Protonation and axial ligation intervened fluorescence turn-off sensing of picric acid in freebase and tin(IV) porphyrins

Abstract

Freebase and tin(IV)-porphyrins are examined for the selective detection of picric acid and their affinity is revealed by spectroscopic titrations and X-ray structures. The sensing is mediated through protonation and axial ligation in 1–5 and 6–9 respectively. Fluorescent lifetime studies show that the quenching is dynamic in the freebase and static in tin(IV)-porphyrins.

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In recent years, trace detection of powerful explosive nitroaromatic compounds (NACs) is an active area of research owing to the increasing concern over environmental and homeland security.¹ In particular, picric acid (PA) is a strong organic acid, extremely hazardous and widely found in explosives.² Among the various analytical methods employed so far to track down NACs, fluorescence based sensing techniques are the most effective tool due to their high sensitivity, specificity and fast response times.³ A number of fluorescent materials have been identified based on macrocycles such as anthryl calix[4]arene,⁴ and porphyrins/phthalocyanines,⁵ small organic molecules,⁶ conjugated polymers,⁷ green materials,⁸ Schiff base complexes⁹ and metal–organic frameworks¹⁰ that show a selective response towards PA. Porphyrin based fluorescent sensors¹¹ are extensively studied because of their ease of synthesis, excellent sensitivity and characteristic photophysical properties like pronounced photostability, high fluorescence quantum yield, tunable fluorescence emission etc. The highly conjugated freebase porphyrin core is able to accommodate various metal ions due to its strong coordination ability and hence it also acts as metal ion sensors. Here, we describe a series of freebase and high valent tin(IV) meso-tetraarylporphyrins (Scheme 1), which exhibit highly selective response towards PA comparing to other NACs, where hydrogen bonding, protonation and axial ligation are the key factors for their sensitivity and selectivity.

Scheme 1 Molecular structure of porphyrins under study.

The freebase and tin(IV) derivatives are synthesized using literature methods^12,13 and characterized by UV-visible, ¹H NMR and single crystal X-ray analysis. The photophysical data of porphyrins 1–9 are given in Table S1.† The optical absorption spectra of freebase porphyrins exhibit an intense Soret band between 419 and 423 nm and four weaker visible (Q) bands whereas tin(IV) porphyrins show strong Soret absorption between 426 and 432 nm with two characteristic weaker Q bands.¹⁴ On excitation at the Soret band, porphyrins 1–9 exhibit two well-defined emission bands between 603 and 722 nm (S₁ → S₀) and a weak emission between 433 and 467 nm (S₂ → S₀).¹⁵

To observe the sensing ability of porphyrins, 1–9 against various NACs, fluorescence titration experiments were performed with porphyrins in CHCl₃ (1 × 10⁻⁸ M) by the addition of micromolar solutions of various analytes (in CHCl₃) such as nitrobenzene (NB), phenol, 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP), 2,4,6-trinitrophenol (PA), 2,4,6-trinitrotoluene (TNT), acetic acid (Aa) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Notably, all the porphyrins show very good sensing performance only towards PA by quenching the emission intensity. Fig. 1 shows the relative fluorescence intensity changes (F₀/F) of freebase as well as tin(IV) porphyrins against various NACs.

Fig. 1 Relative changes in fluorescence intensity (F₀/F) of the fluorophores, (a) 1–5 and (b) 6–9 against various nitroaromatic compounds.

The turn-off fluorescence response of 2 with different concentration of PA on excitation at 420 nm is shown in Fig. 2a and S2.† It is found that the emission intensity of 2 decreases with incremental addition of PA, 90% quenching was observed for 0.1 μM concentration of PA indicating that there is a strong interaction between 2 and PA. Similar spectral pattern is observed for porphyrins, 1 and 3–9 in presence of PA whereas other analytes showed a minor or negligible effect. The selectivity and sensitivity of porphyrins, 1–5 towards PA is possibly attributed to the high polarizability of PA and also the presence of strong hydrogen bonding interactions between them. The presence of hydrogen bonding interaction is evident from the crystal structure of the freebase porphyrin–PA complex (5²⁺·2PA⁻). Notably, the quenching efficiency of freebase porphyrins are high compared to tin(IV) derivatives (Fig. 1), which may be due to the decreased electron density at the porphyrin core upon insertion of the metal ion and a similar trend was also observed in zinc(II) porphyrins.^11b Moreover, the fluorescence quantum yield obtained for freebase porphyrins 2, 4 and 5 are high (ϕ ≈ 0.12), compared to other porphyrins indicating that they may hold promise for sensing applications.

The fluorescence quenching of porphyrins against PA at different concentrations was quantitatively evaluated using the Stern–Volmer (S–V) equation.¹⁶ The S–V plots (Fig. 2b) for PA are linear which indicates the existence of static or dynamic quenching process. The increasing order of quenching efficiency of the freebase porphyrins is 2 ≈ 5 > 1 > 4 > 3 and for tin(IV) porphyrins is 7 > 6 ≈ 9 > 8 (Table S2†). The given order is in concordant with the electron richness of the porphyrin core. Among the porphyrins, 2 and 5 exhibit a high efficiency for the detection of PA with K_SV values of 3.90 × 10⁷ M⁻¹ and 1.32 × 10⁷ M⁻¹ respectively. It is the highest quenching constant values among the reported porphyrin-based fluorophores for selective sensing of NACs¹⁷ and is comparable with the recently reported heavy metal decorated porphyrins (Table S2†).^17c It has been reported that there is no fluorescence quenching for 1 against PA in DMF medium indicating the solvent dependent sensing mechanism.^17c Interestingly, the purple colour solution of freebase porphyrins, 1–5 turned to green upon addition of aliquots of PA, which enables the naked eye detection of picric acid in solution (Fig. 3) whereas tin(IV) porphyrins did not show the colour change. We also performed the control experiments using CHCl₃, acetic acid, nitrobenzene, phenol and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and the freebase porphyrins show negligible fluorescence quenching against them. Whereas the tin(IV) derivative, 6 (Fig. 1) exhibits a lesser magnitude of fluorescence quenching by DDQ and TNT, may be attributed to the hydrogen bonding interactions between the analyte and sensor.

Fig. 2 (a) Fluorescence quenching of 2 (1 × 10⁻⁸ M in CHCl₃) upon incremental addition of picric acid (1.67 × 10⁻⁸ M in CHCl₃) (λ_exc = 420 nm); (b) Stern–Volmer plots for 1–5 against concentration of PA.

Fig. 3 Colorimetric response of 2 (3.3 × 10⁻⁶ M) with various analytes (3.3 × 10⁻⁵ M) in chloroform; (a) under normal and (b) UV-light.

The fluorescence lifetimes of freebase porphyrins and tin(IV) derivatives were measured both in the presence and absence of PA in CHCl₃ (Table S3†) as portrayed in Fig. S4.† In the case of freebase porphyrins, the excited state of the complex (5²⁺·2PA⁻) decays faster than the bare porphyrin suggests that the fluorescence quenching occurs through dynamic mechanism. However, the lifetime is almost invariant at different quencher concentrations for tin(IV) porphyrins, which is possibly due to the axial ligation happened and may correspond to static quenching mechanism occurs through the electron transfer from the electron rich porphyrin to electron deficient PA.

To explore the sensing mechanism involving the protonation of core imino nitrogens, ¹H NMR titrations were further carried out for 1 and 6 with PA (0, 1 and 2 equiv.) in CDCl₃ (Fig. 4, S8 and S9†). Upon titrating 1 with one equiv. of PA, the core protons (–NH) experienced a downfield shift¹⁸ from −2.77 to −1.29 ppm demonstrating the loss of aromaticity leads to nonplanarity. This is due to the protonation occurred in core nitrogen to give porphyrin mono acid observed as a multiplet of β-pyrrole protons specifying the asymmetry nature of the core. Interestingly, the symmetry was regained with the formation of diacid (observed as a singlet for β-pyrrole protons, Fig. 4a) upon addition of one more equivalent of PA, which is subsequently reflected in the increased intensities of –NH peak (Fig. 4a). In the case of 6, the intensity of trans-axial hydroxyl protons signal at −7.42 ppm is disappeared upon increasing the concentration of PA (from 1 to 2 equiv.) indicating the possibility of axial ligation happened in the apex positions (Fig. 4b). These results explicitly corroborate the formation of porphyrin diacid and consistent with the proposed mechanism for the sensing of PA.

Fig. 4 ¹H NMR titration of (a) H₂TPP, 1 and (b) Sn^IV(OH)₂TPP, 6 with picric acid (0, 1 and 2 equiv.) in CDCl₃ at 298 K.

The formation of diprotonated species in 1–5 in the presence of PA are also visualized from UV-visible spectroscopy. As anticipated, PA selectively interacts with 1–5 showing a considerable red-shift of 26 to 36 nm in the UV-visible spectra (Fig. 5, S5†) and no shifts were observed for other analytes. The UV-visible titration of 2 with the incremental addition of PA is shown in Fig. 5b. Remarkably, a decrement in the absorbance of 2 was observed at 419, 514 nm with the concomitant increment at 448 and 658 nm with an isosbestic point. The Hill plot (Fig. 5b inset) shows a straight line between log[PA] and log(A_i − A₀/A_f − A_i) with slope value of ∼2 indicating the stoichiometry ratio between porphyrin to PA is 1 : 2. And, the higher logβ₂ value (9.42) demonstrates the efficient interaction between 2 and PA (2 equiv.) which is comparable with the reported diprotonated planar porphyrins.¹⁹ Moreover, the UV-visible spectrum obtained for 2 with PA resembles the optical absorption spectrum of reported diprotonated planar porphyrins,¹⁹ H₄TPP(X)₄²⁺ (X = H, CH₃, Ph, Br) in presence of TFA clearly indicates the diacid formation.

Fig. 5 UV-visible spectra of 2 (a) with various analytes and (b) with incremental addition of picric acid.

The occurrence of protonation in freebase, 5²⁺·2PA⁻ and axial ligation in tin(IV) porphyrins 6·PA were further unambiguously confirmed by the single crystal X-ray structure data (Table S4, Fig. S6 and S7†). To the best of our knowledge, this is the first report for the crystal structure of porphyrin based chemosensors with PA, 5²⁺·2PA⁻ complex and 6·PA. Single crystals of X-ray quality of 5 with PA was obtained by a slow vapour diffusion of hexane into a chloroform solution of porphyrin that crystallized in orthorhombic crystal system with P2₁2₁2₁ space group. In 5²⁺·2PA⁻ complex, PA exists as picrate ions which are similar to that of reported crystal structure for the tris-imidazolium based sensor.²⁰ The porphyrin core is found to be non-planar (Fig. S6a–c†) and the average mean plane deviation of the β-pyrrole and meso-carbon atoms in 5²⁺·2PA⁻ is found to be ±0.81(2) Å and ±0.062(2) Å respectively, which is much greater than that of the reported H₂T(O–C₄H₉P)PBr₄ (ref. 21) bearing antipodal bromine groups (±0.50(3) Å and ±0.038 (3) Å). The enhanced non-planarity observed in 5²⁺·2PA⁻ without bulky antipodal bromine groups at the porphyrin periphery indicates the effective interaction between PA and 5 due to their close proximity. Moreover, the intermolecular interactions in 5²⁺·2PA⁻ were found to be mainly of hydrogen bonding between the oxygen atom of the 2,4,6-trinitrophenolate and imino hydrogen of the porphyrin core. It is noteworthy that the hydrogen bonding interactions between porphyrin and PA are very strong owing to their short distances, they found on both faces of the core and in the range of 1.565–2.151 Å (N1–H1⋯O11, 2.073 Å; N3–H3A⋯O11, 1.951 Å; N2–H2A⋯O18, 1.565 Å and N4–H4A⋯O18, 2.151 Å; Fig. S6d†). Moreover, the weak intermolecular interactions involving hydrogen is 71% as per the Hirshfeld surface analysis²² (Fig. S10†). But, the co-crystals of 3 with 4-nitrophenol (3·4-NP) does not show any non-planarity (Fig. S7a†) of the core revealed that the analyte is not good enough to bind with porphyrin, which is also reflected in the fluorimetric titrations.

However, the crystal structure of tin(IV) porphyrins, 6 with the 2,4,6-trinitro phenolate (6·PA) and 2,4-dinitro phenolate (6·2,4-DNP) moieties were covalently attached to its apex positions through axial ligation (Fig. S7b and c†). Though the crystal structure of 6·2,4-DNP was successfully achieved, the fluorimetric titration does not show any turn-off behaviour presumably due to the higher pK_a of 2,4-DNP (∼4) compared to PA (∼0.4) as well as the lesser affinity of 2,4-DNP towards axial ligation.²³ Hence, the sensing mechanism of these porphyrin chemosensors for the detection of explosive PA is based on the existence of intermolecular proton transfer from PA to freebase porphyrin results in the formation of diprotonated species and axial ligation succeeded in tin(IV) porphyrins (Fig. 6).

Fig. 6 Proposed mechanism of sensing of PA against freebase and tin(IV) porphyrins along with the ORTEP diagrams of 5²⁺·2PA⁻ and 6·PA.

In summary, freebase and tin(IV) meso-tetraarylporphyrins have been explored for its selective detection of powerful explosive PA. The spectroscopic studies revealed that the protonation and axial ligation lead to high fluorescence quenching in freebase and tin(IV) porphyrins respectively which are further evident from single crystal X-ray crystallography. The crystal structure of porphyrin 5 with PA exhibits strong intermolecular hydrogen bonding interactions between the porphyrin diacid and picrate anions. The quenching process is dynamic for freebase and static for tin(IV)-porphyrins. The porphyrins 2 and 5 exhibit highest K_SV values (3.90 × 10⁷ M⁻¹ and 1.32 × 10⁷ M⁻¹) among any reported porphyrins, for its selective NACs sensing and thus can serve as efficient chemosensors for the detection of PA.

Acknowledgements

Financial support from DST, New Delhi to CA (SB/EMEQ-016/2013) is gratefully acknowledged. We also thank Dr Shibu M. Eappen, STIC, CUSAT, Kochi and Dr Babu Varghese, SAIF, IIT Madras, Chennai for the data collection and structure refinement respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, plots, tables, photophysical data, fluorescence quenching spectra, S–V plots, ORTEP diagrams and X-ray structural data. CCDC 980779 (3·4-NP), 1058018 (5²⁺·PA²⁻), 1058017 (6·2,4-DNP) and 974216 (6·PA). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18310c

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