Processes of molecular association and excimeric emission in protonated N,N-dimethylformamide

Formation of associates of N,N-dimethylformamide (DMF) molecules was studied to clarify their role in photoluminescent activity of protonated DMF solutions. The association of DMF molecules was observed in dilute aqueous solutions at concentrations of DMF above ∼4x10−2 M. The association is enhanced when the CO bond of the DMF molecule is activated by protonation with hydrochloric acid, which leads to appearance of an excimeric emission at ∼530 nm. The excitation spectrum of the excimeric emission showed the excitation maximum in the region of the absorption of DMF associates, which is a first evidence of a more complex mechanism of excimer formation originating from excitation of associated rather than monomeric molecules in the ground state. A simple approach was provided to evaluate a number of molecules in the excimer structure. An original theory has been developed, and it was calculated that the DMF excimer has a dimeric nature. A model of the excimer formation was proposed, which suggests that a hydrogen-bonded associate is an intermediate form leading to the excimeric structure upon excitation. It was observed that DMF possesses also a monomeric emission with the emission maximum at ∼385 nm, which was attributed to the intramolecular charge-transfer process. It has been found that the change in structure of the DMF associates via the liquid-solid phase transition affects both excitation and emission bands of excimers, so that the excimeric emission shifts to the blue region and intermixes with the emission of DMF monomers.