New Charge Transfer Complexes of Self-Assembled TCNQ Derivatives and Picric Acid: Synthesis, Characterization, Crystallography and Sensing Application

Abstract

TCNQ derivatives with primary/secondary amines as substituents are commonly known as diaminodicyanoquinodimethanes (DADQs); a D-π-A molecular system, being zwitterionic are generally stable. Facile and simple charge transfer (CT) complexes were achieved by the reaction between 7,7-bis (N,N-dimethylethylenediamino)-8,8-dicyanoquinodimethane (BMEDDQ),7,7-bis (N,N-diethylethylenediamino)-8,8-dicyanoquinodimethane (BDEDDQ) and picric acid (PA) each respectively; resulting in BMEDDQ-PA [1] and BDEDDQ-PA [2], further followed by effortless purification by crystallization. Acquired CT complexes (CTs) [1], [2] were characterized thoroughly by crystallography, spectroscopic and microscopic techniques. Crystallographic investigations revealed extensive H-bonding, π-π stacking, short contacts (<sum of the Van der Waals radii of atoms) among [1], [2] (molecular dipoles) and PA moieties; leading to the supramolecular self-assemblies. Aggregation-caused quenching (ACQ) was observed among the solid [1] and [2], marking non-emissive feature, when compared to their solutions. Henceforth, BMEDDQ and BDEDDQ were further utilized for PA sensing. The limit of detection (LOD) was ~3.0×10⁻⁶ M for BMEDDQ and BDEDDQ. Field emission scanning electron microscopy (FESEM) manifested varied aggregate formation with increasing PA concentration. Simplicity in synthesis and analysis accompanied by immediate color change i. e. fast response/fluorescence-based quenching was noteworthy. Moreover, selectivity for PA, good sensitivity in solid as well as solution, and solubility in MeCN : H₂O (1 : 9) mixture accomplish adequacy in detection from water bodies, thus BMEDDQ and BDEDDQ manifest as potential chemosensors for the detection of PA.