A novel heteroacene 2-(perfluorophenyl)-1H-imidazo[4,5-b]phenazine for selective sensing of picric acid

Abstract

A novel sensing probe, 2-(perfluorophenyl)-1H-imidazo[4,5-b]phenazine (PFIPZ) containing imidazole and phenazine units, has been successfully synthesized and can identify picric acid over other electron-deficient compounds or strong acids.

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2,4,6-Trinitrophenol, commonly known as picric acid (PA, a strong organic acid), is a nitroaromatic chemical, whose properties are similar to many polynitrated aromatic compounds.¹ Since PA is not only an explosive chemical (its explosive power is superior than that of 2,4,6-trinitrotoluene), but also is recognized as a toxic pollutant (it is a strong irritant to skin/eye and can cause potential damage to organs), it is very urgent to develop an efficient probe to detect PA before it becomes harmful to human beings or the environment.² Although several detecting techniques (such as spectroscopic and electrochemical) involving different types of materials (e.g. small molecules or polymers, nanoparticles/nanofibers, MOFs) have been employed to detect PA in solution or in the vapor phase, searching new materials with optical response (e.g. color changes or fluorescence varying) is still highly desirable because this type of detection has a fast response, high sensitivity, low cost, and simple sample preparation.³ However, many previously-reported sensors with optical response had several drawbacks including low binding affinity to PA or low sensitivity toward PA vapor, which cause poor signal amplification effect. Thus, it is important to develop new probes to address these problems.

It is well-known that azaacenes have been demonstrated to show excellent optical responses to different ions or small molecules.⁴ These results strongly make us believe that azaacenes could be an excellent optical probe for PA molecules. In fact, our group has been working on heteroacenes for a long time and these materials have been widely applied in field-effect transistors, organic light-emitting diodes, memory devices, sensors and so on.⁵ As a member of heteroacenes, phenazine derivatives have been widely used in the field of organic electronics, however, they have seldom been used as chemosensors.⁶ In this work, we designed and synthesized a new sensing probe (2-(perfluorophenyl)-1H-imidazo[4,5-b]phenazine, PFIPZ) containing imidazole and phenazine units, which shows an excellent optical response to PA molecules. As shown in Scheme 1, the synthesis of compound PFIPZ can be finished in one-step and the as-prepared PFIPZ was fully characterized through ¹H and ¹⁹F NMR spectroscopy, mass spectrometry and single crystal analysis. Due to the poor solubility, it is very difficult for us to obtain a reasonable ¹³C NMR spectrum.

Scheme 1 Synthetic route to compound PFIPZ.

The single crystals suitable for X-ray analysis (CCDC number: 1446065) were obtained through slowly cooling the hot dimethyl sulfoxide solution. Fig. 1a shows the single crystal structure of compound PFIPZ, indicating that compound PFIPZ is the targeted material. Fig. 1b shows molecular packing diagram of compound PFIPZ and the intermolecular distance of two neighbouring molecules is 3.24 Å, which is a little smaller than the normal π–π interaction distance (ca. 3.3 Å), suggesting the existence of strong π–π stacking. Compound PFIPZ exhibits a very good thermal stability with an onset decomposition temperature of ∼277 °C (considering the temperature for 5% weight loss, Fig. S3†).

Fig. 1 (a) Single crystal structure and (b) molecular packing diagram of compound PFIPZ.

The normalized optical absorption spectrum of compound PFIPZ in acetonitrile solution is provided in Fig. 2a. The absorption spectrum of compound PFIPZ shows one prominent band (384 nm) and two small bands (433 and 457 nm), which can be ascribed to the π–π* transition and the n–π* transition with less intensity than that of the former. The absorption edge of compound PFIPZ extends to ∼485 nm, from which the optical band gaps (E^opt_g) is estimated to be 2.56 eV.

Fig. 2 (a) Normalized optical absorption spectrum of compound PFIPZ in acetonitrile; (b) cyclic voltammogram curves of compound PFIPZ in acetonitrile.

Cyclic voltammetry measurement of compound PFIPZ was performed in acetonitrile solution (with 0.1 M TBAPF₆ as electrolyte) to investigate the electrochemical properties. As shown in Fig. 2b, compound PFIPZ exhibited one reversible reductive, one quasi-reversible, and one irreversible oxidative waves. The one reversible reductive wave may come from the phenazine while one quasi-reversible and one irreversible oxidative wave could contribute from imidazole. The lowest unoccupied molecular orbital (LUMO)/the highest occupied molecular orbital (HOMO) energy levels of compound PFIPZ were calculated from the onset of the first reduction/oxidation peaks. The LUMO/HOMO energy level of compound PFIPZ were estimated to be −3.78/−5.12 eV from the onset reduction/oxidation potential with reference to Fc⁺/Fc (−4.8 eV) using the equation of E_LUMO/HOMO = −[4.8 − E_Fc + E^onset_re/ox] eV. The band gap for compound PFIPZ was calculated to be 1.34 eV.

To investigate the suitability of compound PFIPZ as a chemosensor for PA, an ethanol solution containing PFTPZ was titrated with electron-deficient or acid compounds to investigate the change of fluorescence. Compound PFIPZ emits strong green light at 524 nm in the ethanol solutions with the fluorescent quantum yield 24% using fluorescein (φ = 0.79 in 0.1 M NaOH) as a standard. As shown in Fig. 3a, the fluorescence emission intensity rapidly disappeared without any shift in the spectral energies with the increasing concentration of PA. As shown in Fig. S5,† the linear regression equation was determined to be y = −10 680x + 647 and then the slope was −10 680. The detection limit of PFIPZ and PA was calculated to be 2.4 × 10⁻⁷ M with the equation: detection limit = 3S_d/ρ, where S_d is the standard deviation of blank measurement (0.88); ρ is the slope between the fluorescence intensity versus PA concentration. Fig. 3b shows that the addition of some other electron-deficient or acid compounds (AcOH, TFA, CSA, m-cresol, BN, NB, 1,2-DBN, 1,4-DBN) has almost no effect on the fluorescence emission intensity except for 2-NP, 3-NP, 4-NP, 2,4-DNP. Although there are ∼4% and 3% fluorescence quenching observed for 1,2-dinitrobenzene and 1,4-dinitrobenzene respectively, fluorescence quenching for 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol are slightly increased to 20%, 18%, and 20%, respectively. Furthermore, obvious fluorescence quenching (∼46% and 80%) can be investigated upon the addition of 2,4-nitrophenol and PA. The high fluorescent quenching efficiency associated with PA over other nitro aromatic compounds suggests that it has the strongest interactions with compound PFIPZ than that of other nitro aromatic compounds. As shown in Scheme 2, PA as an acid can transfer its acidic proton to basic functional group. Compound PFIPZ associated with basic amino group can strongly interact with PA that leads to the fluorescence quenching upon adding PA. To gain further insight into electronic structures, the geometry structures of compound PFIPZ and PA were optimized by using DFT calculations (B3LYP/6-31G*),⁷ and the frequency analysis was followed to assure that the optimized structures were stable states. All calculations were carried out using Gaussian 09 as shown in Fig. 3c. In case of PFIPZ, the HOMO was located on the imidazo and phenazine while the LUMO was located on the whole molecule, suggesting notable interaction between the donor (imidazo and phenazine) and acceptor (perfluorophenyl) entities. The LUMO energy level of PA is between the LUMO energy level and the HOMO energy level of compound PFIPZ, suggesting that the fluorescence quenching of compound PFIPZ in ethanol with increasing concentration of PA is due to photoinduced electron transfer (PET) process.

Fig. 3 (a) Change in fluorescence spectra of compound PFIPZ (10 uM) upon titration with picric acid (0–100 equiv.) in ethanol; (b) fluorescence quenching efficiencies of compound PFIPZ toward different analytes in ethanol. PA = picric acid; AcOH = acetic acid; TFA = trifluoroacetic acid; H₂SO₄ = sulfuric acid; CSA = camphorsulfonic acid; BN = benzonitrile; NB = nitrobenzene; 1,2-DNB = 1,2-dinitrobenzene; 1,4-DNB = 1,4-dinitrobenzene; 2-NP = 2-nitrophenol; 3-NP = 3-nitrophenol; 4-NP = 4-nitrophenol; 2,4-DNP = 2,4-dinitrophenol; (c) electron transfer fluorescence quenching process.

Scheme 2 Schematic diagram for photoinduced electron transfer process between PFIPZ and PA.

In conclusion, we have successfully designed and synthesized a novel heteroacenes, PFIPZ, which can act as an efficient chemosensor for PA over a wide range of electron-deficient or acid compounds (PA, AcOH, TFA, CSA, m-cresol, BN, NB, 1,2-DBN, 1,4-DBN) due to the photoinduced electron transfer (PET) process.

Acknowledgements

Q. Z. acknowledges financial support from AcRF Tier 1 (RG133/14 and RG 13/15) and Tier 2 (ARC 2/13) from MOE, and the CREATE program (Nanomaterials for Energy and Water Management) from NRF, Singapore. Q. Z. also thanks the support from Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant number: sklssm201630), Jilin University, China.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section. CCDC 1446065. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra08547d

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