Simple chloro substituted HBT derivative portraying coupling of AIE and ESIPT phenomenon: Ratiometric detection of S2- and CN- in 100% aqueous medium

https://doi.org/10.1016/j.jphotochem.2019.112177Get rights and content

Highlights

  • A benzothiazole based compound BTCP was synthesized and characterized.

  • BTCP was found to show coupling of AIE and ESIPT.

  • BTCP is used in neat aqueous medium ratiometric detection of sulfide and cyanide.

  • The practical utility has been explored by successful paper strip experiment.

Abstract

A simple hydroxybenzothiazole (HBT) derivative, namely 2-(benzo[d]thiazol-2-yl)-4-chlorophenol (BTCP) was synthesized and its photophysical properties were studied spectroscopically. Detailed steady state and time resolved spectroscopic studies along with Field Emission Scanning Electron Microscope (FESEM) and Dynamic Light Scattering (DLS) analyses established the coupling of Excited State Intramolecular Proton Transfer (ESIPT) and Aggregation Induced Emission (AIE) in BTCP molecule. The current report stands out as AIE and ESIPT coupling observed by introduction of a simple Chlorine atom in an established ESIPT active probe. The utility of BTCP has been demonstrated by successful ratiometric determination of S2− as well as CN- in aqueous medium. Paper strips coated with BTCP could successfully detect the anions by a change in color under UV-light.

Introduction

The phenomenon of excited state intramolecular proton transfer (ESIPT) process has enjoyed tremendous attention from chemists since it was first reported by Weller in methyl salicylate [1]. Moving over methyl salicylate, several similar designed molecules capable of exhibiting ESIPT has been reported by various research groups, including ours [[2], [3], [4], [5], [6]]. ESIPT usually occurs through 4-membered, 5-membered or 6-membered hydrogen bonded network in the molecular systems [[7], [8], [9]]. The principal feature of ESIPT molecules in their emission spectra is the presence of a short wavelength emission band, designated as local emission (LE) along with a large Stokes shifted band, assigned as proton transferred form (PT). Among the various types of ESIPT active molecules reported in the literature, the 6-membered ESIPT systems have been dealt with greater attention by a number of researchers. The main reason is their applicability as bioimaging agents for targeted cell organelles, as proper substitution causes the probes to behave as NIR-emitting probes, an important criterion for biological applications [10]. Of late, there have been reports to tune/monitor ESIPT in molecules capable of showing other excited state phenomena like excited state charge transfer [11]. Since the epoch making work of Tang et.al on aggregation induced emission (AIE) [[12], [13], [14]], attempts are being made to couple ESIPT with AIE with each passing year [[15], [16], [17], [18], [19], [20]]. Simple tetraphenyl ethylene (TPE) derivatives are reported to show simple AIE effect [[12], [13], [14]], whereas hydroxyl containing Schiff bases that can form intramolecular hydrogen bonding with the imine N atom are reported to exhibit ESIPT along with AIE [15,16,18]. Another important class is the protected chalcone derivatives as well as HBT derivative from TPE which show similar spectral properties [17,20]. The ESIPT probes that show coupling with AIE mainly show keto-enol/imine-amine tautomerism. Since AIE-ESIPT coupled probes are reported to be excellent candidates for ratiometric detection of analytes, we decided to develop an ESIPT-AIE coupled probe and check its response towards environmentally hazardous analytes. While doing so, we came across several probes reported to portraying AIE phenomenon. However, most of these probes were equipped with bulky substituents and the synthetic routes were also complex (Scheme 1). While searching for a route which would overcome the complex synthetic route as well as fulfill our requirement, we came across a report by Kim et al. [21] where the photophysics of some simple substituted 2-(2-hydroxyphenyl) benzothiazole (HBT) nanoparticles were studied. HBT derived nano particles showed aggregation effect in water: THF mixtures in the same report. We decided to check whether introduction of a simple halogen moiety in the HBT framework could exert the same effect in the solution phase without having to prepare nanoparticles from HBT. The reason behind choosing a halogen substituent was due to the expectation that the said substituent would provide just enough hydrophobic environments to cause of aggregation in solution, which might not be possible if substituents like single bondOH, single bondOCH3 and single bondCOOH were introduced owing to their tendency of solvation by water molecules. Furthermore, recent reports have proved the positive role of halogen substitution in inducing self assembly caused by π-π interactions of various organic molecules [22,23]. Since self assembly is a prerequisite for AIE effect to be observed, chlorine substituent was chosen. We thus synthesized a simple Chloro substituted HBT derivative, namely 2-(benzo[d]thiazol-2-yl)-4-chlorophenol (BTCP). BTCP shows bright yellowish green emission in the solid state as observable through naked eye under UV light. The same color under UV light is observed in an aqueous solution of BTCP. Since reports on the detailed photophysical survey of BTCP were not available, we carried out the same and experimentally confirmed the coupling of AIE and ESIPT in BTCP. Furthermore, BTCP could successfully detect S2− and CN- in pure aqueous medium upto a detection limit of 1.57 μM and 10.82 μM respectively by ratiometric change in its emission profile. The ratiometric change can be realized by observing the color swing from yellowish green to bright blue under UV light. The detection of the aforesaid anions occurred by deprotonation of the phenolic Osingle bondH in BTCP, thereby hampering the ESIPT process. The merit of BTCP as a fluorescent probe can thus be well understood by considering the detrimental effects the aforesaid anions exert on the human body [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]]. Finally, the real time applicability of BTCP was demonstrated by successful solid state detection of the anions as their aqueous solutions using paper strips coated with the compound. Thus, our current report finds its own merit as we report an extremely easy to synthesize fluorescent probe BTCP where introduction of a Chloro group results in coupling of AIE and ESIPT as also harnessing the aforesaid photophysical properties to achieve aqueous medium detection of toxic S2− and CN- ions.

Section snippets

Materials and methods

5-chlorosalicylaldehyde and 2-aminothiophenol were purchased from Sigma Aldrich and used without further purification. All solvents were spectroscopic grade and purchased from Spectrochem, India. Perchlorate salts of cations and sodium salts of anions were used from Spectrochem. Triple distilled water was used for all solution phase studies. NMR spectra were recorded on a Bruker Advanced Supercon 300 MHz NMR spectrophotometer using TMS as internal standard. Steady state UV–vis absorption

Absorption spectral studies

The absorption spectra of BTCP were recorded in a series of solvents of varying polarity (Fig. 1, Table 1). In almost all solvents, two strong structured absorption bands were observed at ∼290 nm and ∼340 nm respectively. An extra band appeared for dimethylsulfoxide (DMSO) at ∼437 nm. The presence of structured bands in BTCP was concluded to be due to the presence of electron withdrawing chlorine group, which is in accordance to the previous report of HBT derivatives containing other electron

Conclusions

A Chloro substituted HBT derivative BTCP was synthesized and established to show ESIPT coupled AIE. The coupling of the above mentioned phenomena has been established by steady state, time resolved, DLS and FESEM imaging. BTCP acts as ratiometric sensor for sulfide and cyanide in water upon rupture of aggregates in water. BTCP shall be important as synthetically simpler yet useful platform for portraying AIE as well as a practical probe for detecting cyanide and sulfide using paper strips

Declaration of Competing Interest

The authors declare that they have no known conflict of interests reported in this paper.

Acknowledgements

AB and SM would like to thank CSIR and UGC for granting fellowships. NG thanks DST, India (Project No. EMR/2016/004788) and CSIR, India (Project No. 01(2920)18/EMR-II) for financial support. AB thanks Professor Dilip Kumar Maity, University of Calcutta for helping with DLS measurements.

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