title: Unusual, solvent viscosity-controlled tautomerism and photophysics: meso-alkylated porphycenes.
abstract: 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.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.
authors.forename: Michał
authors.initials: M
authors.lastname: Gil
authors.forename: Jacek
authors.initials: J
authors.lastname: Dobkowski
authors.forename: Gabriela
authors.initials: G
authors.lastname: Wiosna-Sałyga
authors.forename: Natalia
authors.initials: N
authors.lastname: Urbańska
authors.forename: Piotr
authors.initials: P
authors.lastname: Fita
authors.forename: Czesław
authors.initials: C
authors.lastname: Radzewicz
authors.forename: Marek
authors.initials: M
authors.lastname: Pietraszkiewicz
authors.forename: Paweł
authors.initials: P
authors.lastname: Borowicz
authors.forename: David
authors.initials: D
authors.lastname: Marks
authors.forename: Max
authors.initials: M
authors.lastname: Glasbeek
authors.forename: Jacek
authors.initials: J
authors.lastname: Waluk