Theoretical study on the sensing mechanism of a fluorescence chemosensor for the cyanide anion

The sensing mechanism of the ratiometric fluorescence chemosensor 2-((6-(diethylamino)quinolin-2-yl)methylene)malononitrile (3A) for the cyanide anion (CN^-) has been investigated theoretically. The calculated results show that the small reaction barrier (8.58 kcal/mol) of 3A implies a rapid response speed to CN^-, and that the large interaction energy (25.75 kcal/mol) between 3A and CN^- indicates a high selectivity to CN^-. The results of condensed dual descriptor calculation confirm that CN^- attacks the C₂ site of 3A rather than other sites. The nucleophilic addition reaction of CN^- breaks the original conjugation structure of 3A and results in the redistribution of the charge. The frontier molecular orbitals (MOs) and the Hirshfeld population analysis demonstrate that the long-rang electronic transition between the diethylamino moiety and the dicyanovinyl group in 3A is cut off after the addition of CN^- and a local electronic transition between the quinoline and diethylamino moiety is formed. These changes lead to a blue shift (63 nm) of the fluorescence emission in the nucleophilic addition product (P) compare to 3A.