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Fluorescent Dye-Based Metal-Organic Framework Piezochromic and Multi-Color-Emitting 2D Materials for Light-Emitting Devices Kartik Maity, Devdeep Mukherjee, Mainak Sen, and Kumar Biradha ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00055 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019
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ACS Applied Nano Materials
Fluorescent Dye-Based Metal-Organic Framework Piezochromic
and
Multi-Color-Emitting
2D
Materials for Light-Emitting Devices Kartik Maity,† Devdeep Mukherjee,‡ Mainak Sen# and Kumar Biradha*† †
Department of Chemistry, ‡Department of Biotechnology and #Department of Metallurgical and
Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India KEYWORDS: metal-organic frameworks, pleochroism, piezochromism, optical bandgap, light emitting.
ABSTRACT: The dye encapsulated metal-organic frameworks are of interest for their practical applications such as light-emitting diodes, temperature sensing and microlasers. However, very few MOFs have been reported to date where the dye moiety is utilized as one of the components of the framework. Fluorescein dye (FSD) doped materials have an ability to emit various colors such as red, green, yellow depending upon their tautomeric form and external stimuli. In anticipation
of
light
emitting
devices,
three
FSD
based
novel
MOFs,
namely
{[Co2(FSD)2(H2O)4]ꞏH2OꞏMeOH}n, FSDCo, {[Mn2(FSD)2(H2O)4]ꞏH2O.MeOH}n, FSDMn and {[Cd2(FSD)2(H2O)3(MeOH)]ꞏ2(H2O)}n, FSDCd have been designed and synthesized. These crystalline materials were shown to exhibit multi color light emitting behaviour and reversible green to brown piezochromism implying their potential to act as fluorescent switches, 1 ACS Paragon Plus Environment
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mechanosensors and security papers. Further, these crystalline MOFs were also shown to exhibit rare optical phenomenon called pleochroism. The green colored crystals displayed multiple colors under polarized light due to the agglomeration of nano and microcrystals at different angles on the surface of macrocrystals.
Introduction: Metal organic frameworks (MOFs) have emerged as multifunctional materials in the last two decades given the immense interest in their porous, optical, luminescence, electronic and catalytic properties.1-5 However, the properties based on morphological aspects including size, shape, surface and growth of crystalline MOFs remained unexplored.6-8 The crystals of MOFs were shown to obtain in various forms such as nanocrystal, microcrystal, macrocrystal and also the agglomerated state of these into a polycrystal.9-11 Recent studies revealed that the nanoscale MOFs can drastically improve the electrochemical performances. For instance, ultra-small CoMOF nanocrystals were shown to be effective for electrochemical energy storage purpose.12 Further, the exfoliation on the surfaces of MOFs macrocrystals by external stimuli was shown to exhibit chromic properties.13 The dye doped MOFs have been exploited for various practical applications such as white-light-emitting diode, temperature sensing and microlasers.14-16
However, the MOFs
containing fluorescent dye molecules as network components were not explored to date. Such MOFs are anticipated to function as opto-electronic devices, sensors and semi-conducting materials. The rarity of MOFs containing dyes as building blocks may be attributed to a lack of suitable functional groups on dye molecules and also due to the difficulties in obtaining their single crystals. Further the chromism of crystalline MOFs has been actively explored in the last
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ACS Applied Nano Materials
decade and they were shown to exhibit thermo, solvato and piezo chromism in reversible manner upon suitable external stimuli.17-24 In particular, the piezochromic materials have been explored for their applications such as fluorescent switches, data storage, mechanosensors and security papers.25-29 Pleochroism is another important aspect of chromism that is exhibited by crystalline materials under polarized light; it literally means “more colors”. In principle, the crystals with non-cubic symmetries are expected to exhibit such phenomenon due to their anisotropic nature. However, most of such anisotropic crystals hardly exhibit pleochroism making this phenomenon special and very less studied compared to other optical properties.30,31 Some of the natural gem stones such as andalusite, tourmaline, tanzanite, alexandrite and cordierite were found to exhibit contrasting color changes with the change of direction of observation.32-34 This unusual property of crystals is being used for the identification of minerals or for the manufacturing of several accessories by trimming the crystals along suitable directions. Fluorescein dye (FSD) is widely used as fluorescence probes given its high fluorescence quantum yield in aqueous media. It may appear as red, yellow or colorless material depending upon its tautomeric form.35 Recently, the fluorescein loaded ZIF-8 spheres were shown to emit green fluorescence in cell medium.36 In a similar line, the fluorescein adsorbed chiral MOFs were found to be yellow in color but appear green under UV-light.37 Herein, we present three fluorescein based novel MOFs which exhibit strong pleochroic properties as well as reversible green to brown piezochromism. To the best of our knowledge, these are the first examples of pleochroic MOFs with reversible piezochromic behaviour. Further confocal microscopy images of these crystals revealed that these materials can emit various colors of light (red, green and blue) depending upon the excitation wavelength. 3 ACS Paragon Plus Environment
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Scheme 1. Self-assembly of luminescent dye fluorescein and metal ions into functional MOFs. Results and Discussion: The solvo-thermal reactions of sodium salt of FSD with Co(NO3)2, Mn(OAc)2 and Cd(NO3)2 in methanol and water solvent system resulted in single crystals of complexes {[Co2(FSD)2(H2O)4]ꞏH2OꞏMeOH}n, FSDCo, {[Mn2(FSD)2(H2O)4]ꞏH2O.MeOH}n, FSDMn and {[Cd2(FSD)2(H2O)3(MeOH)]ꞏ2(H2O)}n, FSDCd respectively.
The careful
examination of the single crystals under microscope revealed some peculiar surface morphologies.
Two types of microcrystal growth were observed on the surface of the
macrocrystals of FSDCo and FSDMn (Figure 1). In case of FSDCo, popped up layers were observed on the surface of its macrocrystals. The growth of micro needles at an angle of
