Excited state intramolecular proton transfer as a consequence of excited state aromaticity changes

In the present work, DFT/TD-DFT methods have been implemented to study the theoretical concept of how a change in aromaticity supports the presence of the ESIPT process. The representative molecule used was 2,2’-bipyridyl-3,3’-diol-5,5’-dicarboxylic acid ethyl ester (BP(OH)₂DCEt), consisting of a double proton transfer via (N–H) intramolecular hydrogen bonding. The solvent effect on the ESIPT process was also investigated with the help of polar Water (H₂O) and non-polar Cyclohexane (CYH) solvent. It was observed that intramolecular hydrogen bonding was strengthening in the S₁ state, which was more prominent for polar solvents. MCI aromaticity index explained the decrease in stability of molecules upon excitation. Further charge redistribution, NICS(1)_zz analysis and HOMA index of the molecule reinforced the occurrence of the ESIPT process supported by the aromatic behaviour of the molecule. Furthermore, the stability of the molecule in S₀ and S₁ states as enol and keto form respectively was confirmed by PES plots.