Multistimuli-Responsive Interconvertible Low-Molecular Weight

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Multistimuli-Responsive Interconvertible Low-molecular Weight Metallohydrogels and the In-situ Entrapment of CdS Quantum Dots Therein Saibal Bera, Amit Chakraborty, Suvendu Karak, Arjun Halder, Soumyajyoti Chatterjee, Subhadeep Saha, and Rahul Banerjee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01698 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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Chemistry of Materials

Saibal Bera+a,b, Amit Chakraborty+b, Suvendu Karaka,b, Arjun Haldera,b, Soumyajyoti Chatterjeea,c, Subhadeep Saha*,b, Rahul Banerjee*,d aAcademy of Scientific and Innovative Research (AcSIR), CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune-411008, India. bPhysical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune-411008, India. cPolymer Science and Engineering Division, CSIR−National Chemical Laboratory, Pune 411008, India. dDepartment of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mohanpur, 741252, India.

ABSTRACT: Two low molecular weight metallohydrogels (ZALA and CALA) have been synthesized from an amino acid-based ligand precursor (LA) and two different metal salts [zinc acetate dihydrate (ZA) and cadmium acetate dihydrate (CA), respectively]. These two hydrogels show a unique chemically stimulated inter-conversion to each other via a reversible gel-sol-gel pathway. This programmable gel-sol reversible system satisfies logic operations of a basic Boolean logic (INHIBIT) gate. Also, these hydrogels can be degraded into different MOF phases at room temperature spontaneously or in the presence of chloride and bromide salts (NaCl, NaBr.). CdS quantum dots can be grown inside the CALA gel matrix (CdS@CALA) in the presence of small amount of Na2S. This CdS doped gel exhibits time dependent tunable emission (white to yellow to orange) as a consequence of slow agglomeration process of the entrapped quantum dots inside the gel matrix. This luminescence property also reflects in the corresponding gel derived MOFs (obtained either by self-degradation of CdS@CALA or via anion induction) as well. This, to the best of our knowledge, is probably the simplest way to make a CdS quantum dot based composite material where CdS is entrapped within the gel and the gel-derived MOF matrix.

Low-molecular-weight gels are formed by the assembly of small molecular precursors into entwined threedimensional networks with solvent molecules entrapped inside the gel matrix.1-16 Hence, these gels are supposed to collapse to sol in the presence of suitable physical or chemical stimuli that destroy the supramolecular interactions responsible for the assembly process of gelator molecules. However, the sol phase can revert to the original gel phase in the presence of specific stimuli and subsequently reform the supramolecular assembly.17-24Such stimuli-responsive reversible gel-sol systems are highly desirable for applications such as drug delivery, sensors and logic gates. 2531There is a plethora of literature reports on supramolecular gels that exhibit reversible gel-sol transitions in the presence of suitable physical or chemical stimuli (e.g., heating-cooling, shaking-resting, oxidation-reduction, acidbase, etc.).17-31However, the inter-conversion of two different supramolecular metallogels via reversible gel-sol-gel transitions has not been reported so far. Herein, we report a simple, one-pot synthesis of two different metallohydrogels ZALA and CALA; [ZA= Zinc Acetate; CA= Cadmium Acetate and LA = Leucine based molecule [4-methyl-2-((pyridine-4-ylmethyl) ammonio) pentanoate] (Scheme 1)] that show a rare inter-conversion to each other via a gel-sol-gel route) (Figure 1a). The LA gels (CALA and ZALA) belong to the community of

metallohydrogels (ZAVA, ZPVA, ZNVA, ZAIL) which can be obtained via room-temperature reaction between similar amino acid-based ligands (VA, IL; derived from valine and isoleucine, respectively.) and Zn(II) salts viz. zinc acetate dihydrate, zinc nitrate hexahydrate, zinc perchlorate hexahydrate. 32-35,49It is noteworthy that, VA and IL cannot form gels with Cd(II). This contradictory behavior (gelation vs. precipitation) can be attributed to the room temperature solubility of the ligands. It has been observed that solubility of LA in water is inferior compared to that of VA and IL ligand. Hence, the kinetics of the formation of the Cd(II)-LA complex (which has limited solubility in water) in water is slower than that of Cd(II)-VA/IL (if the aqueous solutions of the ligands have similar concentration). Hence, the Cd(II)-LA complex gets enough time to self-assemble to form gel fiber rather than precipitate out instantaneously. The addition of Na2S to ZALA or CALA results in the disruption of the gels into corresponding sulfide salts (ZnS or CdS, respectively) and an aqueous solution of the LA ligand. This solution can be converted to either of the gels (ZALA or CALA) in the presence of the corresponding acetate salts (ZA or CA, respectively) in a cyclic pathway (Figure 1a). This cyclic process can start from either the sol (solution of LA) or gel (ZALA or CALA) and can run for several cycles. Metal acetate (ZA & CA) and Na2S have been found to switch the system (ZALA/CALA gel or ligand

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Scheme 1.Schematic representation of the one-pot synthesis of two metallohydrogels, ZALA and CALA, in-situ entrapment of CdS quantum dot in the gels and conversion of gels into MOFs via both self-degradation as well as the anion triggered route.

solution LA) between the gel and sol phase in accordance with two-input INHIBIT gate behavior36 which is rarely found in low molecular weight metallogel systems (Figure S1c).37Both the gels convert to MOFs (ZALA-self and CALA self, within three and five days, respectively) upon stand-

ing at room temperature (Figure S7). Although there are precedents of conversion of supramolecular gels to molecular crystals,38-48spontaneous room-temperature conversion of a low molecular weight metallohydrogel into a three-dimensional MOF is rare.45-48 It is interesting to note


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Chemistry of Materials

Figure 1. a) Inter-conversion of two supramolecular metallohydrogels (ZALA to CALA and vice versa) by the means of chemical stimuli; b) rheology profile of the CALA and ZALA (both pristine and CdS doped); c) multi-responsiveness of ZALA and d) FESEM images, recorded at the different time of interval during the ZALA to ZALA-self conversion.

that these metallohydrogels (ZALA and CALA) can be rapidly converted to other MOFs (ZALA-Cl/Br and CALACl/Br) in the presence of chloride and bromide salts (e.g. NaCl and NaBr) (Figure 2a & Figure S8). Both ZALA and CALA have been found to act as a medium for the synthesis of CdS quantum dots without the aid of any capping agents (Figure 2b). These CdS entrapped gel phases, similar to their pristine analogs, can be converted to CdS doped MOFs spontaneously. All of these materials can be synthesized in bulk scale (3-5 gm) (Figure S22) via one-pot, room temperature methods that make them useful for further applications. ZALA and CALA gel have been synthesized via a simple, room-temperature, one-pot mixing, of aqueous solutions of metal precursor (ZA or CA) and ligand (LA). These gels have been seen to disrupt to sol in the presence of various physical or chemical stimulus (mechanical force, heat, acid/base etc.) and reform upon removal of the stimulus

(Figure 1c). ZALA gel can be disrupted to a white precipitate (ZnS) and a clear solution upon addition of Na 2S (equivalent to the amount of Zn2+ in the gel) solution. The clear solution, after removal of the white precipitate readily reform into the ZALA gel upon the addition of aqueous solution of zinc acetate dihydrate (ZA). This phenomenon indirectly proves that the clear sol is nothing but the solution of ligand LA. The reformed ZALA gel shows all characteristic features of the pristine ZALA gel. However, interestingly if an aqueous solution of cadmium acetate dihydrate (CA) is added instead of ZA another gel forms which has been proved to be similar to the original CALA gel (Figure 1a). These reformed gels showcase characteristics similar to the pristine CALA gels and convert to the identical MOF crystals (spontaneously or in the presence of halide salts). CALA gel also can be converted to the ZALA gel and vice versa (Figure 1a). The mixture of metal salts (ZA or CA) and ligand (LA) doesnot form any gel in most protic



Figure 2. a) Digital photographs for the conversion of the ZALA gel to ZALA-Cl MOF; b) change in photoluminescence color of CdS@CALA gel with time (increment in the size of the entrapped CdS quantum dots causes the red shift in photoluminescence ); c) luminescent CdS loaded CALA-Br MOFs under UV and visible light; d) solid state PL spectra of luminescent CdS@CALA-Br MOF (increment in the size of the entrapped CdS quantum dots causes the red shift in photoluminescence ); e) HRTEM images of CdS@CALA-Br and f) PXRD patterns of the pristine, as well as CdS doped CALA MOFs.

and aprotic solvents other than water (Table S1). The ligand LA forms a precipitate upon reaction with most other metal salts [e.g., salts of Ca2+, Mg2+, Al3+, Fe2+/3+, Co2+/3+, Ni2+etc.]. ZALA and CALA showcase similar physicochemical properties as the metal precursors (Zn and Cd) belong to the same group in the periodic table. This reversible ZALA gel-sol-CALA gel transformation (or the reverse) (Figure 1a) can run for several cycles without any perceivable changes in the system. The inter-conversion phenomenon of these gels (ZALA and CALA), driven by two stimuli (ZA/CA and Na2S), can be utilized to mimic logic operations. It has been seen that this system can be employed as a two-input (ZA/CA and Na2S) INHIBIT gate if either of the gels or LA solution is considered as the initial state (Figure S1c). An INHIBIT gate is basically an AND gate with one input reversed by a NOT function.36When none of the inputs (I1 and I2) is on, the gate remains off. However, input I2 turns the gate on (when the system is gel), but I1 does not turn the gate on. On the contrary, input I1 turns the gate on (when the system is the ligand solution). Also, the combined effect of both the inputs turns the gate off. To the best of our knowledge, the present system is a rare metallogel system to show such kind of logic gate operations.37

The CALA gel can be converted to CdS@CALA gel via a first-of-its-kind, room-temperature, and one-pot recipe in the presence of a catalytic amount of Na2S. To the best of our knowledge, this is one of the most straightforward methods to make a quantum dot composite material. In practice, fabrication of quantum dot-based materials is a two-step process; i) the synthesis of quantum dots in the presence of capping agents that prevent their agglomeration to bigger particles and ii) immobilization of the quantum dots in the desired host matrix (e.g. silica, polymer, MOF etc.).49-62If Na2S (0.25 ml of 0.0125M) is added to the CALA sol (obtained upon mixing of 0.5ml of 0.2 mmol CA and 0.5 ml of 0.4 mmol LA or via a heat-induced gel-to-sol transition), fluorescent CdS@CALA gel forms after few minutes. The free uncoordinated pyridinic nitrogen of the LA ligand of the CALA gelator is assumed to attach to the Cd2+ centers of the CdS quantum dots that, in turn, suppress their agglomeration into bigger particles and stabilize them in the gel matrix. According to recent reports, pyridinic nitrogen atoms of the adenine moieties of nucleotides (e.g., ATP) can act as capping agents for quantum dot nanocrystals.54CdS@CALA exhibits tunable fluorescence (Figure 2b) with time; i.e., a gradual red-shift in fluorescence (Figure 2d) which can be attributed to the continuous growth of the CdS quantum dots with time.


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Chemistry of Materials ZALA and CALA gel and their CdS doped analogues were characterized by an average storage modulus (G’) greater than the loss modulus (G”) within the linear viscoelastic region (pulsation = 10 rad s-1; strain=0.1%) as measured by dynamic strain sweep studies (range=0.1–100%; Figure 1b). The results from this dynamic strain sweep studies clearly indicate that the gels retain their viscoelastic nature even after the entrapment of CdS quantum dots (~1-2%, obtained from HRTEM EDAX) inside the gel matrix. Comparative FTIR analysis of the LA ligand and the xerogels proves the possibility of participation of the carboxylate group as well as the tertiary amine group in the coordination of Zn and Cd metal centers in the gels (Figure S3). PXRD patterns of the xerogels (pristine and CdS doped) exhibit their amorphous nature (Figure S4). Crystalline CdS quantum dots (~1-2% of the mass of the composite gel) do not influence this amorphous nature of the CdS doped xerogels. Furthermore, both the gels (CALA and ZALA) show amorphous nature in their pristine form and carry no signature of the MOFs. However, the addition of chloride or bromide salt to the gels initiate conversion of the amorphous gel phase to crystalline MOFs phase (Figure S9, S11). In the absence of NaX, the MOFs formation occurs at a much slower rate (Figure 1d, S10). Thermogravimetric analysis (TGA, under the N2 atmosphere, Figure S2) of the xerogel materials (both pristine and CdS incubated) exhibit an initial steady weight loss of ~20% up to 270 °C, indicating loss of water followed by degradation of the unreacted ligand [m.p. (LA) = 154 °C]. This initial weight loss follows another weight loss at 270-280 °C till ~500 °C (~40% and ~60% weight loss in the cases of ZALA and CALA xerogels, respectively) depicting thermal degradation of the gel network. LMWGs often show a tendency to the transform themselves thermodynamically more stable 3D extended crystalline state due to the higher number of stabilizing interactions present in the latter than the 1D nano-fibrillar structures (LMWGs). Many LMWGs have been seen to disrupt into molecular crystals spontaneously or in the presence of a suitable stimulus.41,42However, the transformation of LMWGs to co-ordination polymeric structures are rare.44,49Moreover, spontaneous conversion of metallohydrogel into a three-dimensional MOF is even unheard of. ZALA and CALA gels convert themselves to MOF crystals (ZALA-self and CALA-self) within three and five days at room temperature. ZALA gel converts to the 1D coordination polymers ZALA-self and CALA gel converts to 3D CALA-self with unj topology. ZALA-self and CALA-self are two rare MOFs that could be obtained as single crystals from a metallogel at room temperature without the aid of any external stimulus. However, these metallogels produce different MOF phases if they are exposed to chloride or bromide salts (e.g., NaCl, NaBr) at room temperature. CALA-Cl, CALA-self, CALA-Br crystallize in P61 space group (hexagonal), whereas ZALA-Br, ZALA-Cl crystallize in P212121 space group (orthorhombic). ZALA-self crystallizes in P21 space group (monoclinic system) (Section S9). This crystallization process happens in a gel matrix that provides a static environment for single crystal growth. It is intriguing to note that, CdS@ZALA and CdS@CALA gels also convert themselves into corresponding CdS loaded MOFs. To the best of our knowledge, this spontaneous con-

version of the gels to MOFs is the easiest entrapment method of quantum dot nanoparticle into MOF matrices. PXRD patterns (Figure 2f & S20) of the MOFs (ZALA-self, CALA-self, ZALA-Cl, CALA-Cl, ZALA-Br, CALA-Br) show their high crystallinity which remains unaffected upon CdS doping (~1 wt %). Contrary to the xerogels, the MOFs display single step thermal degradation in TGA traces. The MOFs remain stable up to 200-250oC and a further increment of temperature causes framework collapse (Figure S12). Pristine ZALA and CALA gel show blue luminescence, but the emission frequency can be tuned upon encapsulation of the CdS quantum dots in the gel matrices (Figure 2b & Figure S19a). After 2hrs of Na2S addition (during the preparation of gel CALA) the emission color changes from blue to white and eventually turns into yellow. Finally, after 6hrs of addition, the emission color changes into orange and remains stable for several days (Figure 2b). These CdS loaded gels, similar to their pristine analogs, can be degraded (spontaneously or anion induced) to luminescent (white or yellow or orange) CdS loaded MOFs (Figure 2c & Figure S19b). The photoluminescence color of the CdS doped MOFs does not change with time, due to entrapment of the CdS quantum dots between crystallite surfaces.49,50 They also do not adhere on the outer surface of the MOFs crystal as their photoluminescence activity remain intact after repeated washing with water. This shift in the photoluminescence color (blue to white, yellow, orange) of the pristine MOFs (ZALA-Cl, CALA-Cl) upon entrapment of CdS quantum dots is the consequence of the possible energy transfer between the donor (MOFs) and acceptor chromophores (CdS quantum dots) of the composite (CdS doped MOFs).49 To visualize the distribution and the nature of CdS quantum dots in the MOF matrix, we utilized HRTEM techniques which reveal a distribution of sub-10 nm sized particles (3-4 nm) embedded in the MOF crystal matrix, (Figure 2e & Figure S6) having lattice separation of 0.24nm [d-spacing between two (102) planes of the hexagonal CdS crystals; JCPDS Card No.-41-1049]. In summary, we have made two unique supramolecular metallohydrogels (viz. ZALA and CALA) which are interconvertible via a chemically stimulated reversible sol-gel process. These two low-molecular-weight gels can transform themselves into MOFs spontaneously at room temperature. Moreover, these gels have been found to be responsive towards chloride and bromide salts (e.g. NaCl, NaBr) which convert them to chloride/bromide (coordinated to metal center) containing MOFs. These metallogels can be utilized as a medium for synthesis of CdS quantum dots without the aid of any capping agent. These fluorescent CdS doped metallogels also produces CdS doped MOFs similar to their pristine analogues. This unique synthetic procedure has potential to replace multistep, high temperature, inert atmosphere synthesis of an important class of photosensitizer material, metal chalcogenide quantum dots. Hence, this unique procedure pioneer in the fabrication of quantum dots [viz.CdSe, CdTe, PbS, PbSe] @MOF composites. These photosensitizer-based MOF composites would be promising for various applications, e.g., photocatalysis, light emitting diodes, dye-sensitized solar cells. Further developments on this green synthetic procedure as well as the application of these quantum dots@MOF composites are presently underway in our laboratory.



Supporting Information Synthetic procedures, PXRD, FT-IR, TGA, SEM, TEM, PL spectra single crystal structures, crystallographic data (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author [email protected] [email protected] Author Contribution +S.B

and A.C contributed equally.

Notes The authors declare no competing financial interests.

S.B and A.C acknowledge CSIR, India, and SERB-NPDF for a research fellowship. R.B. acknowledges DST Indo-Singapore Project (INT/SIN/P-05) and DST Nanomission Project (SR/NM / NS-1179/2012 G) for funding.

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Multistimuli-Responsive Interconvertible Low-Molecular Weight

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