and Chemo-Electrical Single-Molecule Switches - American Chemical

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Multi-Responsive Photo- and Chemo-Electrical Single-Molecule Switches Nadim Darwish,*,†,‡ Albert C. Aragonès,†,‡ Tamim Darwish,§ Simone Ciampi,*,⊥ and Ismael Díez-Pérez*,†,‡ †

Departament de Química Física, Universitat de Barcelona, Diagonal 645, Barcelona 08028, Spain Institut de Bioenginyeria de Catalunya (IBEC) Baldiri Reixac 15-21, Barcelona 08028 Spain § Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001 Kirrawee DC New South Wales 2232, Australia ⊥ Intelligent Polymer Research Institute, University of Wollongong, Northfields Avenue, Wollongong, New South Wales 2522, Australia ‡

S Supporting Information *

ABSTRACT: Incorporating molecular switches as the active components in nanoscale electrical devices represents a current challenge in molecular electronics. It demands key requirements that need to be simultaneously addressed including fast responses to external stimuli and stable attachment of the molecules to the electrodes while mimicking the operation of conventional electronic components. Here, we report a single-molecule switching device that responds electrically to optical and chemical stimuli. A light pointer or a chemical signal can rapidly and reversibly induce the isomerization of bifunctional spiropyran derivatives in the bulk reservoir and, consequently, switch the electrical conductivity of the single-molecule device between a low and a high level. The spiropyran derivatives employed are chemically functionalized such that they can respond in fast but practical time scales. The unique multistimuli response and the synthetic versatility to control the switching schemes of this single-molecule device suggest spiropyran derivatives as key candidates for molecular circuitry. KEYWORDS: Molecular Electronics, Multi-Responsive Molecular Switches, Photo- and Chemo-Switches Spiropyran, Single-Molecule Conductance, STM Break-Junction

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highly tunable switching capabilities offered by chemical backbones.9,10 Inspired by optically activated dynamic materials, recent directions in the field of molecular electronics are toward using noninvasive external stimuli such as light to modulate the conductivity of molecular junctions.9,11 Photochromic molecules are attractive for such aims due to the ability to reversibly control their structural and electronic properties with light.12 Photochromic compounds are widely used today in numerous applications such as photochromic glasses, chemosensors for metal ions, controlling chemical reactions, molecular motors, drug delivery, traffic regulation in biological

he concoction of chemistry, nanotechnology, and electronics initiated what is known today as the field of molecular electronics, an intense emerging field that offers promise of replacing the current inorganic microsized electronics with nanoscale electronic platforms bearing active molecular components. In addition to their advantages in miniaturization, molecular electronic components provide enormous versatility to tune the electronic properties by the plethora of synthetic tailoring.1−5 Much research effort has been invested on building nanoscale molecular devices that mimic the standard electronic components that resulted in single-molecule wires displaying electrical behaviors such as diodes,6 electrical switches,7 and field-effect transistors.8 Reversible conductivity switching in response to external stimuli currently represents a major aim encouraged by the © XXXX American Chemical Society

Received: September 9, 2014 Revised: November 14, 2014

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dx.doi.org/10.1021/nl5034599 | Nano Lett. XXXX, XXX, XXX−XXX

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Results and Discussion. Molecular Design. For the purpose of building a nanoscale electrical device using spiropyrans as the active component, it is important to design specific spiropyran backbones that can form stable bonds to metal electrodes while fine-tuning their switching capabilities. For this purpose, we have synthesized compounds SP-1, SP-2, and SP-3 containing symmetrical substitutions on both ends of their backbone that have affinity for gold surfaces such that they can stably bridge between two gold electrodes. The compounds contain either −NO2 groups (molecules SP-1 and SP-2) or −CN groups (molecule SP-3), both known to have affinity for gold surfaces,31,32 on both ends of the spiropyran backbone (Figure 2). Detailed experimental procedure and characterization for the synthesis of molecules SP-1, SP-2, and SP-3 can be found in the Supporting Information (section 16). Switching in the Solution Bulk. Compound SP-1 switches from a colorless form in a mesitylene/dichloromethane (10:1 v/v) solvent mixture to the MC-1 merocyanine form (blue solution) upon irradiation with UV light. The photocoloration upon UV irradiation is nearly instantaneous with a low-power UV LED (λ = 365 nm, 0.8−1.2 mW) and the blue solution is reversed back to colorless rapidly when the UV LED is turned off (see the rapid response in Video 1 of the Supporting Information). These visual observations are consistent with the UV−vis absorption spectra collected from the same colorless SP-1 solution that showed an absorption maximum, λmax, at 362 nm and the appearance of a new absorption maximum upon UV irradiation in the visible range at λmax = 610 nm, assigned to the open merocyanine MC-1 state (Figure 3a). The switching between the SP-1 and MC-1 forms is reversible with limited fatigue after continuous cycling (See Supporting Information Figure S4). The observed red shift is a result of the increased conjugation when going from the SP-1 to the MC-1 form. The MC-1 state was also detected in NMR experiments, where the characteristic proton signal of the −NCH3 group in the closed SP-1 (at 2.90 ppm) shifts to higher frequency (3.70 ppm) in the open −N+CH3 form upon an in situ irradiation of UV light inside the NMR tube (see details in section 3 of the Supporting Information). The NMR experiments also allowed the estimation of 13−17% conversion yield of SP-1 → MC-1 (Supporting Information Figure S5). The first order apparent rate constant of ring closure (MC-1 → SP-1 relaxation), after turning the UV LED off, was found to be 1.5 s−1 with a total decay time of 3 s (Supporting Information Figure S3). This MC-1 → SP-1 relaxation time is significantly faster than that of typical spiropyran substituted with one −NO2 group only from the chromene side (ca. 50 min total decay).33 The faster relaxation of MC-1 is attributed to the presence of another −NO2 group on the indoline side of SP-1, which makes the N+ of the open MC-1 state more electrophilic and, consequently, induces a significantly faster recombination of MC-1 back to SP-1. Compound SP-1 also switches rapidly to the open protonated merocyanine state (MC-H-1, yellow solution) upon the addition of equimolar concentration of trifluoroacetic acid (TFA). The MC-H-1 is characterized by an absorption maximum at 420 nm26 and can be fully reversed, as observed in the absorption spectra (Figure 3c) and in the NMR spectra (Figure 3d), to the closed SP-1 state by neutralizing the TFA with triethylamine base (TEA). The base addition first leads instantly to the MC-1 (blue state) through the deprotonation of the phenol proton, leading to the MC-H-1 → MC-1 conversion, which then rapidly undergoes ring closure to the

membranes and dimension/properties control of polymers.13−16 Only a few attempts, however, have demonstrated the feasibility of modulating the conductance of a singlemolecule device with light using porphyrin-fullerene dyads,17 ethene-1,2-diyl derivatives,18 diarylethenes,10,11,19 and the closely related dimethyldihydropyrene photochromic derivatives.9 In this study, we turn our attention to spiropyran,14 a unique class of photochromic molecules owed to the range of stimuli that are able to induce their reversible isomerization, which includes light,20,21 different solvents,22 metal ions,23 gases,24,25 acids and bases,26 force,27 and temperature.28 These unique properties of spiropyrans inspired us to exploit them in developing single-molecule electrical devices whose conductance can be controlled by multiple external stimuli, offering potential applications in nanoscale logic gates. Spiropyrans are colorless, UV-light absorbing, molecules consisting of two heterocyclic parts, an indoline part and a chromene part, linked through a common spiro CO bond (Figure 1).14,29,30 The electronic conjugation between the two

Figure 1. Schematics of the molecular switching. (a) Light-induced isomerization of spiropyran and (b) acid-induced isomerization of spiropyran.

parts, which is broken in the colorless form, can be brought about by a range of external stimuli. Here, we focus on the most common stimuli used to trigger spiropyran: (i) upon irradiation with UV light to form the highly conjugated zwitterionic merocyanine open form (Figure 1a) or (ii) upon solution acidification to form the protonated cationic merocyanine open form (Figure 1b).26 An attractive property is the complete conversion occurring in the latter case, which can be reversed to the closed form by neutralizing the acidic medium.26 In this work, we report the first experimental design and characterization of a dual photo- and chemo-responsive electrical switch in a single-molecule device. We show that suitably functionalized spiropyran derivatives can be reversibly switched between the closed and the open forms in solution, and consequently, the conductance of the molecular junction is reversibly switched between a lower-conducting and a higherconducting form using either light or acid/base stimuli. The operation of the electrical switch is governed by the difference between the broken conjugation in the closed form and the complete conjugation in the open form. The single-molecule electrical characterization is compared to the bulk spectroscopic analysis by NMR and UV−vis spectrophotometry techniques, demonstrating a comparable performance of the molecular switches both in the solution bulk and in the single-molecule device. This study constitutes the first example of a multiresponsive single-molecule electronic device. B


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Figure 2. Molecules investigated in this study along with the observed switching properties under UV light or acid stimuli. Inset photos show the corresponding solutions.

the SP-2 spectra upon the addition of excess TFA (Supporting Information Figure S6). We attribute this unique acid-resistant property of SP-2 to presence of electron-withdrawing nitro (−NO2) groups on both the indoline and the chromene side of the molecule. These two strong electron-withdrawing groups make the oxygen atom at the spirocenter not susceptible to protonation, which is the first step in acid catalyzed ringopening.26 The acid resistant characteristic of SP-2 is in contrast to compound SP-1, whose acid-catalyzed ring opening is facilitated by the presence of the electron-donating −OMe group in close proximity to the oxygen spirocenter enabling its protonation and consequently the ring opening to the protonated MC-H-1 form (Supporting Information Figure S8). Compound SP-3 is inactive in response to both UV and acid stimuli as observed by the absence of visual coloration when the solution is either irradiated with UV light or subjected to TFA (Figure 2) or, likewise, by the absence of absorption peaks in the visible region (Supporting Information Figure S2). The inactive response of compound SP-3 is attributed to the opposite effect of the two (−CN) groups on each side of the spiropyran. The electron-withdrawing −CN group on the chromene side tends to stabilize the open state by making the phenolic oxygen (SP−O−) more nucleophilic. Conversely, the −CN group on the indoline side tends to destabilize the open state by making the (SP−N+) more electrophilic. Thus, the net effect of the two −CN groups lead to a situation similar to that

initial SP-1 state (See Video 2 in the Supporting Information). Unlike the low conversion yield (13−17%) induced by the light stimulus, the conversion yield induced by the acid (SP-1 → MC-H-1) is 100% (Supporting Information Figure S6). This is due to the chemical-trapping nature of the acid stimulus,26 which induces a permanent ring opening as a result of the deactivation of the nucleophilic character of the phenolic oxygen through protonation. The acid-induced opening can be reversed to the closed state by “unlocking” the nucleophilic phenoxide anion through neutralization of the acidic medium with equimolar concentration of triethylamine (TEA). The complete chemical reversibility is evident from the restoration of the UV−vis absorption peak and the characteristic NMR signals of SP-1 after neutralizing the TFA acid (Figure 3c and d). The mechanism of the acid-catalyzed opening is described in more detail in Supporting Information Figure S8. Compound SP-2 switches in a fashion comparable to that observed for SP-1 upon the irradiation of UV light (Supporting Information Figure S2 and S3). However, the absence of the −OMe group on the chromene side in SP-2 makes this compound resistant to the TFA acid-induce opening. Prolonged exposure to a molar excess of TFA did not lead to the appearance of the 420 nm absorption maximum, characteristic of the open MC-H state (Supporting Information Figure S2). This unprecedented feature for a spiropyran molecule is also reinforced by NMR experiments that showed no change in C


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Figure 3. Characterization of the molecular switching in the solution bulk. (a) UV−vis absorption spectra of SP-1 in the absence of UV illumination (black line) and after UV illumination (red line). (b) In situ NMR spectra showing the appearance of MC-1 in the presence of UV light (see Supporting Information for details. (c) UV−vis absorption spectra of SP-1 (black line), SP-1 after the addition of TFA (red line) and after the addition of the neutralizing base TEA (green line). (d) NMR spectra showing the complete conversion of SP-1 to MC-H-1 upon TFA acidification followed by its complete conversion back to SP-1 after the TFA acid is neutralized. Characteristic NMR signals are labeled in panels b and d. The spike in panel a reflects the instance when the UV LED was switched off while recording the spectra to minimize interference with the photodetector of the spectrophotometer.

of unsubstituted spiropyran, which lacks appreciable photochromism.34 Upon UV irradiation, such compounds give rise to a photostationary state with negligible merocyanine concentration.34 When comparing SP-2 and SP-3, −NO2 is a stronger electron withdrawing group than −CN. Thus, the −NO2 on the chromene side of SP-2 makes the phenolic oxygen of the open state less nucleophilic and thus slows the reverse reaction rate to the closed SP-2 form. The −CN group on the chromene side in SP-3, however, is a moderate electron withdrawing group with less withdrawing effect on the phenolate anion, which results in SP-3 favoring the closed state. The bulk characterization of compounds SP 1−3 demonstrates that the switching dynamics of the spiropyran core largely depend on the chemical substituents, and therefore, one can tweak the lifetime of the open MC state to a time scale desirable for the design of an electronic device. For instance,

the total decay time needed for the relaxation of MC-1 to SP-1 is only 3 s, which is exceedingly convenient for implementing such molecules in nanoscale electrical devices. A slower MC to SP relaxation within minutes, as observed for the typical spiropyran derivatives substituted with a single −NO2 group on the chromene side,33 would result in slow molecular switching responses thus compromising the final device bandwidth. Conversely, extremely short MC to SP relaxations within subnanosecond, known for unsubstituted spiropyran,35 would exceed the current detection bandwidth in the electrical device making the molecular switching too fast to be captured before it reverts back to the SP state. From the present bulk characterization of SP 1−3, it is concluded that SP-1 has all required ingredients to operate as the active molecular component in a nanoscale multiresponsive electrical switch: it can (1) respond in a fast but practical time D


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Figure 4. Single-molecule transport characterization of the SP-1 compound. Representative individual current traces, 2D color maps and the corresponding 1D histograms obtained from hundreds of current traces are shown, respectively, for (a, b, c) SP-1 in the presence of UV light; (d, e, f) SP-1 in the absence of any stimuli; and (g, h, i) SP-1 in the presence of TFA acid.

from the gold electrode surface covered by the solution of the SP-1 molecule. Conductance histograms were then built by the accumulation of hundreds of individual current traces and the peak maximum represents the most probable conductance value. Same individual current traces were also employed to build 2D conductance color plots that better show the dispersion of the conductance values as well as their evolution while pulling the molecular junction. Histograms were built before and after (Figure 4) constant irradiation of UV light through a UV LED fitted on top of the STM cell (Supporting Information Figure S1). Under dark conditions (Figure 4d−f, UV LED OFF), the single molecule conductance of the closed SP-1 state showed two clear conductance levels at 0.023 Go and, less frequently, at 0.051 Go, both related to the SP-1 closed form. The presence of multiple peaks is commonly observed in STMBJ experiments and is generally ascribed to multiple molecular junctions or multiple contact geometries at the molecule/electrode interface.36,38 Through DFT calculations, it has been shown that binding geometry of the −NO2 group to the gold electrode can involve one or both oxygen atoms which reinforces the possibility of multiple contact geometries being responsible for the multiple conductance peaks observed for −NO2 contacts.31 When the UV LED is turned-on in the STM cell an additional high conductance state at 0.15 Go is now obtained (Figure 4 a,b,c). This high conductance level is assigned to the open merocyanine MC-1 state whose high conductivity agrees

scale, (2) reversibly respond to multiple stimuli, that is, light and acid/base, and (3) it possesses chemical anchoring groups with a 2-fold aim: (i) possessing mechanical attachment to the electrodes and (ii) enhancing both the switching behavior and the electrical coupling to the device electrodes. These unique properties of SP-1 are owed to the combined presence of −NO2 on the chromene side and an −OMe group on the indoline side, which serve as a chemical “knob” to control the switching kinetics and the response to multiple stimuli. Switching in the Nanoscale Device. The utility of photochromic molecules as active components in electronic circuitry requires testing their performance when incorporated into a device. We used the scanning tunneling microscopy break junction (STMBJ)36 method coupled to a UV LED to electrically characterize single-molecule spiropyran wires in both the SP (closed, colorless solution), MC (open, blue solution), and the MC-H (protonated open, yellow solution) forms. Details of the STMBJ method are published elsewhere.36,37 Figure 4 summarizes the electrical properties of the single-molecule switching operation for molecule SP-1 under both UV and acid stimuli. Details of the experimental procedures can be found in the Experimental Section and in the Supporting Information. Briefly, a dilute solution of SP-1 dissolved in mesityelene/dichloromethane (10:1, v/v) is added to the STM Teflon cell and the molecular conductance is measured by repeatedly forming and breaking the molecular junctions by forcing an STM gold tip in and out of contact to/ E


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with the extended conjugation and the complete delocalization of the frontier orbitals along the entire MC-1 molecule, as evidenced by DFT calculations (Supporting Information Figure S10). This remarkable high conductance indicates an efficient electrical MC-1/electrodes coupling, which allows a high current flow and makes this molecular switch attractive in terms of large bandwidth detection. The high conductance of the merocyanine can be explained by its small band gap (λmax = 610 nm equivalent to a band gap of 2.04 eV), the extended conjugation along the entire molecule and the strong electron withdrawing character of the −NO2 groups which brings the LUMO of the molecule close to the gold Fermi energy.31 Conductivity magnitude within the same range has been previously reported for highly conjugated molecules.39,40 From the area under the conductance peaks of the histograms in Figure 4c, we estimated a yield for the SP-1 to MC-1 conversion of 15−35%. This yield is comparable to that observed in the solution bulk (13−17%) based on NMR experiments. The comparable ratio of the open/closed state both in the single-molecule junction and in the bulk experiments signifies that in the single-molecule junction experiments the gold electrodes are catching the molecules in either their open or closed state. The conductance switching was found to be completely reversible and the 0.15 Go conductance peak completely disappeared when the UV LED was turned off (Supporting Information Figure S9). The acid/base switching was also verified in the singlemolecule conductance experiments. Upon the addition of TFA, the conductance of the single-molecule device switches quantitatively (100% yield) to a high conductance value of 0.11 Go (Figure 4 g−i). This high conductance value is assigned to the open and protonated MC-H-1 state and is comparable to that observed for the MC-1 state (0.15 Go). Importantly, the complete disappearance of both the 0.023 and 0.051 Go conductance peaks reinforces the multipeak assignment of the closed SP-1 form. The single-molecule chemo-electrical switching was completely reversible by the addition of TEA base (Supporting Information Figure S11). Control conductance histograms in the presence of the TFA acid and the TEA base but in the absence of SP-1 showed no conductance peaks confirming that the conductance peak observed comes exclusively from the open and protonated MC-H-1 form (Supporting Information Figure S13). Single-molecule conductance experiments performed on molecules SP-2 and SP-3 further demonstrated that the bulk operability of the spiropyran derivatives are perfectly translated when going to a molecular scale device (Figure 5). Hence, SP-2 showed electrical switching under the UV light stimulus but not with the acid stimulus and SP-3 remained unaltered with both stimuli (Supporting Information Figure S13 and S14). Importantly, the similarity of the electrical switching performance in SP-1 and SP-2 under UV stimulus indicates that the electron donating character of the −OMe group is only involved in controlling the opening and closing of the spiro backbone, but it is not playing a significant role in the singlemolecule transport, as evidenced by the similarity of the conductivity of the MC-1 and MC-2 forms. This property demonstrates that a simple −OMe chemical modification of SP-1 can access the acid-induced electrical switching with limited interference in the conductance output and further highlights the interplay between chemical substituents and the performance of molecular-electrical switches.

Figure 5. Comparison of the single-molecule transport for all compounds. Conductance values obtained for SP-1, SP-2, and SP-3 systems before and after being subjected to the external stimuli.

Conclusions. We have designed new spiropyran derivatives that can respond rapidly and reversibly to different external stimuli and demonstrated their utility as multiresponsive singlemolecule electrical switches in a nanoscale device. The electrical switching is governed by the conductance difference between the broken-conjugation (low-conducting) in the closed form and the complete conjugation (high-conducting) in the open form, responding to both light or acid stimuli. Their optimal conductance switching that tracks dynamics of the bulk material, the control over their switching time scales via classical synthetic chemistry and the multiple stimuli upon which the molecular device electrically switches, place spiropyran derivatives as unique candidates for the design of nanoscale organic circuits. So far the switching process is happening ex situ in solution and the molecules are caught by the gold electrodes either in their open or closed state. Future direction of this work is to build in situ switches such that the switching events occur while the molecule is still attached to both electrodes. Experiments of such characteristics can be achieved in a low temperature UHV system where the timescale of the junctions can exceed the time required for the switching. The fast switching response of SP-1 is promising for such experiments. Experimental Section. The synthesis of molecules SP-1, SP-2, and SP-3 are shown in section 16 of the Supporting Information. Details about the STM-break junction technique are published elsewhere.1,36,41 All STM experiments were carried out with a homemade Teflon STM cell and a PicoSPM I microscope head controlled using a Picoscan 2500 electronics, all from Agilent. Data was acquired using a NI-DAQmx/BNC2110 National Instruments (LabVIEW data acquisition System) and analyzed with own LabVIEW code. When needed, UV light was irradiated with a UV LED of λ = 365 nm of type UVLED365-10E from Roithner Laser Technik (Supporting Information Figure S1). In a typical break-junction experiment, the STM tip is first brought to tunneling distance over a flat clean Au (111) surface area in the presence of a dilute solution of SP molecules. The STM feedback is then turned off and the tip is driven into and out of contact with the substrate at 20 nm/s. During the contact process, molecules can bridge between the tip and the surface through specific interactions with the nitro groups (SP-1 and SP-2) or the cyano groups (SP-3) located at the distal ends of the molecules (see details in the Supporting Information Section 1). NMR experiments in the presence of UV light were carried out with the aid of a 3 m long, single core, 1 mm diameter (inside diameter) optical fiber F


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(Thorlabs Pty, Ltd.) with a flat cleaved end and an SMA connector end was used to direct UV light from LLS-365 nm LED (Ocean Optics Pty, Ltd.) to the inside of the NMR tube while it is inside the NMR probe (see details in the Supporting Information Section 3).

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ASSOCIATED CONTENT

S Supporting Information *

Full experimental procedure including the spectroscopic characerization of the compounds, absorption spectra, and DFT calculations are provided. Videos showing the quick response of SP-1 to the stimuli can be found in the supporting documents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Pawel Wagner for the help in the synthetic part of the project. This research was supported by the MINECO Spanish national project CTQ2012-36090 and the EU Reintegration Grant FP7-PEOPLE-2010-RG-277182. N.D. acknowledges the European Union for a Marie Curie International Incoming Fellowship. I.D.-P. thanks the Ramon y Cajal program (MINECO, RYC-2011-07951) for financial support. S.C. thanks the University of Wollongong for the Vice Chancellor Fellowship and Australian National Fabrication Facility (ANFF) for financial support. A.C.A. thanks the Spanish Ministerio de Educación for a FPU fellowship. T.D. thanks the National Deuteration Facility-ANSTO, partly funded by the National Collaborative Research Infrastructure Strategy, for financial support.

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REFERENCES

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