π–π-Driven aggregation-induced emissive aza-thiazole: a selective fluorescent sensor for anionic surfactants

Abstract

A novel aza-thiazole hybrid tetraphenylethylene conjugate has been designed and synthesized as a donor–acceptor (D–A) hybrid exhibiting strong aggregation-induced emission (AIE), and thermofluorochromism. The compound 4-(4-(1,2,2-triphenylvinyl)phenyl)thiazol-2-amine (TTHI-3) forms bright fluorescent nanoaggregates in aqueous media and selectively interacts with anionic surfactant sodium dodecyl sulfate (SDS) via π–π-driven self-assembly. This unique mechanism produced a pronounced fluorescence enhancement, enabling micromolar-level SDS detection (LOD = 2.73 × 10⁻⁴ M). Its practical utility was demonstrated using low-cost fluorescent test strips for the rapid detection of SDS in home-care products. These results establish TTHI-3 as a promising π–π-driven sensor for environmental and analytical applications.

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Introduction

Novel aza-thiazole luminescent materials with unusual properties have attracted immense interest over the last decade.^1a,b Aza-heterocycles are abundant in natural products and drugs, displaying a broad range of biological activities, including antiviral,^2a antidiabetic,^2b anti-HIV,^2c,d and anticancer^2e activities. Aggregation-induced emission (AIE)-materials are of particular interest due to their enhanced emission in the aggregated state.^3,4 Despite their immense potential, the development of rapid and efficient methods⁵ for sensor synthesis with high sensitivity and selectivity remains a significant challenge.⁶ Conventional synthetic methods involve lengthy and multi-step procedures; therefore, the advancement of rapid and efficient methods is highly sought after.⁷ Sodium dodecyl sulfate (SDS), a widely consumed surfactant, is extensively used in various industries, including food and pharmaceuticals.⁸ Anionic surfactants, characterized by a non-polar hydrophobic alkyl tail and a hydrophilic head,⁹ serve as detergents, lubricants^10a and catalysts, and are employed in biological applications,^10b such as the extraction of DNA and proteins.^10c Given their extensive usage and wide application,^11a,b anionic surfactants, including sodium dodecyl benzenesulfonate (SDBS) and SDS, significantly contribute to drug delivery and environmental pollution. Due to their inherent properties, they are prevalent in numerous commercial surfactant products, such as soap, hand wash, and shampoo,^12a which are directly discharged into aquatic environments, causing harmful effects to humans. Several methods have been developed to detect anionic surfactants, including spectrophotometry.^12b,c However, these methods face limitations, including reproducibility issues, synthetic utility, reliance on colorimetry, paper-based detection, and poor stability. Numerous fluorescent methods have been successfully used to detect anionic surfactants, and the vast majority rely on a simple interaction where a cationic probe attracts the anionic head group, as widely demonstrated in recent literature.¹³ Kaur et al.¹⁴ reported a prominent cationic dye sensor, and an Eosin Y dye with a polyethyleneimine sensor was reported by Weg et al.¹⁵ Despite the traditional method, Wang et al.²⁶ proposed a π–π-driven mechanism to sense 16 types of surfactants. Although these new method-based sensors achieved good sensitivity, they suffered from poor selectivity due to ubiquitous competing ionic species (salts, buffers, etc.) present in environmental or commercial samples. This necessitates the development of probes utilizing distinct sensing to minimize interference. Therefore, the development of rapid, efficient, multifunctional, and simple donor–acceptor-based organic molecules for the selective detection of SDS remains a significant challenge. To address this limitation, we have designed and developed a facile and innovative fluorescence probe, aza-thiazole hybrid tetraphenylethylene (TTHI-3), to determine the anionic surfactants effectively in various industrial and academic contexts. To achieve high selectivity and sensitivity, the current work directly addresses the need for introducing a π–π-driven self-assembly that contrasts sharply with conventional electrostatic systems based on the unique hydrophobic microenvironment of SDS micelles. The π–π stacking interactions with the aza-thiazole conjugate core facilitate pre-micellar interactions with sodium dodecyl sulfate (SDS). This mechanism is attributed to the aza-thiazole, which hybridizes with TPE forming a donor–acceptor (D–A) system. The aggregation enhances detection capabilities by synthesizing a new aggregation-induced emission (AIE)-active¹⁶ aza-thiazole derivative via a rapid microwave-assisted method. This approach reduces the time and also improves the yield. Thereby offering an efficient method for the rapid production of valuable materials. The resultant compound is multi-stimuli-responsive, rendering it a valuable tool for diverse applications.

Results and discussion

The aza-thiazole compound TTHI-3 was synthesized following the steps outlined in Scheme 1. When compound dibromoketone TBr₂1 and thiourea 2 were combined in a clean microwave (M.W.) environment, they produced a high yield. The preparation of 1-(4-(1,2,2-triphenylvinyl)phenyl)ethan-1-one (ATPE) was performed, which served as the starting material. The key step involves the reaction of ATPE with TsNBr₂¹⁷ and thiourea 2 to form the target aza-thiazole TTHI-3 (Scheme 1). Initial synthesis starts with Mc-Murray coupling, followed by Friedel–Crafts acylation to synthesize ATPE. Using ATPE and TsNBr₂ as model substrates, the influence of the TsNBr₂ 2 equivalents was investigated.

Scheme 1 Synthesis of 4-(4-(1,2,2-triphenylvinyl)phenyl)thiazol-2-amine (TTHI-3).

The reaction of ATPE with TsNBr₂ (1.73 mmol) in MeCN¹⁸ at room temperature for 2 h yielded the desired product. Increasing the amount of TsNBr₂ to 2.00 mmol further improved the yield. Interestingly, the presence of water during workup enhanced the product yield. The optimized conditions involved reacting ATPE (1.3 mmol) with TsNBr₂ (2.00 mmol) in 2.5 mL of MeCN at room temperature for 2 h, affording an 87% yield of the novel α-dibromoketone intermediate TBr₂1. The structure of TBr₂1 was confirmed by NMR spectroscopy (see Fig. S10), which showed characteristic signals at 7.76 ppm in the ¹H NMR spectrum and 43.5 ppm in the ¹³C NMR spectrum.

Subsequently, we explored the condensation of Hantzsch aza-thiazoles.¹⁹ Initially, the reactions were conducted at a controlled temperature (140 °C, 10 W) to establish the optimal reaction time (Table 1, entries 1–9). Lowering and increasing the temperature, Watts, equivalents, and time (Table 1, entry 5) to 140 °C with 50 W for 30 s was the optimal. The increase in time led to the formation of an unidentified side product and a slight decrease in yield (Table 1, entry 7).

Table 1 Optimization for the reaction conditions for the synthesis of TTHI-3

Entry	Temp.^c (°C)	Power^d (W)	Time (s)	2 (equiv.)	Yield^a (%)
a Yield of the product. b Thiourea (2) equivalent. c Reaction performed under solvent-free conditions. d Under microwave irradiation. e Reaction performed under reflux. f Minutes under neat conditions.
1	140	10	15	1.0	56
2	140	10	25	1.2	60
3	140	10	30	1	73
4	120	10	25	1	34
5	140	50	30	1.2	86
6	160	10	30	1.2	0
7	140	50	120	1.2	75
8^e	120	—	20^f	1.2	60

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The influence of the thiourea 2 (0.18 mmol) stoichiometry was investigated.²⁰ Varying the amount of thiourea demonstrated that thiourea 2 (0.2254 mmol) and TBr₂1 (0.187 mmol) under solvent-free conditions provided the best yield (86%, Table 1, entry 5). For comparison, the reaction was also performed using a thermostat oil bath under the same conditions but with longer reaction times. Microwave irradiation significantly shortened the reaction time (12 times faster) and improved the yield compared to conventional heating (Table 2).

Table 2 Comparison of microwave and conventional heating for synthesizing compound TTHI-3

Entry	Method^b	Temp. (°C)	Time	Yield^a (%)
a Yield of the pure product. b Reaction performed under solvent-free conditions.
1	Microwave	140	30 s	86
2	Conventional heating (oil bath)	140	20 min	60

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With the optimized conditions in hand, we explored the scope of the reaction mechanism of TTHI-3. The amine group (–NH₂) acts as an electron donor, while the aza-thiazole ring and tetraphenyl rings function as the electron acceptor, forming a classic donor–acceptor (D–A) system. Electron-donating substituents on the tetraphenylethylene moiety led to a marginal increase in yield of up to 86% under the optimized conditions (Table 1, entry 5), indicating their influence on the reaction mechanism (Fig. 1).

Fig. 1 Plausible reaction mechanism for the compound TTHI-3.

The synthesized aza-thiazoles TTHI-3 were purified via recrystallization and characterized spectroscopically. The ¹H NMR spectrum showed a characteristic NH₂ signal at δ 5.51 ppm. The ¹³C NMR spectrum exhibited a signal at 167.4 ppm, characteristic of the C2-thiazole carbon. Mass spectra confirmed the molecular weight and were consistent with the assigned structure. For the ¹H and ¹³C NMR, and (ESI) HRMS spectrum, see the SI.

The synthesis of TTHI-3 was carried out via a classic Hantzsch-type thiazole synthesis.²¹ The reaction proceeds through a multi-step mechanistic pathway (Fig. 1). Initially, the reaction involves a nucleophilic attack (a) by the thiourea 2 on the electrophilic carbon atom of compound TBr₂1, resulting in the displacement of a hydrogen bromide ion (b) with the imine form to eliminate –H₂O, followed by an intramolecular cyclization by attack of [H⁺ Br⁻] and subsequent elimination of –H₂O and –Br₂ to form the product TTHI-3.

Optical properties

The photophysical properties of the compound TTHI-3 were examined using UV-vis absorption and fluorescence emission spectroscopy (Fig. 2). The normalized UV-vis absorption spectrum (red line) shows a strong absorption band (black line) around 330 nm at 24 °C, and an excitation band (red line) around 360 nm, attributed to the conjugation system's π–π* electronic transitions. Upon photoexcitation at 360 nm, a broad fluorescence emission band (blue line) is observed, centred around ∼480–490 nm in THF and ∼450 nm in n-hexane (SI, Fig. S12). Indicative of a large Stokes shift and supporting an intramolecular charge transfer (ICT) mechanism (Table 3).

Fig. 2 UV-vis, excitation and emission spectrum of TTHI-3.

Table 3 Optical properties of compound TTHI-3 in THF solvent

Entry	Code	Σ (mol^−1 dm^3 cm^−1)	λ abs max (nm)	λ em max (nm)	Stoke shift cm^−1 (nm)	ΦSfl (%)
a.	TTHI-3	7 200 000	360	490	7369 cm^−1 (130)	0.55

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Quantum yield values of TTHI-3 (90 : 10) % H₂O/THF, solid state probe TTHI-3 fluorescence spectrum (see SI, Fig. S11), and TTHI-3/SDS are summarized below (Table 4).

Table 4 Fluorescence quantum yield

Entry	Code	ΦSfl (%)
a.	TTHI-3 in (90 : 10) %	2.08
b.	Solid state TTHI-3	2.91
c.	TTHI-3 + SDS	1.76

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Temperature-based fluorescence

The temperature-dependent fluorescence behavior of compound TTHI-3 was systematically studied in tetrahydrofuran (THF) solvent over a narrow temperature range of −5 °C to +5 °C. The emission spectra (Fig. 3) show a clear variation in fluorescence intensity and emission maxima with rising temperature. At −5 °C, the compound reached its maximum emission intensity around 500 nm. A significant hypsochromic shift was observed as the temperature increased gradually, reaching approximately 450 nm at +5 °C. This gradual blue shift of approximately 50 nm and the systematic increase in fluorescence intensity suggest a temperature-driven conformational change in the excited-state species. The shift can be attributed to a twisted intramolecular charge transfer (TICT) effect, often seen in donor–acceptor type fluorophores. At lower temperatures, the molecule likely adopts a more twisted conformation in the excited state, stabilizing the charge-separated TICT state and causing red-shifted emission.

Fig. 3 Temperature-dependent spectra of TTHI-3.

As the temperature rises, thermal energy may limit this twisting, favoring a more planar intramolecular structure. Consequently, a locally excited (LE) state becomes more prominent, resulting in blue-shifted emission. This behavior aligns with previous reports on TICT-active luminophores showing temperature-dependent spectral shifts.²² The increase in intensity with higher temperature may also indicate a reduction in non-radiative decay pathways, such as those related to solvent reorganization or aggregation-induced quenching at lower temperatures. Overall, the data confirmed that compound TTHI-3 exhibits notable thermofluorochromic properties, which could be useful for temperature sensing or molecular logic applications.

Aggregation-induced emission (AIE) studies

The UV–vis absorption spectra of TTHI-3 in different THF/H₂O mixtures are shown in Fig. S6 (SI). In THF, the compound exhibits a broad absorption band centered at 360 nm, characteristic of π–π* transitions. As the water content increases, the absorption intensity gradually diminishes, accompanied by a notable bathochromic red shift. This spectral change indicates the formation of aggregates. The red shift in the absorption band upon aggregation (j-aggregation-like behavior) is consistent with the presence of enhanced intermolecular, most likely π–π stacking or hydrophobic clustering, which typically leads to broadened and bathochromically shifted absorption bands.^22,23a The spectral alterations become significant above 60% water fraction, signalling the onset of aggregation at this composition.

The fluorescence emission spectra (Fig. 4a) reveal the AIE characteristics of TTHI-3. In THF and at low water fractions (below 50%), TTHI-3 shows weak fluorescence with negligible emission intensity. This is due to non-radiative decay pathways facilitated by vigorous intramolecular motions in the molecularly dispersed state. However, a substantial increase in fluorescence intensity is observed as water content exceeds 60%, reaching a maximum in the 90 : 10 water/THF mixture. The emission band was observed at 500 nm, with fluorescence intensity nearly 10 times higher in the aggregated state compared to the dispersed solution. This significant enhancement is attributed to the restriction of intramolecular rotation (RIR) in the aggregated state, a key feature of the AIE mechanism.^23b Importantly, no substantial spectral shift is detected in the emission maxima, suggesting that aggregation boosts emission without greatly altering the emissive excited state. These findings confirm that TTHI-3 functions as an AIE-active luminogen. The molecule is non-emissive in solution but becomes highly fluorescent upon aggregation in the aqueous state. TTHI-3 functions as an AIE-active luminogen. The molecule is non-emissive in solution but becomes highly fluorescent upon aggregation in the aqueous state. Introduction of the aza-thiazole unit establishes a donor–acceptor (D–A) system, where it acts as an electron acceptor and the TPE moiety serves as the donor. This configuration promotes a significant intramolecular charge transfer (ICT) transition, which lowers the energy gap resulting in the desired red-shift in the emission spectrum (∼500 nm). Furthermore, the aromatic, polar surface of the aza-thiazole unit enables the specific π–π stacking interactions. This property makes TTHI-3 a promising candidate for use in aqueous-phase sensing, bioimaging, and solid-state optoelectronic devices. This property makes TTHI-3 a promising candidate for use in aqueous-phase sensing, bioimaging, and solid-state optoelectronic devices.

Fig. 4 (a) Emission spectrum, (b) 3d spectra, (c) liquid state fluorescence of TTHI-3 in (9 : 1% W/THF).

With a well-defined and sharp emission peak centered at roughly 500 nm, the 3D spectrum for the 90 : 10 water/THF combination reveals a sharp rise in fluorescence intensity. Strong proof for the aggregation-induced emission (AIE) phenomenon comes from this clear “turn-on” of fluorescence, as shown by the high intensity of the red and yellow colors in the plot. The 3D spectra in Fig. 4b clearly show that the molecule's fluorescent properties, with Fig. 4c liquid state fluorescence of TTHI-3, are clearly caused by the development of nanoaggregates in the aqueous solution.

Lifetime studies

The excited-state dynamics of the compounds were investigated using time-resolved photoluminescence measurements, and decay profiles (Fig. 5). The prompt fluorescence (black line) exhibits a sharp and fast decay, confirming its short-lived nature in the nanosecond region. In contrast, the pristine TTHI-3 compound (red line) displays a comparatively longer-lived emission, with an average lifetime (τ_avg) of 0.30 ns, indicating stabilization of the excited state due to its molecular framework. The aggregated molecule (90 : 10 H₂O/THF) derivative (green line) shows a distinct increase in lifetime compared to the solution state exhibiting an extended τ_avg of 0.72 ns. This significant-2.4-fold enhancement in lifetime provides quantitative kinetic evidence for the AIE mechanism, as the tight aggregation structure effectively restricts intramolecular rotation (RIR) and suppresses non-radiative decay pathways. Interestingly, the TTHI-3/SDS system (blue line) demonstrates an average lifetime (τ_avg) of 0.30 ns, which is nearly identical to that of the pristine TTHI-3 monomer (0.30 ns). This result suggests that the fluorescence enhancement upon SDS addition is primarily driven by micellar encapsulation which prevents further rotation, rather than an enhancement. Hence, the TTHI-3/SDS lifetime does not reach the value of the AIE aggregate 90 : 10 H₂O/THF (0.72 ns), which indicates a different mode of stabilization than simple aggregation. Overall, these results confirm that molecular modifications and the surrounding environment play a crucial role in tuning the excited-state dynamics. The measured lifetimes are summarized in the SI, Table S1.

Fig. 5 Lifetime spectra of compounds TTHI-3, A-TTHI, and TTHI+SDS.

Computational studies

To investigate the electronic properties of TTHI-3, Density Functional Theory (DFT) calculations (B3LYP-D3/6-31G(d) were performed. The results reveal a HOMO −5.12 eV and a LUMO energy of −1.26 eV, with an energy gap of 3.85 eV (Fig. 6a). The HOMO is localized on the electron-rich aza-thiazole-amine region, while the LUMO resides on the TPE moiety, confirming strong D–A separation and ICT character.

Fig. 6 (a) DFT calculation and (b) MEP of compound TTHI-3.

The distinct spatial separation of the molecular orbitals suggests a tendency for intramolecular charge transfer (ICT). Notably, the large energy gap and the separate localization of orbitals support this behavior. The blue area, especially around the amino group (–NH₂), shows a positive electrostatic potential from a molecular electrostatic potential (MEP) (Fig. 6b) perspective, indicating an electron-poor center prone to nucleophilic attack. The green area, mainly on the aza-thiazole and TPE sections, indicates a neutral charge, confirming the electron density distribution throughout the molecule.

Surfactant sensor

To evaluate the chemical selectivity of TTHI-3, we recorded steady-state fluorescence spectra (λ_em ∼ 500 nm) of the probe in the absence and presence of common metal salts, anions, and surfactants at identical concentrations (Fig. 7a). The response was quantified as a relative intensity change, S = (I−I₀)/I₀ × 100%, where I₀ and I are the emission maxima for TTHI-3 alone and after adding the test species, respectively. SDS (Anionic surfactant) produced a pronounced “turn-on” response, giving the highest emission among all additives (≈ 50–70% enhancement vs. blank across the 480–520 nm band). The spectral shape is retained, indicating microenvironmental rigidification/micellar encapsulation rather than new emissive states. The anionic surfactant (SDS) led to a moderate enhancement, consistent with electrostatic/microenvironment effects that partially restrict non-radiative decay.

Fig. 7 (a) Selectivity spectrum with various analytes and (b) sensitivity spectrum, (c) Stern–Volmer plot, and (d) liquid state fluorescent TTHI-3+SDS.

The non-ionic surfactant (Triton X-100) caused only a small increase, supporting the role of the electrostatic effect in the CTAB, a cationic surfactant. Paramagnetic transition-metal salts (CuSO₄, FeCl₂, NiCl₂) induced clear quenching relative to TTHI-3, attributable to heavy-atom/paramagnetic spin–orbit interactions that promote intersystem crossing and non-radiative pathways. Anions (NaCl, K₂SO₄, NaNO₂, NaNO₃, Na₂SO₄, and NH₄H₂PO₄) showed minor to modest intensity changes without substantial peak shifts, indicating weak or non-specific interactions with the excited state of TTHI-3. Among all tested species, including other alkyl anionic sulfates such as SDBS, SLES (sodium lauryl ether sulfate) (Fig. S7), SDS stands out as a quick and strong enhancer, while Cu²⁺/Fe²⁺/Ni²⁺ are the primary quenchers. The probe therefore exhibits dual functional selectivity, a strong turn-on response to cationic micellar environments and turn-off sensitivity to paramagnetic transition metals, with minimal interference from common anions/salts. This behavior is consistent with a mechanism in which micelle-induced confinement suppresses non-radiative decay of TTHI-3, whereas d-block ions facilitate excited-state deactivation.

As shown in Fig. S8 (SI), the UV-vis absorption spectra of TTHI-3 exhibited a dominant absorption band centered around 320–350 nm. Upon incremental addition of the surfactant, there were no significant shifts in the absorption maxima, indicating that the ground-state electronic structure of the probe remains largely unaffected. However, a gradual decrease in absorption intensity was observed, suggesting weak ground-state interaction or dilution effects, possibly accompanied by slight changes in the environment of the chromophore due to surfactant and probe association. The fluorescence emission behavior of TTHI-3 in the presence of the surfactant is presented in Fig. 7b. The probe displays strong emission centered around 500 nm. With increasing surfactant concentration, the fluorescence intensity initially increases, reaching a maximum at 50 µL addition. Beyond this point, a progressive quenching of fluorescence intensity is observed, especially at higher volumes (100–140 µL). The initial fluorescence enhancement can be attributed to the hydrophobic TTHI-3 core within the surfactant, which restricts non-radiative decay pathways such as intramolecular rotation and aggregation-caused quenching.

This phenomenon is consistent with micelle-induced fluorescence enhancement commonly observed for hydrophobic AIEgens. The maximum fluorescence intensity observed at intermediate surfactant concentration suggests optimal micelle formation or hydrophobic cavity encapsulation. Subsequent fluorescence quenching at higher surfactant volumes could be due to overcrowding of micelles, increased polarity of the local environment, or static quenching via surfactant and probe complexation. Such behavior indicates a two-phase interaction: an initial fluorescence turn-on effect followed by fluorescence suppression at excess surfactant levels. The binding constant (K_b) of TTHI-3+SDS (1.23 × 10⁻⁴ M⁻¹) and Stern–Volmer linear plot (I₀/I − 1) (4.04 × 10⁻⁵ M⁻¹) have been found and showed in the Fig. 7(c), and the limit of detection (LOD) = 2.73 × 10⁻⁴ M was also determined. The linear fit for the K_sv plot showed a strong correlation, R² = 0.95 confirming the high sensitivity and selectivity of TTHI-3 towards SDS micellar environments. Fig. 7d shows a visual fluorescence turn-on effect upon adding SDS to TTHI-3.

Sensing mechanism

The Transmission Electron Microscopy (TEM) image reveals amorphous particles for TTHI-3 in THF (Fig. 8a). The spherical nanoaggregates in 90 : 10 W/THF undergo a distinct morphological transformation forming uniform spherical nanoaggregates (Fig. 8b). This nanoaggregate formation is a direct consequence of the aggregation-induced emission (AIE) process, as the tightly packed structure restricts non-radiative decay pathways. Critically, this nanoaggregation is a separate phenomenon from the highly ordered self-assembly observed upon the addition of the anionic surfactant SDS. As shown in Fig. 8c, the presence of SDS leads to the formation of a distinct, ordered, and chain-like supramolecular morphology at the nanoscale. This TEM result provides direct visual evidence for π–π-driven interactions between the probe molecules, which are templated and stabilized by the SDS micelles. This unique self-assembly restricts molecular motion and produces the observed “turn-on” fluorescence which was crucial for its sensing application²⁷ (SI, Fig. S11b).

Fig. 8 TEM image of TTHI-3 (a), 9 : 1% W/THF (b), and TTHI-3+SDS (c).

The interaction between TTHI-3 and the anionic surfactant SDS was further investigated by ¹H–NMR titration^24,25 in a DMSO-d₆ : D₂O (9 : 1 v/v) solvent mixture (Fig. 9). The exchangeable –NH₂ proton at δ 5.51 ppm disappeared in deuterated solvents. As shown in Fig. 9, upon the sequential addition of SDS (Fig. S9, SI), the aromatic protons of the TTHI-3 core exhibit a consistent and noticeable upfield chemical shift (shielding effect).^28a,b Specifically, the protons of the aza-thiazole core and the internal TPE phenyl rings show shifts (Δδ) up to 0.05–0.15 ppm (protons a, b, and c). This shielding is due to π–π stacking arrangement^28c where the aromatic protons are positioned over the π-electron clouds of neighboring^28dTTHI-3 molecules within the SDS micellar template. This overall shift pattern provides direct evidence that the interaction between the aromatic rings^28e of TTHI-3 is the key driving force for the SDS-templated self-assembly. Also, a schematic representation of TTHI-3 with SDS is shown in Fig. 10.

Fig. 9 The ¹H-NMR of compound TTHI-3+SDS (1.0 mM) dissolved in DMSO-d₆ : D₂O (9 : 1, v/v) as a solvent mixture.

Fig. 10 Plausible sensing mechanism of TTHI-3 with SDS.

Further quantitative insight into the sensing mechanism was provided by DFT-D3 calculations.²⁹ The optimized π–π-stacked structure revealed a highly favourable interaction quantified by E_b (BSSE-corrected binding energy) of −30.63 kJ mol⁻¹ (see the SI, Table S3). This significant negative energy value provides strong computational support for the proposed π–π-driven self-assembly mechanism. These results are entirely consistent with the upfield shift observed in the ¹H-NMR titration and ordered, chain-like morphology visualized by TEM.

Real-time application of TTHI-3

For the practical analytical ability of TTHI-3 as a probe, filter paper-based analysis for on-site detection of anionic surfactant (SDS) was performed using rectangular Whatman paper (4.50 mm height and 2.6 cm width). In this study, the filter paper was soaked in SDS solution (probe + SDS) for 30 minutes and then allowed to air dry. The probe-coated filter paper was then dipped in an SDS solution (30 µL in aqueous solution) and dried again. When analyzed under UV illumination, a significant fluorescence “turn-on” was observed for TTHI-3 after exposure to SDS (Fig. 11). Secondly, the probe's sensitivity was quantitatively validated by assessing its performance in complex matrices. Compound TTHI-3 successfully detected SDS via the spiking method in a variety of challenging home-care products, including shampoo, soap, hand and face wash, (SI, Fig. S15). The analysis demonstrated reliable quantitative performance across all samples, yielding high recovery from 95.8% to 103.3% with a Relative standard deviation (RSD) of ≤4.5% (summarized in SI, Table S4). Notably, the probe exhibited high sensitivity in the Facewash matrix, demonstrating its resilience against interference from common product ingredients. These results rigorously establish TTHI-3 as a viable and effective fluorescent sensor for environmental and safety monitoring applications.

Fig. 11 Paper strip detection of the probe (TTHI-3) and SDS.

Conclusion

In summary, a novel aza-thiazole conjugate TTHI-3, was successfully developed using a highly efficient microwave-assisted synthesis method. Its electronic properties, including its D–A nature, and ICT characteristics, were confirmed by theoretical and experimental methods. The compound exhibits a remarkable AIE phenomenon, with its fluorescence significantly enhanced in the aggregated state, and functions as a highly sensitive and selective fluorescent sensor for the anionic surfactant SDS in aqueous media. The unique sensing mechanism, driven by π–π interactions that lead to the formation of self-assembled micelles by TEM, ¹H NMR, and DFT-D3 distinguished it from conventional electrostatic sensors. This approach enables micromolar detection of SDS, highlighting TTHI-3 as a versatile and powerful tool for synthesis and sensing.

Experimental

Materials and methods

All experiments were conducted in the open air unless otherwise stated. Reagents were obtained from commercial sources (Aldrich and CDH) and used without further purification. The new compounds’ ¹H and ¹³C NMR spectra were measured at 400 and 75 MHz, respectively, using Bruker NMR instruments in DMSO-d₆ or CDCl₃. Chemical shifts are reported as δ values (ppm) relative to tetramethylsilane (δ 0.0) as an internal standard. Splitting patterns were designated: s, singlet; bs, broad singlet; d, doublet; dd, doublet of doublets; t, triplet; m, multiple. Silica gel G plates (Merck) were used for TLC analysis with a mixture of petroleum ether (60–80 °C) and ethyl acetate as the eluent. Mass spectra were obtained using an electrospray ionization (ESI) mass spectrometer and recorded in positive mode. UV-visible spectra in the solution state were recorded using a BioTek spectrophotometer, and fluorescence, temperature-dependent, lifetime, and quantum yields (ΦS_fl) were measured using a HORIBA-Fluoromax, and DFT calculation was done with Gaussian 09 B3LYP/6-31G (d) and DFT-D3 with B3LYP-D3/6-31G(d).

General procedure for the synthesis of compound dibromoketone TBr₂

To a stirred solution of ATPE (0.5 g, 1.2 mmol) in acetonitrile (3 mL) was added TsNBr₂ (1.82 g, 2.00 mmol) portion-wise at room temperature. The reaction mixture was stirred for 3 h. The solvent was then removed under reduced pressure, and the residue was poured over crushed ice. The resulting precipitate was filtered, washed with water, and dried to afford 0.92 g (87%) of the desired dibromoketone 1 as a yellow powder. Yield (87%) (for characterization data, see the SI).

General procedure for the synthesis of compound 4-(4-(1,2,2-triphenylvinyl)phenyl)thiazol-2-amine (TTHI-3)

TBr₂1, 0.1 g (0.187 mmol), and thiourea 2, 0.017 g (0.2254 mmol, see Table 1) were mixed in a flask. The mixture was then subjected to microwave irradiation under neat conditions for a specific time. The resulting solid was washed with water (5–10 mL) and filtered to isolate the desired aza-thiazole product. A bright yellow solid, TTHI-3 (86%), was obtained (for characterization data, see the SI).

Author contributions

Sivakumar Shanmugan: review, editing, validation, supervision, and extensive checks for final draft approval. Ajaydev Rameshbabu: conceptualized the plan, carried out all experiments, collected and curated the data, and prepared the original draft. Preethi Murugesan: performed formal analysis of optical studies, curated data, and contributed to writing, all experiments conducted at the School of Chemistry, Madurai Kamaraj University, Madurai-625021, India.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data is contained within the article and supplementary information (SI).

The data supporting this article have been included as part of the supplementary information (SI). The SI contains detailed experimental characterization and optical data of TTHI-3 (¹H, ¹³C NMR, HRMS, UV-Vis, Fluorescence, Lifetime, ¹H NMR titration spectra, SEM, TEM, DFT-D3, and home care samples recorded spectra). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc03507d.

Acknowledgements

We thank SERB, TANSCHE, and RUSA 2.0 for financial assistance and instrument facility (Fluromax -TCSPC). Also, we thank DST-FIST for the common instrument facilities (400 MHz NMR and Spectrofluorometer).

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