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An Electroactive Metallo-polypyrene Film as a Molecular Scaffold for Multi-state Volatile Memory Devices Megha Chhatwal, Anup Kumar, Satish Kumar Awasthi, Michael Zharnikov, and Rinkoo D. Gupta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12597 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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The Journal of Physical Chemistry

An Electroactive Metallo-Polypyrene Film As A Molecular Scaffold For Multi-State Volatile Memory Devices Megha Chhatwal, Anup Kumar, Satish K. Awasthi, Michael Zharnikov*, Rinkoo D. Gupta* †

Megha Chhatwal, Anup Kumar , Prof. Satish. K. Awasthi Department of Chemistry, University of Delhi, Delhi-110 007, India. Prof. Dr. Michael Zharnikov Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Dr. Rinkoo D. Gupta Faculty of Life Sciences and Biotechnology, South Asian University, New Delhi-110 021, India ABSTRACT: In this report, an anodically electropolymerized smart film is utilized for memory storage up to quaternary states via adjustable electrical commands. The ruthenium-terpyridine complex based polypyrene film is adherent and robust enough to withstand large number of read-write cycles (∼5 × 102) and harsh temperature conditions (∼200oC) for facile chip-lithography. Importantly, this polymeric system is a promising molecular alternative for silicon-based static random access memory (SRAM) and provides data storage density even higher than generated by flip-flop and flip-flapflop logic circuits. The film enables a data storage density of up to ∼ 4 × 1015 bits/cm2, controlled precisely by applied voltage and accessed optically. In this way, it fulfills the essential criteria for successful realization of an economically viable molecular chip with enormous storage capability as compared to analogous silicon based devices. INTRODUCTION As envisaged by Moore’s law,1 the conventional siliconbased devices are continually shrinking and would be reaching their fundamental threshold in near future due to exponentially increasing rate of technological expansion and computational stress. Miniaturization of integrated circuits via top-down photolithographic techniques comes with serious threat to their attributes owing to spatial-resolution constraints.2 Consequently, the present day research is focused on principally new approaches such as molecular electronics, in compliance with what Richard Feynman once suggested.3 The key idea of molecular electronics, which is a bottom-up approach, is to employ molecules as electronic components for fabricating functional devices.4 The molecules are deftly customized for executing Boolean computational tasks in order to meet the essential criteria of an electronic device. In this regard, stimuli-responsive molecular films are ones of the most sought-after and promising systems as they are well-attuned with the existing semiconductor technology.5 These ‘smart’ films embrace redox-active molecular assemblies which respond to electrical stimuli and rapidly switch states at accessible potentials. Scheme 1. Synthesis scheme for complex 1

Furthermore, electrochromic properties of molecular films help in transduction of redox responses into nondestructive optical signals which when articulated as programmed logic operations, eventually allow these films to function as charge-storage devices. In a pursuit to achieve higher memory densities as well as better computation ability with low power consumption and high speed, a variety of significant advances have been recently made in the field of molecular assemblies adept for multi-state information storage.6-7 Our group has also been engaged in the development of several covalently-assembled homo- and hetero-metallic monolayers as well as multilayers which displayed remarkable elec-

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tro-optical properties for the integration in future molecular memory devices.8-11 Alternatively, electropolymerization offers a more convenient and time-saving methodology for the construction of surface-confined architectures. It is a versatile technique to obtain polymers and their electroactive thin films simultaneously on conducting surfaces.12 The family of metalpolypyridyl complexes appended with pyrrole, thiophene, aniline, and vinyl groups has extensively been exploited for the formation of electropolymerized films.13 These metallo-polymeric films are enriched in photophysical and electrochemical assets which unveil a whole new panorama of exciting solid-state applications in the field of electrochromism,14 electrocatalysis,15 sensors16 and many more. Having said this, it becomes imperative to point out that only few of the existing reports have shed light on the potential applications of electropolymerized films as charge-storage memory devices.17-18 Moreover, despite of a profound backdrop on electropolymerization, only a modest thought has been given to anodic polymerization of metalpolypyridyl complexes with pyrene moiety, so far.19 The pyrene-based complexes can offer robust and stable memory systems via a very convenient electropolymerization in a potential window ambient for metallochromophores. On the other hand, in spite of their promising multi-state near infra-red (NIR) electrochromic behavior,18 vinyl-functionalized polypyridyl complexes suffer from tedious and cumbersome synthetic pathways using expensive heavy metals.20 We present here a Ru(II)-terpyridine based adhesive and redox-active polypyrene film on glassy carbon and indium tin oxide (ITO)-coated electrodes. The modified electrode could be logically programmed for information storage up to quaternary states with retention times considerable enough for the construction of static random access memory (SRAM). To the best of our knowledge, this is the first ever report on any polypyrene film demonstrating itself as a propitious molecular memory device. RESULTS AND DISCUSSION

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ble oxidation peak corresponding to pyrene moiety at Eoxd = +1.48 V (Figure S3).21. This pyrene-based oxidation wave could be seen in case of complex 1 as well but only at lower scan rates. At higher scan rates the metalcentered oxidation wave overlapped with that of pyrene. The linear correlation of peak current densities (anodic, Ipa and cathodic, Ipc) with square root of scan rate suggested diffusion-controlled redox process (Figure S4).22

Figure 1. (a) Absorption spectrum of 1 (∼0.55 × 10-5 M, CH3CN). (b) Cyclic voltammetric profiles of 1 (∼0.5 × 10-3 M, vs. Ag/AgCl, 0.1 M Bu4NPF6, CH3CN) at scan rates ranging from 0.1 to 1.0 Vs-1. In order to investigate the memory applications on solid-support, the complex 1 was anodically electropolymerized on freshly cleaned ITO-coated glass electrode by repeatedly scanning the potential between +0.8 V and +2.0 V (vs. Ag/AgCl). The cyclic voltammogram (Figure 2a) showed gradual and continuous increase in the current indicating smooth and uniform formation of film (2 cm × 0.8 cm). Most likely, the pyrene units in 1 underwent radical cation polymerization23-26 to form π– conjugated Wolf type-III27 metallo-polymer having Ru(II) ions incorporated within the polymer backbone (Figure S5). A simple polypyrene film propagates through 1,1’-bi-pyrene bridges because in the radical cation the maximum unpaired electron density is situated at 1, 3, 6 and 8 equivalent positions. Since in our case, position 1 of the pyrene ring is occupied, we anticipate that here the polymer grows via unblocked positions forming either 6, 6’-, or 6, 8’- pyrenediyl units.25 The thickness of the film could easily be controlled by varying the number of polymerization cycles (Figure S6).

Electropolymerization The homoleptic complex 1, Ru(pyrtpy)2.2PF6, (pyrtpy = 4’-(pyren-1-yl)-2,2’:6’,2”–terpyridine) was synthesized by adopting a simple procedure8 (Scheme 1) which involved refluxing of pyrtpy and RuCl3.3H2O in ethylene glycol (EG), followed by anion exchange with NH4PF6 (see experimental section) (Figure S1-S2). The complex exhibits a characteristic metal-to-ligand charge transfer (1MLCT) band at λmax = 490 nm (ε = ∼50,000 M-1 cm-1) in its absorption spectrum in acetonitrile (Figure 1a). The cyclic voltammogram of the complex in the presence of 0.1 M Bu4NPF6 (TBAP) in acetonitrile showed a reversible one-electron redox wave for Ru2+/Ru3+ couple with half-wave redox potential (E1/2) at 1.41 V (vs. Ag/AgCl) (Figure 1b). Notably, ligand pyrtpy showed an irreversi-

Figure 2. (a) Oxidative electropolymerization of 1 (∼0.5 × -3 10 M, vs. Ag/AgCl, 0.1 M Bu4NPF6, CH3CN) on ITO elec-1 trode by 10 successive potential scans at 50 mVs . (b) Cyclic -1 voltammetric profiles (scan rate = 50 mVs ) of 1-ITO polymeric films attained after 5 (olive line), 10 (red line) and 20 (blue line) electropolymerization cycles.


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Figure 3. (a) Cyclic voltammetric profiles of 1-ITO polymeric film (surface coverage = ∼6.12 × 10 mol cm ) at scan rates ranging -1 from 10 to 100 mVs . (b) Linear dependence of anodic (red spheres) and cathodic (blue spheres) peak currents of the film with 2 2 scan rate (R = 0.98). (c) Cyclic voltammetric profiles of the film before (black line) and after (red line) 10 cycles of potential -1 stress at the scan rate 50 mVs .

Figure 2b shows cyclic voltammograms of the films having surface coverage of ∼ 3 × 10-9, 6.1 × 10-9 and 7.9 × 10-9 mol cm-2 obtained after 5, 10 and 20 polymerization cycles, respectively. The surface coverage (Γ, mol cm-2) of the films has been calculated using the equation Γ = Q/nFA where, Q is the charge under the Ru2+/Ru3+ wave, n is the number of electrons per molecule reduced, F is the Faraday’s constant (96485 C mol-1) and A is the area of the electrode in cm2 covered by the polymerized film.18 The polymeric film depicted a well-defined reversible redox peak with E1/2 = 1.39 V for Ru2+/Ru3+ couple and substantiated the characteristics of a typical surface-confined redox-process with peak-to-peak separation (∆Ep) of 50 mV at scan rate 10 mVs-1 (Figure 3a). However, with increasing scan rate the voltammogram became more skewed with ∆Ep reaching up to ∼270 mV (at scan rate 100 mVs-1), probably due to the non-ohmic contact between the polymeric film and the electrode.28 A sharp peak before the metal-based oxidation wave in Figure 3a can be attributed to charge-trapping in the entangled redox sites of the polymeric film which are isolated from the ITO-electrode surface. These sites release their charge just before the oxidation potential of metal ion.29 Further, the current densities exhibited linear fit with the increasing scan rate suggesting that the redox process was not controlled by diffusion of molecules at the solid-liquid interface (Figure 3b).30 Moreover, both the redox states were highly stable and reversible as implied by the ratio of anodic and cathodic peak currents (Ipa/Ipc) which is almost unity and independent of scan rate (Figure S7).31 The film was stable under potential stress up to 102 cycles and showed minimal peak current loss (∼10%) (Figure 3c). Upon imposing 2 × 102 potential cycles, the signal loss was ∼ 30%. Adhesive polymeric films could also be deposited on glassy carbon electrode (GCE) (Figure S8-S9). Significantly, the film on GCE had smaller ∆Ep values (20 mV and 100 mV at scan rates 0.1 and 1.0 Vs-1, respectively) as compared to 1-ITO film and portrayed excellent electrochemical stability for multiple cycles (5 × 102) with peak current plummeting by only ∼10% of its original value.

Surface Characterization of Electropolymerized Films The electropolymerized film of 1 on ITO-coated electrode was analyzed by complementary surface characterization techniques, viz. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The C 1s XP spectrum of the 1-ITO film in Figure 4a shows clear and intense signature of sp2 carbon atoms (C=C) at ~284.73 eV, associated with the phenyl and pyridine rings of 1. This peak also contains the merged Ru 3d3/2 component which is comparatively weak and cannot be distinguished, because there is only one Ru atom for 62 C atoms in 1. However, the second component of the Ru 3d5/2,3/2 doublet, viz. the Ru 3d5/2 emission at ~281.0 eV is well separated from the C 1s feature and well perceptible, evidencing the presence of 1 on the substrate.8 Another evidence is provided by the N 1s spectrum which reveals an intense emission at ~399.8 eV assigned to the N atoms in the terpyridine moiety (Figure 4b).

Figure 4. (a) C 1s/Ru 3d and (b) N 1s XP spectra of 1-ITO polymeric film. The characteristic emissions are marked. Further characteristic features could be observed in the NEXAFS spectra, acquired at the C and N K-edges (Figure 5). The C K-edge spectra in Figure 5a shows the double-peak 1π* resonance at ~284.8 and ~285.6 eV, characteristic of the pyridine rings in the terpyridine moiety and the terminal pyrene groups. Unlike benzene which shows a single 1π* resonance at 285.0 eV,32 the pyridine resonance is split into double peak due to different chemical shifts of the carbon atoms in the ortho positions (C=N) and those at meta and para (C=C) positions.33 An analogous splitting also occurs upon conjuga-



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tion of the phenyl rings in pyrene as this, e.g., takes place for perylene and tetracene.34-35 Finally, the N-edge NEXAFS spectra exhibit the characteristic π1* resonance at ~398.9 eV due to the excitation of N 1s core level electron into the LUMO of pyridine rings.32,36 In addition, both C and N K-edge spectra in Figure 5 exhibit slight linear dichroism i.e., a dependence of the absorption resonance intensity on the angle of X-ray incidence. The intensity of the π* resonances at the grazing incidence of the X-rays (20°; the E vector almost perpendicular to the substrate) is somewhat higher than that at the normal incidence (90°; the E vector parallel to the substrate). This can also be seen in the difference between the spectra recorded at 90ο and 20ο (Figure S10). Considering that the π* orbitals are perpendicular to the pyridine and phenyl rings of the terpyridine and pyrene moieties, this suggests that the molecules in the 1-ITO film are slightly inclined, on the average. Based on the standard theoretical framework for a vector-type orbital37 the respective angle can be estimated at 35-41° with respect to the surface normal. Note, however, that this is just a tentative value describing the average molecular orientation since the dichroism is small, so that the 1-ITO film is assumed to be mostly disordered.

Figure 5. (a) C K-edge and (b) N K-edge NEXAFS spectra of 1-ITO polymeric film acquired at the different incidence angles of the primary X-ray beam (marked at the spectra). Realization of Multi-state Memory and Logic Circuits Metallo-functionalized surfaces are appealing candidates for the construction of miniaturized chargestorage devices owing to the affluent electro-optical properties of metal ions. In view of this aspect, 1-ITO film had been explored for its potency to store multivalued information in the form of discrete optical bits (Figure S11). The sigmoidal shape of absorbance changes of the film (∆A) as the function of applied potential depicted an inflection point at +1.44 V, which corresponds to Eoxd of the film. The derivative of fit is a normal distribution centered at E1/2 of the metallo-polymer and its full-width at half-maximum (FWHM) value came out to be 0.28 V which made it plausible for the metallo-polymeric film to show several memory states.38 Therefore, the electrical stimuli (applied potentials) had been scrupulously modulated to generate optical signals (absorbance changes) for non-destructive and prompt

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data transfer. Preliminary spectroelectrochemical investigations attested the inherent bi-stability of the polymeric system as it could be fully oxidized (∼96%) and reduced afterwards by applying potential of +1.6 V and +1.2 V (vs. Ag/AgCl), respectively (Figure 6). The event of oxidation was marked by a palpable hypochromic shift in the MLCT absorption at λ = 490 nm (Figure 6a) and the obvious bleaching of film from red to pale yellow colour (Figure S12).39 The response time for the charge storage (oxidation, 4.7 s) as well as release (reduction, 4.4 s) was deduced from chrono-coulometry experiment (Figure 6b).

Figure 6. (a) Spectro-electrochemical switching of 1-ITO -9 -2 polymeric film (surface coverage = ∼2.0 × 10 mol cm ) upon applying potential bias of +1.6 V (black solid line) and +1.2 V (blue solid line). b) Chrono-coulometry experiment at 1.2–1.6 V for 20 s.

Furthermore, continuous switching of these double potential steps (+1.6 V, +1.2 V) and monitoring of MLCT absorbance as a function of time allowed read-write cycles with binary states of ‘1’ and ‘0’, respectively (Figure 7a,b). Interestingly, a close inspection of these operations suggests that the system mimics the functioning of a Set/Reset flip-flop device40 which works according to the equation; Q(t+1) =S (t) + R(t)Q(t). S where Q(t) and Q(t+1) refer to the current and next output of the device. This SR-latch is represented by a sequential logic circuit comprising of two cross-coupled NOR gates strengthening each other (Figure S13). The applied potential of +1.6 V (‘S’) writes the information into the system and sets it at ‘state 1’ (oxidized film), whereas the subsequent potential step of +1.2 V (‘R’) erases the stored information and resets the system to ‘state 0’. The threshold limits for state ‘1’ and ‘0’ have been fixed at A ≈ 0.045 and 0.09, respectively. This emphasizes that the surface-confined polymeric system is competent enough to memorize the input-history. When none of the inputs is active, previous state of the system is preserved. Both the inputs could not be active concurrently as it creates an undefined output. Significant absorbance change between the oxidized and the reduced states ensured good on/off ratio (∼3:1) which motivated us to think beyond two-state memory device. The construction of ternary memory is highly sought-after as base three is the most efficient and economical system of information processing.41 Intermediate mixed-valence states between the two extreme states (fully oxidized and fully reduced) could be achieved by applying precisely chosen triple potential steps. Three distinct absorption states were achieved upon applying potential steps of +1.6 V, +1.4 V and +1.2 V (Figure 7c,d).


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Figure 7. Chrono-absorptometry switching experiments of 1-ITO polymeric film (surface coverage = ∼1.8-2.2 × 10 mol cm ) at λ = 490 nm indicating multiple absorbance states. (a) Binary states; ‘1’ and ‘0’ upon applying double potential steps of +1.6 V and +1.2 V, respectively. (c) Ternary states ‘-1’, ‘0’ and ‘+1’ upon applying triple potential steps of +1.6 V, +1.4 V and +1.2 V, respectively. (e) Quaternary states ‘0’, ‘1’, ‘2’ and ‘3’ upon applying quadruple potential steps of +1.6 V, +1.45 V, +1.35 and +1.2 V, respectively. (b), (d) and (f) display enlarged view of a single binary, ternary and quaternary cycle, respectively. Dashed red lines represent the threshold levels of distinct optical states.

The potential bias of +1.6 V almost completely oxidized the film whereas at +1.2 V the film got reduced back. However, at +1.4 V an intermediate state was generated where the film was neither wholly oxidized nor wholly reduced. The perpetual switching of triple potential steps as a function of time signified three quite stable states viz. ‘+1’, ‘0’ and ‘-1’ and the threshold values for these states had been set at A ≈ 0.09, 0.06 and 0.045, respectively. The average switching time between the states was ∼ 4 s. Similar to flip-flop logic circuit, the ternary system can also be represented in terms of a sequential flip-flap-flop device.42 Here, three interconnected OR gates select the proper input sequence from I1 (+1.6 V), I2 (+1.4 V) and I3 (+1.2 V) and convert it into appropriate inputs for the cross-coupled NOR gates. These NOR gates further generate the memory function of the logic system (Figure S14). Undoubtedly, binary and ternary memory bear more closer affiliation with conventional systems, but a larger radix would always allow higher memory storage by utilizing fewer digits to store given information.43 In this context, an attempt was made to realize quaternary memory. Apart from the two extreme states (fully oxidized and fully reduced), two intermediate mixed valence states could also be obtained by imposing segregate quadruple potential steps of +1.2 V, +1.35 V, +1.45 V and +1.6 V (Figure 7e,f). The threshold values had been set at A ≈ 0.09, 0.07, 0.055 and 0.04 for states ‘3’, ‘2’, ‘1’ and ‘0’, respectively. As the potential was switched from

+1.2 V to +1.35 V, the absorbance value for MLCT band considerably dropped indicating ∼25% oxidation of the film. Further potential command of +1.45 V led to another mixed-valence state comprising of ∼53% oxidized film. The final potential pulse of +1.6 V fully oxidized (∼96%) the polymeric film. The average switching time between the states was ∼6s. These four input potentials can be fed into four interconnected OR gates which distribute proper input sequences to four cross-coupled NOR gates, respectively to concatenate a more complex latching circuit.38 (Figure 8) Again, only one input can be active at a time. Notably, charge stored during oxidation of a Ru(II) center can be considered as a single-bit information.8, 10 The selection of appropriate voltage bias (quadruple potential steps) and consequent increase in number of optical states (quaternary), allow our system to behave as a potential memory module which can save up to ∼ 4 × 1015 bits/cm2, more than what a binary memory system is capable of. In open circuit conditions, it becomes difficult to distinguish between the states due to the adventitious moisture from the environment which aids reduction of Ru(III).44 The optical memory effect45 of the film for all the four states deduced that state ‘0’ (Ru3+) retains itself for ∼2.3 min before reverting back to state ‘1’ (mixedvalence) which in turn took ∼3.5 min for getting transformed into state ‘2’ (mixed-valence). The system could memorize state ‘2’ for almost ∼6.6 min after which it solely consisted of state ‘3’ (Ru2+) (Figure S15). However,



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the full conversion of Ru3+ to Ru2+ was quite a sluggish process and took about ∼25 min to get completed. The retention capacity can be further improved by complete exclusion of traces of water from the system. It is worth to mention here, that the retention time for state ‘3’ (fully reduced state) was infinity and the film on ITOcoated glass slide could be stored for numerous days without any sign of degradation. Therefore, the retention periods for the memory states were appreciable enough to avoid instantaneous refreshing voltage corroborating the potential utility of the electropolymerized film in molecular electronics with volatile static random access memory (SRAM).40 The switching experiments had been repeated for three times with same instrumental set-up and signal deviation was estimated to be ∼3-8% (Figure S16). The variations in optical readout upon feeding electrical input have also been demonstrated in solution (Figure S17).

Figure 8. (a) Characteristic truth table for (b) sequential four-input logic circuit competent of storing up to quaternary memory states. IN1 = +1.6 V, IN2 = +1.45 V, IN3 = +1.35 V and IN4 = +1.2 V. The overall output is measured for the absorbance value at λ = 490 nm.

Furthermore, to withstand the wide temperature range during device processing and operation, the electrically addressable systems ought to depict an appreciable stability under thermal stress.48 In this context, the pyrene-based robust 1-ITO polymeric film did not show any significant change in the MLCT absorbance even after 60 h under elevated temperature of 200oC. Additionally, gradual elevation of temperature from 30oC to 200oC with time intervals of one hour, also could not affect the integrity of the film (Figure S18).49

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Undeniably, the recent reports on cathodically electropolymerized films as memories are quite sophisticated owing to their multi-state NIR electrochromic behavior.18 But these are mainly focused on vinylfunctionalized polypyridyl complexes, which are deemed to give stable films since years.46 However, our work is primarily based on the memory competence of a robust polypyrene film which is not a usual case as evident by just a handful of reports on electropolymerization of pyrene.19,26,47 Moreover, despite of the fact that our complex is not cyclometallated, the film shows considerable retention time for a volatile memory module. CONCLUSIONS Fabrication of viable molecular chips for information and data processing is need of the hour and underlies a new and challenging area of research. In summary, we have successfully demonstrated oxidative electropolymerization of an naïve pyrene-appended ruthenium(II)polypyridyl complex to eventually form well-adherent electroactive polypyrene films on glassy carbon and ITO-coated glass electrodes. The films were characterized by surface-analysis techniques viz. XPS and NEXAFS spectroscopy and exhibited characteristic emission and absorption resonances. The innate bistability of Ru(II) center allowed electrical stimuli to have a precise control over its optical properties while creating four distinct memory states with short interstate response times. These well-defined quaternary absorbance states had retention time large enough for the construction of volatile static random access memory devices. However, to sustain the processing and operational environment of commercial device formulation, the films should be electrically and thermally stable. There was a minimal data loss even after subjecting the film to multiple redox cycles and the mechanical heat also could not affect the robustness of the film. To the best of our knowledge, the polymeric film described in this study, is the first ever pyrene-based platform for the construction of molecular alternatives of silicon-based electronics. As compared to monolayerbased devices, multi-state memory modules generated from electropolymerized films have higher signal intensities due to bulk surface density available. Having said that, we believe that there is still room for improvement in order to achieve better signal-to-noise ratio, rapid response, greater stability and larger retention time for real-time fabrication of multi-state molecular dataprocessing and storage devices. EXPERIMENTAL SECTION Synthesis of Ru(pyrtpy)2.2PF6 complex, 1 4’-(pyrene-1-yl)-2, 2’:6’,2” –terpyridyl (pyrtpy) was synthesized by adopting a reported method20b and characterized by 1H-NMR, ESI-MS and UV-vis spectroscopy. Pyrtpy (100 mg, 0.231 mmol) and RuCl3.3H2O (30 mg, 0.115 mmol) were added to ethylene glycol (EG) (20 ml) along with few drops of N-ethylmorpholine. The mixture was refluxed for 5-6 hours under inert atmosphere of nitrogen. Afterwards, the contents of the reaction were allowed to cool at room temperature. The deep red solution was filtered through celite and an excess of


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ethanolic solution of ammonium hexafluorophosphate was added into the filtrate. The resulting dark red precipitate was filtered in a G3 sintered crucible and washed with ample amount of water followed by diethyl ether. The crude compound was further chromatographed on a short silica column using acetonitrile, saturated aqueous KNO3 and water (7:1:0.5 v/v) as the eluent. The collected main reddish-orange band was reduced in volume followed by the addition of excess of aqueous NH4PF6 solution into it to induce precipitation of the required binary ruthenium (II) complex. (∼190 mg, 65% yield). For characterization data see supporting information. XPS and NEXAFS studies of polymeric film The XPS and NEXAFS measurements were performed at the HE-SGM bending magnet beamline of the synchrotron storage ring BESSY II in Berlin, Germany, using a custom-made experimental station equipped with a Scienta R3000 electron energy analyzer.50 The synchrotron light served as the primary X-ray source. The spectra acquisition was conducted in normal emission geometry with an energy resolution of 0.5-0.6 eV at a photon energy of 580 eV selected for the spectra acquisition. The binding energy scale of the XP spectra was referenced to the Au 4f7/2 peak at a binding energy of 84.0 eV.51 NEXAFS measurements were performed at the carbon and nitrogen K-edges in the partial electron yield mode with retarding voltages of −150 V and −300 V, respectively. Linearly polarized synchrotron light with a polarization factor of ~91% was used as the primary X-ray source; the incidence angle of the light was varied following the standard approach.37 The energy resolution was ~0.30 eV at the C K-edge and ~0.5 eV at the N Kedge. The photon energy scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.52 Absorption experiments UV-vis spectra were recorded by JASCO UV-Vis-NIR spectrophotometer (V670) in transmission mode (200– 800 nm) using a quartz cuvette (path length = 1cm, volume = 3 ml). The functionalized ITO-coated glass substrates were fixed in a Teflon holder inside the cuvette and an identical bare ITO-coated glass substrate without any film was used to compensate for the background absorption. Electrochemical experiments All the electrochemical experiments were carried out by using a CH Instruments potientiostat (Model 660D). A three-compartment electrochemical cell was used with ITO-coated glass/glassy carbon as the working electrode, Pt wire as the counter electrode and Ag/AgCl as the reference electrode. Cyclic voltammograms were recorded in 0.5 mM solution of complex 1 in acetonitrile with 0.1 M n-tetrabutylammoniumhexafluoro-phosphate (TBAP) as the supporting electrolyte. For electropolymerization studies, the glassy carbon electrode was polished with alumina powder and rinsed thoroughly with distilled water whereas ITO-coated glass was freshly cleaned by sonication with isopropanol and dried in

oven for few minutes. During electropolymerization, the working electrode was placed parallel to and opposite the counter electrode. The resulting films were briefly rinsed with acetonitrile followed by drying with the stream of nitrogen. The spectro-electrochemical experiments in solution (∼ 10-5 M, 20mM Bu4NPF6, CH3CN, 50 ml) were done with Basi-spectro-electrochemical kit with porous glassy carbon as working electrode, springtype Pt wire as counter electrode and Ag/AgCl as reference electrode. Switching experiments of electropolymerized films on ITO surface The 1-ITO film was fixed in quartz cuvette which was filled with TBAP solution (20mM, CH3CN). The film was set-up as working electrode while aqueous Ag/AgCl and platinum wire were used as reference and counter electrodes, respectively. The background correction was done with a blank ITO slide dipped in TBAP solution in reference cuvette. The experiments were then performed in chrono-absorptometric mode. After fixing the electrodes, the double-step potential range 1.6-1.2V for oxidation and reduction of ruthenium centers was applied and corresponding absorbance changes were recorded using UV-vis instrument. For triple-potential step switching, a potential range of 1.6-1.4-1.2V was applied for segregate oxidation and reduction of ruthenium centers deciphered through absorbance changes. Finally, quadruple potential steps of 1.6-1.45-1.35-1.2V were applied for four-step oxidation and reduction of metal centers. Each potential pulse was applied for 20s.

ASSOCIATED CONTENT Supporting Information. Characterization data of complex 1, electropolymerization studies on glassy-carbon electrode, spectro-electrochemical experiments in solution and thermal and temporal stability studies of 1-ITO film. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Dr. Rinkoo D. Gupta E-mail: [email protected]

*Prof. Dr. Michael Zharnikov E-mail: [email protected]

Present Addresses †Department of Chemical Physics, Faculty of Chemistry, Weizmann Institute of Science, Rehovot 7610 001, Israel.

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT MC thanks CSIR, India for award of Senior Research Fellowship and University of Delhi for technical support. RDG thanks DST (SERB/F/1424/2013-14) and South Asian University, New Delhi, India for financial assistance. MZ thanks S. Schuster for the help during the experiments at



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the synchrotron, Y. L. Jeyachandran for the processing of the XP spectra, A. Nefedov and Ch. Wöll for technical cooperation at BESSY II, and BESSY II staff for the technical support.

DEDICATION A tribute to Late Dr. Tarkeshwar Gupta, Department of Chemistry, University of Delhi, Delhi, India.

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