Norharmane prototropism in choline chloride-based deep eutectic solvents

•Role of H-bond donor (HBD) of a deep eutectic solvent (DES) on prototropism of norharmane is assessed in 8 different DESs.•Low HBD acidity and/or high HBA basicity of the DES support neutral form of the probe in the ground state.•Only cationic form of the probe is present in the excited-state in highly acidic DESs and those with high HBD acidity.•The norharmane prototropic form can be correlated with empirical Kamlet-Taft parameters of DESs.•Both HBD acidity and HBA basicity of the solubilizing media controls the prototropism of norharmane within DESs.