High-Throughput Crystal Engineering Based Synthesis of

Communication pubs.acs.org/crystal

High-Throughput Crystal Engineering Based Synthesis of Supramolecular Gels: Blue-Emitting Fluorescent Gold Clusters Synthesized and Stabilized on the Gel-Bed Tapas Kumar Adalder, Dhurjati Prasad Kumar, and Parthasarathi Dastidar* Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, West Bengal, India S Supporting Information *

ABSTRACT: Crystal engineering approach has been successfully exploited in designing a new series of supramolecular gelators derived from (1R,3S)(+)-camphoric acid (CA) and primary amines; facile synthesis of blue-emitting gold clusters without the use of any external reducing and capping agents was achieved on one such gel-bed.

L

ow molecular weight gelators (LMWGs)1−3 are an important class of compounds capable of gelling wide ranges of solvents, resulting in supramolecular gels (SGs) that offer various potential applications in material science.4 However, designing LMWGs is ever-challenging, as the mechanistic insights of gelation are poorly understood. Nevertheless, there have been efforts by various groups to design LMWGs.5−7 We have been engaged in designing LMWGs following supramolecular synthon8 approach in the context of crystal engineering.9 We have identified a number of supramolecular synthons that promoted gelation (Figure S1 of the Supporting Information). In these studies, we have successfully designed and synthesized a large number of LMWGs based on simple organic salts.1,10 Since SGs are known to impart formation of metal nanoparticles (MNPs) which display unique physical and chemical properties11−14 as compared to the bulk metal, we thought it was worthwhile to study the ability to form MNPs by SGs. MNPs with smaller size range (typically within ∼2 nm) are called metal clusters (MCs),15−18 which are of high demand due to their various applications in catalysis,19,20 bioimaging,21 and sensing.22,23 However, controlling the size of MNPs to yield MCs is a challenging task as they are prone to aggregate to form larger clusters or MNPs. Related literature suggest that the use of weak reducing agents at a very low concentration impart slow nucleation and growth of the MCs.24 Since amines25 and acids26 are known as the reducing agent in synthesizing MNPs, the SGs derived from simple organic salts may be suitable for generating MCs by exploiting the gelbed as the source of slow and sustained release of the components of the gelator salts as possible reducing agents. © 2013 American Chemical Society

In this communication, we exploited a 2D supramolecular synthon, namely primary ammonium dicarboxylate (PAD) synthon, to generate a new series of LMWGs derived from (1R,3S)-(+)-camphoric acid (CA) and primary amines having various chain length reacted in 1:2 (acid:amine) molar ratio in methanol (Scheme 1 and the Supporting Information). Most Scheme 1. Frequently Observed 2d Pad Synthon (Left). Various Pad Salts Studied Herein (Right).

interestingly, as envisaged, we demonstrated that one such SG derived from C14 was capable of forming and stabilizing blueemitting gold clusters (BLE-AuCs). Our studies indicate that the amine component of the gelator salt C14 must have been leached out slowly from the gel bed to reduce the AuIII salt producing AuCs. To the best of our knowledge, this is the first Received: October 3, 2013 Revised: November 25, 2013 Published: December 2, 2013 11

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report wherein a SG is exploited to synthesize and stabilize BLE-AuCs without the use of any exogenous reducing agent. Deprotonation of both the COOH groups of CA moiety in the salts synthesized herein was supported by the appearance of strong FT-IR band at around 1624−1630 cm−1 (for COO−) and disappearance of the stretching frequency band 1699 cm−1 for COOH of the parent acid in all the cases. Remarkably, all the salts studied herein were shown to have good to super gelation ability (Figure 1 and Table S1 of the Supporting

wherein the powder X-ray diffraction (PXRD) patterns obtained from xerogel and single-crystal X-ray data (simulated) are compared; a near superimposable match of these patterns understandably establishes the supramolecular synthon present in the SAFINs of xerogel. Interestingly, PXRD patterns for all the gelator salts except C10 displayed nearly superimposable patterns establishing the existence of 2D PAD synthon in the corresponding xerogels; the slight mismatch of the diffraction patterns in the case of C10 could be due to both poor diffraction at higher angle and possible crystal phase transition during solvent evaporation while transforming gel into xerogel. However, the 2D PAD synthon may still be present in the xerogel network of C10, even if it underwent a crystal phase transition (Figure 2); except for C4 and C5, the PXRDs of the

Figure 1. Representative photographs of the 4.0 wt % gel of various gelator salts in different solvents.

Information). The fact that various supramolecular interactions were responsible for gelation was established from the steady increase in Tgel with the increase in gelator concentration and decrease in Tgel with the increase in alkyl chain length (see Figures S2 and S3 of the Supporting Information). Thermoreversibility, viscoelastic nature of some selected gels were established by differential scanning calorimetry (DSC) and dynamic rheology, respectively (Figures S4 and S5 of the Supporting Information). Various morphologies such as highly entangled helical fibers, spheres, and spherulitic growth of short tapes could be seen in the corresponding xerogels (Figure S6 of the Supporting Information). Our best efforts resulted in X-ray quality single crystals of the gelatros C10, C12, C13, and C14. Interestingly, all the salts were found to be isomorphous crystallizing in the noncentric monoclinic space group P21. All the structures suffered from severe disorder in alkyl chain as well as in the carboxylate moiety; in two cases (C10 and C12), the crystals did not diffract beyond ∼45° 2θ. In all the cases, there were more than one molecule in the asymmetric unit (Z′ problem) that added an extra burden on the data to parameter ratio. All these factors contributed to the high R factors observed (Table S2 of the Supporting Information). Crystallographic analyses suggested that the COOH moieties most likely underwent deprotonation as evident from the near identical C−O bond length [1.244(7) − 1.243(7) and 1.260(7) − 1.264 (7) Å] of the carboxylate moieties and appearance of strong bands at 1624 − 1630 cm−1 (for COO−) in FT-IR. In the crystal structures, the carboxylates were involved in hydrogen-bonding interactions with the ammonium cations via N−H···O interactions [N···O = 2.715(7) − 3.322(9) Å; ∠N−H···O = 126.5 − 177.6°]; a closer look revealed that PAM synthon ‘X’ characterized by columnar network of alternating 8- and 12-membered hydrogen-bonded rings was present in each carboxylate end of the salts, resulting in a 2D network resembling 2D PAD synthon, as depicted in Scheme 1. The 2D networks were packed in parallel fashion, allowing interdigitation of the alkyl chains. To establish the supramolecular synthon present in the self assembled fibrillar networks (SAFINs), we followed a structure−property correlation approach originally proposed by Weiss et al.,27

Figure 2. Illustration of single crystal structure of C10; (A and B) side and front views of the two-dimensional (2D) hydrogen-bonded network (HBN) displaying 2D PAD synthon, (C) interdigital packing, and (D) PXRD patterns of the gelator salts under various conditions.

rest of the salts showed interesting similarities with that of C12 − C14, indicating the existence of similar hydrogen bonded network (HBN) in these cases as well (Figure S7 of the Supporting Information). To explore whether these gels can be used for MCs synthesis, we selected DMF gel of C14. When a DMF solution of HAuCl4 was carefully placed over a bed of 5.0 wt % DMF gel of C14, the pale yellow color of gold salt solution turned to wine red at the interface, and the solution gradually turned to wine red within a few minutes and became almost colorless within two and half hours (Figure 3A). The process was monitored by UV−vis spectroscopy, which showed the presence of peaks at ∼274, ∼370, and ∼430 nm with gradual decrease in OD with time. Importantly, these spectra did not show any surface plasmon resonance peak in the range of 530 − 570 nm, indicating the absence of AuNPs28 (Figure 3B). Field emission gun transmission electron microscopy (FEG-TEM) of the wine red solution revealed the existence of AuCs, having average sizes of 1.9 ± 0.5 nm (as determined from the image analysis of 88 individual particles) along with a few larger clusters (3−10 12

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Figure 3. (A) Synthesis of AuCs over a gel bed 5.0 wt % DMF gel of C14. (B) Time-resolved UV−vis spectra of the solution above the gelbed. (C and D) FEG-TEM of wine-red and colorless solution, respectively. (E and F) XPS of wine-red and colorless solution, respectively.

Figure 4. (A) Fluorescence study of the colorless solution containing AuCs. Fluorescence microscopy images of (B) a drop-casted thin film made from the colorless solution and (C) photobleaching experiment of the same thin film.

nm); interestingly, the FEG-TEM of the colorless solution displayed the presence of AuCs (∼2−2.5 nm) (Figure 3, panels C and D and Figure S8 of the Supporting Information). X-ray photoelectron spectroscopy (XPS) analysis of the wine red solution revealed the presence of AuIII and AuI species in the sample with a quantitative ratio of 1:3; XPS of the colorless solution, on the other hand, confirmed the presence of AuI species (Figure 3, panels E and F, and the Supporting Information). MALDI-TOF data of this solution indicated the existence of clusters containing the signature of 3−8 Au atoms (Figure S9 of the Supporting Information). We further explored the fluorescence property of the colorless solution. It may be worth mentioning here that fluorescent gold clusters (FAuCs) are important technologically as they can be used as imaging probe.29 Thus, excitation of the colorless solution at 310 nm produced an emission peak around 403 nm (Figure 4A, left) with a quantum yield of 0.0643 (Supporting Information). The same solution was found to produce gradually red-shifted photoluminescence spectra with the increase in excitation wavelengths (290−370 nm), indicating wide size distribution of AuCs (Figure 4A, right). Interestingly, via observation under the fluorescence microscope, the colorless solution showed blue fluorescence when excited with the UV light (330−385 nm) (Figure 4B). However, excitation with blue and green light (420−480 and 480−550 nm, respectively) of the same solution produced negligibly weak green and red fluorescence (Figure 4B). These data established that the colorless solution contained mostly BLE-AuCs. Since the resistance of AuCs toward photobleaching is an important criterion in bioimaging applications, we exposed a drop-casted thin film of the colorless solution under UV-light (330−385 nm) and observed that the blue fluorescence of the AuCs remained reasonably stable (Figure

4C). Similar photoluminescence behavior of the gel-embedded AuCs was observed (Figure S10 of the Supporting Information). To probe the reducing agent (whether it is acid or the amine component), we performed controlled experiments; the gold salt was mixed with the acid and the amine component of C14 in DMF in separate experiments. While the acid mixture did not show any characteristic UV−vis spectra (Figure S11 of the Supporting Information), the corresponding amine mixture showed almost similar UV−vis spectra with that obtained from AuCs synthesized over the gel-bed, indicating that the amine component might be the reducing agent. Existence of nonfluorescent AuCs within the size range of 2−4 nm could be seen in the FEG-TEM of the amine mixture. Incapability of these AuCs to fluoresce could be because of the larger size range, as compared to that obtained on the gel-bed. Subsequent XPS analysis clearly established the presence of Au0Cs, indicating that the amine was solely present in a relatively high concentration compared to that leached out into the solution above gel-bed reduced AuIII to Au0 within a short time span (Figure S12 of the Supporting Information). In conclusion, we have reported a highly successful crystalengineering-based design of a new series of gelators derived from a chiral acid CA and primary alkyl amines. All the salts (100%) showed excellent gelation behavior with a number of polar and nonpolar solvents. Most importantly, we could exploit the ionic nature of the gelator molecule in synthesizing and stabilizing AuCs. Since the controlled experiments suggested that the amine component was most likely the reducing agent for forming AuCs, we believe that the individual components of the gelator salt leached out into the solution above the gel-bed containing the Au salt, thereby supplying 13

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(21) Jin, R. Nanoscale 2010, 2, 343−362. (22) Sun, J.; Yue, Y.; Wang, P.; He, H.; Jin, Y. J. Mater. Chem. C 2013, 1, 908−913. (23) Liu, H.; Zhang, X.; Wu, X.; Jiang, L.; Burda, C.; Zhu, J.-J. Chem. Commun. 2011, 47, 4237−4239. (24) Palmal, S.; Basiruddin, S. K.; Maity, A. R.; Ray, S. C.; Jana, N. R. Chem.Eur. J. 2013, 19, 943−949. (25) Huo, Z.; Tsung, C.-K.; Huang, W.; Zhang, X.; Yang, P. Nano Lett. 2008, 8, 2041−2044. (26) Vemula, P. K.; Aslam, U.; Mallia, V. A.; John, G. Chem. Mater. 2007, 19, 138−140. (27) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. 1996, 35, 1324−1326. (28) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Acc. Chem. Res. 2012, 45, 1470−1479. (29) Yang, X.; Gan, L.; Han, L.; Li, D.; Wang, J.; Wang, E. Chem. Commun. 2013, 49, 2302−2304.

slow and steady release of the reducing agent (amine) required for AuC formation.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, physico−chemical data, gelation data, additional figures, crystallographic parameters, molecular plot with hydrogen bonding parameters, and data for the controlled experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and parthod123@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.K.A. and D.P.K. thank CSIR, New Delhi, and IACS for their research fellowships, respectively. P.D. thanks CSIR, New Delhi, for financial support. We thank Professor Gautam R. Desiraju, IISc, Bangalore for allowing us to collect a few singlecrystal X-ray data. DST-funded single-crystal diffractometer facility at the Department of Inorganic Chemistry, IACS, is also thanked for X-ray data collection. Professor Sugata Ray and Dr. N. R. Jana, IACS, are thanked for fruitful discussions.

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