics was also studied for BP(OH)(2) in sol -gel glasses. Residual amounts of methanol and water in the pores of the so l-gel glass are found to still solvate the probe molecule and to affect the proton transfer dynamics. Finally, some results of fluorescence upconversi on experiments for 6-Me-BPOH (6-methyl-[2,2'-bipyridyl]-3'-ol) and BP(OH)(2 )-6-COOMe (methyl-3,3'-dihydroxy- [2,2'-bipyridyl]-6-carboxylate) are prese nted for comparison. It appears that the latter compound in the excited sta te exhibits a single proton transfer only.