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Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐ b ]pyridine: Evidence for Tautomer in the Ground State
Author(s) -
Chipem Francis A. S.,
Malakar Ashim,
Krishnamoorthy Govindarajan
Publication year - 2015
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/php.12411
Subject(s) - tautomer , chemistry , enol , intramolecular force , photochemistry , excited state , ground state , benzoxazole , fluorescence , pyridine , keto–enol tautomerism , stereochemistry , medicinal chemistry , organic chemistry , atomic physics , physics , quantum mechanics , catalysis
Abstract The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐ b ]pyridine ( HPOP ) is studied using UV ‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis ‐ and trans ‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis ‐enol‐ trans ‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.