Collective Hydrogen Bonding between Amides Enhances the Basicity of Their Clustered Form in the Proton Transfer of a Super Photoacid

Abstract

The excited state proton transfer (ESPT) of the strong photoacid N-methyl-7-hydroxyquinolium (NM7HQ) was studied in the presence of N-methylbenzamide (NMB) as a base in the aprotic solvent of acetonitrile. In the ground state, it is found that the complexed form of NM7HQ and NMB exists with a 1:1 ratio with the association constant (K) to 22.2 ± 1.7 M-1, obtained by using the Benesi-Hilderbrand relation. Also, the cationic photoacid fluorescence lifetime Stern-Volmer relation of fast decay time shows that the ESPT of NM7HQ and NMB has the molecularity of two with amides. Therefore, in the excited state, one more molecule of NMB is needed to the ground state NM7HQ-NMB complex for ESPT. It shows that the hydrogen bonded complex of 2 molecules of NMB can enhance the basicity of the NMB molecule.