Distinct Tetracyanoquinodimethane Derivatives: Enhanced Fluorescence in Solutions and Unprecedented Cation Recognition in the Solid State

Abstract

Tetracyanoquinodimethane (TCNQ) is known to react with various amines to generate substituted TCNQ derivatives with remarkable optical and nonlinear optical characteristics. The choice of amine plays a crucial role in the outcome of molecular material attributes. Especially, mono/di-substituted TCNQ’s possessing strong fluorescence in solutions than solids are deficient. Furthermore, cation recognition in the solid-state TCNQ derivatives is yet undetermined. In this article, we present solution-enhanced fluorescence and exclusive solid-state recognition of K+ ion achieved through the selection of 4-(4-aminophenyl)­morpholin-3-one (APM) having considerable π-conjugation and carbonyl (CO) functionality, particularly in the ring. TCNQ when reacted with APM, in a single-step reaction, resulted in two well-defined distinct compounds, namely, 7,7-bis­(4-(4-aminophenyl)­morpholin-3-ono)­dicyanoquinodimethane (BAPMDQ [1], yellow) and 7,7,8-(4-(4-aminophenyl)­morpholin-3-ono)­tricyanoquinodimethane (APMTQ [2], red), with increased fluorescence intensity in solutions than their solids. Crystal structure investigation revealed extensive C–H−π interactions and strong H-bonding in [1], whereas moderate to weak interactions in [2]. Surprisingly, simple mechanical grinding during KBr pellet preparation with [1, 2] triggered unidentified cation recognition with a profound color change (in ∼1 min) detected by the naked eye, accompanied by a drastic enhancement of fluorescence, proposed due to the presence of carbonyl functionality, noncovalent intermolecular interactions, and molecular assemblies in [1, 2] solids. Cation recognition was also noted with various other salts as well (KCl, KI, KSCN, NH4Cl, NH4Br, etc.). Currently, the recognition mechanism of K+ ion in [1, 2] is demonstrated by the strong electrostatic interaction of K+ ion with CO and simultaneously cation−π interaction of K+ with the phenyl ring of APM, supported by experimental and computational studies. Computational analysis also revealed that a strong cation−π interaction occurred between the K+ ion and the phenyl ring (APM) in [2] than in [1] (ΔG binding calculated as ∼16.3 and ∼25.2 kcal mol–1 for [1] and [2], respectively) providing additional binding free energy. Thus, both electrostatic and cation−π interactions lead to the recognition. Scanning electron microscopy of drop-cast films showed microcrystalline “roses” in [1] and micro/nano “aggregates” in [2]. Optical band gap (∼3.565 eV) indicated [1, 2] as wide-band-gap materials. The current study demonstrates fascinating novel products obtained by single-pot reaction, resulting in contrasting optical properties in solutions and experiencing cation recognition capability exclusively in the solid state.

Design and Development of Distinct Tetracyanoquinodimethane Derivatives Exhibiting Dual Effect of Fluorescence and Anticancer Activity

“…Consequently, from the above study we can infer that mono-substituted derivatives [1], [4] showed similar features mainly due to conjugation, and di-substituted (displaying large dipole moments) [2], [3] and [7] exhibited alike attributes primarily owing to zwitterionic characteristic. [33][34]39,44] [2], [3] and [7] demonstrated fluorescence enhancement in solids (Figure 6, Table S5-S9) as well, feature noticed in certain previously reported DADQ's, due to the strong zwitterion nature in di-substituted TCNQ's leading to restricted intramolecular motion in solid/aggregated state. [47][48] Nevertheless, [1] and [4] possess extended conjugation in their structural arrangement leading to the enhancement of fluorescence in solutions.…”

“…A minimal blue shift (∼12 nm) in the λ max emission (Table S4) of [ 7 ] in MeOH, EtOH when compared to the latter, could be due to the restricted non‐radiative decay of the excited states owing to its stabilization. One of our very recently reported BAPMDQ and APMTQ; [44] bearing 4‐(4‐aminophenyl) morpholin‐3‐one (APM) revealed similar blue shift in DMSO and DMF. Consequently, from the above study we can infer that mono‐substituted derivatives [ 1 ], [ 4 ] showed similar features mainly due to conjugation, and di‐substituted (displaying large dipole moments) [ 2 ], [ 3 ] and [ 7 ] exhibited alike attributes primarily owing to zwitterionic characteristic [33–34,39,44] .…”

“…Among the five potent molecules [ 1 ] and [ 4 ] showed immense emission intensity in solutions than their solids, likely due to the extent of conjugation caused on the account of aromatic amines possessing electron withdrawing groups. However, the decreased emission intensity in solutions of [ 1 ], [ 4 ] in contrast to [ 2 ], [ 3 ], [ 7 ] might be due to the overlap of S 1 /T 1 vibronic modes with S 0 [44] . Interestingly, in [ 3 ] the aromatic amine is directly attached to TCNQ; thus the corresponding n‐ π* and π ‐ π * transitions were observed at 382 nm and 302 nm respectively; the charge transfer peak from π ‐donor to π ‐acceptor has also predominantly appeared in the absorption spectrum (Figure 5).…”

“…However, the decreased emission intensity in solutions of [1], [4] in contrast to [2], [3], [7] might be due to the overlap of S 1 / T 1 vibronic modes with S 0 . [44] Interestingly, in [3] the aromatic amine is directly attached to TCNQ; thus the corresponding nπ* and π-π* transitions were observed at 382 nm and 302 nm Crystallographic Investigation: [1], [2] were crystallized as dark red and yellow blocks from acetonitrile and DMF each respectively by slow evaporation at room temperature. [1] had acetonitrile as solvent of crystallization and found to be disordered; using solvent mask, the disordered solvent molecule has been omitted.…”

“…It has been reported that introduction of phenyl rings and electron withdrawing groups on amine substituents has great impact on fluorescence with significant bathochromic shift. [44] Chromaticity output diagrams (Figure 7) revealed the emitted colors in the solids; [2], [3] and [1], [2], [3], [4] and [7] in isopropyl alcohol; images of the same under b) ambient light and c) short UV (254 nm). ( 5) in b), c) refers to compound [7] in Figure 1.…”