The effect of hydrogen bond strength on emission properties in 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines

• The emission maxima of derivatives of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine depends on the strength of the hydrogen bond.
• Improved predictions of the fluorescence characteristics can be achieved using the QTAIM approach.
• Easily calculated HOMA indexes based just on geometry make it possible to predict the emission wavelength of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine derivatives.

The correlation between the structures and molecular properties on the one hand and the fluorescence properties on the other for a set of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines exhibiting excited-state intramolecular proton transfer (ESIPT) was analyzed via comparison of the optical data and computational results. Ab initio calculations were used to characterize the electronic transitions of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine and its three derivatives both in non-polar and polar solvents by using DFT and TDDFT methods and the IEFPCM model. The calculations involving geometry optimization in the excited state allow to precisely predict the emission wavelength from the excited keto state. We constructed and studied the correlations between the positions of the emission maximum for the compounds considered and the hydrogen bond energies values for ρ(r) and ▽2ρ(r) at the hydrogen bond critical point and the aromaticity of chosen structural fragments. It was found that the fluorescence properties of ESIPT molecules could be manipulated by directed modification of their chemical structure, most importantly by the introduction of electron-withdrawing functionalities into their proton donor moieties. The electron-withdrawing CN group hypsochromically shifts the emission from the keto excited state by ∼60 nm and strengthens at the same time the intramolecular hydrogen bond.

Figure optionsDownload as PowerPoint slide