D.R.A. Marks

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

Abstract We report an excitation energy dependence of the dynamics of the excited-state intramolecular double-proton transfer of [2,2′-bipyridyl]-3,3′-diol in liquid solution. By means of femtosecond fluorescence upconversion experiments it is shown that an increase of the vibrational energy in the excited electronic S 1 state results in an increase of the ratio of the reaction yields of the monoketo and diketo tautomeric products. The dominance of the concerted double-proton transfer process at the lower excitation energies is evidence for an energy barrier in the dienol–monoketo reaction pathway.

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

Solvent dependence of (sub)picosecond proton transfer in photo-excited [2,2′-bipyridyl]-3,3′-diol

Abstract We report on (sub)picosecond fluorescence studies of the mono- to diketo tautomer reaction in photo-excited [2,2′-bipyridyl]-3,3′-diol in aprotic and protic solvents. In aprotic solvents no influence of the nature of the solvent, temperature and deuteration of the hydroxyl groups on the proton transfer dynamics was found. In protic solvents, however, the proton transfer time is found to change proportionally with the viscosity coefficient of the solvent. We briefly discuss (i) the formation of the mono- and diketo forms from the dienol form and (ii) the nature of the vibrational modes assisting in the proton transfer process.

Femtosecond intramolecular proton transfer in photoexcited mono- and dienol derivatives of bipyridine.

Abstract A femtosecond study of the intramolecular proton transfer dynamics in photoexcited mono- and dienol derivative compounds of pyridine is reported. It is shown that when a strongly electron-withdrawing substituent is attached to [2,2′-bipyridyl]-3,3′-diol, the excited-state proton transfer of this compound is changed from a double-proton transfer to a single-proton transfer. The excited-state single-proton transfer time in the title compounds is

Femtosecond studies of double-proton transfer in [2,2′-bipyridyl]-3,3′-diol

Abstract Fluorescence up-conversion experiments are reported for [2,2′-bipyridyl]-3,3′-diol with a resolution of 300 fs. The dynamics reveals two excited state intramolecular reactions: (i) concerted double-proton transfer, and (ii) two-step single-proton transfer.

Femtosecond studies of double proton transfer in [2,2'-bipyridyl]-3,3'-diol in sol-gel glasses

Abstract From a femtosecond fluorescence up-conversion study of [2,2′-bipyridyl]-3,3′-diol in the TMOS sol-gel glass, it is shown that the double proton transfer dynamics is significantly influenced when the wavelength of the exciting laser pulses is varied. From the data an energy barrier in the excited-state energy surface of about 300 cm−1 is concluded in the dienol to monoketo reaction.

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

For photoexcited [2,2′-bipyridyl]-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.