Article pubs.acs.org/JPCC
Chromogenic Homo-Dinuclear Ruthenium(II) Monolayer as a Tunable Molecular Memory Module for Multibit Information Storage† Anup Kumar,‡ Megha Chhatwal,‡ Domenico A. Cristaldi,§ Satish Kumar Awasthi,‡ Rinkoo D. Gupta,*,∥ and Antonino Gulino*,§ ‡
Chemical Biology Laboratory, Department of Chemistry, University of Delhi, New Delhi-110 007, India Dipartimento di Scienze Chimiche, Università di Catania, and INSTM Udr of Catania, viale A. Doria 6, 95125 Catania, Italy ∥ Faculty of Life Sciences and Biotechnology, South Asian University, New Delhi-110 021, India §
S Supporting Information *
ABSTRACT: A molecular-module comprising of a surface-confined optically rich and redox active homobimetallic chromophore on siloxane-based templates, has been obtained and characterized via X-ray photoelectron spectroscopy, atomic force microscopy, UV−vis measurements, and cyclic voltammetry. The multimetallic system offers high order optical/redox “writeread” for viable chip-engineering. In fact, this system proposes a potential platform to save a high charge/information density of ∼3.6 × 1014 electrons/ cm2 as a function of the applied potential. Moreover, the convenient synthetic pathway, and the manipulation of Ru2+/Ru3+ redox wave into multiple intermediate mixed valence redox states, are the significant advantages with respect to heterobimetallic analogs toward its integration in “multi-memory” systems. Notably, the module exhibits a high charge density storage capacity, is robust against a large no. of redox cycles and high temperatures.
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INTRODUCTION Stimuli responsive hybrid materials are considered promising alternatives to silicon for miniaturized information storage1 and processing2 devices. Consequently, electrically addressable organic−inorganic hybrid architectures3 that produce distinct electro-optical responses can easily be integrated within the silicon-based electronics4 for the genesis of info-chemistry.5 Though, these modern molecular devices have to meet the function-criteria of silicon based-devices, namely, recognizable write/read, low power consumption, and stability under daunting lithographic procedures/applications before the successful commercialization.6−8 Moreover, it is important that these systems would undergo facile integration within the electronic circuits. In this context, bulk architectures such as multilayered and electro-polymerized systems, despite of the difficulties in their fabrication pathways, have been exploited as molecular-memories in recent times.9,10 Recently, we reported on an heterobimetallic Os and Ru polypyridyl-based monolayer that acts as a ternary memory module using low-voltage control over optical properties.11 So, in the perspective to realize a further system which is more advanced in retaining multimolecular memory, we herein introduce a new monolayer of a novel homobinuclear ruthenium(II) complex with a conducting imidazole backbone for memory storage (Scheme 1). Notably, the charge stored via oxidation of the double-Ru(II) can be considered as double-bit information.2 On the other hand, appropriate selection of voltage would allow the manipulation © 2015 American Chemical Society
Scheme 1. Pictorial View of the Memory-Storage Mode of the Homo-Binuclear Ruthenium(II) Complex (2) on ITO as a Function of the Applied Potential
of the module optical changes into multiple steps for multibit information storage for more efficient ternary/quaternary/ quinary numeral systems, which would not be achievable with the heterometallic analog.12 Received: December 15, 2014 Revised: February 18, 2015 Published: February 18, 2015 5138
DOI: 10.1021/jp5124629 J. Phys. Chem. C 2015, 119, 5138−5145
Article
The Journal of Physical Chemistry C
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EXPERIMENTAL DETAILS Materials and Methods. Most of the chemicals used for the synthesis, namely, p-tolualdehyde, 2-acetylpyridine, RuCl3· xH2O, and N-bromosuccinimide were purchased from SigmaAldrich and used without any further purification. NaClO4· xH2O and hexamethylenetetraamine were purchased from Alfa Aesar Company. Potassium tert-butoxide and 1,10-phenanthroline were bought from Spectrochem (India) Pvt. Ltd. Methanol, ethanol, and acetonitrile were purchased from Merck, India, and were distilled using the reported method13 before use. The solvents used for monolayer fabrication such as dry n-pentane, toluene, 30% aq. ammonia, and H2O2 (all AR grade) were purchased from S. D. Fine Chemicals (India). Double-distilled water was used. Deuterated solvents were purchased from Aldrich. Single-crystal silicon (100) substrates and indium−tin oxide (ITO)-coated glass were purchased from Beschichtungen (Silz, Germany) and Scientific Technologies (India), respectively. Hydrothermal bombs (25 and 50 mL) were purchased from Prakash Scientific Works (India). 4′-(p-Tolyl)-2,2′:6′,2″-terpyridine,14 4′-(phenyl-p-bromomethyl)-2,2′:6′,2″-terpyridine, 15 4′-(phenylcarbonyl)2,2′:6′,2″-terpyridine,16 1,10-phenanthroline-5,6-dione,17 2-(4([2,2′:6′,2″-terpyridin]-4′-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, and ruthenium-mononuclear precursor,18 Ru(bpy) 2 Cl 2 19 and Ru(pytpy)Cl 3 , 20 were prepared using previously reported methods and characterized by 1H NMR, mass, UV−vis spectroscopy, elemental analysis, and cyclic voltammetry (Scheme S1). UV−vis spectra were recorded at room temperature with a JASCO (model No. V-670) spectrophotometer in the range of 200−800 nm. The functionalized transparent substrates were fixed in a custom-made Teflon holder (1.5 cm × 0.75 cm window), and an identical bare substrate (without monolayer) was used to compensate for the background absorption. Mass spectra were recorded on ESI-MS mass spectrometer (Dept. of Chemistry, University of Delhi, India). A CHI-660D electrochemical workstation having conventional three electrode configuration consisting of a glassy carbon as working electrode, a platinum wire as counter electrode and the aq. Ag/AgCl as reference electrode, was used to carry out electrochemical solution measurements. For film experiments, the ITO-coated glass (3.0 cm × 1.0 cm) was used as working electrode (WE), a Pt wire as counter electrode (CE), and the Ag/AgCl as reference electrode. TBAPF6 (20 mM) in dry acetonitrile was used as supporting electrolyte. The electrolyte solution was degassed for 5 min before cyclic voltammetric measurement. The spectro-electrochemical switching experiment was performed with the Basi-spectro-electrochemical kit with a working porous glassy carbon electrode, Ag/AgCl as reference, and a spring-type Pt wire as counter electrode. All 1H NMR spectra were recorded on Jeol JNMECX 400P spectrometer. All chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hz, relative to tetramethylsilane. All measurements were carried out at ambient temperature unless otherwise stated. Characterization of the Designed Monolayers. Monolayer films were analyzed by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), UV−vis spectroscopy, and cyclic voltammetry. XPS and the latter two techniques were also used to confirm the integrity of molecules on the substrates. Angle resolved X-ray photoelectron spectra (ARXPS) were measured at different photoelectron takeoff angles
(5, 15, 45, and 80°) relative to the surface plane with a PHI 5600 Multi Technique System, which gives good control of the electron takeoff angle (base pressure of the main chamber 2 × 10−10 Torr).21−23 Spectra were excited with monochromatized Al Kα radiation. XPS peak intensities were obtained after Shirley background removal.23 Spectra calibration was achieved by fixing the main C 1s peak at 285.0 eV.24 Experimental uncertainties in binding energies lie within ±0.3 eV. Fitting of the N 1s and C 1s spectra was carried out by fitting the spectral profiles with symmetrical Gaussian peaks after subtraction of the background. This process involves data refinement, based on the method of the least-squares fitting, carried out until there was the highest possible correlation between the experimental spectrum and the theoretical profile. The R-factor (residual or agreement factor), R = [Σ(Fo − Fc)2/Σ(Fo)2]1/2, after minimization of the function Σ(Fo − Fc)2 converged to R values ≤ 0.02.21 Atomic force microscopy (AFM) measurements were performed with a Solver P47 NTD-MDT instrument in semicontact mode (resonance frequency 150 Hz). The noise level before and after each measurement was confined within ±0.01 nm. Synthesis and Characterization. Synthesis of the Ruthenium-Mononuclear Precursor (1). An equimolar mixture of Ru(bpy)2Cl2 (50 mg, 103 μmol) and 2-(4-([2,2′:6′,2″terpyridin]-4′-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (54 mg, 103 μmol) were refluxed for 15 h in absolute ethanol (20 mL). The resulting reaction mixture was filtered and precipitated by addition of an excess of NaClO4 after reducing the solution to 5 mL. The complex was filtered and washed with ample amount of water followed by diethyl ether and purified by column chromatography using an alumina (neutral, activity G-III) column and acetonitrile−toluene (70:30, v/v) as eluent. The second orange fraction was collected, dried in vacuo and recrystallized with the vapordiffusion method using diethyl ether (yield: 55 mg, 52%). Solubility: soluble in DMF, DMSO, CH3CN, slightly soluble in ethanol, methanol, and not soluble in chloroform and DCM, alkanes. 1H NMR (400 MHz, DMSO-d6, 25 °C, TMS): δ (ppm): 9.1 (d, 2H, J = 2.5 Hz), 9.0 (s, 2H), 8.87−8.92 (m, 8H), 8.61 (d, 2H, J = 7.96 Hz), 8.43 (dd, 2H, J = 8.2 Hz, J = 8.4 Hz), 8.09−8.05 (m, 8H), 7.94−7.85 (m, 6H), 7.85−7.74 (m, 2H), 7.51−7.56 (m, 2H), 7.49−6.98 (m, 2H). ESI-MS (m/z (%)): 939 (75) [M-2ClO4]+, 470 (100) [M-2ClO4]2+. UV−vis (CH3CN; λ, nm; ε, M−1 cm−1): 286 (102540), 332 (46266), 456 (18862). Elem. Anal.: (%) Observed, C, 56.2; H, 2.8; N, 13.1; % Calcd, C, 56.90; H, 3.27; N, 13.52. CV (vs Ag/AgCl, 10−3 M, CH3CN): E1/2 = +1.44 V, ΔE = 55 mV, at 300 mV·s−1. Synthesis of Dinuclear Ruthenium Complex (2). To a solution of Ru(pytpy)Cl3 (50 mg, 96 μmol) in ethylene glycol (20 mL), an equimolar amount of the Ru-mononuclear precursor (110 mg, 96 μmol) was added with a few drops of n-ethylmorpholine, and the resulting solution was refluxed for 12 h. The reaction mixture was cooled to room temperature and filtered via G-3 crucible and then a saturated solution of NaClO4 (30 mL) was added to precipitate the compound. The complex was filtered and washed with copious amount of water and diethyl ether. The complex was purified by column chromatography using an alumina (neutral, activity G-III) column and acetonitrile−toluene (80:20, v/v) as eluent. The desired fraction was collected, dried under vacuo and recrystallized by slow vapor-diffusion of diethyl ether (yield = 71 mg, 42%). Solubility: soluble in DMF, DMSO, CH3CN, 5139
DOI: 10.1021/jp5124629 J. Phys. Chem. C 2015, 119, 5138−5145
Article
The Journal of Physical Chemistry C
Figure 1. Monochromatized Al Kα excited XPS at 45° photoelectron takeoff angle for 2 on Si(100) in (a) the Ru 3p, (b) N 1s, and (c) C 1s binding energy regions. The experimental spectral data points (open circles) in (b) are fitted with three gaussians at 399.1 eV (blue line), 400.1 eV (magenta line), and 402.7 eV (orange line). The experimental spectral data points (open circles) in (c) are fitted with five gaussians at 281.1−285.2 eV (blue line for the Ru 3d spin−orbit doublet), 285.0 eV (magenta line), 286.5 eV (orange line), and 288.7 eV (green line). The red line superimposed to the experimental profiles (b, c) refers to the sum of the Gaussian components.
slightly soluble in ethanol, methanol, and no soluble in chloroform and DCM, alkanes. 1H NMR (Figure S1; 400 MHz, DMSO-d6, 25 °C, TMS) δ (ppm): 8.94 (s, 4H), 8.76− 8.57 (m, 8H), 8.52 (d, 4H, J = 9.0 Hz), 8.33 (d, 4H, J = 7.7 Hz), 8.06−7.92 (m, 7H), 7.84−7.81 (td, 3H, J = 7.7 Hz, J = 1.9 Hz), 7.8−7.75 (m, 2H), 7.67 (t, 3H, J = 1.3 Hz, J = 7.10 Hz), 7.59 (s, 3H), 7.44−7.31 (m, 7H), 7.24 (td, 3H, J = 6.1 Hz). ESI-MS (Figure S2; m/z): 337.1979 (100%) [M-4ClO4]4+. UV−vis (Figure S3a; CH3CN; λ, nm; ε, M−1 cm−1): 283 (97353), 464 (28958), 492 (32608). Elem. Anal.: (%) Obs., C, 50.1; H, 3.2; N, 11.3; (%) Calcd, C, 50.78; H, 2.94; N, 12.00. CV (Figure S3b; vs Ag/AgCl, 10−3 M, CH3CN): E11/2 = 1.4165 (Ru2+/3+), ΔE = 107 mV at 300 mV·s−1. Ipa/Ipc = ∼2.06 (Figure S4a,b). Activation of the Substrates.25 The Si(100) and ITOcoated substrates were cleaned by subsequent sonication in nhexane, acetone, and 2-propanol for 15 min, followed by drying under a N2 stream. Then, all the substrates were dried at 120 °C in an oven (2 h) before functionalization with linker. Preparation of the Linker Template and Designed Monolayers. The ITO-coated glass and Si(100) substrates were treated in a glovebox under a nitrogen atmosphere11 with a dry n-pentane solution of 3-iodo-n-propyltrimethoxy-silane (200:1, v/v) for 30 min. Then the substrates were rinsed with dry n-pentane, followed by dichloromethane and isopropanol for 3 min, to remove any physisorbed material. The resulting template layers were dried under a stream of N2, followed by heating in an oven at 120 °C for 15 min. Consequently, the functionalized substrates were loaded into a Teflon-lined autoclave and immersed in a dry acetonitrile/toluene (7:3, v/ v) solution of 2 (∼0.5 mM) and heated at 80 °C for 56 h. Then, the substrates were naturally cooled to room temperature. Finally, they were washed with acetonitrile and sonicated for 3 min with acetonitrile and isopropanol, followed by drying under a nitrogen stream.
The resulting 2-based monolayer showed good mechanical stability, since neither washing with common organic solvents nor mechanical abrasion with a solvent-wetted wipe could remove the molecules from the substrates.28 In addition, the monolayer showed excellent thermal, temporal, and electrochemical stability (vide infra). For instance, the monolayers of 2 were found to be stable for months under dark at ambient conditions as judged by UV−vis spectroscopy. Note that reaction times >56 h did not enhance the optical absorption, which indicated the completion of the monolayer formation. X-ray Photoelectron Spectroscopy. A representative monolayer film of 2 on Si(100) was characterized by XPS analysis. The Ru 3p levels (Figure 1a) lie at 462.5 (3p3/2) and 486.7 (3p1/2) eV and, according to already reported results, are consistent with the presence of the Ru(II) states.29 The fitting of the XPS of 2 on Si(100) in the N 1s binding energy region (Figure 1b) shows a main peak centered at 400.1 eV due to the nitrogen atoms coordinating the ruthenium center.30,31 The component at 399.1 eV can be assigned to the two nitrogens of the imidazole function29 and the remaining peak at 402.7 eV is certainly due to the quaternized nitrogen after the surface grafting of 2.30,31 Worthy of note, there is a close correspondence between the intensity ratio of these three components (12:2:1) and the chemical formula of 2. Figure 1c shows the XPS spectrum of a monolayer of 2 on Si(100) in the C 1s binding energy region. The fitting of this spectrum revealed some interesting features. The main peak at 285.0 eV is due to the aliphatic and aromatic carbon backbone.21−23 The Ru 3d levels overlap the C 1s peak and only the Ru 3d5/2 component is already evident in the experimental spectrum at 281.1 eV. The careful fitting of the spectrum allowed to obtain the other Ru 3d3/2 spin−orbit component at 285.2 eV with a spin−orbit separation of 4.1 eV.32 All these results are in strong agreement with Ru(II) 3d states of Ru polypyridyl complexes.11 Two other Gaussians at 286.5 and 288.7 eV were essential for the fit. The 286.5/288.7 eV intensity ratio is 16:1 and is consistent with the CN/CN + (14:1) groups.33−37 Obviously, at 286.5 eV we also expect ionizations of the carbon of the −CH2I groups of the silane that are somewhat buried under the Ru-complex monolayer but contribute to some extent to the 286.5 eV band thus increasing the ratio from 14:1 to 16:1. The XPS atomic concentration analysis revealed a N/Ru ratio of 7.2 ± 0.5 that is in close agreement with the expected ratio.
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RESULTS AND DISCUSSION Preparation Procedure. The novel metallo-chromophore 2 was especially designed for this optoelectronic study because of its octahedral geometry and free terminal pendant pyridine groups26 would allow the tethering of 2 for the formation of a 1D monolayer. Therefore, in the present study, the covalently immobilized monolayer of the bimetallic metallo-chromophore 2 was fabricated on functionalized ITO and silicon substrates.27 5140
DOI: 10.1021/jp5124629 J. Phys. Chem. C 2015, 119, 5138−5145
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The Journal of Physical Chemistry C
Figure 2. AFM image of a representative monolayer of 2 (left) on Si(100) and of the bare Si(100) (right).
Atomic Force Microscopy. The surface morphology investigated by AFM in semicontact mode revealed relatively smooth film surfaces and homogeneous monolayers (Figure 2). The average height is 1.80 nm. The root-mean-square (rms) roughness, Rrms measured for a 5 μm × 5 μm scan area was found to be ∼0.40 nm. Absorption Spectroscopy. The UV−vis measurements for 2 on ITO show the characteristic singlet metal-to-ligand charge transfer (1MLCT) band at λmax = 502 nm (Figures S5a and 3a), which suffers a slight red-shift (Δλ = 10 nm) w.r.t. solution measurements (λmax = 492 nm) possibly due to quaternization of the terminal pyridyl-group and significant intermolecular interaction.38 The average packing density/ footprint was estimated roughly at ∼50−60 Å2/molecule from the UV−vis data assuming for the MLCT band the same molecular extinction coefficient (ε ≈ 29800 cm−1 M−1) obtained in solution. The obtained packing density corresponds to ∼4.4 × 1014 molecules on the ITO-substrate (0.8 × 3.0 cm). The footprint value is in good agreement with the previously reported packing density for the Ru(II)-polypyridyl parent complex and other pyridyl-based covalently assembled monolayers.39 Additionally, the full width at half-maximum (fwhm) bandwidth calculated for the MLCT band marginally broadened by 12 nm w.r.t. solution measurements, and this could be attributed to a much efficient chromophore-tochromophore charge transfer on the solid surface (Figure 3a).2 Cyclic Voltammetry. The cyclic voltammogram of molecular films of 2 (Figure 3b) shows a reversible redoxcouple assigned to Ru2+/3+ with a typical behavior of surface bound species i.e.. The linear correlation (R2 = 0.98−0.99)
between the peak current Ip and the scan rate suggests that the redox process is not controlled by diffusion of molecules at the solid−liquid interface (Figure S6a).40 The half-wave redox potential (E1/2) was observed at 1.43 V (vs Ag/AgCl, aq. 1.0 M KCl). The Ipa/Ipc ratio is almost independent of the scan rate and signifies high stability and reversibility of both redox states (Figure S6b).41 The peak-to-peak separation for anodic and cathodic waves, ΔE, for 2 on ITO increased linearly (21−68 mV) with the scan rate, thus, indicating a more than twoelectron transfer redox process in lower scan rate which reduces to one at higher scan rate. The above observation should accounts for the iR-drop and a heterogeneous electron transfer kinetics (Figure S5b).42 The ratio of oxidation/reduction charge (Qoxi/Qred), evaluated by integration of the respective redox wave, was estimated as ∼3:2, which probably is due to the oxidation of both ruthenium centers and the redox-active imidazole and reduction of both ruthenium centers in return. Notably, the imidazole moiety in the ligand spine demonstrates a oxidative redox wave (E1/2 = 1.2−1.34 V, ΔE = 51 mV) in the same region with no possible reduction wave (Figure S7).43 The fwhm of the oxidation peak was found to be 180−220 mV, and this is another deviation from the ideal Nernstian electrochemical reactions and further supports the contribution of the imidazole ring in redox process as well as the presence of predominant redox site−site interactions and site heterogeneity.41,42 The surface coverage, ΓS, estimated from the peak current density (Ipc, Ipa), was found to be ∼57−64 Å2/molecule, which is in close agreement with the UV−vis data. These overall data suggest that a high charge/information density of ∼3.6 × 1014 electrons/cm2 can be stored using this module. Note that the reversible redox couple on the molecular film is highly advantageous to write/read information using selected potential biases. For instance, imposing an oxidizing potential of 1.6 V causes the transfer of an electron from the molecules to the substrate thus resulting in the storage of information/ positive charge, whereas application of a reduction potential of 1.0 V would reset the initial state/process. Repetitive write− read cycles have been carried out for multiple cycles (102), and only a minimal spectral intensity variation (
