Chemical Science This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online EDGE ARTICLE View Journal | View Issue Exploiting the versatile alkyne-based chemistry for expanding the applications of a stable Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. Cite this: Chem. Sci., 2020, 11, 516 All publication charges for this article triphenylmethyl organic radical on surfaces† have been paid for by the Royal Society of Chemistry J. Alejandro de Sousa, ‡ae Francesc Bejarano,‡a Diego Gutiérrez,a Yann R. Leroux,b Ewa Malgorzata Nowik-Boltyk, c Tobias Junghoefer, c Erika Giangrisostomi,d Ruslan Ovsyannikov,d Maria Benedetta Casu, *c Jaume Veciana, a Marta Mas- Torrent, a Bruno Fabre, *b Concepció Rovira *a and Núria Crivillers *a The incorporation of terminal alkynes into the chemical structure of persistent organic perchlorotriphenylmethyl (PTM) radicals provides new chemical tools to expand their potential applications. In this work, this is demonstrated by the chemical functionalization of two types of substrates, hydrogenated SiO2-free silicon (Si–H) and gold, and, by exploiting the click chemistry, scarcely used with organic radicals, to synthesise multifunctional systems. On one hand, the one-step functionalization of Si–H allows a light-triggered capacitance switch to be successfully achieved under Received 6th September 2019 Accepted 19th November 2019 electrochemical conditions. On the other hand, the click reaction between the alkyne-terminated PTM radical and a ferrocene azide derivative, used here as a model azide system, leads to a multistate DOI: 10.1039/c9sc04499j electrochemical switch. The successful post-surface modification makes the self-assembled monolayers rsc.li/chemical-science reported here an appealing platform to synthesise multifunctional systems grafted on surfaces. context, among the extensive library of functional molecules Introduction investigated, optically-, redox- and magnetically-active stable The impact of (bio)molecular surface modication in different organic radicals are very attractive.9 Nowadays, these materials elds of research is supported by the wide variety of applications have attracted much interest and their exploitation as compo- that derive from it. The goal of the functionalization can be either nents of molecular functional materials is boosting a new transferring specic molecular properties to the substrate, i.e. by generation of devices for applications in OLEDs,10,11 energy graing (multi)functional molecular systems, or generating novel storage and conversion,12,13 molecular spintronics,14 imaging,15,16 interfaces, for example to promote cell-adhesion, (bio)sensing or sensors17 and memory devices.18–20 It has been shown that for the to inhibit undesired reactions, among others.1–7 The molecular device implementation, in many cases their immobilization onto modication of surfaces has also been a determining factor in the surfaces is needed.21 emerging eld of molecular (spin)electronic devices.8 In this To this end, the formation of stable self-assembled mono- layers (SAMs) on substrates typically employed for the fabrica- a tion of electronic components, such as silicon or gold, is a well- Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Networking Research Center on Bioengineering Biomaterials and Nanomedicine (CIBER-BBN), Campus de established strategy.22,23 The hybrid system robustness, and la UAB, 08193 Bellaterra, Spain. E-mail: ncrivillers@icmab.es hence its potential applicability, is equally dependent on the b Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes)-UMR 6226, F- stability of the molecular entity as well as on the stability of the 35000 Rennes, France molecule–substrate interface. For this, beyond the search for c Institute of Physical and Theoretical Chemistry, University of Tübingen, 72076 transferring a specic molecular functionality to the surface Tübingen, Germany d (mainly magnetically-, redox- or optically-active), great efforts Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Albert-Einstein-Str 15, have been focused on: (i) the search for new robust anchoring 12489 Berlin, Germany e Laboratorio de Electroquı́mica, Departamento de Quı́mica, Facultad de Ciencias, groups and (ii) the post-graing modication of SAMs. Universidad de los Andes, 5101 Mérida, Venezuela Terminal alkynes have appeared as an appealing alternative to † Electronic supplementary information (ESI) available: Synthetic details and the thiol group as they spontaneously and covalently react with characterization of the prepared compounds, details of the preparation of SAMs gold.24,25 Besides the stability of the C–Au bond, the high on Si–H and on Au, additional electrochemical characterization of SAMs, survey directionality of the s-bonded C–Au has been demonstrated to and XPS spectra of SAM-3-Rad and SAM-2-Rad together with their provide higher conductance at the single-molecule level,26 stoichiometric and experimental elemental ratios and instrumentation. See DOI: 10.1039/c9sc04499j which makes terminal alkynes very attractive for molecular ‡ These authors contributed equally. electronics. Interestingly, these functional groups can also be 516 | Chem. Sci., 2020, 11, 516–524 This journal is © The Royal Society of Chemistry 2020 View Article Online Edge Article Chemical Science used to covalently react with hydrogen-terminated silicon (Si–H) photochemically switchable systems.29,36,37 Then, silicon itself surfaces.27–29 Concerning post-graing modication, preparing can be used as a light-controlled gate to turn ON/OFF the monolayers with exposed functional end groups which can electronic communication with the graed electroactive react with other (bio)molecular systems is of great importance centers, in this case the PTM radicals. With this aim, we exploit This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. to expand the applicability of these surfaces. Among the several herein the reactivity of the alkyne terminated PTM (1-Rad) to chemical strategies reported to perform interfacial reactions on chemically modify SiO2-free p-type Si–H substrates, forming SAMs,30 the so-called “click chemistry” has been one of the most robust Si–C]C bonds. To date, the functionalization of Si–H employed, in particular the 1,3-dipolar cycloaddition of azides surfaces has been achieved with numerous electroactive mole- and alkynes to form 1,2,3 triazoles.31–34 Hence, alkynes can be cules, among them are ferrocene, quinones, tetrathiafulvalenes considered as very versatile groups to be employed for the (TTF) and metallic complexes.29 However, to the best of our Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. modication and post-modication of substrates. knowledge, before this work, only one example of an organic The functionalization of SiO2-free silicon and gold substrates radical graed on Si–H has been reported. In that case, the with electrical and/or light triggered molecules has been graing was done through a two-step approach, to avoid the pursued with the aim of obtaining switchable surfaces. In the reactivity of the unpaired electron with the silicon surface case of silicon, unlike metals, its electronic properties can be bonds, thus leading to a loss of the radical character.38 Herein, nely tuned by modifying the density and the nature of the the Si–H functionalization was carried out in a single step by the charge carriers (electrons and holes) under light illumination, hydrosilylation reaction of molecule 1-Rad through the terminal which can be used as a second gate for the tuning of the acetylene group leading to the covalently bound PTM- properties of the modied surface.29 terminated monolayer (SAM-1-Rad-Si) (Fig. 2a). In this work, by exploiting the rich chemistry offered by Although the hydrosilylation reaction between alkynes and alkynes and the high chemical and thermal stability of per- Si–H can be performed by either thermal39,40 or photochemical41 chlorotriphenylmethyl (PTM) radicals,35 the redox properties of activation, the instability in solution under light of the radical a PTM radical bearing one and two terminal alkyne groups PTM moiety excluded the possibility of using in this work such (Fig. 1) have been dually exploited as: (i) a capacitance switch on a photochemical route. Several graing attempts were carried Si–H triggered by light and, (ii) as an organic radical-based out by varying the concentration of the PTM radical solution, platform to be further modied by click chemistry giving rise solvent, temperature and immersion time (see the experimental to a multistate electrochemical switch. In both cases, a partic- details in the ESI and Table S1†). PTM radical monolayers with ular focus has been placed on the optimization of the experi- the highest surface coverage of PTM (estimated from the elec- mental conditions to ensure the integrity of the radical and the trochemical measurements, vide infra) and the lowest oxidation terminal alkyne. level of underlying silicon were obtained at 145  C for 20 h using 1,2-dichlorobenzene (DCB) as a high-boiling solvent and a ca. 7– 10 mM concentration of 1-Rad. Results and discussion The SAM-1-Rad-Si monolayer was characterized by X-ray Functionalization of Si–H surfaces with PTM radicals photoelectron spectroscopy, which, additionally to the chem- ical composition, provides information about the oxidation Interfacing technologically important semiconducting surfaces, state of the underlying silicon (Fig. S1†). For the C 1s, two main such as silicon, with high-quality, and stable redox-active lms peaks at, 285.0 and 286.6 eV, assigned to the C]C and the C–Cl has appeared as a promising strategy toward functional devices bonds, respectively, were observed. The Cl 2p spectrum for charge storage. Remarkably, by taking advantage of the fact that the charge transfer characteristics of silicon can be either inhibited or activated upon light illumination, this substrate constitutes a relevant platform for the development of Fig. 2 (a) Sketch of SAM-1-Rad-Si. (b) Cyclic voltammograms under red light illumination of SAM-1-Rad-Si at different scan rates (0.1, 0.2, 0.4, 0.6, 1, 2, 4, 6 and 10 V s1, from brown to purple) in CH3CN + 0.1 M Bu4NClO4. The inset shows comparative CVs at 0.4 V s1 in the dark Fig. 1 Chemical structure of the PTM radicals employed in this work. (dashed line) and under illumination (solid line). This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 516–524 | 517 View Article Online Chemical Science Edge Article displayed a typical doublet with both components at 200.8 of the curve enable E and the dopant density ND to be deter- (2p3/2) and 202.5 (2p1/2) eV, arising from the chlorinated phenyl mined, respectively (Fig. S3†). The calculated ND value (1.2  rings of the PTM unit. Besides, the Si 2p spectrum was decon- 1015 boron atoms cm3) was consistent with the dopant density voluted into two contributions at 99.2 and 99.8 eV assigned to derived from the four-probe resistivity measurements of silicon This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. bulk and interfacial silicon, respectively, in crystalline samples, between 5 and 10 U cm. The extracted value of E was Si(111).42,43 Additionally, a low intense peak appeared at 103 eV, 0.25  0.02 V. Based on this parameter, it can be concluded that which is attributed to the unavoidable oxidation of a certain the PTM(radical) 4 PTM(anion) redox process occurs in content of remaining Si–H sites by water and atmospheric a potential range wherein the semiconductor is in weak deple- oxygen, indicating that, as expected, the surface is not fully tion. The small potential difference between E0 and E (around passivated by the bulky radical 1-Rad. Nevertheless, the low 50 mV) explains, however, why low (but not zero) oxidation Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. contribution of this peak was positively surprising considering currents are observed at SAM-1-Rad-Si in the dark (inset in the bulkiness of the PTM moiety. Fig. 2b). Much higher currents were observed under illumina- SAM-1-Rad-Si was electrochemically characterized in the tion because the redox process could now occur with high rate dark and under illumination through a red lter (l > 600 nm) to thanks to the high concentration of photogenerated minority avoid the possible degradation of the graed PTM radical. As charge carriers (i.e. electrons). can be seen in Fig. 2b, the cyclic voltammograms (CVs) in the In the dark, consistent with the current response, the dark showed negligible oxidation and reduction currents (lower measured capacitance values were small and did not exceed 1 than 10 mA cm2 at 10 V s1), as expected for a semiconductor mF cm2. In contrast, the capacitance curve under red light under depletion conditions,44 i.e., when only a few majority illumination was characterized by a much more intense charge carriers (holes in the case of p-type Si) are available for capacitance peak (enhancement by a factor of 10) at 0.18 V, charge transfer (vide infra). close to the formal potential of bound PTM determined by CV Upon illumination of SAM-1-Rad-Si, an intense reversible (Fig. 3a). This capacitance peak was clearly attributed to the redox wave was observed at E0 ¼ 0.21 V vs. Ag/AgCl, KCl 3 M charging/discharging currents associated with the oxidation/ (average of the anodic Epa and cathodic Epc peak potentials) reduction of the bound PTM centers,48,49 in perfect line with corresponding to the PTM(radical) 4 PTM(anion) process previous reports on ferrocene-modied Si surfaces.36,50 promoted by captured photogenerated electrons (Fig. 2b). The This signicant contrast between the two states (ON and redox response observed indicates that the radical character of OFF) permitted us to exploit this system as a capacitance the PTM remains unaltered upon graing. Additionally, both switch and its cyclability was investigated by carrying out the anodic and cathodic peak photocurrents Ipa and Ipc, corre- consecutive ON/OFF switching cycles by turning on and off the sponding to the reversible redox couple, were found to be light along time (Fig. 3b). A 30% decrease in the maximum proportional to the potential scan rate v, as expected for a surface-conned reversible redox species (Fig. S2a–c†).45 The variation of Epa and Epc with v (Fig. S2b†) enabled us to deter- mine the apparent rate constant for electron transfer at the bound PTM center, ket,ap, using the recent theoretical model developed by Vogel et al. accounting for semiconductor diode effects.46 A value of 90  20 s1 was estimated, in accordance with the literature data reported for other silicon electrodes modied with electrochemically reversible systems.29,46 The surface coverage of attached PTM moieties was electro- chemically estimated from CVs of illuminated SAM-1-Rad-Si (eqn S1†). Indeed, anodic charge integration at several scan rates (between 0.4 and 1 V s1) resulted in an average value of (8.5  0.3)  1011 mol cm2, very close to the analogous SAMs on Au (see Section 2.2.2). To obtain further insights into the light dependence of the redox process in SAM-1-Rad-Si, differential capacitance measurements were performed in the same electrolytic medium (i.e. CH3CN/Bu4NClO4). First, the atband potential (E), i.e. the electrode potential for which there is no space-charge region in the semiconductor, was estimated from the commonly used Mott–Schottky plot (Csc2 vs. E, eqn S(2)†) that gives the space- charge capacitance Csc as a function of the electrode potential E Fig. 3 (a) Capacitance–potential curves of SAM-1-Rad-Si measured at under depletion conditions, i.e., the depletion of valence band 50 Hz in the dark and under red light illumination. (b) Capacitance– holes in the space charge region of the p-type surface.47 In the time profile at 50 Hz measured at 0.18 V during dark (OFF state, 30 s)/ dark, a linear Csc2–E plot was obtained for potentials below illumination (ON state, 30 s) switching cycles. Electrolytic solution: 0.25 V vs. Ag/AgCl, KCl 3 M, in which the intercept and the slope CH3CN + 0.1 M Bu4NClO4. 518 | Chem. Sci., 2020, 11, 516–524 This journal is © The Royal Society of Chemistry 2020 View Article Online Edge Article Chemical Science photocapacitance was observed along the rst 30 minutes For the synthesis of the target radical PTM–Fc dyad (3-Rad) before reaching a situation of higher stability. This loss in the a copper-catalyzed azide–alkyne cycloaddition (CuAAC) click switching ability is believed to be caused by the gradual chemistry strategy was envisaged.53–55 The standard conditions degradation of the electrical properties of the interface due to for the click reaction that generates in situ the copper(I) catalyst This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the oxidation of the underlying silicon. Indeed, owing to the by reduction of CuSO4 using ascorbic acid or ascorbate56 could moderately dense packing of the PTM monolayer (due to the not be used here because the ascorbic acid reduces the radical bulky nature of the PTM head groups), traces of water and/or to the anion that in turn is irreversibly protonated to the aH oxidizing species present in the electrolytic medium form.57 Therefore, as mentioned above, before performing the unavoidably penetrate through the molecular layer via defects reaction on the surface, the synthetic procedure was optimized or pinholes to react with remaining Si–H sites. Despite this, in solution. The successful synthesis of the target molecule 3- Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. the functionalization was found to be remarkably stable. Rad with the highest reaction yield was obtained by using copper(I) iodide as a catalyst, and N-ethyldiisopropylamine Functionalization of PTM radical molecules via click (DIPEA) and triphenylphosphine as ligands (see ESI† for further chemistry, in solution and on the gold surface details on the synthesis). As shown in Scheme 1, two routes were followed. First, the CuAAC reaction conditions starting with the In solution. The click chemistry is used in different elds of non-radical precursor, aH-PTM molecules (2-H), were opti- research, such as biomedical science, chemistry and materials mized. Both mono- and di-cycloaddition compounds, 3-H and science.51,52 Remarkably, the functionalization of stable organic 4-H, were obtained as products of the click reaction between the free radicals via this reaction has been only marginally explored. azidomethyl ferrocene and the PTM derivative with two This is basically attributed to the low stability of the radical acetylene-terminated groups (2-H) even when a 1 : 1 molar ratio character under the most standard reductive “click” experi- of the azide and 2-H was used. Next, the corresponding radicals mental conditions. In this work, we report for the rst time the 3-Rad and 4-Rad were prepared through treatment of the aH experimental conditions to achieve a click reaction between an adducts, 3-H and 4-H, with tetra-n-butylammonium hydroxide alkyne-terminated PTM radical and an azide derivative (Scheme (TBAOH), to generate the corresponding anions, and then the 1). The experimental conditions were rst optimized in solution oxidation of these anions to the PTM radicals with p-chloranil. and then were used to engineer a radical-PTM-based SAM which The second route consisted of directly coupling the open-shell acted as a platform to elaborate more complex multifunctional compound, 2-Rad, with the azidomethyl ferrocene to obtain surfaces. In this work, the focus was placed on the reaction also the two cycloaddition compounds 3-Rad and 4-Rad with between the organic radical (2-Rad, Fig. 1), previously synthe- better yield than in the case of the non-radical derivative. All the sized,26 and an azidomethyl ferrocene (Fc–N3) derivative leading compounds were fully characterized by spectroscopic tech- to a donor–acceptor (D–A) SAM. niques (NMR, FT-IR, and UV/vis), cyclic voltammetry and elec- tron paramagnetic resonance for the radicals (see ESI†). On-surface PTM radical functionalization. PTM SAMs were prepared using a freshly template-stripped Au (AuTS) surface as the substrate.58 Its ultraat topography makes it an ideal bottom electrode for the charge transport measurements, as well as for promoting the formation of higher quality monolayers.59 Compounds 2-Rad, 3-Rad, 2-H and 3-H were used to generate SAM-2-Rad, SAM-3-Rad, SAM-2-H and SAM-3-H, respectively. These SAMs were prepared following a similar previously re- ported methodology,26 working under inert conditions to avoid the oxidation of the alkyne (see ESI† for further details on the SAM preparation). All the SAMs were characterized by different electrochemical techniques: CV, square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS). This allowed us to obtain information about the faradaic transfer process of the anchored molecules, the surface reaction yield and the capacitive behavior at different potentials. SAM-5-Rad was obtained by the on-surface post-modication of SAM-2-Rad. The determination of the Fc/PTM ratio for the SAM-5-Rad by SWV and its control were essential to (i) estimate the yield of the on-surface click reaction, (ii) improve the faradaic-background current ratio, and (iii) decrease both capacitive currents and the double layer impact. It is important to mention that there is an intrinsic error in the estimation of Scheme 1 Synthetic route to the preparation of the target radical the Fc/PTM ratio due to the different electron transfer rates of PTM–Fc dyad 3-Rad. the electrochemical processes of both redox components but This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 516–524 | 519 View Article Online Chemical Science Edge Article the values were validated by comparison with those of SAM-3- Interestingly, the PTM radical surface coverage estimated for Rad (Fig. S8†) in which the Fc/PTM ratio should be 1 : 1. the SAM-3-Rad, which was obtained by direct Au functionali- Following the conditions used in solution, an 30% reaction zation with the PTM–Fc dyad 3-Rad, was very similar to that of yield (Fc vs. PTM radical) was obtained (Fig. S4 and S5†). This the SAM-2-Rad (7.9 and 8.9  1011 mol cm2, respectively) This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. value was enhanced to approximately 50% when a toluene–THF (Table S2†). Such a result denotes that the self-organization and mixture was changed for acetonitrile as the solvent (Fig. S6 and the reactivity of the molecules for both SAMs were quite similar. S7†). This gain was attributed to both the better solubility of CuI These values further support that only one of the terminal and stability of Cu(I) in solution. Other parameters such as alkynes of the 2-Rad reacted with the Au surface. temperature and/or reaction time did not lead to a higher Fc To back up these ndings, we investigated SAM-2-Rad and surface coverage. The bulkiness of the PTM moieties, which can SAM-3-Rad by XPS62 which is an effective tool for studying Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. induce certain disorder of the layer, as was observed before by organic radical thin lms.63 It was found that the ratio of the STM for a previously reported thiolated PTM SAM,60 would be integrated signal intensities of the different lines of the XPS responsible for the yield decrease. curves agrees to a good extent with the theoretical percentages Fig. 4 shows the CV response of SAM-3-H, SAM-2-Rad, SAM-3- indicating that SAM-2-Rad and SAM-3-Rad were effectively ob- Rad and SAM-5-Rad at different scan rates. The stability of the tained. The carbon concentration slightly exceeds the theoret- different monolayers was demonstrated by performing consec- ical values (see Tables S3 and S4†). This is the usual case in utive voltammetric scans (Fig S9† to S14). Further, an Ipa/Ipc samples prepared ex situ with wet-environment techniques. ratio very close to 1 and a Ipa, Ipc vs. scan rate linear relationship Molecular orbitals are highly directional and, consequently, were determined, evidencing both the reversibility of the redox they are perturbed if a different/new chemical interaction process and the connement of the molecules on the surface occurs. In this case, no deviation from the expected shapes was (Fig. S15 and S16†). The reversible process observed at around observed from the XPS main lines. Furthermore, a best t 0.23 V (vs. Ag/AgCl, KCl 3 M) corresponds to the PTM(radical) procedure applied to the XPS spectra allows identifying the 4 PTM(anion) redox process while the redox signature of the contributions from atomic sites having slightly different Fc+/Fc couple is clearly visible at +0.60 V (vs. Ag/AgCl, KCl 3 M). binding energies due to variations in the chemical environ- It is worth noting that the redox potential of the PTM radical ment.62 Several constraints based on electronegativity and bond bound to gold was ca. 400 mV lower than that observed for SAM- strength were applied64,65(see ESI† for details). The comparison 1-Rad-Si under illumination. This trend is not surprising and is of the tted results with the theoretical stoichiometry of the usually observed for electroactive molecule-modied photo- carbon atoms supports the successful preparation of SAM-3-Rad electrodes.29 In the case of silicon, the reduction of the PTM (Fig. S18† and Table S3†). In fact, any perturbation of the radical is easier owing to the photogenerated electron-induced chemical environment of the carbon atoms that would hint at activation of the redox process.61 As expected, for SAM-3-H different chemical congurations would deliver a different best and SAM-2-Rad, only a single redox process attributed to either t curve. Fc or the PTM radical, respectively, was observed. In the case of As depicted in the CVs shown in Fig. 4c and d, due to the SAM-3-Rad and SAM-5-Rad, two redox processes were present presence of two electroactive moieties, an enhanced number of owing to the double functionality of the graed monolayer. stable redox states can be realized, which is of interest for molecular memory devices. In particular, the PTM–Fc dyad displays two redox processes and hence, three distinct redox states, [PTManion–Fc], PTMradical–Fc and [PTMradical–Fc]+, in the 0.6 V to 0.8 V potential range. Such an electrochemical response prompted us to characterize SAM-3-Rad by EIS, using the applied potential as a perturbation signal and the capacitive real component as an output, characteristic of the interface at each redox state. This methodology was employed before in Fc- based and tetrathiafulvalene (TTF)-based SAMs by some of us.66,67 It is important to emphasize that the three redox states are well differentiated in a narrow and stable potential window which makes the system very appealing.68 The measurements were acquired in the frequency range from 100 kHz to 0.1 Hz with a 20 mV amplitude and the applied potentials were 0.5, 0.3 and 0.7 V. These values were chosen from the CV results to eliminate the contribution of the faradaic processes and thus to only consider the capacitive behaviour of the double layer. The Fig. 4 CV curves of SAMs on gold: (a) SAM-3-H, (b) SAM-2-Rad, (c) Nyquist plots showed a common shape for a SAM-based inter- SAM-3-Rad and (d) SAM-5-Rad in CH2Cl2 + 0.2 M Bu4NPF6 at different face with one semicircle without the diffusional process scan rates (0.05, 0.1, 0.25, 0.5, 1, 2, 3 and 4 V s1). The color code used for SAMs is the same as that used in Scheme 1. (Blue) PTM aH; (Fig. S17†). The difference between the [PTManion–Fc], (magenta) PTM radical; (yellow) triazole ring; (green) Fc and (violet) PTMradical–Fc and [PTMradical–Fc]+ species was evidenced in terminal alkyne moieties. the Cole–Cole plot, where the imaginary capacitance (Cim) is 520 | Chem. Sci., 2020, 11, 516–524 This journal is © The Royal Society of Chemistry 2020 View Article Online Edge Article Chemical Science plotted vs. the real capacitance(Cre)48 (Fig. 5a). The obtained Cre values (indicated in the plot) were: Cre1 (PTManion–Fc) ¼ 6.5 mF cm2, Cre2 (PTMradical–Fc) ¼ 8.0 mF cm2 and Cre3 (PTMrad- ical–Fc+) ¼ 12.8 mF cm2. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Clearly, the higher Cre value for the positively charged SAM (Cre3) is attributed to a well-dened charged double layer at the interface. The charged ferrocenium species are exposed to the top solid–electrolyte interface favoring the formation of a tight ion pair with the electrolyte anion (Fig. 5b, Cre3). In the case of the PTManion–Fc SAM, the negative charge of the PTM anion is Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. located in the central carbon atom, which is shielded by the bulky chlorinated triphenyl rings. So, it forms a loose ion pair with the electrolyte cation.69 Therefore, this ill-dened double layer generated at the PTManion–Fc SAM/electrolyte interface is Fig. 6Histograms and Gaussian fits of log|J| at 1 V for junctions AuTS/ expected to be very similar to that of the neutral PTM–Fc SAM SAM-2,3 and 5-Rad//GaOx/EGaIn. Inset: Plots of mean log|J| versus E(V). (Cre2). Charge transport measurements across PTM monolayers brought into contact with the sample surfaces was around bound to gold 1000–2500 mm2 in order to obtain stable measurements (see Charge transport measurements across the AuTS/SAM-N-rad/ ESI† for details on the experimental setup).75 liquid metal were performed in order to evaluate the SAMs as Fig. 6 shows that the three monolayers display a different molecular wires, and to examine the inuence of the D–A dyad. molecular wire behaviour, in terms of the measured current Moreover, the electrical output of the junction could be used to through the junction, showing a current density (J) (SAM-2-Rad) evaluate the second moiety introduced through the click > J (SAM-5-Rad) > J (SAM-3-Rad) (see ESI† for details on the chemistry on the PTM radicals (Fig. 6). The eutectic gallium statistical analysis). The differences between the three layers indium alloy (EGaIn) was chosen as the top electrode,70,71 since were mainly attributed to the different tunneling distances it has been successfully used previously to investigate different between the two electrodes arising from the thickness of the PTM-based SAMs and lms.72–74 The spontaneously formed molecular layer. Taking into account that an approximately 50% oxide skin (mainly Ga2O3) allows the electrode to be shaped as of Fc incorporation was achieved to form SAM-5-Rad through a cone. In this work, the area of the GaOx/EGaIn tip that was the click reaction on SAM-2-Rad, the electrical response measured for this SAM was considered to be an average contribution of SAM-2-Rad and SAM-3-Rad. Current rectica- tion was previously reported for SAMs incorporating Fc as the redox-active moiety.73,76 Here, in the 1 V range, we did not observe such a behaviour (inset in Fig. 6). This could be attributed to a poor molecular order within the layer and hence a poor directionality of the Fc moiety towards the top electrode, compared to well-ordered Fc-thiolated SAMs presenting a well- dened interface with the liquid-metal electrode.76 XPS and Near Edge X-ray Absorption Fine Structure (NEX- AFS) measurements helped to conrm this interpretation. NEXAFS measurements of SAM-5-Rad show a clear Fe signal (Fig. 7d), however, the survey XPS and the Fe 2p core level spectra at 800 eV photon energy (Fi. S18 in ESI†) are charac- terized by a signal that is proportional to a lower concentration of iron atoms in comparison with the theoretical stoichiometry. These ndings can be taken as an indication that SAM-5-Rad was obtained, however, the monolayer has a mixed nature: also molecules that do not carry the ferrocene unit are present, i.e., SAM-2-Rad, and they contribute to the signal. NEXAFS measurements also allow determining the average molecular arrangement of the molecules in the lms.77 The C–K edge of Fig. 5 (a) Cole–Cole plots recorded for the SAM-3-rad at different SAM-5-Rad shows typical features due to transitions from the C applied potentials to obtain three different redox states. The electro- 1s core levels to the unoccupied states, p* and s* (Fig. 7b). In lytic medium (CH2Cl2 + 0.2 M Bu4NPF6) was used and the frequency analogy with previous NEXAFS measurements of derivatives of was varied from 100 kHz to 0.1 Hz with a 20 mV amplitude. (b) Scheme of the three different interfaces at the specific applied voltage. the PTM radical, two main regions can be identied in the This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 516–524 | 521 View Article Online Chemical Science Edge Article character. The electronic properties of Si have allowed us to fabricate a light-triggered capacitance switch exploiting the redox properties of the graed radicals. In the case of gold, not only the formation of the SAM but also its post-modication This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. with other functional molecules through click chemistry has demonstrated the potential of such a versatile platform for achieving multifunctional layers displaying the fascinating properties of the attached organic radicals. In particular, here a radical donor–acceptor dyad has been on-surface synthesized following optimized conditions that enable the spin to be Open Access Article. Published on 20 November 2019. Downloaded on 3/12/2020 2:50:57 PM. unaltered. For all this, we believe that the ndings reported in this manuscript are a signicant step forward in the imple- mentation of organic radicals in molecular based devices with different properties and applications in elds such as energy storage and conversion, sensing, imaging, memory devices, and spintronics. Fig. 7 SAM-5-Rad (a) Cl–L edge, (b) C–K edge (the arrows indicate the transitions as discussed in the text) and (c) N–K edge in grazing (30 to the substrate, red circles) and normal incidences (blue circles). Conflicts of interest (d) Fe–L edge NEXAFS spectrum at grazing incidence. There are no conicts to declare. spectra in Fig. 7: the p* region up to around 290 eV and the s* region in the photon energy range above 290 eV.73,78,79 We can Acknowledgements assign the feature at around 285.4 eV to contributions due to S. Ababou-Girard and C. Meriadec (Institut de Physique de transitions from the C 1s core levels located in the carbon atoms Rennes, UMR6251, France) are fully acknowledged for XPS of the aromatic rings and of the perchlorinated benzene rings to measurements on the Si–H surfaces. Dr Vega Lloveras and p* orbitals. The feature at 287.2 eV (see Fig. 7b, grazing inci- Amable Bernabé are acknowledged for the EPR and MALDI-ToF dence) is typical of ferrocene C–K NEXAFS spectra and is due to measurements, respectively. We also would like to thank the C 1s to Fe 3d/p* transitions.80–82 We observe a shoulder at Thomas Chassé for allowing us to access the photoelectron 285.9 eV that is also assigned to contributions belonging to the laboratory at the University of Tübingen, Hilmar Adler for ferrocene moiety.80–82 The intensity at around 282.2 eV corre- technical support, and Helmholtz-Zentrum Berlin (HZB) for sponds to transitions from the C 1s core levels to the singly providing beamtime at BESSY II. This work was funded by the unoccupied molecular orbital (SUMO).60 Spanish Ministry project FANCYCTQ2016-80030-R, the Gen- The determination of the average molecular orientation of eralitat de Catalunya (2017SGR918) and the Spanish Ministry of SAM-5-Rad is very challenging, the Cl–L edge signal shows a very Economy and Competitiveness, through the “Severo Ochoa” weak dichroism (Fig. 7a), and the resonances in the C–K edge Programme for Centers of Excellence in R&D (SEV-2015-0496), spectra due to transitions from aromatic carbon atoms are all the CSIC with the i-Link+ 2018 (Ref. LINKA20128) and CIBER- very close in energy (Fig. 7b). Thus, we have focused on the BBN. J. A. de S. is enrolled in the Materials Science PhD single building blocks. We have calculated an average molec- program of UAB. J.A. de S. thanks the Spanish Ministry for the ular orientation of the ferrocene moiety of 22 with respect to FPI fellowship. Financial support from the German Research the substrate using the intensities for the two polarizations at Foundation (DFG) under the contract CA852/11-1 and from 285.9 eV and cross-checking with the intensities at 287.2 eV Helmholtz-Zentrum Berlin is gratefully acknowledged. (21 ).77,83,84 The relative torsion of the component blocks in SAM- 5-Rad together with the presence of SAM-2-Rad might hinder a good interaction with the top contact and, thus, contribute to Notes and references the absence of a current rectication behaviour. 1 K. T. Butler, G. Sai Gautam and P. Canepa, npj Comput. Mater., 2019, 19, DOI: 10.1038/s41524-019-0160-9. Conclusions 2 M. S. Maglione, S. Casalini, S. Georgakopoulos, M. Barbalinardo, V. Parkula, N. Crivillers, C. Rovira, In summary, we have demonstrated that the functionalization P. Greco and M. 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