A Thiophene-Based Dicyanovinyl as an Anion Chemosensor †

: In recent years, push–pull dicyanovinyl derivatives have been reported for several optical applications, including as ﬂuorescent probes for cyanide ion detection. In the presence of cyanide ions, this type of compound exhibits an ICT mechanism associated with the addition of the cyanide ion to the β position of the dicyanovinyl group, due to the strong electron-withdrawing effect of the cyano groups. These structural changes caused by the addition reaction are accompanied by changes in the probe’s absorption and emission. In this work, we report a thiophene-based dicyanovinyl derivative and its prospective application as an anion chemosensor. The photophysical characterization was carried out in acetonitrile and aqueous acetonitrile solutions. Preliminary sensing assays in the presence of various anions of biological, medicinal, and environmental relevance were conducted, and the interaction with CN − was further explored through spectrophotometric titrations.


Introduction
Anions are involved in several important biological processes that play numerous roles, as well as being involved in environmental and industrial applications. The development of small molecules for anion recognition is an area of research in constant development. The study of anion recognition in the 1960s and 1970s became a fundamental area of supramolecular chemistry with a wide range of applications [1][2][3][4]. The design of anion receptors is a challenge because anions have a small charge/radius ratio and are sensitive to pH, solvents, and solvation [5].
The cyanide ion is a highly toxic substance for the environment and mammals. Small amounts of cyanide can affect various functions in the human body, attacking the vascular, visual, central nervous, cardiac, endocrine, and metabolic systems. The cyanide anion can be absorbed by the lungs, gastrointestinal tract, and skin, causing vomiting, convulsions, unconsciousness, and possibly leading to death. It is lethal to humans at concentrations in the range of 0.5-3.5 mg/kg of body weight. In addition to being found in many foods and plants, cyanides are used industrially in the synthesis of organic chemicals, polymers, and metallurgy, as well as in gold mining. Several strategies for the colorimetric and fluorimetric detection of cyanide ions have been reported in recent years, involving various mechanisms, such as coordination, CdSe quantum dots, the displacement approach, the deprotonation of hydrogen bond interactions, and nucleophilic addition reactions [6,7].
Dicyanovinyl derivatives are widely used as colorimetric and fluorescent chemosensors. Based on the structural properties of this group, the occurrence of nucleophilic addition reactions, at the β position of this group, by chemical species with a high electron density is possible. In the literature, the addition of a cyanide ion to the β position of the dicyanovinyl portion is widely reported, causing significant colorimetric and fluorimetric changes associated with intramolecular charge transfer (ICT), thus allowing for its detection [8][9][10][11][12].
The use of five-membered rings, such as thiophene, allows for the establishment of an electron donor-acceptor relationship with chromophores by increasing π-conjugated systems. In addition to providing thermal and chemical stability, these heterocyclic structures also improve the delocalization of electrons, revealing improved optical properties [13][14][15][16].
In this communication, we report a thiophene-based dicyanovinyl derivative and its prospective application as a cyanide chemosensor. The photophysical characterization was carried out in acetonitrile and aqueous acetonitrile solutions. Preliminary sensing assays, in the presence of various anions of biological, medicinal, and environmental relevance, were undertaken, followed by spectrophotometric titrations with cyanide ion.

Methods and Materials
All reagents were purchased from Sigma-Aldrich, Acros, and Fluka and used as received. UV-visible absorption spectra were obtained using a Shimadzu UV/2501PC spectrophotometer. Fluorescence spectra were collected using a FluoroMax-4 spectrofluorometer. The relative fluorescence quantum yields were determined using a quinine sulfate in 0.05 M H 2 SO 4 as fluorescence standard. The synthesis of thiophene dicyanovinyl derivative 1 was reported by us elsewhere [15].

Photophysical Studies of Thiophene Dicyanovinyl Derivative 1
Thiophene dicyanovinyl derivative 1 was studied in an ACN solution with concentrations between 1.0 × 10 −6 and 1.0 × 10 −5 M. Quinine sulfate (Φ F = 0.546) in 0.05 M H 2 SO 4 was used as fluorescence standard with a concentration of 1.0 × 10 −5 M. Compound 1 was excited at 421 nm, as was the fluorescence standard solution, in 10 mm optical path quartz cells, and the respective fluorescence area was determined. The calculation of the relative quantum fluorescence yield of the dicyanovinyl derivative was performed using Equation (1).
where A stand and A comp are the absorbances of the solutions at the excitation wavelengths of the standard and the compound under study, respectively; F stand and F comp are the areas under the fluorescence plot of the standard and the compound; n stand and n comp represent the refractive index value of the standard and the compound under analysis, respectively.

Preliminary Chemosensing Studies and Spectrophotometric Titrations
Evaluation of compound 1 as a colorimetric and fluorimetric chemosensor was carried out in the presence of several anions with environmental and biomedical relevance ( As a result of the preliminary study, a spectrophotometric titration of dicyanovinyl derivative 1 was performed in the presence of CN − . The solution of the dicyanovinyl derivative (8.0 × 10 −6 M) was prepared in ACN/H 2 O (8:2) and the solution of the anion under study (1.0 × 10 −2 M) was prepared in ACN. The spectrophotometric titration was performed through the sequential addition of the CN − solution to the dicyanovinyl derivative solution.

Photophysical Characterization of Thiophene Dicyanovinyl Derivative 1
The synthesis of thiophene dicyanovinyl derivative 1 (Figure 1) was achieved through a previously published procedure involving a Knoevenagel reaction of the corresponding aldehyde precursor and malononitrile, catalyzed by piperidine in refluxing ethanol [15].
under study (1.0 × 10 M) was prepared in ACN. The spectrophotometric titration was performed through the sequential addition of the CNsolution to the dicyanovinyl derivative solution.

Photophysical Characterization of Thiophene Dicyanovinyl Derivative 1
The synthesis of thiophene dicyanovinyl derivative 1 (Figure 1) was achieved through a previously published procedure involving a Knoevenagel reaction of the corresponding aldehyde precursor and malononitrile, catalyzed by piperidine in refluxing ethanol [15].    under study (1.0 × 10 M) was prepared in ACN. The spectrophotometric titration was performed through the sequential addition of the CNsolution to the dicyanovinyl derivative solution.

Photophysical Characterization of Thiophene Dicyanovinyl Derivative 1
The synthesis of thiophene dicyanovinyl derivative 1 (Figure 1) was achieved through a previously published procedure involving a Knoevenagel reaction of the corresponding aldehyde precursor and malononitrile, catalyzed by piperidine in refluxing ethanol [15].

Spectrophotometric Titration of Thiophene Dicyanovinyl Derivative 1 with Cyanide
From the previous tests and considering the colorimetric and fluorimetric response of compound 1 to the presence of CN -, a spectrophotometric titration of dicyanovinyl derivative 1 with CNwas performed in ACN/H2O (8:2), which revealed a very sensitive colorimetric response toward this anion. The UV-visible spectra revealed that the intensity of the absorption band at 422 nm decreased rapidly and progressively upon the addition of the anion. The number of CNequivalents for the total loss of absorbance was about four equivalents in ACN/H2O (8:2) solution ( Figure 4). A spectrofluorimetric titration of compound 1 was also performed in ACN/H2O (8:2) with CN − . The fluorescence spectra revealed that the intensity of the emission band at 521 nm decreased progressively upon the addition of the anion, in accordance with the results from the spectrophotometric titration. The number of CNequivalents to achieve an almost complete fluorescence quenching (>90%) was about five equivalents in ACN/H2O (8:2) ( Figure 5).

Spectrophotometric Titration of Thiophene Dicyanovinyl Derivative 1 with Cyanide
From the previous tests and considering the colorimetric and fluorimetric response of compound 1 to the presence of CN -, a spectrophotometric titration of dicyanovinyl derivative 1 with CNwas performed in ACN/H2O (8:2), which revealed a very sensitive colorimetric response toward this anion. The UV-visible spectra revealed that the intensity of the absorption band at 422 nm decreased rapidly and progressively upon the addition of the anion. The number of CNequivalents for the total loss of absorbance was about four equivalents in ACN/H2O (8:2) solution ( Figure 4).

Conclusions
The thiophene dicyanovinyl derivative 1 was characterized by absorption and emission spectroscopies, establishing its photophysical properties. A preliminary chemosensory study in ACN and ACN/H2O (8:2) of dicyanovinyl derivative 1 in the presence of selected anions showed a colorimetric and fluorometric response of this compound in the presence of cyanide, through loss of color and fluorescence. Therefore, more detailed spectrophotometric and spectrofluorimetric titrations were conducted, which revealed that the number of equivalents necessary for achieving a plateau was four and five equivalents, respectively, which can be considered a very sensitive response. Based on the preliminary chemosensory studies and the spectrophotometric titrations, this novel thiophene dicyanovinyl derivative is a promising candidate as a colorimetric and fluorimetric probe for cyanide ion recognition.

Conclusions
The thiophene dicyanovinyl derivative 1 was characterized by absorption and emission spectroscopies, establishing its photophysical properties. A preliminary chemosensory study in ACN and ACN/H 2 O (8:2) of dicyanovinyl derivative 1 in the presence of selected anions showed a colorimetric and fluorometric response of this compound in the presence of cyanide, through loss of color and fluorescence. Therefore, more detailed spectrophotometric and spectrofluorimetric titrations were conducted, which revealed that the number of equivalents necessary for achieving a plateau was four and five equivalents, respectively, which can be considered a very sensitive response. Based on the preliminary chemosensory studies and the spectrophotometric titrations, this novel thiophene dicyanovinyl derivative is a promising candidate as a colorimetric and fluorimetric probe for cyanide ion recognition.