Femtosecond Intramolecular Double Proton Transfer in Solution

Abstract A review is given of our recent femtosecond fluorescence upconversion studies of photoexcited [2,2′‐bipyridyl]‐3,3′‐diol (BP(OH) 2 ), and a few derivative compounds thereof. For BP(OH) 2 in liquid solution, concerted and consecutive intramolecular double proton transfers are found to occur concurrently. 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 dependent. From the results we infer that the excited‐state double proton transfer in BP(OH) 2 is promoted by low‐frequency vibrational stretching and bending 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 sol‐gel glass are found to still solvate the probe molecule and to affect the proton transfer dynamics. Finally, some results of fluorescence upconversion 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 presented for comparison. It appears that the latter compound in the excited state exhibits a single proton transfer only.