The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin and Alizarin-S: A Comparative study

Abstract

Chemosensing properties of Alizarin (A3) and Alizarin S (AS3) towards anions and cations in acetonitrile are reported. The absorption and fluorescence properties of the two molecular entities were investigated in CH3CN. Based on the excited state intermolecular proton transfer system (ESIPT), the probes were able to collectively discriminate specific cations and anions via colorimetric observations and spectrometric activities. The investigation revealed that A3 was selective to Cu2+, Fe3+, and Fe2+, compared to Cu2+, Zn2+, Fe3+, and Ni2+ for AS3. The disagreement in spectral responses were ascribed to the strong electron withdrawing group present in AS3, hence the difference in behaviors. Moreover, the emission properties displayed by A3 and AS3 upon molar titrations with cations, were closely similar for all cations, which all nearly experienced fluorescence quenching, except for Zn2+ with A3, which exhibited fluorescence enhancement. Similarly, a two-step fluorescence effect was observed in A3 towards anions, which experienced both fluorescence quenching and enhancement, with incremental additions. The simultaneous fluorescence effects were ascribed to the deprotonation activities experienced by A3, as excess anion quantities were added. Thus, the sulfonyl electron withdrawing group had an effect on the Alizarin structure, towards the discrimination of anions and cations, both colorimetrically and fluorometrically.

1. Introduction

Alizarin is a stable organic compound, prominently known as a red dye with significant industrial applications, particularly its use in dying textile fabrics. The application in textile coloration industry is inspired by the fact that alizarin is a natural compound often referred a natural dye, initially extracted from the roots of plants of the madder genus [1], before it was synthetically made [2], thus, it is a molecular species from nature, exhibiting and portraying green chemistry properties. The molecular structural framework of alizarin is characterized by the anthraquinone moiety bearing two para-positioned intermolecular hydroxyl groups both on one carbocyclic ring adjacent to the quinone ring. A typical excited state intermolecular proton transfer system (ESIPT), alizarin is a natural dye which has been widely used in pigments, as anticancer agents as well as chemical reagents for use in data recording and storage materials due to its tunable electronic properties [3, 4]. In addition, the natural dye has strong antigenotoxic activity, ascribed to the transfer of ultrafast electrons to TiO₂-based materials, which can also perfectly fit as an excellent photosensitizer in dye-sensitized solar cells. Thus, alizarin chromophore has been favored by many researchers, both experimentally and theoretically [5, 6, 7, 8, 9, 10].

Ideally, alizarin forms an intramolecular hydrogen bond between a hydroxyl and a carbonyl group, in the ground and excited states, whereby upon photoexcitation, a proton transfer from the hydroxyl to the carbonyl group is observed, which normally results in dual emission bands of the locally excited (LE) and proton-transferred (PT) tautomers [9, 11, 12, 13]. Characteristically, this process in known as ESIPT, which is viewed as a very fast photo-tautomerization process taking place along an intramolecular hydrogen bond between two atoms that are significantly tuned by electronic excitation. In recent studies, the practical and applications of the ESIPT mechanism based on their photophysical characteristics and properties have been extensively explored and investigated, especially in laser dyes, OLEDs, molecular switches, fluorescence sensors, and particularly biological systems [14, 15, 16, 17, 18, 19, 20]. More importantly, the ESIPT based reactions increase the acidity of the proton donor groups, due to the change of electron density after electron excitation, and the basicity of the acceptor groups is significantly increased to promote the formation of tautomer by intramolecular proton transfer [4, 21, 22, 23, 24, 25, 26].

On the other hand, molecular recognition has been the epic center of supramolecular chemistry due to its significant role in biological and environmental systems, through the host-guest interaction chemistry. Consequently, chemosensors are designed for specific target analytes based on their chemical make-up and complementarity towards each other. The impact of sensing biologically important anions such as acetate, cyanide, fluoride, dihydrogen phosphate, etc., have been receiving attention in literature and many industrial applications. A large volume of colorimetric and fluorometric probes for anions such as fluoride (F-), cyanide (CN-), acetate (AcO-), dihydrogen phosphate (H₂PO₄⁻), hydroxide (OH-) and others have been developed. Hydroxide ions play a very significant role in environmental and physiological systems, thus monitoring its concentration in these systems must be highly prioritized [27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37]. Moreover, the presence of soft (donor) atoms such as oxygen from hydroxyl and quinone groups of the carboxylic ring raises the prospect of dual sensing, for both cations and anions, which stems from the presence of both, the anion receptors (-OH) and the cationphilic groups, through coordination induced interaction [38, 39, 40, 41].

Herein, we have conducted a comparative study for the two alizarin-based derivatives, A3and AS3(Figure 1), to investigate their chemosensing properties. The study literally focuses on the effect of the sulfonyl hydroxide group (-SO₃H) present in AS3, which is the only difference between the two chemical entities. The two entities are highly stable and available commercially, which are very rich in hydroxyl groups, paving ways for possible hydrogen bonding based charge transfer bonding. The two dyes displayed interesting behaviors in the presence of anions and cations, in water-soluble acetonitrile (CH₃CN) solvent, with certain commonalities and variations upon interacting. Thus, the two probes (A3and AS3) can be used as colorimetric/fluorometric probes for discriminating specific cations and anions, with distinctive color changes in organic-aqueous solvents.

Figure 1.

The molecular 2-D structures of (a) alizarin (A3) and (b) alizarin S (AS3).

2. Experimental section

2.1 Materials and apparatus

Compound A3and AS3were obtained from a commercial source. All the chemicals and reagents used in this study were all of analytical reagent grade. The anions of Cl⁻, CN⁻, OH⁻, AcO⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻, N₃⁻, NO₃⁻, OCN⁻ and F⁻ were purchased as tetrabutylammonium salts from Sigma–Aldrich. Absorption measurements were performed on Perkin Elmer Lambda 35 spectrophotometer in a standard 3.0 ml quartz cuvette with 1 cm path length at room temperature. Fluorescence measurements were carried out on a steady state excitation and emission spectra on the Molecular Device SpectraMax M2, Plate Reader.

2.2 Procedures for UV: Vis experiments

All UV–Vis spectra were recorded in acetonitrile (CH₃CN) solvent on a Perkin Elmer Lambda 35 spectrometer by adding Tetrabutylammonium salts while keeping the concentration of ASor AS3constant (1 × 10⁻⁵ M). Tetrabutylammonium salt (TBA) anions of Cl⁻, CN⁻, OH⁻, AcO⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻, N₃⁻, NO₃⁻, OCN⁻ and F⁻ were used for UV–Vis experiments. In addition, a range of heavy metal cation salts (AgNO₃, Al(NO₃)₃, Co(NO₃)₂, Cr(NO₃)₃, Cu(NO₃)₂, Fe(NO₃)₂, Hg(NO₃)₂, MnCl₂, Ni(NO₃)₂, Pb(NO₃)₂, SnCl₂, Zn(NO₃)₂, CdCl₂) were used for UV–Vis titrations.

3. Results and discussions

3.1 Photophysical studies of A3 and AS3 with anions and cations

3.1.1 Visual observation of A3 and AS3 with anions and cations

In order to establish the occurrence of chemical interactions between A3and the anions, a series of prepared anionic and cationic solutions (0.03 M) in CH₃CN were tested separately, about 3 ml of A3(1 x 10⁻³ M) in CH₃CN, at room temperature. The colorimetric activities observed were recorded (Figure 2). The addition of anions (Cl⁻, CN⁻, OH⁻, AcO⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻, NO₃⁻, ClO₄⁻, N₃⁻, and F⁻) to A3, were investigated through naked eye observable color changes. The dropwise addition of anions (TBA salts) to the shinny-yellow colored A3solution, resulted in a series of naked eye observable color changes, ranging from deep blue (CN⁻), blue-violet (AcO⁻), blue-brownish (H₂PO₄⁻), brown-yellowish (N₃⁻), deep blue (OH⁻) and (blue-violet (F⁻) as displayed (Figure 2). The intensity of the colors is an indication that A3is a color-based indicator, with such unique concentrated color change. Clearly, the color changes were due to chemically associated interaction between A3and the anions. However, none of the other anions (Cl⁻, Br⁻, HSO₄⁻, NO₃⁻ and ClO₄⁻) used could induce any significant color changes, even when large quantities were added. Interestingly, A3was discriminatingly selective and sensitive to N₃⁻, turning the yellow color A3to dark-yellowish A3-N₃complex. Probes sensitive and selective to N₃⁻ are extremely rare in literature, with this being the least in literature reports. In addition, the addition of OCN⁻ to A3displayed a light brownish color of the complex A3-OCN. The two anions formed distinctive colors on interaction with A3, defying the trend with other anions above, which formed strong blue to violet color upon complexed (Figure 2a).

Figure 2.

Observable colorimetric changes of different anions upon interacting with (a)A3and (b)AS3both (1 x 10⁻⁵ M) in CH₃CN at room temperature.

Moreover, comparative colorimetric studies were conducted for AS3and anions (Figure 2b), were visual observation clearly showed the variation between the two-alizarin derivatives (A3and AS3). Like with A3, the addition of anions to yellow AS3resulted in a variety of colorimetric observations, such as F⁻ (deep purple), AcO⁻ (pink), H₂PO₄⁻ (dark yellow), N₃⁻ (reddish-brown), OCN⁻ (reddish-brown), OH⁻ (deep-blue), CN⁻ (purple) and Br⁻ (no change) as displayed (Figure 2b). The effect of the sulfonyl group on the alizarin molecule was clearly visible upon interacting with anions, giving different colors between A3and AS3. Apart from the OH⁻ and F⁻ ions, which displayed deep-blue and violet colors for both A3and AS3respectively, the rest of the anionic interactions displayed different colors. However, the pattern in colors are still displayed among the two probes, but clearly the intensities differed perhaps due to different association constants, where some are more strongly associated comparing to others.

Furthermore, the multi-colorimetric sensor (A3) was also able to selectively and sensitively detect the presence of cations in acetonitrile. The gradual addition of heavy metal cations, of divalent in nature, to the A3solution saw the yellow color changes to varieties of colors ranging from light-green (Cu²⁺), deep yellow (Zn²⁺), dark brown (Fe³⁺) and light orange (Ni²⁺) as displayed (Figure 3a). The range of different colors could mean diverse interacting modes with the probe, or dissimilar geometrical complementarities towards each cation. In addition, the presence of the sulfonic acid group in AS3could induce mole or less similar cation interactions, even though different colors, with Cu²⁺ (light green), Zn²⁺ (crimson red), Fe³⁺ (brown) and Ni²⁺ (light orange). In addition, there was a noticeable additional color activities when Hg²⁺ was added to AS3, the color from light yellow (AS3) to slight deep yellow of the complex formed (AS3-Hg) as displayed (Figure 3b). The variations in colors of the complexes formed upon the two probes interacting with cations, signifies their differences in coordination and the effect of the sulfonic acid group present in AS3.

Figure 3.

Observable colorimetric changes of different cations upon interacting with (a)A3and (b)AS3both (1 x 10⁻⁵ M) in CH₃CN at room temperature.

3.1.2 Absorption properties of A3 and AS3

Spectrally, the two probes were characterized by more or less similar absorption spectra, both of them defined by the π → π* transitions in the ultraviolet region, as well as the internal charge transfer band (ICT) in the visible region. Specifically, AS3displayed a high-energy intense peak centered at 249 nm and another moderate absorption band at 427 nm (Figure 4a), with a visual yellow color of the probe, (Figure 4a inset). Similarly, A3was characterized by an intense π → π* band at 246 nm, as well the internal charge transfer band at 420 nm (Figure 4b), with the color displayed (Figure 4b). In addition, in both cases (A3and AS3) each probe has a light hump at 328 nm and 330 nm respectively, ascribed to the admixture of the π → π* and the ICT transition overlaps. The only difference in the two spectra is the slight red shift of the spectra of AS3with about 2 nm of all the bands.

Figure 4.

Absorption spectra of (a)AS3(1 x 10⁻⁵ M) and (b)A3(1 x 10⁻⁵ M), both in CH₃CN. The insets are colorimetric signatures under daylight conditions.

3.1.3 Absorption studies of A3 and AS3 on interaction with anions

The interaction of A3with anions was characterized by similar changes in all four titrations. For instance, the molar addition of 5 equiv. of CN⁻ to A3, resulted in the gradual disappearance of the ICT band at 420 nm, concomitant with the appearance of a band associated with 328 nm peak centered now at 350 nm, as well as a completely new band deep in the visible light region centered at 570 nm (Figure 5a). In addition, an intense π → π* band at 246 nm disappeared followed by another new peak at 260 nm, upon the molar addition of CN⁻. Evidently, several isosbestic points were observed at 222 nm, 254 nm, 292 nm, 390 nm and 466 nm, which testimony to the co-existence and formation of new complexes, at equilibrium (Figure 5a). Similar patterns were observed upon gradual addition of AcO⁻, F⁻ and OH⁻ (Figure 5b–d), which were all due to the hydrogen bonding interaction of these anions with the hydroxyl groups of A3. It is evident that the interactions were similar in nature, given the similarities of the absorption spectra in all four cases, which are all via hydrogen bonding and suspectedly deprotonations.

Figure 5.

The absorption titration spectra ofA3(1 x 10⁻⁵ M) in CH₃CN, with 5 equiv. of (a) CN⁻, (b) AcO ⁻, (c) F⁻ and (d) OH⁻ at room temperature.

On the other hand, the molar titrations of AS3with anions resulted in similar patterns as those of A3for the three anions (F⁻, CN⁻, OH⁻) as displayed (Figure 6a,c, and d). The molar titration of AS3with F⁻ saw a gradual disappearance of the ICT peak at 427 nm, concomitantly with the appearance of new peaks at 350 nm and 550 nm respectively (Figure 6c). The spectral activities are ascribed to the hydrogen bonding induced charge transfer upon bonding has taken place, to form complex pedants of AS3-F, AS3-CNand AS3-OH. Like in A3, the spectral activities were accompanied by several isosbestic points, at 238 nm, 300 nm, 400 nm and 463 nm, which proves the formation of new pedants co-existing with other species at equilibrium. The other two titrations have resulted in similar patterns like that of F⁻ (AS3-F), with spectral shifts precisely resembling each other (Figure 6a and d). Uniquely, the molar addition of N₃⁻ to AS3resulted in a slightly different spectra, comparing to the previous three (F⁻, CN⁻, OH⁻), with the disappearance of the ICT band at 427 nm, simultaneously with the formation of new bands at 330 nm and 526 nm (Figure 6b). This was accompanied by isosbestic points observed at 308 nm, 373 nm and 466 nm respectively, signifying the formation of the pedant complex, and the co-existence of the probe and the complex at equilibrium. The interaction between N₃⁻ and AS3is still suspectedly through hydrogen bonding, even though it slightly differs from the rest, the binding position might be different.

Figure 6.

The absorption titration spectra ofAS3(1 x 10⁻⁵ M) in CH₃CN, with 5 equiv. of (a) CN⁻, (b) N₃⁻, (c) F⁻ and (d) OH⁻ at room temperature.

Notingly, among the rest of the anions, H₂PO₄⁻ was still able to induce changes when added to A3, same way like F⁻, CN⁻ & OH⁻, with all spectral characteristics (Figure 7a). Several isosbestic points were observed indicating that two species were co-existing in equilibrium. In addition, presence of AcO⁻ when molar added to AS3, spectral changes were observed, displaying similar behavior to AS3-F, AS3-CNand AS3-OHas indicated (Figure 7f). This illustrates that the interaction between AS3and AcO⁻ was of hydrogen bonding nature, through the hydroxyl groups of the probe. Other anions used could not induce any significant changes when added to A3(Figure 7b and 7c) and AS3(Figure 7d) respectively. Thus, the main variation of the two probes (A3& AS3) towards anions was observed with the discrimination of N₃⁻, which AS3was able to detect in addition to the other anions. Moreover, unlike A3, AS3was unable to detect the presence of H₂PO₄⁻ as indicated (Figure 7a), probably due to the presence of the sulfonic acid group in AS3.

Figure 7.

The absorption titration spectra ofA3(1 x 10⁻⁵ M) in CH₃CN, with 5 equiv. of (a) H₂PO₄⁻, (b) N₃⁻, (c) other anions, andAS3with 5 equiv. of (d) other anions, (e) H₂PO₄⁻, (f) AcO⁻, at room temperature.

3.1.4 Absorption studies of A3 and AS3 on interaction with cations

Complementary to colorimetric experiments, UV–Vis spectroscopic experiments were conducted to investigate how absorption properties of A3and AS3were influenced by the presence of cations in the given solvent system. Subsequently, the molar addition of Cu²⁺ to A3resulted in spectra shifts of significant distinction. The molar titration of A3with Cu²⁺ resulted in the gradual disappearance of the ICT band at 423 nm concomitant with the appearance of a new band at 383 nm, accompanied by an isosbestic point at 383 nm (Figure 8a). The isosbestic point clearly shows the formation of a complex (A3-Cu) from the probe (A3), the mechanism attributable to electronic energy transfer caused by the coordination between the guest (Cu²⁺) and the host (A3) species. The chelation-induced spectral changes were due to the coordination of the p-orbitals of the oxygen atoms (hydroxyl and/or carbonyl group) of A3to the empty d-orbitals of Cu²⁺ to form a copper complex (A3-Cu). The chelation-induced change in spectra were in agreement with the light green color displayed upon introducing Cu²⁺ to A3, which was distinctively different from the interactions with other cation (Figure 8). However, the characteristics of the spectra as a result of titration, are suggestive that a two-stage interaction was possible, where Cu²⁺ was interacting with A3in two different sites (in stages), given the geometrical identity of the molecule.

Figure 8.

The absorption titration spectra ofA3(1 x 10⁻⁵ M) in CH₃CN, with 5 equiv. of (a) Cu²⁺, (b) Fe ²⁺, (c) Fe³⁺, and (d) Zn²⁺ at room temperature.

Furthermore, the only other cations that could induce significant changes when introduced to A3were Fe²⁺ and Fe³⁺. For instance, the molar titration of Fe²⁺ with A3resulted in the emergence of two peaks in the UV-region at 309 nm and 360 nm, as well as the hyperchromic shift of the π → π* transition band at 243 nm (Figure 8b). The formation of a new peak, complemented by the color change, was ascribed to the coordination of Fe²⁺ to A3forming a complex (A3-Fe). It was also noticed that no change was observed in the ICT band at 423 nm. Interestingly, the addition of Fe³⁺ was distinctively different, displaying a slight hypochromic shift of the ICT band at 423 nm, and the appearance of two new broader peaks in the regions of 300 nm to 360 nm, as well as 500 nm to 600 nm (Figure 8c), with two weakly identifiable isosbestic points, at 397 nm and 452 nm, respectively. The spectral behaviors are completely different from those of Fe²⁺, which signify the possible paramagnetic (Fe³⁺, d⁵) and diamagnetic (Fe²⁺, d⁶) property influence for the two cations. The difference could also stem from the fact, the two have different atomic sizes (varying atomic radii), which could play a significant role into geometrical complementarity between the guest (Fe) and host (A3), in terms of shape and size, let alone the electrostatic force. The addition of other competitive cations, including Zn²⁺ could only induce slight changes (Figure 8d).

Contrastingly, the introduction of cations to AS3displayed slightly varying outputs as compared to A3. In addition to Cu²⁺, Fe²⁺ and Fe³⁺ discriminated by A3, the probe (AS3) was able to detect the presence of other cations such as Ni²⁺ and Zn²⁺ in the same solvent medium. However, similarly to A3, the probe could discriminate the presence of Cu²⁺, Fe²⁺ and Fe³⁺ in the same manner, with the disappearance of the peak at 423 nm, with the appearance of a new peak with maximum absorption at 299 nm upon the molar addition of Cu²⁺ (Figure 9a), with an isosbestic point at 388 nm. The characteristics of the spectra were still suspect that there exists a two-stage interaction between the guest (Cu²⁺) and host (AS3), which translates into Cu²⁺ interacting with probe through site 1, before interacting again at site 2moment later, chronologically. Moreover, the molar titration of Fe²⁺, resulted in more distinctive and intense peaks, such as the hyperchromic shift experienced by the π → π* band at 240 nm, followed by two new peaks at 310 nm and 361 nm (Figure 9b). These peaks were followed by a new broader chelation-induced metal-to-ligand charge transfer (MLTC) band in the visible region (450 nm to 650 nm), with two weakly recognizable isosbestic points, at 422 nm and 440 nm, respectively.

Figure 9.

The absorption titration spectra ofAS3(1 x 10⁻⁵ M) in CH₃CN, with 5 equiv. of (a) Cu²⁺, (b) Fe ²⁺, (c) Fe³⁺, (d) Ni²⁺ (e) Zn²⁺ and (f) Hg²⁺ at room temperature.

Moreover, it was noticeable that the interaction behaviors of Fe³⁺, Ni²⁺ and Zn²⁺ are of the same nature, based on the characteristics of their respective absorption spectra. The molar introduction of these cations to AS3displayed similarities in spectral characteristics, for example, all three had experience a new ICT band in the UV-region, a hypochromic shift at 425 nm and chelation-induced MLTC band in the visible region (Figure 9c–e). In both cases, three isosbestic points were observed, indicating the formation of complex pedants, upon interacting with the host species. In addition, other cations used did not induce any significant spectral shift, such Hg²⁺ as displayed (Figure 9f). It is worth noting, molecular identities of alizarin nature (A3& AS3) are highly likely to undergo ESIPT mechanism, which could describe varying characteristics of the formed complexes with different cations.

3.1.5 Selectivity studies of A3 and AS3 on towards cations and anions

The selectivity of A3towards cations observed was relatively indistinguishable among several cations (Zn²⁺, Fe³⁺, Ni²⁺, Hg²⁺), except for Cu²⁺ and Fe²⁺, which displayed well-resolved spectra (Figure 10a). The addition of 5 equiv. each of cations demonstrated that A3was mostly selective only to Cu²⁺ and Fe²⁺, however, in likely different modes of interaction, thus resulting in varying spectra. The color and spectral changes observed are resulting from a coordination induced charge transfer upon the interaction between the host (A3) and the guest (cations). Moreover, similar patterns were observed for AS3upon interacting with cations (Figure 10b). The combined addition of 5 equiv. each of cation to AS3saw Cu²⁺ and Fe²⁺ behaving differently from others, more similar to what was observed with A3, however, in addition Zn²⁺ and Ni²⁺ showed significant response in terms of spectral variation (Figure 10b). The sensitivity and selectivity variation of AS3over A3towards cations, displayed the effect of a sulfonic acid group (–SO₃H) has on the structure in terms of electronic transitions or charge transfers. Predictably, the interaction of A3and AS3is via coordination through the oxygen donor atoms (of hydroxyl groups and the ketones), thus with the additional oxygen donor atoms of the sulfonic acids, more coordination were possible. Therefore, recognition of Ni²⁺ and Zn²⁺ was observed, in addition to Cu²⁺ and Fe²⁺ for AS3, as compared to A3.

Figure 10.

The combined absorption titration spectra of (a)A3(1 x 10⁻⁵ M) with 5 equiv. of each cation, (b)AS3(1 x 10⁻⁵ M) with 5 equiv. of each cations, (c)A3(1 x 10⁻⁵ M) with 1 equiv. of each anion and (d)AS3(1 x 10⁻⁵ M) with 1 equiv. of each anion, all in CH₃CN.

Furthermore, the two sensors were responsive commonly to four anions (F⁻, CN⁻, OH⁻, AcO⁻) as displayed above. However, upon the addition of 1 equiv. of each anion, the spectra intensities of AcO⁻ and OH⁻ were similarly high than all others for A3(Figure 10c), while for AS3it was OH⁻ and CN⁻ which were higher than others (Figure 10d). Thus, the information displayed is informative about the selectivity of the two sensors towards the anions. Evidently, OH⁻ and AcO⁻ exhibit high affinity towards A3, while OH⁻ and CN⁻ have highest binding affinity to AS3. The interaction of anions with the sensors is through hydrogen-bonding with the hydroxyl groups of A3and AS3, in most cases leading to deprotonation. The presence of the sulfonic acid group in AS3has harnessed the sensitivity the sensor to further discriminate CN⁻ in comparison to A3.

3.2 Fluorescence studies

In previous studies, an alizarin molecule (A3) has been found to exhibit fluorescence emission in the region of 600–620 nm, excited at 457 nm, using a range of selected solvents [4]. Thus, based on the existing data, fluorescence studies of A3and AS3were performed in CH₃CN, in order to compare and contrast the effect of cations and anions upon interaction, to complement information observed in absorption studies. Fluorescence analysis of A3has shown that the emission spectrum was characterized by a single moderate energy peak centered at 612 nm (λ_ext = 457 nm), while AS3was defined by a single emission peak at 600 nm (λ_ext = 440 nm). The structural variations of A3and AS3have resulted in slight emission spectral variation, hence different excitation wavelengths of the two structures. The blue shift exhibited by AS3as compared to A3, is attributed to the presence of a strong electron withdrawing group of sulfonyl group. In essence, there exists two possible charge transfer mechanisms in each structure, the locally excited state and the proton transfer state, which coexist. Mostly in such cases, the proton transfer state normally occurs faster, thereby offsetting the locally excited state one, resulting in a single peak [4], in this case exhibited by both A3and AS3moieties.

Upon the molar titration with cations, the emission spectrum of A3gradually experienced quenching process, varying from individual cations. For instance, the addition of Cu²⁺ (3 equiv.) resulted into a significant quenching process of more than 98% (Figure 11a), ascribed to the coordination interaction of A3with Cu²⁺ resulting in complex pedant (A3-Cu). Similar quenching trends were observed upon titration with Ni²⁺, where the emission spectrum gradually decreased with increasing molar addition (Figure 11b) of the cation. Furthermore, the molar titration of A3with 3 equiv. of Cu²⁺, resulted in 100% quenching effect (Figure 11a). Interestingly, the addition of Zn²⁺ displayed completely opposite behaviors of fluorescence enhancement (Figure 11c). Contrary to all other cations used, which have all resulted in emission quenching, the addition of Zn²⁺ yielded fluorescence enhancement, signifying that the nature of interaction was perhaps different from those of other cations. The quenching effect was suspectedly due to the combination of chelation-enhanced fluorescence (CHEF) and internal charge transfer (ICT) mechanisms. The addition of other cations, including Fe³⁺ did not induce any significant change to the spectrum (Figure 11d). Generally, the interaction modes of cations towards A3and AS3are via coordination through hydroxyl oxygen donor atoms within the structures [42, 43, 44, 45, 46, 47, 48, 49, 50, 51]. It has also been established that the binding modes of particularly Zn²⁺ and Cu²⁺ among other transition metals towards A3and/or AS3entities is via hydroxyl groups [42, 43, 46, 47, 50], inducing ESIPT process, in a 2:1 interaction ratio (Figure 12a and b).

Figure 11.

The fluorescence titration spectra ofA3(1 x 10⁻⁵ M) in CH₃CN, with (a) Cu²⁺ (3 equiv.), (b) Ni²⁺ (20 equiv.), (c) Zn²⁺ (30 equiv.), and (d) Fe³⁺ (20 equiv.), atλ_ext = 457 nm.

Figure 12.

Proposed binding mechanisms for (a)A3with cations, and (b)AS3with cations.

Furthermore, the interaction of cations with AS3showed closely similar results as in A3, however, a distinctive variance was observed in the association with Zn²⁺ and Fe³⁺. The addition Cu²⁺ to AS3resulted in a 100% quenching process, showing that a strong association of AS3-Cu(Figure 13a), more like in A3-Cupedant. A similar trend was observed upon the addition of Ni²⁺ to AS3, where 63% quenching was observed (Figure 13c). Interestingly, unlike in A3, the addition of Zn²⁺ to AS3resulted in a significant fluorescence quenching (Figure 13b), with a quenching of 83% attained after adding 10 equiv. The different behavior of AS3-Znis ascribed to the presence of a strong electron withdrawing sulfonyl group, which has a suppressing or disruptive effect on charge transfer mechanisms, the coordination-based charge transfer and the ICT. Moreover, another new trend observed for AS3, was the interaction with Fe³⁺, unlike in A3, the molar addition of Fe³⁺ to AS3resulted in a significant quenching of 67% (Figure 13d) due to chelation effect, even though about 20 equiv. The interaction modes of cations with AS3were similar to A3, through coordination with the hydroxyl groups of the AS3, mostly in a 2:1 interaction ratio (Figure 12a and b).

Figure 13.

The fluorescence titration spectra ofAS3(1 x 10⁻⁵ M) in CH₃CN, with (a) Cu²⁺ (3 equiv.), (b) Zn²⁺ (10 equiv.), (c) Ni²⁺ (20 equiv.), and (d) Fe³⁺ (20 equiv.), atλ_ext = 440 nm.

The interaction of biological anions with A3and AS3were studied fluorometrically, in CH₃CN. The molar addition of anions (F⁻, CN⁻, OH⁻, AcO⁻ and N₃⁻) resulted in fluorescence quenching (Figure 14). However, the incremental addition of F⁻, CN⁻, OH⁻ and AcO⁻ to A3, suggested a two-step interaction behavior, where initial molar addition resulted in obvious fluorescence quenching at 612 nm until a certain quantity was added, then interestingly, the continual addition suddenly gave rise to fluorescence enhancement (Figure 14a–d) with a new peak at 656 nm. The first step of fluorescence quenching was ascribed to hydrogen bonding interaction between anions and A3, while the subsequent enhancement observed shortly, with incremental excessive addition of the anions, was attributed to the deprotonation effect. The divorce of the hydrogen ion off the structure of A3via ESIPT mechanism, resulted in the emissive properties of A3restored, hence a new fluorescence enhancement spectra at a different wavelength (656 nm), with increasing addition of anions. The new emission peak at 656 nm signifies a change in the structure upon the removal of a hydrogen ion through deprotonation, thus red shifting the spectra from 612 nm (A3) to 656 nm (deprotonated A3). The other anion which induced fluorescence quenching upon interacting with A3, was N₃⁻ (Figure 14e), however, no deprotonation effect was suspect, even when large quantities were added, ascribe to perhaps weaker interactions with A3.

Figure 14.

The fluorescence titration spectra ofA3(1 x 10⁻⁵ M) in CH₃CN, with (a) F⁻ (20 equiv.), (b) OH⁻ (10 equiv.), (c) CN⁻ (20 equiv.), (d) AcO⁻ (40 equiv.) and (e) N₃⁻ (30 equiv.), atλ_ext = 457 nm.

Moreover, the effect of an electron withdrawing sulfonyl group was apparent from the activities of emission spectra of AS3upon interaction with anions (F⁻, CN⁻, OH⁻, AcO⁻ and N₃⁻). The interactions of all anions used induced significant fluorescence quenching (Figure 15a–e) due to strong association with AS3, through hydrogen bonding mechanism, however, no deprotonation process seemed to have taken place. Unlike in A3where deprotonation was observed, AS3did not display any signal of change in fluorescence behavior (red or blue shift), apart from quenching. The difference between the two probes (A3and AS3) was the sulfonyl group, which is a strong electron-withdrawing species, thereby polarizing the molecule, thus inhibiting fluorescence enhancement due to the deprotonation activity. It is obvious, the addition of 10 equiv. of the anions resulted in the decrease of fluorescence spectrum at 612 nm, with the quenching strength depending on the nature of anions, F⁻ (75% quenching), CN⁻ (79% quenching), OH⁻ (79% quenching) and N₃⁻ (79% quenching). There was no significant fluorescence change signals upon the addition of other anions, including H₂PO₄⁻ (Figure 15f).

Figure 15.

The fluorescence titration spectra ofAS3(1 x 10⁻⁵ M) in CH₃CN, with (a) F⁻ (10 equiv.), (b) CN⁻ (10 equiv.), (c) OH⁻ (10 equiv.), (d) AcO⁻ (5 equiv.), (e) N₃⁻ (30 equiv.) and (f) H₂PO₄⁻ (50 equiv.), atλ_ext = 440 nm.

4. Conclusion

Conclusively, the comparative studies on the chemosensing property studies of the two Alizarin probes (A3and AS3) were successfully conducted. The two entities displayed closely similar behaviors towards the discrimination of cations and anions in CH₃CN. However, variations in behaviors was observed in cations towards A3and AS3, where each probe was able detect certain cations, by means of UV–Vis absorption titrations, as well as fluorometric responses. The UV–Vis titration of A3with cations resulted in selective detection of Cu²⁺, Fe³⁺, Fe²⁺ and Zn²⁺, while AS3was responsive towards Cu²⁺, Fe³⁺, Fe²⁺, Ni²⁺ and Zn²⁺ respectively. The behaviors of anions towards A3and AS3in UV–Vis titrations (so as colorimetrically) and fluorometrically were consistently similar. On the hand, fluorometric titrations resulted in distinctive behaviors, where mostly fluorescence quenching effect was observed upon adding cations to A3or AS3in CH₃CN. However, distinctive features were experienced upon adding Zn²⁺ to A3and AS3, where chelation fluorescence enhancement (CHEF) and chelation enhanced fluorescence quenching (CHEQ) were observed, respectively. Another distinctive feature between the two probes observed was the deprotonation effects exhibited by A3upon interacting with anions (F⁻, CN⁻, OH⁻ and AcO⁻), where at first a quenching effect was observed, then further addition resulted in the enhancement effect (due to deprotonation effect). However, this was not the case for the with AS3, where all anions were restricted to fluorescence quenching ONLY, even after large quantities were added. The differences in fluorometric properties of A3and AS3towards Zn²⁺ and anions, were influenced by the presence of the sulfonyl group in AS3, which was the main determining factor, since it is a strong electron withdrawing group. Thus, it can be said that the two probes have displayed interesting features in applications towards the detection of cations and anions, which can be used in the development of analytical probes for real time applications.

Acknowledgments

This work was supported by the Department of Physics, Chemistry & Material Science, University of Namibia, Namibia. The work was also partly supported by the Royal Society-DFiD Africa Capacity Building Initiative, New Materials for a Sustainable Energy Future.

Conflicts of interest

There are no conflicts of interest to declare.