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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Inkless Writing and Self-Erasing Security Feature of (Z)-1,2Diarylacrylonitrile-Based Materials: A Confidential Data Communication Tamas Panda,† Dilip K. Maiti,*,‡ and Manas K. Panda*,§,∥ †
New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700009, India § Photosciences & Photonics Section, Chemical Science & Technology Division, CSIR-National Institute for Interdisciplinary Science & Technology (NIIST), Thiruvananthapuram, Kerala-695019, India ∥ Academy of Scientific and Innovative Research (AcSIR), New Delhi 110025, India
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‡
S Supporting Information *
ABSTRACT: Development of a novel organic luminescent material for inkless writing, and self-erasing application is a remarkable solution to reduce paper waste, recycling cost in the printing industry. These innovative materials also offer to reduce global warming due to the lower consumption of paper made from plants. To this endeavor, herein we report the design, synthesis and simple material fabrication of a donor−acceptor type (Z)-1,2-diarylacrylonitrile (1) compound, which in solid state displayed highly contrast and reversible vapochromism under the visible light as well as UV light. We found a unique multiphase luminescence switching from green (λmax = 535 nm) to yellow (λmax = 566 nm) and to orange-red (λ = 580, 640 nm) in solid state. This multiphase switching is induced through the gradual exclusion of entrapped DMSO molecules from the crystalline rod-like materials. Utilizing this purely organic material, we have demonstrated reversible inkless writing/printing on a cellulose strip by employing photothermal effect of sunlight in which sunlight acts as an “inkless pen”. We were able to print complex designs utilizing this technique, which is invisible in ambient light and brighter in UV light. To our delight, the writing is self-erasable on keeping at sunlight for the prolonged period, upon keeping at ambient temperature, or instantly on warming. This remarkably smart function of our material offers cost-effective and environmentally benign technique for security data communication and confidential data printing. KEYWORDS: organic material, vapochromic luminescence, sunlight mediated inkless printing, self-erasing, confidential data printing
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INTRODUCTION
self-sustained protection can be achieved if the printed content is self-erasable under the ambient conditions without the help of any specific stimuli. Organic materials that switch luminescence color in response to light, heat, pressure, solvent vapor, and/or external stimuli has excellent potential for full range of technological uses in optoelectronics, imaging, and sensing.11−19 These materials may also be utilized to develop security ink for anticounterfeiting, confidential data encryption, and other innovative application.20−25 However, despite several reports, the examples of fluorescent materials employing eco-friendly approach for data printing and exhibiting self-erasable features are scarce. For instance, sunlight mediated printing on inorganic materials is promising, but it lacks security feature because of being non-fluorescent.9,10 On the other hand,
Inkless writing or printing on a reusable medium offers a great solution to preserve green plants as well as to reduce paper waste and global warming.1,2 Unlike conventional paper with ink-based technology, a writing media made of stimuliresponsive color switching material that can perform a writeerase-write function with the aid of external stimuli is very much desirable.3,4 Such process would significantly reduce the paper recycling cost in the printing industry. Moreover, from a greener perspective, it would be ideal if abundant stimuli such a sunlight, waste heat, moisture or water can be used as tools for data encryption, printing and erasing. An approach to this direction provide not only a pollution-free printing technique, but also a solution toward sustainable technology.5−10 Moreover, from the point of confidential data communication, it would be advantageous if the printed content has some security features, that is, it should be nonrecognizable in ambient light but recognizable under UV light or after application of specific stimuli. Additionally, an extra layer of © XXXX American Chemical Society
Received: May 19, 2018 Accepted: August 1, 2018
A
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces currently available photoswitchable fluorochromic materials have the limitations using a particular technique like focusedUV beam for writing/printing, which restrict their easy access and viability for practical utility.26−28 Moreover, the majority of these materials were prepared through time-consuming and costly multistep synthetic procedure, using precious metal precursors and/or laborious fabrication of materials. Thus, the intelligent design of small molecular fluorophores armed with appropriate functional groups, development of an easy route to access the designed material in pure form, establishing writing−erasing function under the eco-friendly conditions and carrying out specific security features for secret data communication remains as a priority task in this area of research. Herein, we demonstrate a smart strategy that features the reversibility of dynamic interaction between the fluorophore and entrapped solvent molecules to achieve tunable fluorescence for their application in inkless writing/self-erasing utilizing the photothermal effect of sunlight. The designed donor−acceptor (Z)-2-(3,5-bis (trifluoromethyl)phenyl)-3-(4diphenylamino)phenyl)acrylonitrile (1, Supporting Information (SI) Scheme S1) was synthesized by a simple method and the single molecular material displayed multistage luminescence switching from green → yellow → orange-red because of structural change due to gradual exclusion of entrapped DMSO from the lattice. The sunlight printed content was barely visible in the naked eye whereas clearly visible under UV light, which demonstrated its potential for security printing application. More importantly, the written or printed content can be self-erased by exposure to sunlight for certain period, keeping it at ambient conditions for 15−20 days, or instantly warming with hot air. To our knowledge, this is the first example of a purely organic crystalline material displaying such multistage fluorescence switching and has been successfully demonstrated for inkless writing/erasing and security application under environmentally benign conditions.
Figure 1. (a,b) Digital camera snapshots of the microcrystals of 1G and 1R under ambient light. (c,d) Fluorescence microscopic images of 1G and 1R. (e) Reflectance spectra of 1G and 1R transformed by Kubelka−Munk method. (f) Solid-state fluorescence spectra of 1G, 1Y and 1R crystals. (g) TGA plots of 1G, 1Y and 1R crystals. (h) PXRD pattern of 1G, 1Y, and 1R ground powder.
ambient temperature (∼33 °C) in the open air for 15−20 days also showed a color change to red crystallites 1R through yellow-orange intermediate 1Y. This color change is visible in the naked eye (Figure 1a,b). However, it is brighter under UVlight (Figure 1c,d). Solid state absorption spectra (reflectance spectra converted by Kubelka−Munk method) of both 1G and 1R showed a charge transfer absorption band at 430 nm (Figure 1e). Moreover, a prominent low energy absorption tail at 508−570 nm was observed in 1R, which could be attributed due to the electronic transition corresponds to the newly generated intermolecular electronic state. This low energy absorption band was not present in 1G. Accordingly, solid state fluorescence spectra of 1G showed emission maxima at λem, max = 535 nm (λex = 430 nm), while 1R exhibited a red-shifted less emissive band with (λem, max = 580 nm) along with a shoulder band at 640 nm (λex = 430 nm, Figure 1f). The intermediate 1Y displayed an emission at λmax = 566 nm. Solid state excitation profile of 1G (monitoring emission at 550 nm) was different from that of 1R (monitoring the emission at 580 and 640 nm, SI Figure S6). Thus, the emission bands in two solid forms originated from different excited states, which could be attributed due to their difference in molecular orientation of the self-aggregated solid-state packing. In order to verify whether the color change of 1G and 1R was associated with loss of solvent DMSO molecule, we have carried out 1H NMR, elemental analyses and thermogravimetric analyses (TGA). 1H NMR of 1G fibers in CDCl3 showed a sharp peak at 2.62 ppm confirming the presence of entrapped DMSO (SI Figure S1b). As expected, this DMSO peak is not present in the 1H NMR spectra of 1R. Both 1H NMR and elemental analyses suggest that approximately one DMSO molecule was present per molecule of 1G. Moreover, TGA trace of 1G display ∼13% weight loss near to 100 °C that corresponds to the exclusion of one molecule of DMSO per molecule of 1 (Figure 1g), further supporting the above
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RESULTS AND DISCUSSION Compound 1 was synthesized through one step Knoevenagel condensation29 of 4-(diphenylamino)benzaldehyde with 3,5bis(trifluoromethyl)phenylacetonitrile in 70% yield (SI Scheme S1). Compound 1 was thoroughly characterized by 1H NMR, 13 C NMR, and Mass (SI Figure S1−S3). UV−vis spectra of 1 in chloroform (1 × 10−5 M) showed a band at 413 and 420 nm in DMSO, which corresponds to intramolecular charge transfer (ICT) interaction involving diphenylamine (DPA) donor and −CN/CF3 acceptor group present in the molecule (SI Figure S4). Fluorescence spectra of 1 in DMSO (1 × 10−6 M, SI Figure S5a) showed significantly red-shifted band (λmax = 614 nm, Stokes shift = 7132 cm−1) compared to that in nonpolar solvent (SI Figure S5b). The large solvatochromic shift could be attributed due to considerable differences in dipole moments (in a polar solvent) between the ground state and Franck−Condon excited state that occurred possibly because of strong dipole−dipole interaction between solvent and solute molecules. Interestingly, on the crystallization of compound 1 in hot DMSO resulted in thin fibers like crystals (1G), which were green emissive (Figure 1a,c). To our surprise, slow warming of these fiber crystals on a hot plate at 60 °C for 3−4 h resulted in a complete conversion to red colored solid 1R, which displayed orange-red emission (Figure 1b,d). Keeping 1G fibers at B
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces observations. TGA plot of 1Y showed only 3% weight loss that can be attributed to approximately 1/4 DMSO molecule per molecules of 1. In contrary to 1G and 1Y, 1R did not show any weight loss upon heating (Figure 1g), which confirms the absence of any solvent molecule in the lattice. Infrared (ATR) spectra of 1G crystal displayed characteristic DMSO peaks at 953 cm−1 (νCH3 bending) and 1050 cm−1 (νS=O, stretching, SI Figure S7).30 As anticipated, these frequencies were not present in the spectra of 1R. PXRD pattern of 1Y and 1R (ground form) was very different from the DMSO containing 1G powder, suggesting a significant lattice transformation ongoing from 1G to 1R (Figure 1h). Specifically, upon going from 1G to 1R, the rise of several sharp peaks (at 2θ = 4.82°, 10.4°, 13.4°, 21.5°) are indicative of greater crystallinity of the later. Moreover, higher peak density in the PXRD plot of 1Y and 1R indicates that molecules in 1R adopt a tighter packing structure upon removal of solvent from 1G lattice. Such densely packed materials are expected to have higher density and melting point than loosely packed ones. Indeed, differential scanning calorimetry (DSC) study revealed that 1R has a higher melting point (131 °C) than 1G (110 °C, SI Figure S8). All these results indicate that the inclusion/or loss of guest (DMSO) into/from the crystal lattice is responsible for molecular reconfiguration or reorientation in the selfaggregated solid state. It is associated with visual or emissive color change in a solid state. Several attempts were made to have a single crystal X-ray structure of 1G, however, because of very weak diffraction spot pattern in each of the XRD measurements, the efforts remain unsuccessful. On the other hand, the mosaicity of thermally converted 1R was too high to obtain a good diffraction data. Mechanical stimulation of 1G fiber showed loss of DMSO with a concomitant change in color (Figure 2a−c, SI Figure S9,
Figure 3. (a) SEM micrographs of 1G microcrystals and (b) 1R crystal. (c) AFM image and height profile of 1G and (d) partially converted 1Y form showing roughening of surface. (e,f) TEM images of 1G and 1Y microcrystals, SAED pattern is given in the inset.
existence of its smooth surface. AFM height profile revealed that the surface height varied in the range of 0.03−0.06 μm (Figure 3c). However, the SEM image of 1R displayed a drastic change in the surface morphology. The surface of 1R was rougher (Figure 3b) with the formation of smaller crystallites. Visual inspection by fluorescence microscope equipped with the hot stage was also supportive of this observation. Heating (10 °C per minute to 100 °C) of 1G crystal led to a drastic change in the morphology along with emissive color change (SI Figure S10). This transformation is expected to occur because of rapid rearrangement of molecules and lattice transformation during the exclusion of DMSO molecules. Because of the much rougher surface in 1R materials, our attempts to record AFM height profile were unsuccessful. However, on keeping the 1G crystal for 1 week at room temperature provided partially a converted yellow intermediate crystal (1Y), which was suitable for AFM analysis. A clear change in morphology was observed (Figure 3d) even upon partial removal of the solvent. AFM height profile plot revealed an average height difference in the range of 0.1−0.17 μm (Figure 3d), which was considerably higher than 1G crystal surface. Moreover, TEM images of drop-casted 1G micro fibers (Figure 3e) and 1Y form (Figure 3f) clearly demonstrated the difference in the morphology. Further, it was supportive of the observations from SEM and AFM images. The selected area electron diffraction (SAED) patterns confirmed the existence of materials in the crystalline form. From Figure 3e,f inset and SI Figure S11a,c, Supporting Information, it is observed that the interplanar distances (dspacing) of 1G are 3.69 and 3.82 Å which correspond to the PXRD peaks having 2θ values of 24.1° and 23.1°, respectively.
Figure 2. (a,b,c) Fluorescence color change of 1G-grinded upon DMSO removal by heating with a light bulb. (d,e,f) Gradual change in fluorescence color change of1G-grinded on a filter paper upon exposure to sunlight.
SI). Gently pressed 1G fragments were green emissive, which were gradually transformed into greenish yellow to the dark yellow emissive state upon prolonged (hard) grinding. Upon DMSO fuming the dark yellow emissive state returned to the green emissive state. Precisely same changes were observed when a cellulose filter paper containing 1G ground layer was exposed to intense sunlight (Figure 2d−f). The dark yellow emissive state might be reversibly transformed into the green emissive state through DMSO fuming. We were curious to see the morphological change of the materials upon chromic transformation. The SEM imaging study (Figure 3a) of pristine 1G needle crystals confirmed the C
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
1R (Figure 4c,d). In the presence of entrapped DMSO in the lattice, the molecules of 1G are loosely packed and distant from each other where interchromophoric charge transfer is very weak. In this stage, intramolecular charge transfer fluorescence of the monomer is prevalent, which results in a green emission. Fluorescence excitation spectra of 1G displayed a small band at 430 nm which may be assigned as an intramolecular charge transfer band in the solid state (SI Figure S6). This band was absent in the excitation spectra of 1R. Upon removal of solvent DMSO from the lattice, molecules rearrange to a tighter packing which facilitates intermolecular communications such as weak π−π stacking and excimeric interactions. Such rearrangement is accompanied by the conformational change of the diphenylamine residue that permits closure proximity of the molecules. In fact, drastic change in the infrared and Raman bands also supports this hypothesis. Moreover, lower quantum yield and higher excited state lifetime of 1R (compared to 1G), is also indicative of the presence of excimer type interactions (SI Table S1−S2). The appearance of a new low energy band at 508−570 nm in the absorption spectra of 1R, (Figure 1e) indicates the possibility of forming a preorganized structure that facilitates excimeric interaction in the excited state. The reversibility of emission switching between 1G and 1R phases was tested up to 4 cycles (SI Figure S15). As can be seen from SI Figure S15, the 1G and 1R can retain its emission behavior even after third cycles. The stacked infrared spectra in SI Figure S16, suggest that molecules retain their characteristic conformation and symmetry during switching. To check the aqueous stability, 1R solid was suspended in distilled water, shook well and kept at room temperature for 5 days. And subsequently collected the infrared spectra. As can be seen from the infrared spectra in SI Figure S17, 1R solid retains its conformational stability and symmetry even after keeping in aqueous media for 5 days. Excellent vapochromic features of 1G, 1Y, and 1R system were demonstrated for its applications in inkless writing and erasing. First, we examined the fluorescent printing utilizing the photothermal effect of sunlight. In this method luminescence color of the dye was manipulated by differential exposure to sunlight heating using a plastic stencil or mask (Figure 5a,b). The writing pad with of area 1.5 × 4 cm2, was made on a cellulose filter paper simply by mechanically pressing and rubbing pristine1G fibrous materials (80 mg) with a spatula, which transformed into a layer of green fluorescent film (Figure 5c). The stencil was placed on the top of the pad and exposed to sunlight. After 2 h of exposure, the stencil was removed to obtain the printed content. The printed content (Figure 5d,e) was invisible under ambient light, but nicely visible under UV light. The luminescence color contrast between background pad and the printed content was sufficient to provide good legibility under UV light, but not in visible light. It provides an extra security feature of the printed content and can be utilized for confidential data printing. The printing content is self-erasable on keeping under intense sunlight for 4 h (Figure 5f,g) or at ambient temperature for 10−15 days. As observed from Figure 5g, the legibility of the printed text very much diminished upon 4 h exposure of sunlight. This self-erasable feature offers another layer of security for the purpose of confidential data management or temporary communication. Alternatively, the printed content can also be erased instantly by warming with a hot air gun (90 °C for 10−15 s.). Rapid heating under dynamic
Similarly, SAED pattern of 1Y also exhibits diffraction spots having d values 3.10 and 3.60 Å that correspond to 2θ values of 28.9° and 24.1° (SI Figure S11b,c and SI Table S3). The loss of DMSO from 1G and coordinate transformation to 1R may have a profound effect on the rheology of the materials. As we know, the storage modulus (G′) of a soft material corresponds to the solid-like behavior, while loss modulus (G″) is the parameter of viscous behavior. The storage modulus (G′) and loss modulus (G′’) as a function of % strain is shown in SI Figure S12. Compared to 1G, 1R exhibits slightly wider linear viscoelastic region (1.0% in 1G vs 1.59% in 1R) indicating increased elastic response range in the latter. In this domain, at a particular strain value, the storage modulus (G′) of 1R is always higher than 1G, which is supportive of more solid-like behavior in the former. For example, at strain values of 0.25% and 1%, the G′ values of 1R are 7.84 × 105, and 4.14 × 105 while, the same for 1G are 7.17 × 105 and 2.63 × 105 Pa, respectively. This result is in line with tighter packing and higher crystallinity of 1R than 1G material. To elucidate the molecular level mechanism of vapochromic switching, we carried out infrared and Raman spectroscopy of 1G and 1R (Figure 4a,b, SI Figure S13−S14, Supporting
Figure 4. (a) Infrared spectra of 1G and 1R showing changes in bond frequencies. (b) Raman spectra of 1G and 1R. (c,d) Possible mechanistic model for the transformation of 1G to 1R.
Information). Both the infrared and Raman spectra showed prominent changes in stretching frequencies ongoing from 1G to 1R. Specifically, the downshift of νC=C of aromatic ring stretching frequency in 1R (1572 cm−1) and 1G (1576 cm−1) is suggestive of the close proximity of the molecules and possible weak π−π stacking interactions between the phenyl rings.31 Moreover “out of plane” vibrations of aromatic C−H bonds also shifted to lower wavenumber upon 1G to 1R transformation (831 and 758 cm−1 in 1G vs 825 and 755 cm−1 in 1R) in line with the above observation (SI Figure S13a). A slight increase in the frequency of νC≡N (2212 cm−1, Figure 4a inset) and νCF3 (1330 cm−1 in 1R vs 1328 cm−1 in 1G is an indication of rupturing/weakening of intermolecular interactions involving these groups. The same trend was also observed in Raman active vibrations of 1G and 1R (Figure 4b, SI Figure S14), further substantiating the above observation. Based on all the above observations we propose a possible mechanism of vapochromic luminescence switching of 1G to D
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
carefully moved along the X-Y direction for writing. We can write the word “CSIR” employing this method. Next, we turned out attention for instant writing through employing a hot object (smooth tip metal rod or glass rod, T ∼ 90 °C) as “inkless pen” (Figure 7a). Interestingly, writing with
Figure 5. (a) Schematic diagram of the sunlight mediated printing. (b, c) Printing pad made from 1G. (d, e) Printing pad under sunlight and UV light. (f, g) Self-erasing under sunlight and UV light. (h−k) Printing of QR codes.
Figure 7. (a) Schematics of reversible inkless writing with a hot pen. (b, d) Writing pad made by mechanically pressing 1G material on a cellulose filter paper. (c, e) Writing 123 by a hot glass rod that acts as a pen. (g, (i) Erasing with a hot air gun heating. (f, h) Recovered writing pad by DMSO fuming.
air flow caused a quick removal of entrapped DMSO with concomitant structural relaxation to a dark yellow emissive state, and the writing became indistinguishable. The same writing pad can be recycled to initial green emissive state through fuming with DMSO vapor. A typical set up for fuming procedure is given in SI Figure S18. Interestingly, this reversal of emission switching is very specific to DMSO, other common organic solvents (hexane, DCM, chloroform, THF, toluene, DMF, pyridine, etc.) cannot induce this switching (SI Figure S19). Utilizing the above technique, we have successfully printed several images as well as complex designs, which are shown in Figure 5h−k and SI Figure S20−S21. Notable to mention here that the sunlight mediated emission change is purely due to photothermal effect. No photoinduced change was observed for 1G material even after keeping the same in UV-365 nm light for 6 h (SI Figure S22−S23). Utilizing the above photothermal technique, we tested instant writing using intense sunlight as an “inkless pen” (Figure 6). In a manual set up, the commercially available convex lens was fitted with a fixed z-distance (focal length) and
hot glass rod instantly change the color to dark orange because of the conversion associated with partial loss of DMSO molecules under hot tip (Figure 7b-e). The legibility of written content is higher under UV light compared to visible light. This method may be utilized for advanced writing through a programmed “hot air blow pen” with micrometer-sized focal radius in which hot air acts as ink. Apart from inkless writing, the 1G or 1R materials can also be used as a dye-ink for reversible multicolor writing. To test this, we have written “A” by gently pressing and spreading 1R powdery material on a cellulose filter paper using a smooth glass rod tip. Upon fuming with DMSO, it reversibly transformed into green fluorescent “A” (Figure 8). This color reversibility can be achieved for multiple cycles without a noticeable change in the wavelength (SI Figure S15).
Figure 8. Reversible writing of 1R (a) “A” written by mechanical pressing. (b) DMSO fuming to green emissive “A”.
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CONCLUSION In conclusion, we report an unprecedented highly contrast vapochromic system that efficiently switches its visible and fluorescence color. A unique multiphase fluorescence response was observed from green to orange-yellow through a yellow emissive state. We have demonstrated the application of this material for smart inkless writing process utilizing environ-
Figure 6. Fluorochromic writing by focused sunlight using convex lens. Writing pad with 5 × 2.3 cm2 area (on which CSIR wrote) was made from approximately 170 mg of 1G material. E
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
used as cantilevers at a resonant frequency range of 266−326 kHz. Scan arrays were 256 × 256 points, and the scan rate was 0.62 Hz. PXRD. Powder X-ray Diffraction (PXRD) was measured by a XEUSS SAXS/WAXS system by Xenocs, operated at 50 kV and 0.60 mA. The X-ray radiation was collimated with FOX2D mirror and two pairs of scatter less slits from Xenocs. The XRD data were recorded in the transmission mode geometry utilizing Cu Kα radiation (wavelength λ = 1.54 Å). The fiber diagrams were recorded using an image plate system (Mar 345 detector) and processed using Fit2D software. Rheometer. Rheological properties of 1G and 1R were measured using Anton Paar Modulated Compact Rheometer-MCR 102. Parallel plate sensor with a diameter of 50 mm and a gap size of 0.105 mm was used. The angular frequency was 10 rad s−1 to determine the storage modulus and loss modulus. Synthetic Procedure. Synthesis of 1. The compound 1 was synthesized according to literature procedure.29 Briefly, A 100 mL round-bottom flask fitted with a reflux condenser and magnetic stirrer was charged with 4-(diphenylamino)benzaldehyde (0.153 g, 0.87 mmol), 3,5-bis(trifluoromethyl)phenylacetonitrile (0.1 mL, 0.87 mmol) in 30 mL of methanol solution. Potassium tert-butoxide (0.112 g, 1.0 mmol) and tetrabutyl ammonium hydroxide (25% methanol solution, 1 mL, 1.0 mmol) was added to it and the resulting mixture was heated at 60 °C for 4 h. The product mixture was cooled and kept for crystallization for 2 days. Orange colored plate shaped crystals of the product were obtained from mother liquor which was isolated by filtration (0.198 g, 70%). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.05 (s, 2H, ArH), 7.83 (d, 3H, J = 8 Hz, Ar-H), 7.51 (s, 1H, Ar-H), 7.35 (t, 4H, J = 8 Hz, Ar-H), 7.19−7.15 (m, 6H, Ar-H), 7.05 (d, 2H, J = 8 Hz, Ar-H), 13C NMR (125 MHz, CDCl3) δ (ppm) 151.10, 146.19, 144.58, 137.54, 132.96, 132.69, 132.43, 132.12, 131.37, 129.72, 126.13, 125.53, 124.98, 124.15, 121.96, 121.78, 120.05, 117.90, 104.02. HRMS: calculated for C29H18F6N2 is 508.1374, obtained m/z = 509.1452 (M+H). Elemental analysis: calculated for C29H18F6N2 C; 68.50, H; 3.57, N; 5.51, obtained C; 68.29, H; 3.40, N; 5.45. Preparation of 1G and 1R. 1G fiber crystals were obtained by crystallization of 1 from hot DMSO solution. The crystals were collected by filtering and then dried in open air for 24 h. On keeping at room temperature (∼33 °C) for several days, the initial yellow color of the 1G fibers gradually changes to yellow-orange 1Y form after 10 days and finally transform into orange-red 1R form after 15− 20 days. Elemental analysis (CHN). 1G fibers: calculated for C29H18F6N2· DMSO: C; 63.47, H; 4.12, N; 4.78, S; 5.47, obtained C; 64.68; H;3.630, N; 4.59, S; 5.12.
mentally abundant and benign sunlight stimuli. The written content can be self-erased in multiple ways such as exposure to sunlight, keeping at ambient temperature, vapor fuming or warming. We believe that the results demonstrated herein have significant implications for developing cost-effective and environmentally friendly printing/writing technology for temporary communication that are confidential and multiphase luminochromic behavior for security and anticounterfeiting application.
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EXPERIMENTAL SECTION
Materials. 4-(Diphenylamino) benzaldehyde was purchased from Sigma-Aldrich and used without further purifications. 3,5-Bis(trifluoromethyl)phenyl acetonitrile was purchased from TCI India, and tetrabutylammonium hydroxide (TBAH) from Spectrochem, and used as commercially received. Merck ACS grade solvents were used for synthesis and spectroscopy grade solvents were used for crystallization and spectroscopy studies. Instruments and Methods. NMR. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were carried out on Bruker ASCENDTM 500 spectrometer using CDCl3 as a solvent. The 1D-NMR chemical shifts (δ) are reported in ppm in reference to the internal standard tetramethylsilane (Si(CH3)4). HRMS. High-resolution mass spectra (HRMS) of the compound was recorded on Thermo Scientific Exactive Benchtop LC/HRMS Orbitrap mass spectrometer using electrospray ionization (ESI) mode. UV−Vis and Fluorescence. Solution and solid-state UV−Vis absorption spectra were recorded in a Shimadzu UV-2401C spectrophotometer. Solid state emission spectra of the crystals and ground samples were recorded in a HORIBA SPEX Fluorologspectrofluorimeter FL-1039. The samples were glued to the quartz plate with nonfluorescent grease, placed on the optical path and spectra were recorded in front face mode. For Absolute quantum yields (QY) measurement, the 1G and 1R crystals were sandwiched between two glass plate, placed in the optical path inside a calibrated integrating sphere in a HORIBA Fluorologspectrofluorimeter (SPEX) employing a Xe arc lamp as the excitation source in the sphere using specific excitation wavelengths. Before the experiment, the integrating sphere was calibrated using tris(8-hydroxyquinolinato) aluminum (Alq3). The absolute quantum yield was calculated using the method reported previously.32 Fluorescence lifetime measurements were carried out on a Modular Time-correlated single photon counting (Horiba) spectrometer equipped with Delta Flex detector: PPD850. Fluorescence image was captured using a Leica DM 2500P microscope. IR and Raman. IR spectra were recorded in an Agilent Cary 600 series IR spectrometer. Raman Spectra were recorded on a WiTec α300R Confocal Raman Microscope. Thermal Study. Differential scanning calorimetric (DSC) measurements were carried out on a TA Instruments DSC Q2000 model with a refrigerated cooling system with heating and cooling rate of 10 °C min−1 under a nitrogen atmosphere. Thermogravimetric analyses (TGA) were performed on a TA Instruments SDT Q600 V20.9 Build 20, with a heating increment of 10 °C per min using nitrogen (50 mL/min) as purging gas. SEM and TEM Imaging. Scanning Electron Microscopy (SEM) images were obtained from JEOL-JSM5610 instrument using 8−10 kV of energy. The sample was coated with gold before to the SEM imaging study. Transmission Electron Microscopy (TEM) measurements were performed in an FEI Tecnai T30 with EDAX microscope using an accelerating voltage of 300 kV. The samples for TEM were prepared by drop casting of 1 from DMSO solution (for 1G) on a carbon-coated copper grid. The same grid was kept at room temperature (∼33 °C) for 10 days (for the transformation of 1G to 1R), and then images were recorded for 1Y. AFM. Atomic force microscopy (AFM) measurements were carried out with Bruker NANOSCOPE instrument with a msimple tip (Veeco RTESP tips, 1−10 Ohm. cm−1 phosphorus-doped Si) were
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ASSOCIATED CONTENT
S Supporting Information *
These materials are available free of cost via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08279. Synthesis and characterization data and additional figures are provided (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(M.K.P.) E-mail:
[email protected]; mannup25@ gmail.com. *(D.K.M.) E-mail:
[email protected]. ORCID
Tamas Panda: 0000-0002-7456-9000 Dilip K. Maiti: 0000-0001-8743-2620 Manas K. Panda: 0000-0002-6297-2070 Author Contributions
The manuscript was written through contributions of all authors. F
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Notes
Crystal-to-Single-Crystal Phase Transitions with Alterations of the Luminescence Color. J. Am. Chem. Soc. 2018, 140, 2875−2879. (18) Fujii, K.; Sakon, A.; Sekine, A.; Uekusa, H. Reversible Color Switching of an Organic Crystal Induced by Organic Solvent Vapors. Cryst. Growth Des. 2011, 11, 4305−4308. (19) Cho, S.; Kim, G.; Lee, S.; Park, J.; Shim, W. Molecular-Printed Thermochromic with Fast Color Switching. Adv. Opt. Mater. 2017, 5, 1700627. (20) Hariharan, P. S.; Mothi, E. M.; Moon, D.; Anthony, S. P. Halochromic Isoquinoline with Mechanochromic Triphenylamine: Smart Fluorescent Material for Rewritable and Self-Erasable Fluorescent Platform. ACS Appl. Mater. Interfaces 2016, 8, 33034− 33042. (21) Thirumalai, R.; Mukhopadhyay, R. D.; Praveen, V. K.; Ajayaghosh, A. A slippery Molecular Assembly Allows Water as a self-Erasable Security Marker. Sci. Rep. 2015, 5, 09842. (22) Hou, K.; Ke, C.; Bruns, C. J.; McGonigal, P. R.; Pettman, R. B.; Stoddart, J. F. Tunable Solid-State Fluorescent Materials for Supramolecular Encryption. Nat. Commun. 2015, 6, 6884. (23) Yoon, B.; Lee, J.; Park, I. S.; Jeon, S.; Leea, J.; Kim, J. − M. Recent Functional Material-Based Approaches to Prevent and Detect Counterfeiting. J. Mater. Chem. C 2013, 1, 2388−2403. (24) Sun, H.; Liu, S.; Lin, W.; Zhang, K. Y.; Lv, W.; Huang, X.; Huo, F.; Yang, H.; Jenkins, G.; Zhao, Q.; Huang, W. Smart Responsive Phosphorescent Materials for Data Recording and Security Protection. Nat. Commun. 2014, 5, 3601. (25) Qi, Q.; Li, C.; Liu, X.; Jiang, S.; Xu, Z.; Lee, R.; Zhu, M.; Xu, B.; Tian, W. Solid-State Photoinduced Luminescence Switch for Advanced Anticounterfeiting and Super-Resolution Imaging Applications. J. Am. Chem. Soc. 2017, 139, 16036−16039. (26) Wu, H.; Chen, Y.; Liu, Y. Reversibly PhotoswitchablesuPramolecular Assembly and Its Application as a Photoerasablefluorescent Ink. Adv. Mater. 2017, 29, 1605271. (27) Wang, J.; Jin; Wang, N.; Peng, T.; Li, X.; Luo, Y.; Wang, S. Organoboron-Based Photochromic Copolymers for Erasable Writing and Patterning. Macromolecules 2017, 50, 4629−4638. (28) Li, F.; Zhuang, J.; Jiang, G.; Tang, H.; Xia, A.; Jiang, L.; Song, Y.; Li, Y.; Zhu, D. A Rewritable Optical Data Storage Material System by [2 + 2] Photocycloreversion-Photocycloaddition. Chem. Mater. 2008, 20, 1194−1196. (29) Panda, M. K.; Ravi, N.; Asha, P.; Prakasham, A. P. High Contrast Mechanochromic and Thermochromic Luminescence Switching by a Deep Red Emitting Organic Crystal. CrystEngComm 2018, DOI: 10.1039/C8CE00696B. (30) Skripkin, M. Y.; Lindqvist-Reis, P.; Abbasi, A.; Mink, J.; Perssone, I.; Sandstrom, M. Vibrational Spectroscopic Force Field Studies of Dimethyl Sulfoxide and Hexakis(dimethyl sulfoxide)scandium(III) iodide, and Crystal and Solution Structure of the Hexakis(dimethyl sulfoxide)scandium(III) Ion. Dalton Trans. 2004, 4038−4049. (31) Profit, A. A.; Felsen, V.; Chinwong, J.; Mojica, E.-R. E.; Desamero, R. Z. B. Evidence of π-Stacking interactions in the SelfAssembly of hIAPP. Proteins: Struct., Funct., Genet. 2013, 81, 690− 703. (32) deMello, J. C.; Wittmann, H. F.; Friend, R. H. An improved Experimental Determination of External Photoluminescence Quantum Efficiency. Adv. Mater. 1997, 9, 230−232.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.K.P. thanks Council of Scientific and Industrial Research (CSIR, Govt of India) for fellowship. This research was carried out with financial support from CSIR Lab Fund MLP 07 availed at NIIST Thiruvanthapuram. M.K.P. is also thankful to Director CSIR-NIIST for providing facilities. M.K.P. extends his thank to Dr. U.S. Hareesh, CSIR NIIST for his help with Rheometer instrument. Research fundings from SERB, India (project no. EMR/2017/005028) is gratefully acknowledge.
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REFERENCES
(1) Munger, R. Go Green: Making a case for Eco-friendly Printing at Your Organization, Professional Communication Conference, 2007. IPCC 2007. IEEE International, DOI: 10.1109/IPCC.2007.4464096. (2) Bajpai, P. Management of Pulp and Paper Mill Waste; Springer International Publishing: Cham, Switzerland, 2015. (3) Khazi, M. I.; Jeong, W.; Kim, J.-M. Functional materials and systems for rewritable paper. Adv. Mater. 2018, 30, 1705310. (4) Müller, V.; Hungerland, T.; Baljozovic, M.; Jung, M.; Spencer, N. D.; Eghlidi, H.; Payamyar, P.; Schlüter, A. D. Ink-Free Reversible Optical Writing in Monolayers by Polymerization of a Trifunctional Monomer toward Rewritable “Molecular Paper. Adv. Mater. 2017, 29, 1701220. (5) Ma, Y.; She, P.; Zhang, K. Y.; Yang, H.; Qin, Y.; Xu, Z.; Liu, S.; Zhao, Q.; Huang, W. Dynamic Metal-Ligand Coordination for Multicolour and Water-Jet Rewritable Paper. Nat. Commun. 2018, DOI: 10.1038/s41467-017-02452-w. (6) Kumar, P.; Sakla, R.; Ghosh, A.; Jose, A. D. Reversible Colorimetric Sensor for Moisture Detection in Organic Solvents and Application in Inkless Writing. ACS Appl. Mater. Interfaces 2017, 9, 25600−25605. (7) Singh, V. K.; Chitumalla, R. K.; Ravi, S. K.; Zhang, Y.; Xi, Y.; Sanjairaj, V.; Zhang, C.; Jang, J.; Tan, S. C. Inkjet-Printable Hydrochromicpaper for Encrypting Information and Anticounterfeiting. ACS Appl. Mater. Interfaces 2017, 9, 33071−33079. (8) Sheng, L.; Li, M.; Zhu, S.; Li, H.; Xi, G.; Li, Y.-G.; Wang, Y.; Li, Q.; Liang, S.; Zhong, K.; Zhang, S. X.-A. Hydrochromic Molecular Switches for Water-Jet Rewritable Paper. Nat. Commun. 2014, 5, 3044. (9) Kan, W. Q.; Wen, S.-Z.; He, Y.-C.; Xu, C.-Y. Viologen-Based Photochromic Coordination Polymers for Inkless and Erasable Prints. Inorg. Chem. 2017, 56, 14926−14935. (10) Garai, B.; Mallick, A.; Banerjee, R. Photochromic Metal− Organic Frameworks for Inkless and Erasable Printing. Chem. Sci. 2016, 7, 2195−2220. (11) Parthenopoulos, D. A.; Rentzepis, P. M. Three-Dimensional Optical Storage Memory. Science 1989, 245, 843−845. (12) Kunugi, Y.; Miller, L. L.; Mann, K. R.; Pomije, M. K. A Vapochromic Photodiode. Chem. Mater. 1998, 10, 1487−1489. (13) Gorai, T.; Maitra, U. Supramolecular Approach to Enzyme Sensing on Paper Discs Using Lanthanide Photoluminescence. ACS Sens 2016, 1, 934−940. (14) Gao, M.; Li, S.; Lin, Y.; Geng, Y.; Ling, X.; Wang, L.; Qin, A.; Tang, B. Z. Fluorescent Light-up Detection of Amine Vapors Based on Aggregation-Induced Emission. ACS Sens 2016, 1, 179−184. (15) Mishra, P. K.; Saha, T.; Talukdar, P. Hydrogen Sulfide Mediated Cascade Reaction Forming an Imino Coumarin: Applications in Fluorescent Probe Development and Live-Cell Imaging. Org. Biomol. Chem. 2015, 13, 7430−7436. (16) Wenger, O. S. Vapochromism in Organometallic and Coordination Complexes: Chemical Sensors for Volatile Organic Compounds. Chem. Rev. 2013, 113, 3686−3733. (17) Jin, M.; Sumitani, T.; Sato, H.; Seki, T.; Ito, H. MechanicalStimulation-Triggered and Solvent-Vapor-Induced Reverse SingleG
DOI: 10.1021/acsami.8b08279 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
