Fluorometric Chemosensor: Structural Implications - American

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1,9-Pyrazoloanthrone as a Colorimetric and “Turn-On” Fluorometric Chemosensor: Structural Implications Karothu Durga Prasad, N. Venkataramaiah, and Tayur N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: A colorimetric and “turn-on” fluorescent chemosensor based on 1,9-pyrazoloanthrone specifically for cyanide and fluoride ion detection shows a remarkable solid state reaction when crystals of tetrabutylammonium cyanide and fluoride are brought in physical contact with 1,9pyrazoloanthrone. X-ray crystal structures of 1,9-pyrazoloanthrone and complexes have been determined, and the ion sensing activity (detection limit of 0.2 and 2 ppb) has been inferred based on spectroscopic and structural features.

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he design and synthesis of molecular systems that are capable of sensing various anions from industrial effluents, environmental contaminants, and the potential for applications in biology and medicine have received momentous attention.1 Various novel macrocyclic compounds, neutral and/or charged, mono and multitopic receptors designed for high degree of selectivity are currently in use.2 Hydrogen bonding in anion chemosensors with chromophore units such as phenols, amides, urea, and pyrroles for selective binding and electrostatic interactions serve as a key design element.3 Among different anions, cyanide and fluoride have received special attention due to acute toxicity in physiological systems4 and several inexpensive colorimetric sensors for naked-eye detection5 have been devised. However, colorimetric as well as “turn-on” fluorimetric probes for cyanide and fluoride ions are limited to calix[4]pyrrole derivatives,6 dipyrrolylquinoxalines7 and borondipyrromethene (BODIPY)8 dye-based molecules. In addition, these compounds exhibit low binding affinities with anions in − the order of 104−105 M 1 and involve laborious synthetic efforts.9 Design of novel chemosensors with high sensitivity and specificity with high association constants for anions is still a challenge in this area. Naphthalene-based derivatives for fluoride ion detection with an association constant (105 − M 1)10 and donor−acceptor-based molecules containing anthraquinone with imidazole ring as a neighbor11 are some examples among small molecules. In this work, we subject 1,9-pyrazoloanthrone (SP600125, hereafter abbreviated as DP, Figure 1) as a colorimetric and “turn-on” fluorescent probe for the detection of cyanide and fluoride ions by exploiting the intramolecular charge-transfer (ICT) process. It should be noted that DP is being used as a specific kinase inhibitor for inhibition of c-Jun N-terminal Kinase (c-JNK) in cellular signaling pathway.12 To establish the significance of the acidic −NH protons (which is reported in the cases of other sensors in earlier studies9) in DP and to further establish the structural requisites to result in a highly selective and sensitive colorimetric and turn-on fluorimetric © XXXX American Chemical Society

Figure 1. Chemical structures of DP and DP-M.

sensor (at ppb levels), DP-M, where the −NH proton is replaced by a methyl group, is probed. 1,9-Pyrazoloanthrone (DP) was synthesized by condensation of 1-chloroanthraquinone with hydrazine13 (Scheme S1 of the Supporting Information). DP-M was synthesized by Nalkylation of 1,9-pyrazoloanthrone. Needle-shaped crystals of DP and DP-M were obtained from a solution of ethyl acetate and hexane (3:7 v/v) mixture, respectively, by slow evaporation. DP crystallizes in a trigonal space group P3121 (Z = 3 and Z′ = 1/2), forcing a 2-fold symmetry onto the molecule, thereby generating a positional disorder at the NH group of the pyrazole ring, effectively mimicking a pyrazolo[2,3-b]pyrazole in which the terminal nitrogen atoms are of 50% occupancy. This in addition pushes a disorder at the phenyl ring carbon atoms in the anthraquinone ring (50% occupancy), as shown in Figure 2a (also Figure S1 of the Supporting Information). The hydrogen attached to the disordered atoms is therefore not included during the refinements. The presence of the 31 screw axis leads to the formation of helical motifs along the c axis (Figure 2b). It is to be pointed out that there is no disorder in the crystal structure of DP-M, which crystallizes in the monoclinic space group P21/ n with one molecule in the asymmetric unit (Figure S2 of the Supporting Information) and is devoid of any helical symmetry. Received: February 18, 2014 Revised: March 22, 2014

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dx.doi.org/10.1021/cg5002489 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 2. (a) ORTEP diagram of DP (H’s removed for clarity). (b) Packing diagram of DP along the c axis.

cation form a layer-wise packing in the crystal structure with extensive hydrogen bonding (Figure 4b). DP-TBAF also exhibits a disorder at nitrogen and fluorine atoms similar to the structure of DP-TBACN (Figure 4c). DP-TBAF also appears with a disordered water of hydration. The occupancies of both the disordered fluorine and oxygen (water) were refined with respect to fixing the nitrogen position on the host to values close to 0.5 and later were fixed at this value during the rest of the refinement protocol. DP-TBAF also packs in a similar fashion resulting in layers (Figure 4d). The acidic −NH group on the pyrazole ring of DP provides the binding site for CN− and F− anions resulting in the corresponding intramolecular charge transfer (ICT) complex (Scheme 1), thereby leading to a colorimetric and fluorometric response both in solid and solution states. The change in the color of DP upon treating with anions in solution (acetonitrile) is analyzed to obtain insights into the stoichiometry and detection limits. The absorption spectra of compound DP (3.3 × 10−7 M) in acetonitrile exhibits two bands (283 nm n → π*; 392 nm π → π* transitions). Upon addition of different concentrations of CN− (as TBACN), the 392 nm band gradually decreases, while a new band (intramolecular charge transfer band) appears at 508 nm, corresponding to the formation of the red complex (Figure 5a). The intensity of the peak at 508 nm gradually increases with an isosbestic point at 424 nm. It was found that the intensity of the peak at 508 nm gradually increases until the formation of a 1:1 mol ratio between DP and CN− ion and reaches equilibrium with an association constant of 4 × 106 M−1. TBAF also shows a similar behavior, though the intensities are lower (Figure 5b). Insets of Figure 5 reveal that the reactivity of CN− with DP is higher than the response of F− (about 2.6 times). The detection limits of the CN− and F− ions with DP are found to be in the range of 0.2 and 2 ppb (Figure S3 of the Supporting Information), which are significantly superior to the detection limits reported for other compounds.16 The change in the color of DP upon treating with anions (CN− and F−) was studied both in solid and solution state. The solid state fluorescence emission spectra of the compound DP (Figure 6) before and after addition of CN− and F− shows variation in the fluorescence intensity. The solid state and solution state quantum yields of DP, DP-TBACN, and DPTBAF was tabulated in Table 1. The fluorescence spectra of DP in acetonitrile exhibits a weak emission band at 450 nm (λex = 360 nm). Figure 7a shows the emission spectra of DP upon addition of different concentrations of CN−. The emission intensity of DP at 450

A remarkable solid state reaction resulting in an instantaneous change in color (from pale yellow to intense red; video file of the Supporting Information) occurs when crystals of tetrabutylammonium cyanide (TBACN) and tetrabutylammonium fluoride (TBAF) are brought in contact on a TLC plate coated with DP (Figure 3). In fact bringing independent

Figure 3. (a) Compound DP (yellow) coated on a silica plate. DP reacts with (b) TBAF and (c) TBACN (visible color change to intense red) and not other ions in the solid state.

crystals of DP and TBACN or TBAF into contact results in an instantaneous change in color (even without grinding!). The reaction products are found to be molecular complexes containing DP and TBAX (X = CN and F) salts by single crystal X-ray diffraction. However, it is noted that other tetrabutylammonium salts containing Cl−, Br−, I−, ClO4−, NO2−, NO3−, and OAc− anions did not undergo any reaction and, hence, no change in color is observed. The specificity for CN− and F− anions appears to be due to their higher basicity among the anions. On the other hand, coated plates of DP-M are nonreactive to all anions including CN− and F−, which shows that proton on pyrazole N is important for reaction and therefore sensing. Red plate-shaped crystals of the complex DP-TBAX (X = CN and F) suitable for X-ray diffraction were grown from acetonitrile. Both complexes (DP-TBACN and DP-TBAF) crystallize in a triclinic space group P1̅ (Z = 2 and Z′ = 1) and are both isomorphous14 and isostructural15 to each other. Crystallographic parameters are given in Table S1 of the Supporting Information. The disorder in DP-TBACN at the NH group is still maintained; however, the anthracene moiety is devoid of any disorder. The cyanide ion is also disordered in tandem with equal occupancy corresponding to the two disordered nitrogen sites (Figure 4a). The anion and the B



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Figure 4. ORTEP of (a) DP-TBACN showing the disordered cyanide ion enclosed in green circles. (b) Packing diagram of DP-TBACN complex along the c axis. (c) ORTEP of DP-TBAF showing the disordered fluoride ion as green spheres. (d) Packing diagram of the DP-TBAF complex along the c axis.

Scheme 1. Formation of Intramolecular Charge-Transfer (ICT) Complex upon Reaction of DP with CN− and F− Anionsa

a

The proton can switch between pyrazole N and the anion in the molecular complex (TBA cation is omitted).

nm elicited a weak decrease with the emergence of a new redshifted emission band at 578 nm attributed to the ICT complex, and the emission intensity increases linearly with addition of anions (CN− and F−). Addition of other anions (Cl−, Br−, I−, ClO4−, NO2−, NO3−, and OAc−) did not show any obvious change in fluorescence (Figure 7b), which brings out the affinity of DP for CN− and F− ions. The emission intensity of the ICT band (578 nm) for the CN− ion is 1.5 fold higher than F− ions. The association constants obtained from Job’s plot further reveals the formation of a 1:1 stoichiometry complex with Kf = 1.2 × 106 M−1 (Figure S4 of the Supporting

Figure 5. Absorption spectra of DP (3.3 × 10−7 M) in acetonitrile treated with different concentrations of (a) TBACN and (b) TBAF. Insets show the higher sensitivity of TBACN over TBAF with respect to color and different concentrations at 508 nm.

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fluorescence behavior of DP by addition of different concentrations of CN−. We have studied the effect of different anions on the stability of the DP-TBACN complex. It is to be noted that there is no significant variation in emission intensity of the complex at 578 nm upon addition of other anions, except in the case of F−, which results in enhanced intensity to the complex (Figure S6 of the Supporting Information). The 1H NMR spectra of DP in CDCl3 shows the aromatic −NH protons at 10.38 ppm. Upon addition of TBACN (Figure 8), the intensity of the −NH signal

Figure 6. Solid state fluorescence spectra of DP, DP-TBACN, and DP-TBAF.

Table 1. Quantum Yields (in %) of DP, DP-TBACN, and DP-TBAF solid state (Φ) (%) DP

DP+F

0.00

2.15

−

solution state (Φ) (%)

DP+CN 5.73

−

DP

DP+F−

DP+CN−

0.03

18.51

30.02

Figure 8. 1H NMR spectra of DP (5 mM) in the presence of (a) 0, (b) 0.2, (c) 0.4, (d) 0.6, and (e) 1.0 equiv of TBACN in CDCl3.

Information). This is consistent with the results obtained from the absorption spectra and also corroborated by the crystal structure. Figure 7b shows relative fluorescence intensity of DP with different anions at different concentration portraits indicating that DP is selective for CN− and F− ions (Figure S5 of the Supporting Information). Figure 7c shows the turn-on

gradually decreases and broadens with concentration of the anion due to rapid proton exchange leading to a complete absence of the signal, indicating deprotonation. This was also observed for F−; however, with a relatively higher molar ratio than that of CN− (Figure S7 of the Supporting Information).

Figure 7. (a) Change in the fluorescence spectra of DP (3.3 × 10−7 M) upon addition of different concentrations of TBACN at λex = 360 nm. (b) Plot of relative fluorescence intensity of DP for different anions at different concentration levels. (c) Change in the fluorescence intensity of DP upon addition of different concn of TBACN, while irradiating with a UV lamp (λex = 365 nm). D



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Quality; World Health Organization: Geneva, 1996. (f) Sohn, H.; Létant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399−5400. (g) Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L. L. Angew. Chem. 2005, 117, 3672−3675. (4) (a) Qu, Y.; Hua, J.; Tian, H. Org. Lett. 2010, 12, 3320−3323. (b) Sui, B.; Kim, B.; Zhang, Y.; Frazer, A.; Belfield, K. D. ACS Appl. Mater. Interface 2013, 5, 2920−2923. (5) (a) Black, C. B.; Andrioletti, B.; Try, A. C.; Ruiperez, C.; Sessler, J. L. J. Am. Chem. Soc. 1999, 121, 10438−10439. (b) Piatek, P.; Jurezak, J. Chem. Commun. 2002, 20, 2450−2451. (6) (a) Jimenez, D.; Manez, R. M.; Sancenon, F.; Soto, J. Tetrahedron Lett. 2002, 43, 2823−2825. (7) (a) Sunahara, H.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem. Soc. 2007, 129, 5597−5604. (b) Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Org. Lett. 2008, 10, 4065−4067. (c) Bozdemir, O. A.; Guliyev, R.; Buyukcakir, O.; Selcuk, S.; Kolemen, S.; Gulseren, G.; Nalbantoglu, T.; Boyaci, H.; Akkaya, E. U. J. Am. Chem. Soc. 2010, 132, 8029−8036. (d) Yin, S. C.; Leen, V.; Snick, S. V.; Boens, N.; Dehaen, W. Chem. Commun. 2010, 46, 6329−6331. (e) Guliyev, R.; Ozturk, S.; Sahin, E.; Akkaya, E. U. Org. Lett. 2012, 14, 1528−1531. (8) (a) Swamy, P. C. A.; Mukherjee, S.; Thilagar, P. Chem. Commun. 2013, 49, 993−995. (b) Guliyev, R.; Ozturk, S.; Sahin, E.; Akkaya, E. U. Org. Lett. 2012, 14, 1528−1531. (9) Xu, Z.; Chen, X.; Kim, H. N.; Yoon, J. Chem. Soc. Rev. 2010, 39, 127−137 and references therein. (10) Lee, J. Y.; Cho, E. J.; Mukamel, S.; Nam, K. C. J. Org. Chem. 2004, 69, 943−950. (11) (a) Batista, R. M.; Oliveira, E.; Costa, S. P.; Lodeiro, C.; Raposo, M. M. M. Org. Lett. 2007, 9, 3201−3204. (b) Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Org. Lett. 2004, 6, 3445−3448. (12) Bennett, B. L.; Sasaki, D. T.; Murray, B. W.; O’Leary, E.; Sakata, S. T.; Xu, W.; Leisten, J. C.; Motiwala, A.; Pierce, S.; Satoh, Y.; Bhagwat, S. S.; Manning, A. M.; Anderson, D. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 13681−13686. (13) (a) Showalter, H. D. H.; Johnson, J. L.; Hoftiezer, J. M.; Turner, W. R.; Werbel, L. M.; Leopold, W. R.; Shillis, J. L.; Jackson, R. C.; Elslager, E. F. J. Med. Chem. 1987, 30, 121−131. (b) Bradley, W.; Geddes, W. K. J. Chem. Soc. 1952, 1630−1645. (14) Online Dictionary of Crystallography: Isomorphous crystals. http://reference.iucr.org/dictionary/Isomorphous_crystals (accessed Feb 18, 2014). (15) Online Dictionary of Crystallography: Isostructural crystals. http://reference.iucr.org/dictionary/Isostructural_crystals (accessed Feb 18, 2014). (16) (a) Robbins, T. F.; Qian, H.; Su, X.; Hughes, R. P.; Aprahamian, I. Org. Lett. 2013, 15, 2386−2389. (b) Kumari, N.; Jha, S.; Bhattacharya, S. J. Org. Chem. 2011, 76, 8215−8222.

In conclusion, a remarkable solid state reaction resulting in an instantaneous change in color occurs when compound DP is brought in contact with crystals of tetrabutylammonium cyanide (TBACN) and tetrabutylammonium fluoride (TBAF), and it is nonreactive with other tested (Cl−, Br−, I−, ClO4−, NO2−, NO3−, and OAc−) anions. DP is found to detect trace amounts of CN− and F− ions in the solution state. The addition of cyanide or fluoride ions to DP in acetonitrile results in colorimetric and fluorometric response due to ICT. The presence of the −NH group in the molecule is undoubtedly the center of recognition of the anion as evident by the absence of sensing by the methyl derivative. The structural features of molecular complexes (DP-TBACN/F) and compound DP have been explored by single crystal studies. The sensor property is rationalized by spectroscopic analysis of the compound as well as by the crystal structures of the molecular complexes.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

General and experimental procedures; absorption, fluorescence, and NMR spectra; calculation of detection limits; and a video clip. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel: +91-80-22932796. Fax: +91-080-23601310. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.D.P. thanks the CSIR for the Senior Research Fellowship. N.V. thanks the DST for the Postdoctoral Fellowship. T.N.G. thanks DST for the J. C. Bose Fellowship. We would also like to thank Prof. Satish Patil, Dr. S. Cherukuvada, and Mr. M. S. Pavan for useful discussions.

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Fluorometric Chemosensor: Structural Implications - American

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