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Excited State Intramolecular Proton Transfer (ESIPT) in (2-hydroxyphenyl)pyridines

We have recently reported several new examples of ESIPT between phenol as an acidic site and carbon atom of an adjacent carbocyclic aromatic ring as the basic site.[1-3] These ESIPT reactions require initial hydrogen bonding interaction between the phenol OH and the π -system of the adjacent ring that is possible only in twisted biaryl structures.[1] In the parent biphenyl system (1) short-range ESIPT gives rise to quinone methide (QM) 1 while long-range water-mediated ESIPT gives rise to QM 3. In continuation of our research on ESIPT we replaced the adjacent phenyl ring by a pyridine ring. Potential ESIPT photochemistry of 2- (4), 3- (5) and 4-(2’ -hydroxyphenyl)pyridine (6) was investigated by deuterium-exchange experiments (photolysis in D2O-CH3CN) and laser flash photolysis. Whereas ESIPT in 4 [4] results in exclusive protonation of the pyridine nitrogen, 5 and 6 undergo protonation of carbon atoms of the pyridine ring. Without water in the system, ESIPT leads to the protonation of 2- and 4- positions of the pyridine ring in 5 and 3(5)-positions in 6. Upon addition of water, besides short-range ESIPT, competing water-mediated long-range ESIPT results in protonation of position 6 of the pyridine ring in 5, and nitrogen in 6.

photochemistry; ESIPT; phenoles; pyridines; laser flash photolysis

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