Thioxanthone-based organic probe with aggregation enhanced emission and exceptional mineral acids sensing abilities
Graphical abstract
Introduction
The last few decades have witnessed a tremendous interest in the development of functional luminescent chromophores [1], [2], [3], [4], [5], [6]. The fact that such chromophores have wide-ranging applications, several organic, inorganic, and hybrid molecular systems with varying sizes have been investigated [7], [8], [9]. Especially, there is an upsurge in the design of small organic fluorophores for the selective recognition of analytes of environmental and biological interests [10], [11], [12], [13]. The main advantage of such probes is their ability to quickly and accurately detect deleterious analytes without requiring special instrumentation. Moreover, such probes' optical properties can also be smartly fine-tuned by varying the spacers, functionalities over the backbone, and conjugating units [13]. For example, probes with unique absorption/emission properties (such as large Stokes shift, NIR-FIR emission, etc.) can be achieved via judicious installation of the donor (D) and acceptor (A) groups over the chromophoric framework [13,14]. Despite these features, π-conjugated organic fluorophores are often non-emissive or exhibit weak emission in the solid-state or concentrated solutions. This phenomenon, which is ascribed to the strong intermolecular interactions and termed as aggregation-caused quenching (ACQ), poses a big challenge towards the use of organic fluorescent dyes [15]. Fortunately, the intermolecular interactions are largely controlled by molecular packing and conformation and thus offer a possibility to control such interactions. For instance, an organic chromophore in the aggregated state may exhibit restricted intramolecular rotation and reduced π-π interactions, leading to emission intensity enhancement, generally known as aggregation-enhanced emission (AEE) [16], [17], [18]. AEE is commonly observed in twisted organic molecules and has been exploited to develop luminogen showing enhanced emission in the aggregated state [19]. It has been demonstrated that organic probe with D-A architecture exhibit AEE characteristic [20,21], which can be utilized in different areas including, sensing, bioimaging, and so on [12,13].
The detection of acids (organic/inorganic) is a hot area of research, as they play an important role in the living systems and the environment [22]. A slight perturbation in the acid level can significantly alter a system's pH, leading to a change in the system's features and properties. Compared to the traditional analytical techniques, fluorescent probes are more desirable due to their high selectivity and sensitivity, non-invasive detection, rapid response, simple operation, and extreme signal-to-noise ratio. So far, several acid-sensitive probes based on carbocyclic/ heterocyclic fluorescent probes have been reported [23], [24], [25]. For instance, Mishra and corkers reported anion sensing properties of AEEgens [26].
Among several small organic chromophores, thioxanthone (TX)-based cores are particularly intriguing [27]. Its small aromatic butterfly-shaped core with several functionalization sites have been exploited to develop materials with low singlet-triplet energy gap (ΔEST), high intersystem crossing rate (KISC), and photoluminescence quantum yield (Ф) [27], [28], [29], [30], [31], [32]. For example, Wang and co-workers [27] found that D-A type TX-based dyads (i-ii, Chart 1) are efficient emitter for OLEDs. Similarly, Ye et al. [32] fabricated OLEDs using carbazole/sulfone hybrids (iii-iv, Chart 1), which exhibited the highest values of reported blue-violet/violet OLEDs with CIEy below 0.06. Besides, several other derivatives based on the xanthene core with excellent emission properties, stability, and applications have been reported [1,33,34].
Recently, Liu and co-workers [35] reported thioxanthone derivatives and demonstrated its potential application in NIR imaging of celullar organelle. However, one significant feature of this small class of molecules remained unnoticed, i.e., AEE and pH-sensitive emission features. While preparing new generation analogs of thioxanthene-S,S-dioxide, we noted that 3,6-bis(dimethylamino)-9H-thioxanthen-9-one 10,10-dioxide (3, Scheme 1) exhibited exceptional mineral acids sensing and AEE abilities. The fact that pH plays an essential role in many physiological processes such as cell proliferation and apoptosis, ion transport, enzyme activity, and protein degradation; compound 3 was used to screen commonly used mineral acids in aqueous solution as we all reversibly sense acid and base vapors. The findings of the studies are presented herein.
Section snippets
Synthesis and spectroscopic characterization
Compounds 2 and 3 were obtained by following a procedure reported by Liu and co-workers [35]. Briefly, 4,4′-methylenebis(N,N-dimethylaniline) 1 (2.0 g, 7.8 mmol) was stirred in a mixture of fuming nitric acid and sulphuric acid (Scheme 1). The precipitate obtained was washed thoroughly with a copious amount of water and used for further reactions. The oxidation of 2 was carried out using FeCl3/HCl in DCM to yield a bright fluorescent compound 3. We noted that the oxidant KMnO4/MnO2 was equally
Mechanism of pH sensing
To obtain a clear picture of the mechanism of pH sensing, (i.e., the red shift in the peaks as the pH is lowered), spectra of neutral as well as protonated (at amino, carbonyl, and SO2 moieties) species were computed by TD-DFT (B3LYP/6-311++G(d,p) level in THF and compared (Figs. 10 and 11). As can be seen, protonation at the carbonyl groups in 3 leads to a redshift of the peaks. This is consistent with the proposed mechanism (3-H+, Chart 2) with protonation at the carbonyl group causing the
General procedures
All chemicals, except where stated otherwise, were obtained from Sigma-Aldrich and used as received. NMR spectra were recorded in CDCl3 using a Bruker Advance III HD 700 MHz spectrometer equipped with 5 mm TCI H/C/N cryoprobe. The 1H and 13C NMR spectra were referenced to solvent resonances. IR spectra were recorded directly on the sample as attenuated total reflectance (ATR) on Diamond using Cary 630 FT-IR spectrometer. Mass spectra were acquired using a Kratos MS 890 spectrometer using
Conclusion
In this study, we examined the solvent and pH-dependent photophysical properties of 3,6-bis(dimethylamino)-9H-thioxanthen-9-one 10,10-dioxide (3). The reported probe has donor-acceptor-donor (D-A-D) type architecture with N,N-dimethylamino as donor and thioxanthone as an acceptor units. Compound 3 exhibited a red-shifted absorption band in polar protic solvent compared to polar aprotic and non-polar solvents. An intramolecular charge transfer (ICT) process between the donor and acceptor units
CRediT authorship contribution statement
Syed Imran Hassan: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Ashanul Haque: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Yassin A. Jeilani: Data curation, Formal analysis, Writing - review & editing. Rashid Ilmi: Data curation, Formal analysis. Md. Serajul Haque Faizi: Formal analysis, Writing - review & editing. Imran Khan: Data curation, Formal analysis, Writing - review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
AH and YAJ acknowledge the Department of Chemistry, University of Hail, Kingdom of Saudi Arabia, and SIH acknowledges the Department of Chemistry, Sultan Qaboos University, Oman, respectively, for providing infrastructural support during the preparation of this manuscript. The authors like to acknowledge Dr. Nawal K. Al-Rasbi at the Department of Chemistry, Sultan Qaboos University, Oman, to perform X-ray crystallography. SEAGrid (http:www.seagrid.org) is acknowledged for computational
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