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Home > Substitution Effect on Near Infrared Absorbance Based Selective Fluoride Sensing of Indole Functionalized Thiourea Molecules
Substitution Effect on Near Infrared Absorbance Based Selective Fluoride Sensing of Indole Functionalized Thiourea Molecules
Bijit Chowdhury,[a] Sanghamitra Sinha,[a] and Pradyut Ghosh
Introduction
Chemical sensing of fluoride is of high importance due to its applications in industry, food, toxicity and the major role played by it in a wide range of biological and environmental processes.[1,2] Several research groups are actively working on the area of designing fluoride sensors based on different mecha- nisms.[3–10] Literature reports are well enriched with colorimet- ric detection of fluoride utilizing urea, thiourea amides, sulfon amides, pyrroles, indoles etc. units having acidic hydrogen at- oms.[7–11] On the other hand, a chemosensor with a near-infra- red (NIR) optical response which could avoid interference from endogenous chromophores are useful in biological systems and hence emerge high demand in practical purposes. Our group has shown a characteristic NIR signal based sensing of fluoride along with the change in color using indole functionalized urea/ thiourea receptors.[7e,10d] These reports ascertain that, ind- ole conjugated urea/ thiourea units are the key to achieve the NIR signature. Thus, to design a class of sensors which can be useful for sensing of fluoride in broad NIR window, the indole conjugated urea/ thiourea platform could be the choice, where the indole unit can be functionalized with the substituents and would have an effect on the NIR signature by virtue of position, conjugation, steric and inductive/ mesomeric effects. We envis- age that the planarity of the system and electronic effect of the functional groups in the indole moiety might play an important role on the NIR signatures. Thus 2 and 5 positions of the indole moiety could be cosidered to functionalize with suitable sub- stituents having different steric and electronic effects for tuning the NIR sensing behaviour by fluoride. Here we report six such simple indole conjugated thiourea systems eg. S1–S6 which show fluoride sensing within a broad NIR window i.e. from 825 nm to 1160 nm, depending on the nature and position of the substituent in the indole moiety. To the best of our knowledge, this is the first ever report of such a huge red-shifted (≈ 800 nm) NIR signal at ≈ 1160 nm in the presence of fluoride.
Results and Discussion
Synthesis and Characterization: All the ligands S1-S6 are syn- thesized from the corresponding indole-3-carboxaldehyde de- rivatives via Schiff base condensation reaction with thiocarbo-hydrazide in ethanol/water (2:1; v/v) mixture under reflux for 24 h and followed by the collection of precipitates by filtration (Scheme 1). All the ligands S1-S6 are fully characterized by NMR, ESI-MS, elemental analysis (Figure S1-S17, Supporting In- formation) and single-crystal X-ray structural analysis wherever possible.
Fluoride sensing in NIR region: To understand the electron induction, mesomeric, size and conjugation effects of the sub- stituents attached with the indole unit, optical properties of S1–S6 are studied in the presence of different anions in ACN/ DMF (98:2; v/v) mixture. All the indole conjugated thiourea de- rivatives show maximum absorbance at ≈ 350 nm (for S1-S6, except S4) because of the Ar–CH=N–NH conjugation frame- work. Less basic anions e.g. Cl–, Br–, I–, NO – etc. do not cause any noticeable change in the UV/Vis spectra of the receptors.
Addition of AcO–, BzO–, H PO – and HP O 3– show a red shift in the UV/Vis spectrum (Figure 2). For all the sensors, S1–S6 two steps of change in the UV/Vis spectrum is observed upon addition of fluoride. Initially, addition of 0–4 equiv. of F– leads to slight red shift in the absorption maxima of the S1-S6 along with the concomitant color change from colorless to yellow. Further addition of fluoride (4–12 equiv.) leads to the appear- ance of NIR absorption band at ≈ 825–1160 nm for different substituted indoles along with the changes in color accordingly depending on the substitution in the Indole moiety. The first step (0–4 equiv. of anions) of color change for all the sensors may be due to the binding of anions (F–, AcO–, BzO–, H2PO – and HP O 3–) with thiourea and indole -NH protons via hydrogen bonding interactions. Further addition of fluoride (4–12 equiv.) causes deprotonation of the indole and thiourea-NH functionalities and NIR signature is observed in the absorp- tion spectrum. Different substitutions on the indole derivative at different positions lead to different shifts in UV/Vis spectra. For example, 2-Methyl and 2-Phenyl-substituted indoles show NIR signature at 942 nm and 956 nm, respectively, in the pres- ence of fluoride (Figure 2a and Figure 2b). Upon increasing the conjugation by introducing one more alkene unit in the recep- tor backbone, the optical spectra (λmax at 358 nm for S6) is shifted to 1160 nm (Figure 2f) in the presence of F–. The NIR peak position of S1-S6 in the presence of F– can give clear idea about electron induction effect, mesomeric effect of substitu- ents in the series. Sensors S1 and S2 having 2-methyl and 2- phenyl substitution in the indole moiety show quite similar red shift in the absorption wavelength along with the generation of the NIR peak in the presence of F– as that was obtained for previously reported sensor (S1, provided in Scheme S1 sup- porting information). Other sensor molecules (S4 and S5) in this series are synthesized by substituting 5-position of the ind- ole unit by -MeO and -NO2, respectively, considering their in- ductive and resonance effect. In case of S5, electron donating methoxy group makes the -NH proton of the thiourea moietyless acidic and as a result the interaction with anion is expected to be less. However, 5-methoxy substitution does not alter the UV/Vis-NIR absorption spectra of the deprotonated species obtained from the unsubstituted indole conjugated thiourea de- rivative (S1). Methoxy substitution in the 5-position have nega- tive inductive effect (-I) and positive mesomeric effect (+R) and as a whole it turns out to add no extra effect compared to the unsubstituted indole conjugated thiourea derivative (S1). Thus 5-MeO substituted derivative (S5) show similar color change and UV/Vis-NIR spectral pattern as that of S1. Substitution of-NO2 group at the 5-position of indole unit in the case of S4, where is -NO2 can exert both -I as well as -R effects and this eventually increases the acidity of the -NH proton of the thio- urea unit. Thus this sensor S4 is expected to interact better with anions. However, the NIR peak position is blue shifted com- pared to that of unsubstituted one (S1). This may be due to the cross conjugation of the deprotonated species with the competitive nitro group which in turn affects its conjugation in the entire structure as the case normally obtained for other sensors S1-S3, S5 and S6. As the normal conjugation is affected by the cross conjugation with the nitro group the sensor S4 shows weak absorbance in the NIR region. In case of S6, ab- sorption peak of the deprotonated species formed in the pres- ence of F– appears at around 1160 nm (Figure 2f) along with dark greenish yellow coloration and visible peaks at 412, 597 and 670 nm. This can be justified from the fact that in case of sensor S6, the presence of extra conjugation in the structural framework of the sensor increases to some extent compared to that of unsubstituted one which allows more stabilization of the deprotonated species via delocalization of the negative charge throughout the entire structure compared to that of the unsubstituted deprotonated species where the extra conjugative dou- ble bond is absent. Binding affinities of sensors S1-S6 with F–, AcO–, BzO–, H2PO – and HP O 3– are determined using UV–vistitration experiments. From UV/Vis titration of S1-S6 with F–, it is clear that upon gradual addition of F–, the absorption peak for the sensors S1-S6 at ≈ 350 nm decreases with concomitant appearance of a new absorption peak at ≈ 400–435 nm with one clear isosbestic point. All the titrations reach saturation limit at ≈ 4 equiv. F– concentration for all these sensors (Fig- ure 3). As the concentration of fluoride increases (>4 equiv.), the absorbance of S1-S6 at ≈ 400–435 nm gradually decrease, with the concomitant appearance of NIR peak ≈ 942, 956, 825, 948, 890 and 1160 nm, respectively. This process continues till the addition of 12 equiv. F– and no more changes in absorb- ance takes place upon further addition of F–.
The apparent binding affinities towards fluoride are calcu- lated for sensors S1-S6 from the UV/Vis titration data using non-linear fitting equation, considering a 1:1 binding pattern (Figure 4). The calculated association constants with F– for sen- sors S1-S5 are enlisted in Table S1, Supporting Information. Here it has to be mentioned that, the deprotonation equilib- rium which takes place at higher fluoride concentration, has not been taken into account for calculation of association constants. Only the equilibrium for the adduct formation between ligand and fluoride has been considered.
To ensure the photostability of the sensors, which is a common issue for thiourea based systems, the photophysical prop- erty and colorimetric sensing phenomenon for each ligand is done repeatedly which gives the same response. The colors of the solutions after fluoride addition also remain intact for several days.
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