A π-extended luminogen with colorimetric and off/on fluorescent multi-channel detection for Cu²⁺ with extremely high selectivity and sensitivity via nonarylamine-based organic mixed valence

Abstract

A luminogen (ITP-TPE) comprising isothianaphthene-bridged tetraphenylethene units was efficiently synthesized. Its unique extended π-conjugated structure allows for colorimetric and off/on fluorescent detection for Cu²⁺ with extremely high selectivity and sensitivity, by the formation of nonarylamine-based organic mixed-valence state.

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Introduction

Owing to the industrial environmental monitoring and biological importance of Cu²⁺,¹ a lot of researches have been mainly focused on design and synthesis of functional chromophores to monitor Cu²⁺ changes. In this aspect, organic luminogens have become very effective sensors due to their high sensitivity, selectivity and quick response, in particular, their versatile molecular structures associated with rich functionalities. Among them, the colorimetric sensors offer low cost and portable detection through “naked-eye” observation. And the emission-based chemosensors provide very rich spectral information via various fluorimetric techniques (e.g. choice of excitation and emission wavelengths), and are thus applicable for detection in a complicated system. Currently, there are two approaches to the detection of Cu²⁺ contents by using chemosensors through metal coordination or arylamine-based organic mixed-valence states. Compared with the former method that has been extensively studied,² the latter one is relatively little studied and has not aroused attention until very recently. Generally, for an arylamine with relatively low oxidation potential, its radicals can be generated by electrochemical oxidation or by chemical oxidants such as metal ions.³ This mechanism offers a simple way to design functional chromophores to detect Cu²⁺, such as tris(4-anisyl)amine,⁴ arylaminofluorene derivatives,⁵ thiazolothiazole-based derivatives,⁶ N,N-dimethylaminophenylenediamine,⁷ naphthalenediimides,⁸ indoline benzothiadiazle derivatives,⁹ and their corresponding arylaminium cation radicals, which are readily produced by Cu²⁺ in CH₃CN. However, there are still few reports about nonarylamine-based organic mixed-valence luminogens for probing Cu²⁺.

Isothianaphthene is widely used as a key building block in organic functional materials applicable for various optoelectronic devices, such as organic field effect transistors, organic solar cells, and organic light-emitting devices,¹⁰ because it has promising electronic properties, associated with its ready conversion between aromatic and quinoid resonance structures (Scheme 1).¹¹

Scheme 1 Conversion between aromatic and quinoid structures for isothianaphthene.

Because of their excellent aggregation-induced emission (AIE) effect, tetraphenylethene (TPE) and its derivatives have drawn a great deal of interest in a broad range from small-molecule luminogens to functional materials.¹² However, in most cases, TPE typically as a key functional group offers strong fluorescent emission in the aggregated state.¹³ While using TPE for achieving π-extended conjugation systems seems not to be fully explored so far, owing to its being less rigid with respect to its four freely rotatable bulk phenyl groups. With these considerations in mind, our effort is to integrate isothianaphthene and TPE units for building up an extended π-conjugated system. In this work, we report the synthesis of a novel luminogen comprising one isothianaphthene core and two TPE units in the lateral sides, for achieving isothianaphthene instead of triphenylamine as the reactive platform for generating nonarylamine radical cations by Cu²⁺ and with the two TPE ligands giving stability to corresponding radical cations via extending spin density and changing charge distribution. Its π-extended conjugation was verified by the two successive reversible redox processes in cyclic voltammetry. And combined with its rich photophysical properties, its unique colorimetric and off/on fluorometric sensing of Cu²⁺ through the formation of the relatively stable radical cation has also been revealed.

Results and discussion

Synthetic procedures

The synthetic approach to the target luminogen (ITP-TPE) is illustrated in Scheme 2. First, the key intermediate 1,2-di[S-(2-pyridinyl)]benzenedithioate (1) was easily accessed according to our previous report.¹⁴ Afterwards, 4-(1,2,2-triphenylvinyl)phenyl magnesium bromide was added into a suspension of 1 in THF at 0 °C and stirred overnight, affording 2 in a yield of 85%. Finally, upon the treatment of 2 with Lawesson's reagent,¹⁵ ITP-TPE was achieved in 80% yield. All new compounds were fully characterized by ¹H and ¹³C NMR spectroscopy, and high resolution mass spectroscopy, verifying the chemical identity of the new compounds.

Scheme 2 Synthetic route towards ITP-TPE.

The optical properties of ITP-TPE were investigated by UV-visible and fluorescence spectroscopies. The UV-visible spectrum of ITP-TPE (2 × 10⁻⁵ M) revealed two main absorption bands at 327 nm and 420 nm in the high-energy regions, respectively, assignable to the π–π* transition of the aromatic skeleton (Fig. S1†) and it exhibits good photostability (Fig. S2†). Typically, the TPE-based luminogens undergo strong fluorescence emission quenching due to the non-radiative transition from the free rotation of TPE unit in the non-aggregation state. Interestingly, in our case, the fluorescence spectrum of ITP-TPE showed visible green fluorescence in CH₃CN (Fig. S3†) and the fluorescence quantum yield was up to 1.8%. Such a phenomenon is probably attributed to the promising isothianaphthene unit as a bridge favourable for π-electron delocalization over the whole molecular skeleton, which somehow might weaken the free rotation of the phenyl groups in TPE units, and thus decrease the corresponding non-radiative decay. As expected, the as-prepared luminogen in a H₂O–CH₃CN mixture with the water fraction above 50% shows a markedly enhanced fluorescence intensity due to the classical AIE effect (Fig. S2†). When the water content reaches a maximum value at 80% (v/v), the fluorescence quantum yield increases to Φ_F = 10.45%.

Electrochemical properties

Moreover, the electrochemical behaviour of ITP-TPE was studied by cyclic voltammetry measurement (Fig. 1). Two consecutive one-electron reversible oxidation processes can be observed for this molecule. Accordingly, the two oxidation potentials of the waves were calculated as E_ox1 = 0.45 V and E_ox2 = 0.85 V, with respect to the formation of the stable radical cation and dication, respectively. Two such successive oxidation processes also manifest that such kind of luminogen possesses an extended π-conjugated system over the whole molecular skeleton.

Fig. 1 Cyclic voltammogram of ITP-TPE (1 × 10⁻³ M) measured in CH₂Cl₂ (0.1 mol L⁻¹ n-Bu₄NPF₆) at a scan rate of 100 mV s⁻¹.

Cu²⁺ detection

These unique photophysical properties and electrochemical behaviours of ITP-TPE encouraged us to further explore its application in the examination of metal ions because of their important roles in environmental protection and biological metabolism.

The sensing by ITP-TPE of metal ions was evaluated by adding 1.0 equiv. of various ions, namely Li⁺, Na⁺, Al³⁺, Ag⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, and Zn²⁺, in CH₃CN, with monitoring by UV-visible spectroscopy. As shown in Fig. 2, two new absorption peaks at 692 nm and 1097 nm appeared after the addition of Cu²⁺ (1.0 equiv.). The NIR absorption band may be attributed to the generation of corresponding radical cations. Accordingly, a significant change of the solution colour from yellow to colourless was observed within 5 seconds by the naked eye. On the contrary, by comparing with the blank sample, there is no obvious change of either the absorption profiles or the solution colour in the presence of the other metal ions, indicative of the highly selective colorimetric detection for Cu²⁺.

Fig. 2 Upon the addition of various metal ions (1 equiv.), UV-vis-NIR spectra of ITP-TPE (2 × 10⁻⁵ M) in CH₃CN solution (top), and colorimetric changes of ITP-TPE (2 × 10⁻⁴ M) (bottom). Wavelength unit: nm.

Fluorescence response behaviours of ITP-TPE were also investigated by the addition of various metal ions in CH₃CN (Fig. 3). 1 equiv. Cu²⁺ results in almost complete fluorescence emission quenching as shown by fluorescence spectroscopy and the image under UV light. Whereas the addition of the other ions did not markedly affect the green fluorescence emission relative to a blank sample. Reasonably, the quenching effect could be attributed to the formation of radical cations with respect to the mixed-valence state of ITP-TPE upon Cu²⁺ oxidation. Regarding the aforementioned AIE effect of this luminogen, a back titration was further conducted. To a solution of ITP-TPE (2 × 10⁻⁴ M) containing different metal ions (1 equiv.) was added water in a ratio of H₂O : CH₃CN = 9 : 1. Interestingly, a strong fluorescence emission immediately turned on in the case of the Cu²⁺-containing sample. It is worth noting that the released pale green emission is significantly different from that of the blank sample (Fig. S4†). As a contrast, the other metal ion-containing samples did not show any obvious difference in both colour and intensity of the fluorescence emission with respect to a blank sample. This unexpected phenomenon might be related to the formation of some new TPE-derived AIE-active species under Cu²⁺ oxidation followed by a water treatment. Such unique optical response of ITP-TPE to a synergetic stimuli performance might also allow us to develop an anti-counterfeiting technique with increased security.^12b

Fig. 3 Fluorescence spectra of ITP-TPE (2 × 10⁻⁵ M) in CH₃CN solution upon addition of 1 equiv. of various metal ions. (a). Fluorescence images of ITP-TPE (2 × 10⁻⁴ M) upon the addition of various metal ions (1 equiv.). Wavelength unit: nm. (b). Fluorescence images of metal ion-containing ITP-TPE samples (2 × 10⁻⁵ M) in a mixed solution of water : CH₃CN = 9 : 1 under 365 nm (c).

To gain a better understanding of the impact of Cu²⁺ on the optical properties of ITP-TPE, the UV-vis-NIR absorption spectra of ITP-TPE titrated with Cu²⁺ in CH₃CN are shown in Fig. 4. With the addition of Cu²⁺, the absorption peak at 420 nm gradually decreased, and two new bands with progressively enhanced intensities at 692 and 1097 nm, respectively, can be observed, which may be characteristic of the cation radicals. Moreover, the addition of Cu²⁺ caused the fluorescence intensity to successively decline, and was nearly quenched at 1.0 equiv. due to the formation of the ITP-TPE radical cations, suggesting the extremely high sensitivity of the as-made luminogen for Cu²⁺. Accordingly, a detection limit of 4 × 10⁻⁷ M was calculated, which is the lowest value for the organic mixed-valence type of Cu²⁺ sensors (Fig. S5†).⁸

Fig. 4 UV-vis-NIR absorption spectra (a) and fluorescence spectra (b) of ITP-TPE (2 × 10⁻⁵ M) in CH₃CN with the addition of Cu²⁺. Wavelength unit: nm.

These phenomena can be essentially elucidated by the combination of the aforementioned electrochemical measurement of ITP-TPE and the redox properties of Cu²⁺. Given that the oxidation potential of Cu(CF₃SO₃)₂ in CH₃CN is 0.80 V vs. Fc/Fc⁺ (Fig. S6†), higher than the first oxidation potential of ITP-TPE (E_ox1 = 0.45 V) and lower than the second oxidation potential (E_ox2 = 0.85 V), the free energy change was calculated from the Rehm–Weller equation ΔG₀ = E_ox − E_red − e²/dε, where E_ox is the oxidation potential of the electron donor (ITP-TPE in this case), E_red is the redox potential of the electron acceptor (Cu²⁺ in this case), d is the center-to-center distance between the ITP-TPE and Cu²⁺ in the collision, and ε is the dielectric constant of CH₃CN (=37).¹⁰ In polar solvents like CH₃CN, the coulombic term is neglected. The free energy change (ΔG_ET) in the exergonic thermal electron transfer from ITP-TPE to Cu²⁺ is −0.35 (ΔG_ET1) and +0.05 eV (ΔG_ET2), respectively, which is sufficiently high to readily oxidize ITP-TPE to form the corresponding monocation radical (ITP-TPE˙⁺), but not a dication radical (ITP-TPE²⁺).

The formation of a cation radical (ITP-TPE˙⁺) and the oxidation state of Cu²⁺ during the oxidation processes were further supported by electron spin resonance (ESR) spectroscopy (Fig. 5). On addition of 1 equiv. of Cu²⁺ to a solution of ITP-TPE in CH₃CN, an ESR signal centered at g = 2.0022 appears, which can be attributed to the formation of the mono-radical cation of ITP-TPE, concomitant with a reduction of Cu²⁺ to Cu⁺. Such a result is consistent with the above photophysical and electrochemical analyses.

Fig. 5 ESR spectra of ITP-TPE (1 × 10⁻³ M) in CH₃CN solution with different equivalents of Cu²⁺.

All these results seem to point to a possible mechanism (Scheme 3). At first, ITP-TPE was converted to a monocation radical in a single-electron oxidation process by Cu²⁺. This open-shell species (2) can be stabilized through the formation of a series of resonance structures, associated with the quinoid structures favourable to maximally delocalize the electron over the molecular skeleton. On the other hand, the open-shell resonance structures seem to lack stability, and are subject to form other kinds of new compounds, which might directly lead to the unusual optical properties, such as water-stimuli fluorescence response in the aforementioned back titration.

Scheme 3 Plausible mechanism for the formation of the organic mixed-valence states of ITP-TPE by Cu²⁺ oxidation.

Conclusions

In conclusion, we have efficiently synthesized a new luminogen consisting of an isothianaphthene core and two tetraphenylethene units in the lateral sides, which features a π-extended conjugated structure, very rich photophysical properties, AIE activity and electrochemical behaviour with two successive one-electron reversible oxidation processes. This kind of luminogen enables colorimetric and off/on fluorimetric multi-channel detection for Cu²⁺ with very high selectivity and sensitivity, with the lowest detection limit up to 4 × 10⁻⁷ M, representing a new type of non-arylamine chemosensor. This new approach also might be able to be used for developing a high-safety anti-counterfeiting technique associated with synergetic stimuli response, or for building up other functional materials.

Experimental

Synthesis

All solvents and reagents employed were purchased from Sigma-Aldrich and Adamas-beta. CHCl₃ and CH₃CN were distilled from calcium hydride. THF was distilled from Na. And all procedures were carried out using standard Schlenk techniques.

Instruments

¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded with a Mercury Plus 400 (400 MHz for proton, 100 MHz for carbon) spectrometer with tetramethylsilane as the internal reference using CDCl₃ as solvent in all cases. Mass spectrometry was conducted with an ultra-performance liquid chromatography and quadrupole time-of-flight mass spectrometer. UV-visible spectra were recorded with a Hitachi U-4100 spectrophotometer. Fluorescence spectra were obtained with a FluoroMax-4 spectrophotometer. Electron paramagnetic resonance spectra were recorded with a Bruker BioSpin Corp., Germany. Cyclic voltammetry was performed with a Chenhua 650D electrochemical analyzer in anhydrous CH₂Cl₂ containing recrystallized tetra-n-butylammonium hexafluorophosphate (TBAPF₆, 0.1 M) as supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.3 mm²) and a platinum wire as the counter electrode. The Pt working electrode was routinely polished with a polishing alumina suspension and rinsed with acetone before use. The measured potentials were recorded with respect to Ag/AgCl reference electrode. All electrochemical measurements were carried out under nitrogen at atmospheric pressure.

Synthetic procedures

Synthesis of 1,2-[4-(1,2,2-triphenylvinyl)]diphenylmethanone (2). A solution of 4-bromo(1,2,2-triphenylvinyl)diphenyl (1742 mg, 4.25 mmol) in THF (10 mL) was slowly added to a mixture of iodine activated magnesium (122.0 mg, 5.1 mmol) in THF (5 mL) to form a Grignard reagent in 4 h. Afterwards, the cold Grignard reagent was slowly added to a solution of 1 (600.0 mg, 1.70 mmol) in dry THF (50 mL) at 0 °C, and stirred overnight. Then, the reaction was quenched with 10% HCl (50 mL) and extracted with CH₂Cl₂ (50 mL × 3). The combined organic fractions were washed with 1.0 M NaHCO₃ and water, and dried over MgSO₄. The solution was filtered, and then the solvent was removed under reduced pressure. The resulting residue was further purified by column chromatography using CH₂Cl₂/petroleum (2 : 1) as eluent, to afford the product 2 as a yellow solid (1150.0 mg, 80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.56 (d, 4H, J = 8.0 Hz), 7.09 (m, 18H), 7.00 (m, 16H). ¹³C NMR (100 MHz, CDCl₃): 196.3, 149.1, 143.4, 143.3, 143.2, 142.8, 140.1, 135.4, 131.4, 130.5, 129.7, 129.5, 128.1, 128.0, 127.9, 127.2, 127.0. HRMS (C₆₀H₄₂O₂, ESI⁺): calculated for [M + H]⁺ 795.3258, found: 795.3246.

Synthesis of 1,3-[4-(1,2,2-triphenylvinyl)]isothianaphthene (ITP-TPE). In a Schlenk flask containing a solution of 2 (794 mg, 1.0 mmol) in dry toluene (15 mL), Lawesson's reagent (808 mg, 2.0 mmol) was added in one portion, and then the mixture was heated to 110 °C and stirred overnight. The reaction mixture was poured into brine and extracted by dichloromethane (50 mL × 3). The organic phase was dried over MgSO₄ and the solvent was evaporated in vacuum. The product ITP-TPE was purified by chromatography (CH₂Cl₂/hexane = 1/1) on silica gel to afford a yellow solid (635 mg, 80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.75 (dd, 2H, J = 8.0 Hz, J = 4.0 Hz), 7.39 (d, 4H, J = 8.0 Hz), 7.10 (m, 36H). ¹³C NMR (100 MHz, CDCl₃): δ = 143.7, 143.6, 143.6, 142.8, 141.4, 140.4, 135.2, 134.0, 132.0, 131.5, 131.4, 131.4, 128.1, 127.8, 122.7, 127.6, 124.1, 121.3. HRMS (C₆₀H₄₃S, ESI⁺): calculated for [M + H]⁺ 795.3080, found: 795.3046.

Acknowledgements

We thank the National Basic Research Program of China (973 Program: 2013CBA01602, 2012CB933404), the Natural Science Foundation of China (21574080), and the Shanghai Committee of Science and Technology (15JC1490500).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements and instruments. See DOI: 10.1039/c6ra16631h

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