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Chem., 2009, 33, (2) 254-263 It’s not just about chemistry… The New Journal of Chemistry (NJC) publishes research where chemistry connects with other areas of science creating an interface of many disciplines, inspiring biology, nanomedicine, nanotechnologies, materials science, catalysis etc. Jerry Atwood Editor in chief Michael Scott Associate Editor Pascal Le Floch Editor in chief Peter Junk Associate Editor Submit your work today! ISSN 1359-7345 COMMUNICATION M. R. Ajayakumar and P. Mukhopadhyay Naphthalene-bis-hydrazimide: radical anions and ICT as new bimodal probes for differential sensing of a library of amines www.rsc.org/chemcomm Number 25 | 7 July 2009 | Pages 3641–3812 Chemical Communications FEATURE ARTICLE Kristian Strømgaard et al. Recent advances in the understanding of the interaction of antidepressant drugs with serotonin and norepinephrine transporters Pu bl ish ed o n 16 A pr il 20 09 . D ow nl oa de d by D uk e U ni ve rs ity o n 30 /0 9/ 20 13 1 8: 47 :4 3. View Article Online / Journal Homepage / Table of Contents for this issue Naphthalene-bis-hydrazimide: radical anions and ICT as new bimodal probes for differential sensing of a library of aminesw M. R. Ajayakumar and Pritam Mukhopadhyay* Received (in Cambridge, UK) 13th February 2009, Accepted 20th March 2009 First published as an Advance Article on the web 16th April 2009 DOI: 10.1039/b903097b A new strategy of a bimodal sensing for amines and the first applications of radical anions as probes to colorimetrically differentiate a library of amines is reported. Chromogenic molecular sensors provide the most inexpensive and rapid route to sense a variety of analytes. There has been a tremendous progress in the development of smart chromo- genic sensors for anions and metal ions.1 Although a wide range of receptors are known to bind amines,2 only a very few molecular sensors for these neutral organic analytes have been explored up to now.3 Amines are crucial since they have a wide ranging presence in foodstuffs, drugs, pesticides, paints, etc.; are related to the freshness of foods; and inhibition of its catabolism causes allergy, headache, hypertension, etc. Given the expansive range of class, size, shape, isomers, etc. in which the amines exist, it becomes an arduous task to design sensors that can distinguish between these subtle molecular informations. The molecular amine sensors discovered so far function mainly through intramolecular charge transfer3a–f (ICT) as the singular signalling unit (SSU).3 This produces a single colour tone, which significantly limits its applicability to demarcate closely related amines. Molecular and polymer sensor arrays4b,c for amines have been employed as an alter- native to molecular sensors. However, this approach requires a large number of receptors.4 In order to circumvent this problem, it is essential to have more than one chromogenic SU incorporated in the same molecular sensor. This demands the discovery of new probes that can operate independently of the already well-established pathways such as ICT, MLCT, PCT, etc. and further integrate them in a single molecular sensor to achieve a combination of SUs. We envisioned that radical anions of aromatic chromo- phores can be attractive candidates as new probes for amines, since they have well-defined spectroscopic features, intense colour in the visible region, and are adaptable to solvent polarity. As a proof of principle, herein we report the first example of a chromogenic bimodal signalling strategy for amines utilizing a new class of molecule, denoted naphthalene-bis-hydrazimides. We have explored the usage of radical anions as new probes and conventional ICT as optically distinct signalling processes to generate an information-rich colour palette that aids in sensing a particular amine and also to address critical issues related to size, shape, isomerism, lipophilicity and minimal pKa differences within a library of amines (Scheme 1). This new strategy, therefore, enables a single molecular sensor to even play the role of a complex sensor array with multiple receptors. The naphthalenediimide (NDI) moiety has been extensively used to design elegant supramolecular materials.5 However, to the best of our knowledge, radical anions of the NDI moiety with a characteristic spectroscopic signature have so far not been applied as sensor probes. Towards the design, we have integrated two electronically distinct signalling units, namely the NDI moiety and donor–acceptor functionalized phenyl hydrazine rings (Scheme 2). This is based on the premise that the NH groups in 1a, 1b positioned in between the imide functionality and the phenyl rings would trigger a charge transfer interaction in the presence of amines. Theoretical studies have also confirmed that electron withdrawing inductive effects in 1a, 1b substantially lower the LUMO level of the NDI moiety, which we anticipate would facilitate the recogni- tion of amines through electron acceptance (Scheme 2, right).6 The naphthalene-bis-hydrazimides (1a, 1b) were synthesized in moderate to good yields and characterized by NMR, MALDI-TOF MS, IR and UV spectroscopy (see ESIw). We have carefully chosen the individual amines for the library so that it comprises of 11, 21 and 31 amines; linear, branched, aliphatic, aromatic and biogenic amines and amines having minimal pKa differences. Upon combination of 1a with the 23 closely related amines, we observed spectacular chromo- genic changes that encompass a broad spectral range (Fig. 1). 1a in dry THF with straight chain (SC) 11 amines (2–6), show a spontaneous color change from pale yellow to various shades of blue. This allows us to differentiate amines having subtle changes in chain length and lipophilicity by the naked eye. Importantly, 1a also demarcates the sterically hindered 11 amines 7 and 8 from the straight chain amines. Further, aromatic amine 10 and biogenic diamine 12 could readily be distinguished by the naked eye from their closely related Scheme 1 Supramolecular & Material Chemistry Lab, School of Physical Sciences, JNU, New Delhi, India. E-mail:
[email protected] w Electronic supplementary information (ESI) available: Experimental details, synthesis and characterization. See DOI: 10.1039/b903097b 3702 | Chem. Commun., 2009, 3702–3704 This journal is �c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm Pu bl ish ed o n 16 A pr il 20 09 . D ow nl oa de d by D uk e U ni ve rs ity o n 30 /0 9/ 20 13 1 8: 47 :4 3. View Article Online partners 9 and 11, respectively. Interestingly, 1a with the same set of amines but in the presence of water e.g., THF–H2O (9 : 1) and (1 : 1), shows a spontaneous and dramatically changed colour tone to dark red from the initial pale yellow colour (Fig. 1(b) and (c)). 31 amines 21, 22 do not show any colour changes and hence facilitates their easy detection. This differential response of 1a in dry and aqueous THF generates a sprawling colour palette that aids in identifying a particular amine from a diverse library. For instance, amines 7, 13 and 20 which remain difficult to recognize in dry THF as a result of non-coloration could easily be identified in THF–H2O (9 : 1) and (1 : 1), due to their distinct colour changes. Similarly, the amines belonging to C-4 isomers 3, 13 and 7, C-6 isomers 4, 14, 20 and 21, C-8 isomers 5 and 15 could be differentiated with the aid of this colour palette. The utility of this differential response can be gauged by the fact that a single amine e.g., 5 can generate 12 or more colour shades with distinct RGB values (ESIw) easily induced by small changes in solvent polarity (Fig. 1(d)). UV-vis studies confirmed the operation of the two distinct optical processes in non-polar versus polar solvents. 1a in dry THF, in the presence of amines shows the appearance of a broad band at 590–600 nm (Fig. 2(a)), attributed to ICT from donor N to acceptor CF3 groups, as also confirmed by ZINDO calculations. On the other hand, 1a in THF–H2O (9 : 1) and (1 : 1) (or in DMSO, DMF, etc.) with various amines shows a new set of bands at 470 (highly intense), 609, 690 and 765 nm (Fig. 2(b)), which fully matches with the signature peaks of radical anions of the NDI moiety, as reported earlier.7 Notably, this is the first example of radical anion generation in the NDI moiety with organic analytes. The sensor 1a with amines shows large bathochromic shifts of 230 nm (ICT) and 100–400 nm (radical anions) compared to the original 370 nm p–p* band of the NDI moiety. The large red shifts, adaptability of our sensor to produce colours of different intensities with various amines, and the exclusive formation and stability of radical anions in polar solvents, explains the differential response and generation of the colour palette with colour tones ranging from blue, yellow to red. While applying radical anions as probes for amines, there exists the possibility of interference from other reducing agents. However, such interference can be easily overcome by sensing amines under the solvent conditions where ICT predominates. The selectivity profile of 1a with amines could be established using the spectral response, defined by: ((A � A0)/A0) (Fig. 3(a)). The spectral response for the SC 11 amines 2, 4 and 6 increases by 19-, 53- and 72-fold, respectively, when compared to 31 amine 21. Under similar conditions, 21 amines 13, 15 and 17 show an increase of only 4-, 7- and 10-fold, respectively. This increase of the ((A � A0)/A0) values with increase in chain length of the 11 and 21 amines provides a direct correlation with the molecular length and lipophilicity of the amines. Among the isomeric amines, the spectral response of 4, 14, 20 increases by 53-, 7- and 1.5-fold, respectively wrt isomer 21. The spectral response along with the molecular size and volume of amines allowed us to assign each amine a specific site in a 3-D space (Fig. 3(b)). Scheme 2 Left: Structures of 1a–c. Right: HOMO (red) and LUMO (blue) levels (in eV) of 1a–c calculated by B3LYP/6-31G (d). Fig. 1 Photographs of solutions of 1a and 1a with amines (2–23) showing differential response in solvents [1a, 2.2 mM; all amines 2.6 M]: (a) dry THF, (b) THF–H2O (9 : 1) and (c) THF–H2O (1 : 1). (d) A colour array depicting 12 different colour shades of 1a with 5, generated by tuning the solvent polarity [1a, 1.5 mM, 5, 1.5 M]. Amines 5, 6, 8, 14–17 and 23 could not be studied in some of the solvent mixtures due to solubility problems. Fig. 2 UV-vis spectra of 1a with various amines: (a) in dry THF, (b) THF–H2O (9 : 1) [1a, 2.6 mM; all amines; 677 equiv.]. This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 3702–3704 | 3703 Pu bl ish ed o n 16 A pr il 20 09 . D ow nl oa de d by D uk e U ni ve rs ity o n 30 /0 9/ 20 13 1 8: 47 :4 3. View Article Online It is known that macrocyclic receptors signalling with ICT requires more than 103 equiv. of amines for chromogenic sensing of amines.3a,d On the contrary, 1a requires only 25 and 250 equiv. to sense 12 and SC 11 amines, respectively, by the naked eye. This selective and sensitive chromogenic response of 1a towards putrescine8 12 in solution, encouraged us to explore its potential as a test kit for identifying vapours of putrescine. Two types of sensor strips were developed: (a) soaking and drying a 2 mM solution of 1a on a filter paper and (b) vacuum deposition of a thin film (100 nm) of 1a on a glass surface. In both the cases, vapours of putrescine could be easily demarcated by the naked eye from the other closely related diamines (Fig. 4). To elucidate the bimodal sensing, we synthesized a key control compound 1c. Importantly, 1c in the presence of amines in dry THF, does not show any colour, even on long standing. This highlights the key role NH centres play in 1a in triggering the ICT, which is not feasible in the case of 1c. Subsequently, we explored the radical anion generation ability of 1c in presence of amines in aqueous THF. Light red coloration due to the radical anion could be seen only after a 2–3 h period. Cyclic voltammetry (CV), IR and B3LYP/6-31G (d) studies explained the spontaneity in the radical anion generation in 1a, 1b compared to 1c (ESIw). The E0 value of 1a occurs at �0.396 V cf. for 1c at �0.474 V in degassed THF. This lowering of the first electron reduction by 0.078 V explains the ease of radical anion generation of 1a compared to 1c. These set of results enlighten the special role the naphthalene-bis-hydrazimides 1a and 1b show vis-a-vis 1c in generating the ICT and radical anion states. Our studies have ascertained that steric interactions of the amines play a dominant role during sensing rather than pKa of the amines e.g., the highly basic but bulkier amines (BA) 13, 14, 18, 20, 21 bind very weakly to 1a (ESIw). Gratifyingly, this also allowed us to demarcate amines even with DpKa B 0.01 units e.g., 3 & 8, 3 & 12, etc. The strong discrimination towards BA and their consequent weak sensing suggests that these amines face considerable steric hindrance while approaching the electron accepting carbonyl centers. This points towards an inner-sphere mechanism for the radical anion generation. Molecular modeling studies support the cleft formation in 1a and hence the size recognition of the amines (ESIw). In conclusion, we have developed a new bimodal strategy for sensing of amines that enables parallel processing of multiple molecular information in a diverse library of amines, without the use of a complex sensor array comprising of multiple receptors. We thank DST for financial support and Prof. S. Ghosh, SPS, JNU, Prof. A. K. Singh, IIT Delhi and Dr. S. K. Dhawan, NPL, New Delhi for extending their laboratory facilities. Notes and references 1 (a) P. A. Gale, S. E. Garcia-Garrido and J. Garric, Chem. Soc. Rev., 2008, 37, 151; (b) T. Gunnlaugsson, M. Glynn, G. M. Tocci, P. E. Kruger and F. M. Pfeffer, Coord. Chem. Rev., 2006, 250, 3094; (c) R. Martı´nez-Ma´nˇez and F. Sanceno´n, Chem. Rev., 2003, 103, 4419; (d) J. L. Sessler and D. Seidel, Angew. Chem., Int. Ed., 2003, 42, 5134; (e) P. D. Beer and P. Gale, Angew. Chem., Int. Ed., 2001, 40, 487; (f) A. P. de Silva, H. Q. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97, 1515. 2 (a) J.-M. Lehn, in Supramolecular Chemistry: Concepts and Perspectives, Wiley-VCH, 1995; (b) R. J. Hooley, P. Restorp and J. Rebek Jr, Chem. Commun., 2008, 6291; (c) J. Lagona, P. Mukhopadhyay, S. Chakrabarti and L. Isaacs, Angew. Chem., Int. 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Commun., 2009, 3702–3704 This journal is �c The Royal Society of Chemistry 2009 Pu bl ish ed o n 16 A pr il 20 09 . D ow nl oa de d by D uk e U ni ve rs ity o n 30 /0 9/ 20 13 1 8: 47 :4 3. View Article Online