Thermochromic, Solvatochromic, and Piezochromic Cd(II) and Zn(II

Aug 27, 2018 - Synopsis. The Cd(II) and Zn(II) two-dimensional porous and multichromic coordination polymers have been synthesized which consist of an...
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Thermochromic, Solvatochromic and Piezochromic Cd(II) and Zn(II) Coordination Polymers: Detection of Small Molecules by Lumines-cence Switching from Blue to Green Avishek Dey, Abhijit Garai, Venkatesh Gude, and Kumar Biradha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00924 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Crystal Growth & Design

Thermochromic, Solvatochromic and Piezochromic Cd(II) and Zn(II) Coordination Polymers: Detection of Small Molecules by Luminescence Switching from Blue to Green Avishek Dey, Abhijit Garai, Venkatesh Gude and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. KEYWORDS. Multichromism, Solvatochromism, Thermochromism, Piezochromism, Luminescence switching ABSTRACT: Two novel Cd(II) and Zn(II) based multichromic coordination polymers (CPs) containing 2,4,5-tri(4-pyridyl)imidazole, L, with general formulas {[Cd2L4(SO4)(H2O)]·(SO4)·21H2O}n, 1 and {[Zn2L4(SO4)(H2O)2]·2(MeOSO3)·25H2O}n, 2 have been synthesized which consist of two-dimensional layered structures with continuous channels. These CPs are found to exhibit instant and reversible solvatochromic properties via visual color change in the presence of various solvent molecules such as DMF, DMA, CH3CN, CH3COCH3, DMSO and Et3N as well as quick response for the absorption/detection of small molecules such as formaldehyde, acetaldehyde compared to higher aldehydes. The thermochromic and solvatochromic properties are associated with a color change from blue to green under UV light with significant changes in their emission properties and lifetimes. Further, piezochromic properties are associated with color change from blue to cyan under UV. These materials also found to exhibit greater preference for adsorption of water vapor (200-300 cc/g) over N2 and CO2 gases and also anionic dye sorption such as congo red from aqueous solution.

The design of novel materials with chromic properties had gained importance given their various applications such as sensors, non-linear optics, photo optical switches, displays and optical memories.1-7 These ‘smart materials’ are developed by the ability of a material to switch between two distinguishable states by an external stimulus. Based on the nature of stimuli these materials are categorized as photochromic, thermochromic, solvatochromic, and piezochromic/mechanochromic.8 The photochromism is known with several materials containing simple organic molecules that can change their conformations or the molecular structures upon photochemical stimuli.9-10 Whereas solvatochromism that is triggered by the removal/absorption of solvents is mostly exploited with many metal organic frameworks (MOFs) or porous coordination polymers (PCPs).11 The solvatochromism receiving an immense attention for the detection of explosive materials, various amines and volatile organic compounds. This type of detection method is advantageous as it is easy to use without going through traditional and tedious methods such as GC and GC-MS and also it is useful in the fields of defence, industry, safety and security.12 This type of chromism is generally observed due to the reversible absorption/desorption dynamics of solvent molecules by the CPs which causes either change in the interactions between the network components or change in the coordination environment of transition metals. This phenomenon is usually observed in CPs containing Cu(II), Co(II) and Ni(II) transition metals due to their distinct colors.13-19 However, the CPs of cadmium or zinc are rarely explored in this direction given their colorless to light yellow colors. The piezochromic (mechanochromic) materials are different class of compounds and have wide range of applications as mechanical sensors, organic light emitting diodes (OLEDs)

and optical memories.20-23 This type of chromism is generally exhibited by the materials such as small organic molecules, liquid crystals and organic polymers24-27 and few reports exist with the materials containing metal complexes. Copper iodide cluster, Au-complexes and cationic iridium complexes were shown to exhibit reversible mechanochromic luminescence behaviours.28-32 To the best of our knowledge among the CPs, only few examples are reported so far to exhibit piezochromism and they contain Cd(II), Zn(II), Cu(II) and Bi(III).33-36 Although there are many examples in the literature that can exhibit one type or two types of chromism but no example of CP based material that can exhibit more than two types of chromism with the ability for visual detection and separation of small organic molecules is reported to date. Herein, we would like to report our studies on Cd(II) and Zn(II) based CPs that exhibit solvatochromism, thermochromism and mechanochromism. In addition, these materials are also found to be useful for the visual detection of DMF, DMA, CH3CN, acetone, DMSO and Et3N as well as for the absorption/detection of small molecules such as formaldehyde, acetaldehyde from benzaldehyde or other higher aliphatic aldehydes. In this study, 2,4,5-tri(4-pyridyl)-imidazole, L, was chosen as a complexation ligand for the synthesis of CPs as it contains extended π system of hetero-aromatic rings in anticipation of good luminescence properties of its CPs and also as many as five nitrogens of L are expected to create hydrophilic cavities/channels in the CPs for the inclusion of water or solvent molecules. Recently, couple of reports appeared on the CPs of L and one of those reports shows that CPs of L exhibit piezochromic properties.36-39

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a)

Scheme 1. Chemical drawing of ligand L.

(b)

N

N N N H N

Results and Discussion The complexation reaction of L with CdSO4 and ZnSO4 by layering the EtOH solution (2 mL) of L (10 mg, 0.0333 mmol) over the aqueous solution (2 mL) of corresponding metal salts (0.0333 mmol) resulted in the single crystals of CPs 1 and 2. The single crystal X-ray diffraction analyses reveal that the complexes 1 and 2 have the formulae of {[Cd2L4(SO4)(H2O)]·(SO4)·21H2O}n and {[Zn2L4(SO4)(H2O)2]·2(MeOSO3)·25H2O}n, respectively.40 CP-1 is found to exhibit two types of layers, namely (6,3) brick wall and (4,4) rectangular grid network, in the same crystal lattice, while CP-2 is found to exhibit only (6,3) brick wall network. The pertinent crystallographic information is given in the table 1 (SI). We note here that the CP-1 and -2 can also be synthesized through the solvothermal and waterassisted mechanochemical grinding of the two components (Figure S14). CP-1 crystallizes in a C2221 space group and the asymmetric unit contains three Cd(II) atoms: one with full occupancy (Cd1) and other two with half occupancy (Cd2 and Cd3), four ligands, two coordinated sulphate anions each with half occupancy, one coordinated water, one uncoordinated sulphate anion with full occupancy and lots of uncoordinated solvent molecules. The three crystallographic independent Cd(II) centers exhibited two different coordination environments although all three show distorted octahedral coordination. The equatorial positions of Cd1 are occupied by four pyridine groups of L (dCd-N = 2.310, 2.319, 2.374 and 2.397 Å) while the apical positions are occupied by one water molecule and sulphate ion (dCd-O = 2.320 and 2.227 Å) (Figure 1a). Whereas in case of Cd2 and Cd3, the equatorial positions are as usual occupied by four pyridine groups (dCd-N=2.303 and 2.313 Å) but apical positions are occupied by sulphate ion that bridges these two Cd atoms (dCd-O=2.327 Å) (figure 1b). The ligand L acts as an angular exo-bidentate ligand as one of the pyridine moieties does not participate in coordination and forms 1Dchains which are linked by sulphate ions. The 1D-chains that are formed by Cd1 & L are linked by sulphate ions into 2Dbrick wall type (6,3) network (Figure 1c), while the chains that are formed by Cd2 & L and Cd3 & L are linked by sulphate ions into rectangular grid type (4,4) network (Figure 1d). The brick wall network and grid network exhibit cavities of dimension 20.8×6.8 Å2 and 10.4×7.3 Å2 respectively. The two topologically different concomitant 2D-layers are alternatively packed via π···π stacking interactions between ligands (Figure 1e). Such packing of the layers generate two types of channels with a difference in size (Figure 1f). These channels are occupied by solvents and free sulphate anions. Platon calculations reveal that CP-1 contains 32.5% solvent accessible voids in the crystal lattice. We note that the CP-1 reported here is different from the previously reported structures by Lee et al.37

(c)

(d)

(e)

(f)

Figure 1. Illustrations for the crystal structure of 1: (a-b) distorted octahedral coordination environment around Cd(II) ion; (c) (6,3) brick wall network; (d) (4,4) rectangular grid network; (e) alternative packing of (4,4) and (6,3) layers via π···π stacking; (f) two types of channels along the packing of layers.

CP-2 crystallizes in a C2/c space group and the asymmetric unit contains one Zn(II) with full occupancy, two ligands, one coordinated sulphate anion with half occupancy, one uncoordinated methyl sulphate anion with full occupancy, one coordinated water and some uncoordinated disorder solvent molecules. The Zn center exhibits distorted octahedral coordination environment. Like CP-1, here also four equatorial positions are occupied by pyridine N-atoms and two apical positions are occupied by water and sulphate anion. The ligand L acts as an angular exo-bidentate ligand as one of the pyridine moieties does not participate in coordination and forms 1Dchains which are linked by sulphate ions. The 1D-chains that

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(a)

Solvatochromic Properties

The emission studies have been carried out to understand the further changes that are accompanied with the color change due to the solvent exchange. These studies indicate that the solvent exchange is also accompanied with the change in emission properties. The complex 1 (pale-yellow) exhibits an emission maximum at 412 nm upon excitation at 350 nm. The solvent exchange materials (dark-yellow) are found to exhibit dual emission maximum at 404 and 496 nm (1@acetone), 402 and 502 nm (1@acetonitrile), 412 and 501 nm (1@DMF), 413 and 495 nm (1@DMA), 412 and 482 nm (1@DMSO), 415 and 486 nm (1@Et3N) upon excitation at 350 nm (Figure 2b). Similar results for solvent exchange with CP-2 are observed with minor differences. Interestingly, the lifetime measurements obtained from the best fitting to a single exponential kinetics were found to vary with respect to included solvent. The parent complexes 1 and 2 are found to exhibit lifetimes of 4.95 and 5.29 µs respectively. The solvent exchange was found to significantly enhance the lifetimes as follows: 9.35 µs (1@acetone), 9.99 µs (1@acetonitrile), 9.96 µs (1@DMF), 10.91 µs (1@DMA), 10.23 µs (1@DMSO), 10.21 µs (1@Et3N) (Figure 2c). This suggests that emissive states were different in nature; therefore their deactivation pathways are also different for different solvent included materials. This emission lifetime is relatively long and certainly high compared to other reported MOFs.33 These observations clearly imply that the presence of above said solvent molecules in the frameworks of 1 or 2, in place of water, generate new type of host-guest materials due to change in the noncovalent interactions between host and guest molecules.

1 1@acetone 1@ CH3CN

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Solvatochromism is the ability of a material to change its color upon exposure to different solvents. The solvent exchange experiments with 1 and 2 clearly indicate that the encapsulated solvent molecules can easily be exchanged by small volatile organic solvent at ambient conditions. This exchange process was found to be accompanied with visual color change in case of some solvents such as DMF, DMA, DMSO, CH3CN, acetone and Et3N, but not in case of exchange with water, alcohols (methanol, ethanol etc.), DCM, CHCl3, THF etc (Figure 2a). The immersion of parent crystals in the above listed solvents resulted in the visual color change from very light yellow to dark yellow within a minute (Figure 2a). Further, it was found that the color change is reversible that is when the solvent exchanged crystals (dark yellow) are dipped in water they regained their original color (pale yellow). In order to understand the effect of size of the solvent molecules the exchange reactions are carried out with toluene, xylene, benzonitrile, benzene, aniline etc. These studies suggest that a bigger aromatic guest solvent does not have any tendency to change the color. The 1H NMR analysis of the CDCl3 extract of the dark yellow materials confirms that the change of the color is associated with the inclusion of the relevant solvent molecules in the crystal lattice. The comparison of PXRD patterns before and after exchange confirms that the process occurs via the retention of the networks throughout the exchange process (Figure 6c).

(b)

CP-1

are formed by Cd1 & L are linked by sulphate ions into 2Dbrick wall type (6,3) network with grid dimension of 20.8×6.8 Å2 (Figure S21). The packing of the layers generate two types of channels with a difference in size like CP-1. These channels are occupied by solvents and free sulphate anions. Platon calculation reveals that it contains 34% solvent accessible voids in the crystal lattice.

Lifetime (µs)

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Crystal Growth & Design

Normalized Intensity (a.u.)

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A

Figure 2. Illustrations for the solvatochromic properties of CP-1: (a) photograph of vials to depict the changing/unchanging of color of crystals upon dipping in solvents; (b) emission spectra of the CP-1 with the inclusion of different solvents (λex= 350 nm), notice the differences; (c) histogram depicting the lifetime of CP-1 containing different solvents.

Thermochromic Properties Thermochromism is the ability of a material to change its color in reversible manner in response to heating and cooling cycles. This color change is accompanied by the dehydration and hydration cycle of the material with respect to temperature. Interestingly, the pale-yellow crystals of 1 are heated in an oil bath at 100-120 ºC by placing them in RB, the crystals are found to change the color gradually from pale-yellow to dark yellow (1TC) with the breaking of single crystals to polycrystalline material (Figure 3a). Further, such heating of 1 and 2 is also found to result in the weight loss of 20.2 and 19% respectively. Further, the immersion of dark yellow apo-host materials (1TC and 2TC) found to regain the original color of the parent CPs indicating reversible nature of hydration. The luminescence measurements also indicate that the heated samples have different properties from the parent samples. The emission maximum of 1 with excitation wavelength of 350 nm was found to be 412 nm which is shifted to 514 nm upon heating (for 1TC). The large difference in emission wavelengths (102 nm) indicates that removal of the solvent from 1 created a new phase 1TC (Figure 6a), which may contain better π···π interactions between the 2D-layers. Similar observation was also found in case of CP-2. The emission

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Crystal Growth & Design maximum of 2 and 2TC (Zn-complex) are found to be 415 and 527 nm respectively (Figure S5). These observations suggest that reversible crystal-polycrystalline phase transformation occurs upon dehydration and rehydration accompanied by under uv or naked eye color change. To get further insight into the thermochromic properties, their fluorescence spectra at different times are recorded and corresponding emission peaks are red shifted from 412 nm to 514 nm (Figure 3b). The excited-state lifetimes (τ) are found to be different and after heating increases from 4.95 µs for 1 to 8.22 µs for 1TC and 5.29 µs for 2 to 8.85 µs for 2TC, which may be ascribed to the differences in the of excited states of 1 and 1TC or 2 and 2TC due to the differences in the intermolecular interactions and π···π stacks. The band gap values for these materials have been evaluated using diffuse reflectance spectroscopy (DRS) which indicates that 1TC and 2TC have lower values (2.25 and 2.38 eV) than their parent materials 1 and 2 (3.10 and 3.07 eV) (Figure S12). CP-1

Heating 15 min 100-120 °C

(a)

(b)

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Dipped in water 7 days CP-1 1 min (1TC) 2 min (1TC) 3 min (1TC) 4 min (1TC) 5 min (1TC) 15 min (1TC)

1.0 0.8 0.6 0.4 0.2 0.0 400

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states compared to the parent CPs. Further 1PC and 2PC found to have lower band gap of values (2.72 and 2.80 eV) than the parent complexes 1 and 2 (3.10 and 3.07 eV) (Figure S12). Visual Detection of Aldehydes The capability of 1 and 2 for visual detection of aldehydes from the mixtures has been examined as these materials have shown propensity to do reversible absorption of water and certain solvent molecules with or without change of color. The immersion/suspension of complexes 1 or 2 in the formaldehyde resulted in a quick color change from pale yellow to bright yellow within less than a minute (Figure 4 and S10). Similar experiments with higher aldehydes such as acetaldehyde, propanaldehyde, butanaldehyde and benzaldehyde resulted in a light color change (20 mins, 5 hrs, 24 hrs and 24 hrs respectively) but found to require a more time. These results suggest that the increase in size of aldehydes resulting in the increase in absorption times could be given the better penetration of small molecules into the channels compared to the bigger ones. The repeat of experiment with the mixture of all aldehydes was found to change the color of the suspended solution changes quickly indicating the selective absorption of formaldehyde over other aldehydes. The formaldehyde and acetaldehyde absorbed complex of 1 was found to exhibit red shift in emission maxima about 100 and 80 nm respectively (Figure 5c). A significant increase in the life times of 1@HCHO and 1@CH3CHO are observed, that is from 4.95 µs (for 1) to 10.52 and 8.64 µs respectively. These observations suggest that the CPs have a quick response to absorb formaldehyde or acetaldehyde compared to other higher aldehydes. These results imply that 1 and 2 can act as visual sensor for the detection of formaldehyde and acetaldehyde from the mixture of aldehyde solutions. (a)

Figure 3. Illustrations for depicting the thermochromic properties of CP-1: (a) snap shot of the materials 1 and 1TC; (b) change in fluorescence spectra w.r.t. time (λex= 350 nm).

Piezochromic Properties Piezochromism is a phenomenon in which a material undergoes color change due to mechanical grinding or external pressure. Generally, the induced color change reverts to the original color when the material is kept in dark or dissolved in a solvent. It occurs due to the change in molecular arrangement and crystalline phase transition in response to the external stimuli such as hydraulic pressure or gentle grinding. The grinding or applying hydraulic pressure on complexes 1 or 2 are found to change the color of the complexes from pale yellow to fluorescent type greenish yellow (1PC and 2PC) (Figure S8). The change in color is found to be reversible that is dipping of 1PC or 2PC in the water changes from greenish yellow to pale yellow. The complexes are also shown similar color change during making a palette. After mechanical grinding, the luminescence maxima of the complexes 1PC and 2PC have been red shifted compared to that of parent CPs (1 and 2). The emission maxima for CPs 1 and 2 are 412 nm and 415 nm respectively which are red shifted to 490 nm (1PC) and 484 nm (2PC) respectively at excitation wavelength of 350 nm (Figure 5c and S6). The lifetimes of 1PC and 2PC are found to be 6.26 and 6.98 µs respectively. The increase in lifetimes suggests that 1PC and 2PC have different excited

(b)

Figure 4. Illustrations for the visual detection of formaldehyde and acetaldehyde by CP-1: photographs of the vials with suspended solid of 1 in aldehyde solutions (a) under visible light; (b) under UV light.

Luminescence Switching from Blue to Green and Chromaticity Diagram The visual color change from pale yellow to dark/bright/greenish yellow depending on the external stimuli prompted us to investigate their light emitting behavior under UV light. The pale-yellow materials (1 and 2) are found to

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exhibit blue emission while the dark yellow materials 1TC/1SC and 2TC/2SC are found to exhibit green emission under UV light (Figure 5a and S7). Interestingly, the materials that are produced by grinding (1PC and 2PC) exhibited cyan emission under UV light (Figure 5b, S8). These differences in change in emissions under UV light from blue to green and blue to cyan were further supported by CIE chromaticity diagram (Figure 5d and S9).

0.4

CO2 at 77 K and 273 K respectively (Figure S18 and S19). However they are found to exhibit good water vapor sorption with step wise adsorption-desorption profile. The profile shows gradual uptake (150 cc/g) up to P/P0 = 0.6 and then rapid uptake (280 cc/g) up to P/P0 = 0.8. From P/P0 = 0.8 to 0.95, the uptake increases gradually to 300 cc/g, for CP-1, which corresponds to 13 mmol/g of water adsorption (Figure 7a). The complex 2 is also found to follow similar water sorption profile with lower uptake (200 cc/g) value than 1 (Figure 7b). The stronger affinity, stepwise adsorption and high uptake of water could be due to hydrophilic nature of the pores, effective pore size, and adsorption sites. The large gap between adsorption and desorption profile could be due to strong interaction between the framework and the absorbed water molecules inside the crystal lattice.

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1 1TC 1PC 1@CH3CHO 1@HCHO

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PXRD Analysis for Different Types of Materials The PXRD analysis for transformed materials of 1 and 2 indicate some minor differences from parent materials as well as from each other. The powder patterns of 1 and 2 are found to be almost identical with differences in intensities. The materials that are produced through solvatochromism also found to be identical with their parent complexes in terms of peak positions and intensities except in case of DMSO where some slight shifting of peak position has been observed (Figure 6c). Similarly the materials that are produced through piezochromism are found to also identical with parent complex albeit with reduced intensities (Figure 6b). However, in the PXRD of the materials that are produced through thermochromism some new peaks appeared and some old peaks disappeared indicating some phase change (Figure 6a). Water Vapor Sorption Studies As these materials exhibited reversible solvent exchange or reabsorption properties, their gas sorption abilities were examined with N2, CO2 and water vapors. The materials are activated by immersing them in acetone for few days and then evacuated the solvents by in situ heating at 120 °C for 2 hrs. They are found that they have less propensity to adsorb N2 and

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Figure 6. PXRD patterns of CPs to illustrate the changes by (a) thermochromism; (b) piezochromism; (c) solvatochromism. (a)

(b) 300 240 180 120 60 Adsorption Desorption

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Figure 5. Illustrations for the multichromic properties of CP-1: (a) switching of blue to green emission with thermal/ solvent inclusion stimuli; (b) switching of blue to cyan emission by mechanical stimuli; (c) fluorescence spectra for CP-1 before and after transformation through corresponding stimuli (λex= 350 nm); (d) chromaticity diagram for depicting the color change of materials.

Volume adsorbed (cc/g)

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Crystal Growth & Design

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Figure 7. Water vapor adsorption-desorption isotherm at 298 K for (a) CP-1 and (b) CP-2.

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Crystal Growth & Design

Dye Adsorption Studies The ground water is contaminated by dyes which are released by many industries, such as plastics, textile, rubber, paint, pharmaceuticals etc.41,42 Therefore, the effective removal of toxic dyes from polluted water is a major concern in the recent days. MOFs or PCPs are regarded as promising candidates for selective adsorption of dyes given their nature of frameworks, channels, and active metal sites.43-45 The cationic nature of the framework prompted us to study the dye adsorption property of these complexes. Dye sorption experiment was carried out by the immersion of freshly prepared crystals of 1 and 2 (50 mg) in aqueous solutions of CR dye in 10-4 molar concentration. The adsorption of CR dye by these complexes was monitored by UV-vis spectroscopy at different time intervals (Figure 8a). It was found that CR could be efficiently adsorbed by 1 and 2 in a short period of time with a color change of crystals from pale yellow to red. In the first 5 min, the complexes 1 and 2 could adsorb 65% and 59% of the dye, respectively. The adsorption 98 and 92% of dye was observed for complexes 1 and 2 in 20 min and 40 min respectively. This observed sorption of CR by CPs 1 and 2 by the exchange of sulphate anion which is uncoordinated. Further, the dye adsorbed 1 was found to have a lower band gap ((1.85 eV) value compared to that of 1 (3.10 eV) (Figure 8b). The percentage (%) of removal of dye was calculated using the following equation:

Ci − Ce × 100 Ci

Dye removal (%) =

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1 αΕ p)2 / eV2 nm2

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(

αΕ p)2 / eV2 nm2

Band gap= 3.10 eV

2

*E-mail: [email protected]. Fax: +91-3222282252. Tel.: +91-3222-283346.

We acknowledge SERB, New Delhi, India, for financial support, DST-FIST for the single crystal X-ray diffractometer. AD acknowledges UGC and IIT KGP and AG, VG thank IIT KGP for research fellowship. We thank Venkata N. K. B. Adusumalli, IISER Kolkata for the lifetime measurement.

16

1

Corresponding Author

ACKNOWLEDGMENT

ABBREVIATIONS

650

600

TC, Thermochromic; PC, Piezochromic; SC, Solvatochromic; CR, Congo red.

Wavelength (nm) 20

Electronic Supplementary Information (ESI) available: details of experimental procedure, PXRD, TGA, 1H NMR, FT-IR, crystallographic table, luminescence graphs, lifetime data, color change photographs, gas sorption data, dye adsorption data, excitation spectra etc.

The authors declare no competing financial interest.

1.0

0.0 400

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ASSOCIATED CONTENT

Notes

CR dye (1) 5 min 10 min 20 min

1.5

ZnSO4. The crystal structure analyses reveal that CP-1 is found to exhibit two types of layers, namely (6,3) brick wall and (4,4) rectangular grid network, which pack alternatively in the crystal lattice such that there exists continuous channels, while CP-2 is found to exhibit only one type of layer, namely (6,3) brick wall that packs in the crystal lattice such that there exists similar type of continuous channels. These CPs are also found to exhibit multichromism such as solvatochromism, thermochromism and piezochromism when subjected to various external stimuli such as solvent, heat, and mechanical grinding or pressure. Interestingly, these framework materials are found to be useful for the visual detection of DMF, DMA, CH3CN, CH3COCH3, DMSO and Et3N as well as greater propensity towards formaldehyde, acetaldehyde compared to higher aldehydes. The luminescence studies of these chromic materials indicate that emission properties have red shifted and significant change of lifetimes with the change in colors (blue to green for 1TC/2TC and solvatochromic, and blue to cyan for 1PC/2PC). The greater amount of water vapor adsorption suggests that pores are hydrophilic in nature. Further, the rapid adsorption of anionic dye such as congo red by 1 and 2 suggests their utility for waste water treatment.

AUTHOR INFORMATION

Where Ci and Ce are the initial and final concentration of the dye solution.

(

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3

4

Ep in eV

5

6

7

Band gap= 1.85 eV

REFERENCES

40 30 20 10

Dye doped 1

0 1

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3

4 5 Ep in eV

6

7

Figure 8. (a) UV-vis spectra for the uptake of CR by CP-1 from the aqueous solution at various time intervals; (b) change in band gap for CP-1 and dye doped material of CP-1.

Conclusions Two novel multichromic CPs (1 and 2) have been synthesized containing 2,4,5-tri(4-pyridyl)-imidazole, L, with CdSO4 and

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Crystal Growth & Design SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).

Thermochromic, Solvatochromic and Piezochromic Cd(II) and Zn(II) Coordination Polymers: Detection of Small Molecules by Luminescence Switching from Blue to Green Avishek Dey, Abhijit Garai, Venkatesh Gude and Kumar Biradha*

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The Cd(II) and Zn(II) two-dimensional porous and multichromic coordination polymers have been synthesized which consist of alternate (6,3) brick wall and (4,4) rectangular grid network such that there exists continuous channels. These materials are also found to exhibit multichromism properties such as solvatochromism, thermochromism and piezochromism when subjected to various external stimuli.

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