Thermal and optical control of electronic states in a single layer of switchable paramagnetic molecules

Thermally and optically induced Valence Tautomeric interconversion has been observed for a monolayer of a cobalt–dioxolene complex on gold.


Introduction
Reversible conversion between two electronic states of bistable coordination complexes at the single molecule level promises breakthrough outcomes for information storage and processing technologies. 1 In particular, the control of length and direction of the spin in paramagnetic switchable molecules represents a key feature to be used in quantum computation and molecular spintronic applications. [2][3][4][5] The retention of switchability in molecular-inorganic architectures obtained by the regular assembling of molecules on conductive surfaces is a mandatory step towards their integration in hybrid devices. The possibility of obtaining bistable nano-assemblies as thin lms and nanoparticles comprising Fe(II)-based spin-crossover (SCO) materials, where switchability is due to an externally controllable spin state, has been suggested by several recent reports. [6][7][8][9][10][11][12][13][14][15] These studies, however, highlighted that the direct molecularsubstrate interaction can signicantly modify the thermodynamics of the SCO equilibrium. While an HOPG (highly oriented pyrolytic graphite) supported SCO system has been reported to preserve its bulk phase switchability features, 16 other sub-monolayer deposits of several Fe(II) complexes show the "pinning" of their spin statesand loss of their original SCO behaviourwhen evaporated on metallic surfaces. [8][9][10][11][12][13][14] Promising alternative switchable systems for nanoscale investigation are those exhibiting a thermally or optically induced intramolecular charge transfer between an acceptor and a donor unit. 17 This class of materials includes cyanometallates [18][19][20][21] and metal complexes showing Valence Tautomerism (VT from now on), an interconversion between redox isomers caused by electron transfer between the metal ion and an organic ligand. [22][23][24] In order to investigate the VT behaviour on a metallic surface, we focused our interest on cobalt-dioxolene complexes, 25 which can be considered as class II mixed valence systems, according to Robin and Day classication. 26 These coordination compounds are the simplest and most widespread molecular systems displaying VT and rely on a [CoLdiox] core, L being a tetradentate N-donating ancillary ligand and diox a chelating ligand belonging to the orthoquinone (dioxolene) family. Proper chemical tuning of the reduction potentials of Co 3+/2+ couple allows to prepare structurally related systems with different charge distributions, either ls-Co III Cat (ls ¼ low-spin, Cat ¼ catecholato), hs-Co II SQ (hs ¼ high-spin, SQ ¼ semiquinonato radical), or exhibiting VT. 27 In the latter case, at low temperatures the diamagnetic ls-Co III Cat redox isomer is the ground state, but entropy driven intramolecular electron transfer triggers the reversible formation of the paramagnetic hs-Co II SQ species upon heating the system (Scheme 1). 25 At cryogenic temperatures the same interconversion can be induced by visible 17 and so X-ray irradiation; 28 once the stimulating source is removed, a slow decay to the ls-Co III Cat ground state occurs with a temperature dependent characteristic time. 29 DFT calculations have recently proposed that switching can be induced by the application of an electric eld, suggesting that detection and modulation of the magnetic state of these complexes can be obtained at the single molecule level by using Scanning Tunnel Microscopy-based techniques. 30 Whatever the external stimulus chosen to control the charge distribution, the perspective of integration of these molecules in spintronic devices obviously requires the development of a reliable method to obtain a monolayer of VT complexes which retains their switchable properties. As a rst step in this direction we have recently shown that a sulfur-based functionalisation can be introduced in a [CoLdiox] unit maintaining the switchability of the core, 31 thus opening the way to a wetchemistry approach for the deposition of VTs on noble metal surfaces. This technique 32 has afforded successful examples of bidimensional structuration for different classes of molecular materials, including molecular nanomagnets, 33 redox switches based on organic radicals, 34 and organic photochromic materials. 35 In the case of charge transfer systems, to the best of our knowledge, self-assembling from solution has not yet been carried out.
This work reports on the deposition on a metallic surface of a monolayer of a charge-transfer based molecular switch. Functionalisation of the molecular core with a thiol-bearing moiety allowed the chemisorption through self-assembly from a diluted solution. A multi-technique (X-ray photoelectron spectroscopy, XPS, and time-of-ight secondary ion mass spectrometry, ToF-SIMS) study proved the formation of a single layer of intact molecules on top of polycrystalline gold surface. Moreover, XPS and X-ray absorption spectroscopy (XAS) analysis provided the experimental proof of the retention of both entropy driven and light induced interconversion of molecules graed to the gold surface.
Electronic (UV-Vis) spectroscopy, elemental analysis and mass spectrometry ( Fig. 1 and S1 †) indicate that the complexes share the molecular structure reported in Scheme 1 differing only for the number of methyl groups present in the ancillary ligand. UV-Vis, magnetometry and XPS data (Fig. S1, S2 and S3 †) evidence a room temperature charge distribution of ls-Co III Cat for 1, hs-Co II SQ for 3, and a mixture of the two for 2, as expected on the basis of previous reports on other [CoLdiox] complexes featuring the same ancillary ligands. 27,31,36,37 The temperature dependence of the molar magnetic susceptibility c M T occurring in 2 ( Fig. S2 †) points out a reversible interconversion between the two redox isomers in the solid state, in line with an entropy driven VT process. Using the c M T values of 1 and 3 as references for ls-Co III Cat and hs-Co II SQ phases (eqn S(1) †), it is possible to quantitatively describe the thermal distribution prole of 2. The VT interconversion has a very gradual character: at low temperatures it displays a remaining 40% hs-Co II SQ fraction that reaches 66% at 300 K, occurring in a temperature range broader than the experimentally accessible one in our setup. Broad thermally induced electronic transitions in switchable molecular materials are usually related to the absence of intermolecular interactions in the solid state, 38,39 and have been previously found for other sulfur-functionalised [CoLdiox] switchable complexes. 31,40 The VT conversion of 2 can also be triggered at low temperature by light (Fig. S4 †): excitation of the ligand to metal charge-transfer band at 10 K turns 15% of the ls-Co III Cat content to the hs-Co II SQ metastable phase. Upon heating at a 0.3 K min À1 rate, the initial state is recovered at 75 K, while the temperature with the highest measurable relaxation rate (T LIESST ) is 55 K. Compared with previously analyzed [CoLdiox] VT systems, 28,29,31 these data conrm that light-Scheme 1 Synthetic pathway to the thiol-functionalised DBCatSH ligand and to the 1-3 complexes used in this study, highlighting their different charge distributions. triggered bistability is a strictly molecular phenomenon, suggesting a possible exploitation at the nanoscale.
Once the VT behaviour of 2 in the solid state has been conrmed, we prepared monolayers of 1-3 complexes by selfassembly from solution and checked the integrity of the surface supported molecules by ToF-SIMS and XPS. Fig. 1 compares the positive ions mass spectra of bulk and monolayer samples of 2 (for complete peak assignation see Table S1 † 41 In order to further investigate structural and electronic properties of surface supported molecules, an XPS investigation has been performed on monolayer and bulk samples of 1-3 complexes. The direct comparison of the S2p regions in the solid state and monolayer of 2 (Fig. 2a) proves the chemisorption on Au (similar results obtained for 1 and 3 are reported in Fig. S6 †). In the bulk phase, the S2p region can be reproduced using a photoemission peak centered at 162.7 eV, with a spin-orbit splitting (DE SO ) of 1.2 eV, as expected for the very similar chemical environment of the two sulfur atoms. 42 In the monolayer sample, an additional signal appears at 161.7 eV, integrating 46% of the overall S2p photopeak area: this component is directly attributed to the thiol group covalently bound to the Au surface, in analogy with previous reports on thiol based monolayers. 43,44 A minor (8%) fraction at 168.4 eV is assigned to spurious oxidation of the sulfur atoms, usually related to defects in the thiol-based molecular packing on the surface. 41 The semiquantitative analysis and the stoichiometric ratios (Table 1 and Fig. S7 †) are in accordance with the calculated and ToF-SIMS data, suggesting that the majority of surface supported molecules retains the [CoLdiox] cation structure found in the solid state. Moreover, the absence of counter-anion for the surface-supported molecules, monitored by the disappearance of F1s photopeak in monolayer samples (Fig. 2b), is an experimental evidence of the absence of physisorbed molecules on the Au(111) surface.
Besides providing stoichiometric information, Co2p XPS spectra yield clear ngerprints of charge distributions in cobaltdioxolene adducts, 31 and have been thus acquired for monolayer samples of 1-3 to analyze the effects of surface deposition on the electronic state of the metal ion (Fig. 2b). The Co2p spectrum of a monolayer of 1 has the typical lineshape of ls-Co III photoemission: 31,45,46 it shows a main peak at 781.5 eV, integrating about 50% of the overall signal area, and minor satellites at 784.7, 788.0 and 792.2 eV, with DE SO contributions    (Table S2 †) and literature data. 31,46 The Co2p photoemission of both samples does not display any temperature dependence in the 170-300 K range. Thus, surface deposition did not affect the electronic ground states of these complexes, as expected because of the large energy difference between the two possible redox isomers in both 1 and 3. The room temperature XPS Co2p spectral features of a monolayer of 2 are different from those reported above: even displaying a closer resemblance with the latter, 2 shows a slightly lower DE SO (15.8 eV), suggesting the coexistence in the sample of both charge distributions. Moreover, on cooling from 283 K to 173 K, an increase in the main peak contribution to the overall signal intensity (from 40% up to about 46%, respectively) parallels a reduction in the DE SO (15.8 to 15.4 eV), indicating the presence of a thermally driven VT conversion in the monolayer.
To investigate VT equilibrium at the nanoscale with higher accuracy, XAS has been used. Synchrotron-based absorption techniques are unmatched tools to analyze submonolayer deposits of bistable molecular systems, providing the requested sensitivity to monitor oxidation 47,48 as well as spin states 9,13,14 and molecular orientation 49 on surfaces. Fig. 3 displays the temperature evolution of the Co L 3 edge absorption spectrum of a monolayer of 2 in the 100-300 K range (for the corresponding L 2 edges spectra see Fig. S8, † while data taken at additional temperatures are reported in Fig. S9 †). The 300 K spectrum conrms the coexistence of ls-Co III Cat and hs-Co II SQ redox isomeric forms in the monolayer, closely resembling the bulk phase behaviour. Isothermal time dependence of the Co L 2,3 XAS spectra acquired on the same spot of the sample (see Fig. S10 †) discards X-ray irreversible alterations of the electronic structure of the graed molecules. The observed edge-jump (Fig. S11 †), moreover, corresponds to about 10% of what is observed in the bulk samples of structurally related [CoLdiox] VT systems taken in similar experimental conditions. 28,31 Considering that only the rst few nm of a bulk sample are investigated in this experiment, the much weaker signal detected for the lm provides an additional proof of the monolayer thickness of the studied deposit. 41 Upon cooling from 300 to 100 K the spectral features related to the ls-Co III isomer gain intensity while the signal coming from the hs-Co II one weakens; moreover, heating the sample back to 300 K the initial spectral lineshape is restored.
These experimental evidences are in line with the occurrence of a reversible entropy driven VT interconversion in the monolayer. In order to quantitatively analyse the hs-Co II thermal distribution prole, the experimental Co L 3 spectra have been tted using the experimental spectra of reference complexes 31 for the limiting redox isomeric distributions ls-Co III Cat and hs-Co II SQ (for tting details see the ESI †). The results, displayed in Fig. 3, evidence a VT conversion that is comparable with that observed by traditional magnetometry for the bulk phase, the major difference being the 20(2)% increase in the remaining hs-Co II SQ molar fraction at low temperature found for the monolayer. This feature has been oen documented for entropy driven interconversions of bistable molecular nano-assemblies, where the ratio between surface and bulk molecules becomes physically relevant, like thin lms and nanoparticles of SCO systems. 7 The reduction of intermolecular elastic interactions for surface-supported molecules, leading to lowered enthalpic and entropic changes accompanying the thermally driven SCO equilibrium, is the generally accepted explanation for the presence of an unconverting fraction at low temperature. 39 For electron transfer processes, a similar behaviour has been theoretically described 50 and experimentally observed for solid solutions of VT systems, 51 and is thus extendable to our case, where we move from a 3D network of elastic interactions (solid state) to a monolayer. Our analysis points out that the employed chemical graing protocol has no signicant effect on the charge distribution of 1-3 complexes and in particular that it is possible to quantitatively retain the switchability of 2 at the monolayer level. The preservation of ls-Co III Cat and hs-Co II SQ charge distributions for monolayers of 1 and 3 complexes, respectively, is not surprising, once evidenced their intact deposition on the Au(111) surface. Indeed, the charge distributions of similar systems have been shown to be scarcely affected by environmental effects in bulk phases. 48,52 The experimental evidence of the entropy driven VT interconversion of 2 at the monolayer level, on the other hand, could have not been safely a priori predicted. In fact, if thermally induced VT processes have been observed in the past in diluted phases (like glass, 53 solutions, 22,54-56 polymeric dispersions, 57 nanoparticles 58,59 ), they have also been shown to be strongly dependent on the crystal packing in the solid state. 60,61 In particular, transition temperatures of [CoLdiox] systems are known to depend on the available volume per molecule in the lattice and display large changes as a function of the nature of the counter-ions 62 and the crystallisation solvent. 29,52 By employing a VT system displaying a gradual transition, we have demonstrated that entropy driven equilibria can be safely reproduced even on single molecular layers on top of metal surfaces.
The last issue of our analysis concerned the capability to photo-induce the electronic bistability in a monolayer (Fig. 3c). To investigate this point, we have irradiated the monolayer of 2 with a 904 nm laser diode at 10 K and then measured the temperature dependence of the XAS spectra up to 100 K (Fig. S12 †). The coupled effect of the X-ray beam (SOXIESST effect, 28 reported in Fig. S13 †) and the laser light irradiation optically populates the hs-Co II metastable state at 10 K, reaching a hs-Co II SQ percentage of 70(2)%. This value corresponds to a 19% overall conversion of the ls-Co III Cat phase present a 10 K, similarly to what is found for the powder sample (see Fig. S4 †). Upon heating to 100 K, the hs-Co II SQ content decreases to 63(3)%, in line with a thermally activated relaxation of the photo-induced metastable phase to the thermodynamically more stable ls-Co III Cat. Direct comparison of the relaxation proles for the monolayer and bulk phase of 2 points out that, in analogy to entropy driven interconversion, the possibility to optically trigger Valence Tautomerism of Co-dioxolene complexes is retained aer their surface graing, and that the energy barrier to thermally activated relaxation remains substantially unaffected by surface deposition.

Conclusions
To conclude, this work provides the rst evidence of thermal and optical control of the electronic state in a monolayer of switchable paramagnetic units anchored to a metallic substrate.
To this aim, a chemical functionalisation protocol has been developed to couple the interconverting molecular core with a linker group able to covalently bind on gold surface. ToF-SIMS and XPS investigations clearly point out the retention of the bulk phase chemical and electronic structures aer the deposition on surface for all the investigated systems and allow us to exclude the presence of physisorbed material. The XAS analyses of the thermally and optically driven interconversion processes in the monolayer evidence a quantitative transposition of the switchable capabilities from the bulk phase up to the surface deposit, even though the employed molecular system displays an incomplete conversion in the experimentally accessible temperature range. These results hold great promise for VT systems in the perspective of novel molecular-based devices that can be thermally and optically controlled.