Self-assembled tetracyanoquinodimethane derivatives: differential fluorescent responses on sensing copper and mercury ions in an aqueous medium†

Copper and mercury metal ions are known to cause serious harm to biological and environmental systems, and therefore the development of new efficient sensory systems for the detection of copper and mercury ions in aqueous media is always encouraged and in need. Herein, we present the employment of di-substituted tetracyanoquinodimethane (TCNQ) derivatives for metal ion recognition. Owing to their fluorescent properties, single-step synthesis and easy purification, small molecules, namely 7,7-bis(1-(2-aminoethyl)pyrrolidino)-8,8-dicyanoquinodimethane (AEPRDQ) [1], 7,7-bis(1-(2-aminoethyl)piperidino)-8,8-dicyanoquinodimethane (AEPIDQ) [2] and 7,7-bis(N,N-diethylethylenediamino)-8,8-dicyanoquinodimethane (BDEDDQ) [3], have been utilized for Cu²⁺ and Hg²⁺ sensing applications. [1]–[3] considered in this study showed the formation of self-assembled nanoaggregates in aqueous media. A thorough investigation on the effect of the microenvironment, such as temperature, pH, polarity, etc., on the extent of self-agglomeration was done spectroscopically. Interestingly, in an acetonitrile–water (1 : 9) solvent mixture, the compounds showed diverse responses to Cu²⁺ and Hg²⁺ ions. [1] manifested fluorescence quenching with Cu²⁺ ions, while enhanced emission was noted with Hg²⁺. Hg²⁺ induced fluorescence quenching with [2] while no prominent quenching is seen on the addition of Cu²⁺. Adding Cu²⁺ and Hg²⁺ resulted in a turn-off fluorescence signal in [3]. Hence, alteration by simply varying the terminal functional groups among the said [1]–[3] resulted in varied metal ion sensing responses.