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Sustainable Electronic Materials: Reversible Phototuning of Conductance in a non-Covalent assembly of MWCNT and Bioresource Derived Photochromic molecule Kizhisseri Renuka Devi, C Lalitha Lekshmi, Kuruvilla Joseph, and Sankarapillai Mahesh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10752 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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ACS Applied Materials & Interfaces

Sustainable Electronic Materials: Reversible Phototuning of Conductance in a non-Covalent Assembly of MWCNT and Bioresource Derived Photochromic Molecule Kizhisseri Devi Renuka,Ύ, Ψ C. Lalitha Lekshmi,Ύ, ΨKuruvilla Joseph,* Ύ, ΨSankarapillai Mahesh* Ύ, Ψ Ύ

Department of Chemistry, Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram, 695547, Kerala, India. Ψ Centre of Excellence in Nanoscience and Technology, Department of Chemistry, Indian Institute of Space Science and Technology (IIST),Thiruvananthapuram,695547, Kerala, India.

ABSTRACT: Tuning the microstructure, conductance,

band gap of a single molecule with an external stimuli such as light have vital importance in nanoscale molecular electronics. Azobenzene systems are inimitable light responsive molecules suitable for the development of optically modulated materials. In this work we have demonstrated the development of an optically active Multi-walled Carbon Nanotube (MWCNT)-hybrid material by the non-covalent functionalization of azo based chromophore derived from cardanol, a bioresource material. This photo-responsive non-covalent hybrid shows trans-cis photo-isomerization induced switching of conductance. We report this as the first example in which the photochromic assembly developed from a bioresource material exhibited tunable conductivity. We expect that this novel photo-switchable hybrid with reversible conductance may have potential applications in nanoscale molecular electronics, solar cells, OLEDs etc.

Keywords: Photoisomerisation, Bioresource material, Functionalized MWCNT, Reversible Conductance, Azobenzene Ability to tune the microstructure, conductance, band gap of a single molecule will have major importance in nanoscale molecular electronics.1One of the most fascinating inspirations of molecular electronics is to provide distinctive and low-cost solutions for electronic functions centered on molecules, towards the development of devices such as diodes, transistors, switches, and memristors. This is due to the facts that the molecules are probably the smallest units still capable of offering a rich structural variety and more

over molecular electronic components may be integrated with conventional microelectronics2 or assembled into true molecular circuit.3 Conductance switching is the foundation of many potential molecular electronic devices, and has been the motivation of numerous research efforts in recent years.4 However, the ability to control the conductance of molecules at the molecular level by an external mode is still a formidable challenge in this field. Here we report the observation of reproducible conductance switching triggered by external light on an innovative platform of Carbon Nanotube– molecule non-covalent junctions, where Cardanol derived photochromic molecule have a key role in the conductance switching. Moreover we can tune the orientation of molecule in the hybrid by means of non-covalent interactions. Recently our group have reported that a photo-switchable molecule with azo group derived from Cardanol possess the ability to show light induced size variation at the nanoscale.5 Due to the recent importance of ‘Sustainable Electronics’ an emerging area of research aimed at identifying compounds of natural origin and establishing economically efficient routes for the production of synthetic materials that have applicability in electronic materials/Devises.6 We have focused our attention towards developing a non-covalent hybrid of Multi-walled Carbon Nanotube (MWCNT) with photo-responsive molecule (1) derived from Cardanol (Component of cashew nutshell liquid (CNSL). Cardanol stands out as an exciting candidate due to its exceptional properties.7, 8. To make the innovative hybrid, Multi-Walled Carbon Nanotubes (MWCNTs) were selected due to their wide acceptability in various fields and app lications.9 Noncovalent synthetic approach was adopted for the realization of the 1.MWCNT.10 This approach has many advantages over

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Scheme 1 Synthesis of acid functionalized MWCNT and Compound 1 covalent ones as non-covalent modification preserves the structural and electrical properties. The adoption of noncovalent approach helps to orient the molecules using the non-covalent interactions. In the present case directionality of non-covalent interactions help in the perpendicular orientation of azo molecules as the preferred one. Also the non-destruction of π-conjugated structures open ways of electronic interactions like π-π stacking, polymer wrapping etc.11 Among various external stimuli’s, light is a perfect choice since it features high spatio-temporal resolution and it is non-invasive over a wide range of wavelengths.12,13 Among various photochromic molecules, azobenzene systems are the simplest ones that do not have any intrinsic current switching properties, but their covalent14,15 and noncovalent16,17,18 assemblies with different carbon nanomaterials have shown how the photoisomerization ability of the azo groups triggers the modulation of conduction properties of the assembly. These non-covalent hybrids of carbon nanomaterials19 will possess excellent properties inherited through both counterparts. The combination of photochromic carbon-based nanomaterials is

therefore being pursued to develop bi- or multi-functional molecular materials.20 These hybrid systems will not only possess the unique properties of each component, but shall also feature the emergence of new properties that can potentially be used for specific applications in various fields like opto-electronic devices, 21, 22 field effect transistors (FETs), 23 sensing applications24 etc. In view of their outstanding light responsive nature, photochromic carbon-based nanomaterials are particularly suitable for these broad applications.25 Here we discuss the development of an optically active Multi-Walled Carbon Nanotube (MWCNT)-hybrid material by the non-covalent functionalization of azo based chromophore derived from Cardanol, a bioresource material with switchable conductance. In this work we have synthesized compound 1 (Scheme 1) from Cardanol in three steps.26 The compound 3 is esterified to give compound 2 which on further alkylation of the hydroxyl group and by the subsequent deprotection of carboxyl group gave rise to 1. It is then characterized by techniques such as UV, IR, NMR and Mass Spectra. (See Supporting Information). In order to study the photoresponsive behavior of the sample, we have carried out UVVisible absorption measurements. The UV

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Figure 1 (a) UV-Visible absorption spectra of functionalized MWCNT, 1 and Non-covalent hybrid 1.MWCNT at 30°C (b) Temperature dependent UV-Vis absorption spectra changes of 1 (1×10-4M) in Toluene from 20 °C to 60 °C (black), in Toluene at 70 °C (blue), in Chloroform (red). Inset shows the plot of fraction of aggregates vs temperature

Figure 2 Spectral changes upon photo-irradiation in Toluene (1×10-4M) using 354 nm band pass light at 25 °C of (a)1& (b) Non-covalent hybrid 1.MWCNT.The inset plot, gives the rate constant for the trans-cis isomerization.

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spectrum of 1 (1×10-4 M) recorded in toluene shows an absorption maxima corresponding to π-π* band at 368 nm and n-π* band at 458 nm (Figure 1a). Unlike the 3 which is partially soluble in toluene, 1 is completely soluble in toluene which clearly indicates that the introduction of dodecyl chain induces the solubilization in the non-polar solvent. In CHCl3 solvent, 1 showed an absorption maxima at 365 nm corresponding to π-π* band. It has been observed that the molar extinction coefficient of 1 in Chloroform (ε= 48170 M-1 cm-1) (red line) is more than that in Toluene. This clearly indicates that 1 more self-assembles in non-polar solvent Toluene than in Chloroform. The self-assembly behavior of 1 in Toluene is further investigated by temperature dependent absorption studies. On increasing temperature from 20 °C to 60 °C the absorbance (ε) increases from 0.34 to 0.40, because of the breaking of hydrogen bonded aggregates (Figure 1b).The melting transition temperature of the aggregates is 52 °C.12 In order to study the photo-response, we have carried out UV irradiation experiments (Figure 2) to monitor the isomerization behavior of our systems in Toluene. 1 is subjected to UV irradiation using a 354 nm band-pass filter (λ band-pass=354±20 nm, LOT Oriel 200 W high pressure mercury lamp).The irradiation continued for 25minutes with the simultaneous recording of the UV spectra at different time intervals. Upon UV irradiation, there is a decrease in intensity at 368 nm (π-π*band) (ε=0.31) with time and the appearance of the band at 458nm (n- π*) (ε=0.02) whose intensity increases with time confirms the trans to cis isomerization. The Photo-stationery state (PSS) is reached at 10 minutes and the yield of cis isomer at PSS is 70%. (From the 1H NMR irradiation experiments the yield of cis isomer at PSS is 64.5%) (Supporting Information, S6 &S7). The reversibility of above transformation (cis-trans) is also investigated (Supporting Information, S2-S5).The selfassembly of 1 in Toluene is observed through Dynamic Light Scattering (DLS) studies, where 1 showed a size increase from 140 ± 5 nm to 207 ± 3 nm upon UV irradiation at 365 nm. Atomic Force Microscopic (AFM) investigations also correlates the same result. (Size varies from 151 ± 3 nm to 211 ± 4 nm) (Supporting Information Figure S8 & S9).The morphology of 1in Toluene is also given by AFM.1 assumes a disc like morphology initially and with UV illumination at 365 nm forms spherical aggregates. Then we prepared Non-covalent assembly of 1 with functionalized MWCNT and characterized by various techniques. (See Supporting Information).UV-Visible absorption spectra of acid functionalized MWCNT shows the absorption maxima around 279 nm. Formation of the non-covalent hybrid 1.MWCNT is supported by the appearance of the additional peak at 295nm whereas the π-

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Figure 3 Transmission Electron Microscopic images (TEM) of (a) & (c) Functionalized MWCNT and (b) & (d) Non-covalent hybrid 1.MWCNT in Toluene. π* band at 368 nm retained its position but exhibited greater intensity (ε=0.62) compared to 1 (Figure 1a). Photoresponsive behavior of the non-covalent hybrid material 1.MWCNT is also studied using UV-Visible absorption studies. The hybrid exhibits a different photo-responsive behavior compared to that of 1, with a decrease in yield of the cis isomer at PSS to 53.1% (1trans=70 % at PSS).This decrease in the percentage conversion is due to the strong interaction of 1transwith MWCNT by means of non-covalent interactions such as hydrogen bonding and π-π sacking (Figure 2b). The time required to achieve the photostationery state (TPSS) is increased from10 minutes to 15 minutes in the case of hybrid as a result of the strong interaction between the counterparts. The formation of 1.MWCNT again confirmed by Transmission Electron Microscopy (TEM) (Figure 3).The non-covalent attachment of 1 to MWCNT with width 23 ± 4 nm results in an increase in width to 30 ± 6 nm approximately. This increase in width around 7 nm shows that when 1 is added to MWCNT it forms a dense layer over it due to the non-covalent interactions such as hydrogen bonding as evident from absorption.27 The Scanning Electron Microscopy (SEM) images also supports the hybrid formation (Supporting Information, S12). The thermal analyses like Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) along with X–ray diffraction (XRD) studies further supports the strong interaction between 1 and MWCNT to form the noncovalent hybrid 1.MWCNT(Supporting Information, S10,S11,S13).

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Figure 4 Linear current-voltage (I-V) curves of the Non-covalent hybrid 1.MWCNTwithout and with UV irradiation. With increase in time period of irradiation, current increases. The inset shows the log scale of I-V curve.

Figure 5 (a) Photocurrent response of 1 (blue line) and the non-covalent hybrid 1.MWCNT (red line) with time upon alternate On/Off cycles of UV irradiation at a +0.5 bias in 0.1M KCl solution. (b) Photo-current response ratio of hybrid in one on-off cycle.

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Then we investigated the light controlled reversible conductance of 1.MWCNT through various techniques. First we fabricated a conductance switch using this hybrid by coating on Indium tin oxide (ITO) glass substrate by wet etching process (See Supporting Information). The ITO plate with hybrid film is utilized for the conductance measurements in a 2-probe system. The Figure 4 shows the linear current-voltage (I-V) curves of the hybrid at a voltage ranging from -2 to +2 volts (V) with UV irradiation at intervals 0,5,10 and 15 minutes. It is observed that the hybrid shows a remarkable current increase upon UV irradiation. The log scale at the inset clearly shows increase in conductance with irradiation time. This result is in accordance with that obtained for Feng et al where they fabricated a conductance switch with a covalently linked few-walled carbon-nanotube and azobenzene through a flexible spacer.28 This result was further confirmed by using Keithly 4-probe station. Here also the cis-hybrid showed more conductivity than trans-hybrid (Supporting Information, Table S1). The reversibility of conductance switching in hybrid is also examined by using the electrochemical workstation in which there is a three electrode configuration. The photocurrent measurement is done through chronoamperometric experiment in which current is determined as a function of time. From the Figure 5, it is clear that the non-covalent hybrid 1.MWCNT shows a much better tuning of photocurrent when compared to 1 alone. When the UV light is switched on there is an increase in photo-current, whereas upon cutting Off the UV source causes a drop in it. The 1 gives a very small photo-response compared to hybrid. The photocurrent response ratio for the hybrid is 466%, (See Supporting Information) which is a quite larger percentage ratio. This efficient non-covalent hybrid from Cardanol shows potential current switching with larger stability for multiple cycles.29 The plausible mechanism for this conductance modulation with UV light is the variation of tunneling distance with the photo-

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isomerization of azobenzene. Upon UV irradiation the planar trans molecule isomerizes to sterically constrained cis conformation. This eventually leads to the decrease in overall length of the azo molecule resulting in a reduction in tunneling barrier length of the nanotube. There are reports which support that photo-induced geometrical transformation at the nanotube-azo junction result in the changes in conductance.30 The mechanism is depicted in Scheme 2.The flexibility of the nanotube-azo junction is another factor determining the conductance modulation in such systems. Feng et al reported that the introduction of a flexible alkyl spacer between nanotube and azo moiety causes a better conductance tuning due to much easier conformation changes in azomolecule with a good relaxation of nanotubes. Here in our work Compound 1 contains two long alkyl chains one of which is an ether linkage and other one containing a double bond. So the system is very much flexible as evident from the photoisomerization studies the hybrid 1.MWCNT, since the yield of cis isomer at PSS is 53% which is not much bad compared to that of 1 alone (70%). The reversibility of conductance switching as illustrated in Figure 5 supports that with UV irradiation there is an increase in current due to trans-cis isomerization of azomolecule in the hybrid. The fall in current when the UV source is removed is due to the thermal cis-trans back isomerization. In conclusion a non-covalent hybrid assembly is constructed using an azobenzene derivative developed from a bioresource material, Cardanol and an acid-functionalized MWCNT. The azo system efficiently controlled the conduction properties of MWCNT through its photoisomerization ability. This is a first example of such a conductance switch developed from a bioresource material. It is observed that with UV irradiation conductance increases and when the irradiation is stopped, conductance drops down. The reduction in tunneling distance with trans-cis isomerization is responsible for the increased current flow upon UV irradiation as the azo system counterpart transforms from a planar trans to steric cis form. The hybrid

Scheme 2. The electronic switch fabricated using ITO glass plate over which the Non-covalent hybrid 1.MWCNT is coated

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showed a good photo-response ratio also. In future this material can find applications in developing various devices like OLEDs, Solar cells, LCD.

ASSOCIATED CONTENT Supporting Information. The details of the synthesis of compound 1, functionalized MWCNT and non-covalent hybrid 1.MWCNT with characterization including IR, Mass, NMR, XRD, DLS, TGA and DSC The additional morphological studies with SEM, AFM. The visible light and thermal irradiation experiments carried out using UV-Visible spectroscopy and also switching cycle experiment. Photo-irradiation experiment with 1H NMR. Table of conductivity studies in 4-probe station. Table with comparison of different works involving CNT-AZO hybrids. Figure showing pattern of hydrogen bonding in heterodimer. Visible light photographs of solutions. Details for the calculation of photo-response ratio, molar extinction coefficient and fraction of aggregates. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

*E-mail: 1) [email protected] 2) [email protected] Funding Sources

Authors deeply acknowledge IIST, DST for funding. Notes

Authors declare no competing financial interests.

ACKNOWLEDGMENT S.M thank Department of Science and Technology, Govt. of India, New Delhi for the DST-Inspire Faculty Award(DST/INSPIRE FACULTY AWARD/2012-IFACH15).S.M,K.D.R,C.L.L and K.J., acknowledges Indian Institute of Space Science and Technology (IIST),Department of space, Govt. of India. Authors are thankful to NIIST, Trivandrum, IISER, Trivandrum and CUSAT, Cochin for various experiments. Authors are thankful to Dr. K.B.Jinesh, Department of Physics, IIST for the 2- probe experiment and also Dr.J.D.Sudha, NIIST, Trivandrum for 4-probe experiment.

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