Molecular materials with contrasting optical responses from a single-pot reaction and fluorescence switching in a carbon acid

Abstract

Three for the price of one: The reaction of tetracyanoquinodimethane with an aromatic amine is shown to follow an unusual course leading to three products in a single pot. The product molecules form a family of materials with contrasting optical responses, in that they exhibit light emission of different colors and emission switching that is sensitively triggered by acidic/basic environments (see scheme). A wide variety of amines are known to react with 7,7,8,8-tetracyanoquinodimethane (TCNQ) to yield push-pull diaminodicyanoquinodimethanes with a strongly zwitterionic structure and significant optical and nonlinear optical properties. A novel course of reaction is observed now with the amine 2-methyl-4-chloroaniline, which leads to three well-defined products, A-C, in a single pot. A and B are formed through the replacement of one cyano group in TCNQ by the amine; A is a carbon acid and B is its corresponding salt. C is the conventional product in which two cyano groups in TCNQ are replaced by the amine. The products are characterized structurally and spectroscopic studies reveal contrasting optical responses. A is nonfluorescent, whereas B and C show red and green emission, respectively, in the solution and solid states. The acid/conjugate-base pair A and B can be interconverted through facile, reversible, and repeated deprotonation/protonation cycles, which are accompanied by instantaneous switching of the fluorescence. The current study illustrates an interesting case of a single-pot reaction yielding different optical materials with attributes that can be switched through simple approaches such as protonation or tuned through modification of the push-pull characteristics.