**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 **A3** and 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

#### **Figure 2.**

*Observable colorimetric changes of different anions upon interacting with (a)* **A3** *and (b)* **AS3** *both (1 x 10−5 M) in CH3CN at room temperature.*

(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 **A3** solution, resulted in a series of naked eye observable color changes, ranging from deep blue (CN<sup>−</sup> ), blue-violet (AcO<sup>−</sup> ), 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 **A3** is a color-based indicator, with such unique concentrated color change. Clearly, the color changes were due to chemically associated interaction between **A3** and 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, **A3** was discriminatingly selective and sensitive to N3 − , turning the yellow color **A3** to dark-yellowish **A3-N3** complex. 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 **A3** displayed 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**).

Moreover, comparative colorimetric studies were conducted for **AS3** and anions (**Figure 2b**), were visual observation clearly showed the variation between the two-alizarin derivatives (**A3** and **AS3**). Like with **A3**, the addition of anions to yellow **AS3** resulted 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<sup>−</sup> (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 **A3** and **AS3**. Apart from the OH− and F− ions, which displayed deep-blue and violet colors for both **A3** and **AS3** respectively, 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

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

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 **A3** solution 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 **AS3** could 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**.

#### *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,

#### **Figure 3.**

*Observable colorimetric changes of different cations upon interacting with (a)* **A3** *and (b)* **AS3** *both (1 x 10−5 M) in CH3CN at room temperature.*

**AS3** displayed 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, **A3** was 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 (**A3** and **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 **AS3** with about 2 nm of all the bands.

#### *3.1.3 Absorption studies of* **A3** *and* **AS3** *on interaction with anions*

The interaction of **A3** with 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<sup>−</sup> , 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.

On the other hand, the molar titrations of **AS3** with anions resulted in similar patterns as those of **A3** for the three anions (F− , CN− , OH− ) as displayed (**Figure 6a, c,** and **d**). The molar titration of **AS3** with 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-CN** and **AS3-OH**. Like in **A3**, the spectral activities were accompanied by several isosbestic points,

#### **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.*

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

#### **Figure 5.**

*The absorption titration spectra of* **A3** *(1 x 10−5 M) in CH3CN, with 5 equiv. of (a) CN− , (b) AcO <sup>−</sup> , (c) F− and (d) OH<sup>−</sup> at room temperature.*

at 238 nm, 300 nm, 400 nm and 463 nm, which proves the formation of new pedants coexisting 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 **AS3** resulted 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 **AS3** is still suspectedly through hydrogen bonding, even though it slightly differs from the rest, the binding position might be different.

Notingly, among the rest of the anions, H2PO4 − was still able to induce changes when added to **A3**, same way like F<sup>−</sup> , 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-CN** and **AS3-OH** as indicated (**Figure 7f**). This illustrates that the interaction between **AS3** and AcO<sup>−</sup> was of hydrogen bonding nature, through the hydroxyl groups of the

#### **Figure 6.**

*The absorption titration spectra of* **AS3** *(1 x 10−5 M) in CH3CN, with 5 equiv. of (a) CN− , (b) N3 − , (c) F− and (d) OH<sup>−</sup> at room temperature.*

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 **AS3** was able to detect in addition to the other anions. Moreover, unlike **A3**, **AS3** was unable to detect the presence of H2PO4 − as indicated (**Figure 7a**), probably due to the presence of the sulfonic acid group in **AS3**.

#### *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 **A3** and **AS3** were influenced by the presence of cations in the given solvent system. Subsequently, the molar addition of Cu2+ to **A3** resulted in spectra shifts of significant distinction. The molar titration of **A3** with 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 ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

#### **Figure 7.**

*The absorption titration spectra of* **A3** *(1 x 10−5 M) in CH3CN, with 5 equiv. of (a) H2PO4 − , (b) N3 − , (c) other anions, and* **AS3** *with 5 equiv. of (d) other anions, (e) H2PO4 − , (f) AcO− , at room temperature.*

the coordination of the p-orbitals of the oxygen atoms (hydroxyl and/or carbonyl group) of **A3** to 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

#### **Figure 8.**

*The absorption titration spectra of* **A3** *(1 x 10−5 M) in CH3CN, with 5 equiv. of (a) Cu2+, (b) Fe 2+, (c) Fe3+, and (d) Zn2+ at room temperature.*

as a result of titration, are suggestive that a two-stage interaction was possible, where Cu2+ was interacting with **A3** in two different sites (in stages), given the geometrical identity of the molecule.

Furthermore, the only other cations that could induce significant changes when introduced to **A3** were Fe2+ and Fe3+. For instance, the molar titration of Fe2+ with **A3** resulted 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 **A3** forming 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

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

force. The addition of other competitive cations, including Zn2+ could only induce slight changes (**Figure 8d**).

Contrastingly, the introduction of cations to **AS3** displayed 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 twostage interaction between the guest (Cu2+) and host (**AS3**), which translates into Cu2+ interacting with probe through **site 1**, before interacting again at **site 2** moment 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.

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 **AS3** displayed 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 **A3** towards 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 **A3** was 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 **AS3** upon interacting with cations (**Figure 10b**). The combined addition of 5 equiv. each of cation to **AS3** saw 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 **AS3** over **A3** towards 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 **A3** and **AS3** is 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 9.**

*The absorption titration spectra of* **AS3** *(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.*

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 **AS3** it was OH− and CN− which were higher than others (**Figure 10d**). Thus, the information displayed is informative about the selectivity of

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

#### **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.*

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 **A3** and **AS3**, in most cases leading to deprotonation. The presence of the sulfonic acid group in **AS3** has 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 **A3** and **AS3** were 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 **A3** has shown that the emission spectrum was characterized by a single moderate energy peak centered at 612 nm (*λ*ext = 457 nm), while **AS3** was defined by a single emission peak at 600 nm (*λ*ext = 440 nm). The structural variations of **A3** and **AS3** have resulted in slight emission spectral variation, hence different excitation wavelengths of the two structures. The blue shift exhibited by **AS3** as 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 **A3** and **AS3** moieties.

Upon the molar titration with cations, the emission spectrum of **A3** gradually 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 **A3** with 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 **A3** with 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

#### **Figure 11.**

*The fluorescence titration spectra of* **A3** *(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.), at λext = 457 nm.*

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

**Figure 12.**

*Proposed binding mechanisms for (a)* **A3** *with cations, and (b)* **AS3** *with cations*

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 **A3** and **AS3** are via coordination through hydroxyl oxygen donor atoms within the structures [42–51]. It has also been established that the binding modes of particularly Zn2+ and Cu2+ among other transition metals towards **A3** and/or **AS3** entities is via hydroxyl groups [42, 43, 46, 47, 50], inducing ESIPT process, in a 2:1 interaction ratio (**Figure 12a** and **b**).

Furthermore, the interaction of cations with **AS3** showed closely similar results as in **A3**, however, a distinctive variance was observed in the association with Zn2+ and Fe3+. The addition Cu2+ to **AS3** resulted in a 100% quenching process, showing that a strong association of **AS3-Cu** (**Figure 13a**), more like in **A3-Cu** pedant. 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 **AS3** resulted in a significant fluorescence quenching (**Figure 13b**), with a quenching of 83% attained after adding 10 equiv. The different behavior of **AS3-Zn** is 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 **AS3** resulted in a significant quenching of 67% (**Figure 13d**) due to chelation effect, even though about 20 equiv. The interaction modes of cations with **AS3** were similar to **A3**, through coordination with the hydroxyl groups of the **AS3**, mostly in a 2:1 interaction ratio (**Figure 12a** and **b**).

The interaction of biological anions with **A3** and **AS3** were 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 **A3** via ESIPT mechanism, resulted in the emissive properties of **A3** restored, hence a new fluorescence enhancement spectra at a different wavelength (656 nm), with increasing addition of anions. The new emission peak at 656 nm

**Figure 13.**

*The fluorescence titration spectra of* **AS3** *(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.), at λext = 440 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**.

Moreover, the effect of an electron withdrawing sulfonyl group was apparent from the activities of emission spectra of **AS3** upon 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 **A3** where deprotonation was observed, **AS3** did not display any signal of change in fluorescence behavior (red or blue shift), apart from quenching. The difference between the two probes (**A3** and **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

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

#### **Figure 14.**

*The fluorescence titration spectra of* **A3** *(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.), at λext = 457 nm.*

#### **Figure 15.**

*The fluorescence titration spectra of* **AS3** *(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.), at λext = 440 nm.*

at 612 nm, with the quenching strength depending on the nature of anions, F<sup>−</sup> (75% quenching), CN<sup>−</sup> (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**).
