M. Glasbeek
1999
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Journal
Biochimica et Biophysica Acta
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.