Abstract
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.
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