Evidence of lactim-lactam photo-tautomerization through four-member intramolecular hydrogen bonded network in 5-(4-fluorophenyl)-2-hydroxy-nicotinonitrile

Lactim-lactam isomerisation behavior through proton transfer process at the strained four-member intramolecular H-bonded ring in 5-(4-fluorophenyl)-2-hydroxy-nicotinonitrile (FP2HN) has been elaborately investigated by steady state absorption and emission, time-resolved fluorescence spectroscopy and quantum chemical calculations by the Density Functional Theory (DFT) method. Irrespective of the nature of the solvents, FP2HN exists as lactim (FP2HN) and lactam form (FP3PN) in the ground state. The observed large Stokes shifted emission band corresponds to the spectroscopic signature of lactim→lactam conversion by excited state intramolecular proton transfer (ESIPT) reaction across the four member H-bonded network. The ESIPT reaction is found to be suppressed in the basic medium due to the formation of anionic species. The effect of increase of temperature on the spectral behavior and hence the calculated thermodynamic parameters (Ktau0, ΔG0, ΔH0, ΔS0) indicate spontaneous lactim→lactam isomerisation process. The spectral behavior of the studied molecule has been compared with its parent molecule, 2-hydroxypyridine (2HP) and 5-(4-fluorophenyl)-2-hydroxypyridine (FP2HP). Structural calculations and potential energy curves along the proton transfer coordinate by the DFT method have been successfully employed to correlate the experimental findings.