title: Three modes of proton transfer in one chromophore: photoinduced tautomerization in 2-(1H-pyrazol-5-yl)pyridines, their dimers and alcohol complexes.
abstract: Studies of 2-(1H-pyrazol-5-yl)pyridine (PPP) and its derivatives 2-(4-methyl-1H-pyrazol-5-yl)pyridine (MPP) and 2-(3-bromo-1H-pyrazol-5-yl)pyridine (BPP) by stationary and time-resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited-state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited-state intermolecular double-proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent-assisted double-proton transfer in hydrogen-bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited-state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground-state syn-anti equilibria are detected and discussed. The fraction of the higher-energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen-bond donor or acceptor abilities.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland.
authors.forename: Volha
authors.initials: V
authors.lastname: Vetokhina
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Dobek
authors.forename: Michał
authors.initials: M
authors.lastname: Kijak
authors.forename: Izabela I
authors.initials: II
authors.lastname: Kamińska
authors.forename: Keven
authors.initials: K
authors.lastname: Muller
authors.forename: Werner R
authors.initials: WR
authors.lastname: Thiel
authors.forename: Jacek
authors.initials: J
authors.lastname: Waluk
authors.forename: Jerzy
authors.initials: J
authors.lastname: Herbich
mesh_terms: D000438:Alcohols / Q000737:chemistry*; D019281:Dimerization; D006860:Hydrogen Bonding; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D011522:Protons; D011720:Pyrazoles / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry
chemical_list: D000438:Alcohols; D011522:Protons; D011720:Pyrazoles; D011725:Pyridines