# Supramolecular and electrical properties of ruthenium-based organometallic gels

Ilknur Hatice Eryilmaz, William Gallonde, Elsa Caytan, Olivier Jeannin, Franck Camerel, Stephane Rigaut, Olivier Galangau, Emanuele Orgiu

► To cite this version:

Ilknur Hatice Eryilmaz, William Gallonde, Elsa Caytan, Olivier Jeannin, Franck Camerel, et al. Supramolecular and electrical properties of ruthenium-based organometallic gels. Journal of Materials Chemistry C, 2025, 13, pp.7393-7401. 10.1039/d4tc04486j. hal-05033680

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Submitted on 16 Nov 2025

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# Supramolecular and electrical properties of ruthenium-based organometallic gels

Ilknur Hatice Eryilmazª, William Gallondeᵇ, Elsa Caytanᵇ, Olivier Jeenninᵇ, Franck Camerelᵇ, Stéphane Rigautᵇ,*, Olivier Galangauᵇ,*, Emanuele Orgiuª,\*

A novel organometallic gel whose molecular building block contains either one (Ru₁) or two Ru (Ru₂) atoms is presented and for the latter molecule, its supramolecular properties in solution investigated. Its electrical characterization revealed that when Ru₁ is used to form an organometallic gel, such a gel becomes highly sensitive to the presence of common organic solvent, while when a Ru₂ gel is employed the solvent modified electrical performances are preserved over days.

## Introduction

Organic conjugated semiconductors, in contrast to typical metals and inorganic (semi)conductors, have notable physical and mechanical features such as lightweight and flexibility which makes them appealing for novel applications. In addition, their simple and inexpensive synthesis and processing is particularly suited for producing large-area and low-cost electronic devices.¹⁻⁶ Organic semiconducting molecules can be chemically functionalized in order to form organogels which are stable viscoelastic soft materials composed of a low molecular weight molecule called gelator (typically few wt%) embedding a solvent.⁷⁻⁸ The gelation process of such compounds generally occurs upon cooling from a warm solution containing a given concentration of the gelator, which may self-assemble and form ordered supramolecular architectures (fibres, ribbons etc.) thanks to non-covalent interactions such as hydrogen bonding, π-π stacking, van der Waals or electrostatic interactions.⁹⁻¹⁵ These supramolecular nanostructures form a 3D network that traps solvent molecules forming the organogel that can exhibit unique properties such as high surface-area, high porosity, and a given mechanical strength with flexibility. In addition, they may be self-healing,¹⁸⁻¹⁹ which makes them technologically relevant for a variety of applications.²⁰⁻²¹ While recent examples also showed the potential of organogels for charge transport, those were however almost based on purely organic organogelators in their xerogel (dry) form,⁶ but not in their pristine form.

Organometallic gels —which entail an organometallic moiety as a basic molecular building block that introduces novel optical, magnetic or redox properties while preserving the softness of a gel²²⁻²⁴— might be advantageous in a wide range of functional electronic devices such as memristors, batteries, or for applications in optoelectronics, catalysis, or sensing, to name a few.²⁵⁻²⁷ In particular, their nano-to-micrometer range fiber networks do hold

promise for improved charge transport, exciton confinement, and thermoelectrics. Although, platinum,²⁸⁻³⁰ lanthanides,³¹ ruthenium,³²⁻³³ and iron³⁴ based organometallic gels were previously reported in the literature but none of them were investigated for their charge transport properties so far.

Recently, some of us reported on the formation of ordered supramolecular gels in aromatic solvents using an organometallic compound (Ru₁, Fig. 1a) featuring a Ru(II) bis-acetylide motif with the 1,2-bis(diphenylphosphino)ethane (1,2-dppe) ligands leading to molecular solubility and gelation in aromatic solvents.³⁵ Ru₁ was also found to form an air-stable, lyotropic liquid-crystalline gel with fibrous aggregates made of self-organized anisotropic metallomesogens which turn into a liquid crystal state as well as self-assembled lamellae structures in a xerogel.³⁵, ⁴³ Importantly, this ruthenium moiety displays excellent chemical and electrochemical stabilities, because of the strong *d*/π orbital mixing that characterizes their HOMO level that leads to a rather low oxidation potentials (0.43 V vs SCE) and efficient charge transport in molecular junctions.³⁶⁻⁴¹ Even lower oxidation potentials (ca. 0.2 V vs SCE) can be obtained in bimetallic Ru-acetylide compounds of the kind [Ru]-C≡C-Ph-C≡C-[Ru]⁴² which is promising for p-type semiconducting materials as it would reduce the energy mismatch with the work function of the metal electrodes. In this respect, bimetallic Ru-acetylide organogelators are yet to be synthesized and their gel formation ability to be investigated.
<figure><img src="image_2.png" /><figcaption>**Fig. 1** Molecular structure of a) Ru₁ and Ru₂ complexes, b) SEM images of Ru₂ xerogel illustrating the formation of large 2D nanosheets, c) VT-NMR experiment performed on Ru₂ gel, d) Chemical shift variation (orange arrow in c)) vs T plot with an isodesmic simulation.</figcaption></figure>

In this article, we report on the material characterization of supramolecular gels formed by Ru-acetylide based gelators and focus on their electrical properties. Following our previous report³⁵ on monometallic Ru₁, we designed a new bimetallic compound, (Ru₂) (Fig. 1a), flanked by the assembling moieties which allow to form supramolecular gels in aromatic solvents.

We anticipate that the charge transport in those organometallic gels should occur throughout nanostructured supramolecular aggregates that will strongly depend on the chemical structure of the molecular building block, their oxidation potential, as well as on the mutual interaction of these structures with the solvent molecules. Indeed, (i) electron delocalisation is expected to occur between the metal and ligand of the conjugated path within the molecule, and (ii) the supramolecular nanostructures should control charge transport via hopping between molecules.

## 2. Results and discussion

(Supramolecular properties of Ru₂. Synthesis of compound Ru₂ has been achieved using conventional synthetic routes, as described in the SI. The compound was obtained with a 62 % yield in 2 steps and characterized by ¹H, ¹³C, ³¹P-NMR and HR-MS. in solution (CH₂Cl₂ Bu₄NPF₆/Ru₂ features two reversible oxidation processes located at 0.18 V (vs SCE) and 0.48 V (vs SCE). These values compare well with

the ones found in the literature. Most importantly, the insertion of a second metal center has lowered the first oxidation potential, as expected.

While Ru₁ gels in toluene at a Critical Gelation Concentration (CGC) of 5 mM, a toluene solution of Ru₂ requires higher concentrations (9 mM) to form a supramolecular gel at room temperature (see Table S1). The lower efficiency of Ru₂ to gel in toluene most likely stems from its higher solubility in this aromatic solvent with respect to its monometallic counterpart. The significant increase of phenyl rings of Ru₂ with respect to Ru₁ could be the reason for such pronounced differences in solubility. To the best of our knowledge, this is the only example of Ru-acetylide bimetallic species reported to form supramolecular gels.

In a previous work³⁵ SEM images of xerogel Ru₁ revealed the tendency of this compound to form mesoscopic fibers. However, when SEM images of Ru₂-based xerogel were taken, this compound was found to aggregate in the form of micrometric sheets (Fig 1b), highlighting that placing a second metal center in the molecular core profoundly modifies structural and mesoscopic properties of the gels. We further performed FTIR measurement (Figure S14) on Ru₁ and Ru₂ gels. In both cases, our organometallic compounds would feature spectral signatures with a stretching vibration of the N-H bond at 3226 cm⁻¹ (3229 cm⁻¹) and a stretching vibration of the C=O bond at 1638 cm⁻¹ (1639 cm⁻¹). Such values clearly indicate the formation of a dense network of H-bonds responsible for the self-assembly. However, the slightly lower N-H stretching
vibration frequency for Ru₁ suggests a slightly weaker supramolecular packing than with Ru₂ gel.

To gain insights into the supramolecular packing of Ru₂ derivative we conducted SAXS investigations onto its gel state (see dedicated section in the SI and Fig. 2). The scattered intensity vs scattering modulus (Figure S15) clearly indicates a global q⁻² dependence, which is the indicative signature of the formation of 2D objects. Furthermore, an oriented 2D SAXS pattern recorded on a dried gel displays two orthogonal preferential directions confirming the formation of nanosheets rather than fibers. These results are in line with our SEM observations. A molecular stacking model can reasonably be proposed in which the molecules are organized in 2.8-nm-thick layers (Fig. 2). The distance of 1.24 nm is attributed to the intramolecular ruthenium-ruthenium distance. This distance, which is orthogonal to the stacking of the layers, indicates that the molecules are arranged flat inside the layers. The distance of 0.98 nm is attributed to the head-to-tail stacking of the aromatic amide unit within the molecular layers. Thus, in the proposed model, the molecules are arranged parallel to each other in stripes. These stripes interpenetrate at the edges, intercalating the organic flat unit head-to-tail to form a 1D stack along the layers, which is stabilized by hydrogen bonding. In this model, the carbon chains are distributed over the entire surface of each face of the nanosheet lamella. Altogether, these finding tend to demonstrate that the bimetallic compound self-assembles into nanosheets rather than into fibers, thanks to strong H-bonds.

Eventually, we examined the self-assembly process by Variable Temperature NMR (VT-NMR) on a gel sample (10 mM) in tolu-d₈. The sample temperature was slowly varied (0.5 K/min) from 300 K to 370 K. At the lowest temperatures, the NMR signals are unreliable because of large signals broadening, which is indicative of strong aggregation following gel formation. Upon warming, above 310 K, aggregates start disassembling which leads to gel destruction, which allows to achieve a better resolution of the NMR signals. At 370 K, the solution is translucent and the signals are well resolved. Of interest are the observed chemical shift variations. The broad singlet signal that shifts upfield (located at 7.3 ppm at 370 K) upon warming is assigned to the N-H amido protons. Its variation is most probably due to the formation of H-bonds, in line with our FTIR data. The singlet signal indicated by an orange arrow on **Fig. 1d** is assigned to the protons of the toluene fragment (**Fig. 1a**). This singlet experiences a downfield shift upon warming from 7.86 to 7.93 ppm, consistent with a previous study for a monometallic mono-acetylide Ru compound.⁴³ This variation follows a sigmoidal profile (**Fig. 1d**) and possesses a melting temperature (temperature at which 50% of the aggregate remain) of 321.3 K. Below 308 K, gelation occurs leading to unresolved signals (vide supra).

Identification of the supramolecular growth mechanism requires the knowledge of the so-called aggregation parameter (α<sub>agg</sub>) variation as a function of the sample temperature. Therefore, we calculated α<sub>agg</sub> using the latter chemical shift signal (Figure S6) and obtained the related supramolecular growth thermodynamic parameters by

fitting with an isodesmic model.⁴⁴b The simulation revealed a growth enthalpy (entropy) of -91 kJ mol⁻¹ (-241 J K⁻¹ mol⁻¹) indicating an enthalpically controlled self-assembly process. Interestingly, these results also match with previous findings.⁴³

<figure><img src="image_3.png" /><figcaption>**Fig. 2** (Top) Oriented 2D-SAXS pattern recorded on a dried toluene gel directly in the glass capillary (sample to detector distance = 280 mm). The halo at 0.495 nm is assigned to the aliphatic chains. (Bottom) Proposed molecular stacking model of the Ru₂ molecule inside the gel (a: top view; b: side view).</figcaption></figure>

**Charge transport properties of Ru₁ and Ru₂ gels.** The organometallic gels with Ru₁ and the newly designed Ru₂ complexes were electrically characterized on a SiO₂ substrate with pre-patterned interdigitated Au electrodes as seen in **Fig. 2a** that depicts the device architecture (channel length ranging from 20 µm to 2.5 µm). The translucent 3D
<figure><img src="image_4.png" /><figcaption>**Fig. 3** a) Optical microscope image of Ru₂ gel with microscope images of drying gel b) two-terminal device schematic. c) I-V curves of pristine Ru₁-based gel (wet) prepared from toluene and its *dry* state on different channel lengths devices in their wet (line) and dry state (dot), d) Absolute current values (with error bars) of wetted gels at -30V with different solvents [the box includes the 25-75%, the line in the middle is the median, while the small square represents the mean]</figcaption></figure>

casted gels appear smooth at the optical microscope, allowing to image the electrodes (Fig. 2a and Fig. 5c). Ru₁ and Ru₂ gels used as the active layer in representative devices have similar currents at applied bias voltages reaching 10⁻⁸ A at 15/-15 V (Fig. 2c and 3a). At first glance, some tentative explanation behind these findings could be put forward by considering the two following factors: a) based on the oxidation potentials measured by CV, the injection barrier for hole injection from Au (work function sitting approximately at -5.1 eV) into Ru₂ (HOMO[Ru₂] = -4.51 eV) is higher than that of Ru₁ (HOMO[Ru₁] = -4.82 eV). This should lead to a higher current measured in the Ru₁ when the same device geometry and voltages are employed; b) Given the chemical structure of the two compounds, one would intuitively think that extending the conjugation over a “longer” molecule (Ru₂) is generally a good strategy to maximise intramolecular transport while also favoring the intermolecular charge transport by increasing the mutual overlap between molecules. In addition, the mesoscale organization in the form of 2D nanosheets of Ru₂ vs. 1D fibers in Ru₁ should promote charge transport. Therefore, a tentative explanation to justify the similar current intensity in these compounds could be that the more unfavorable charge injection into Ru₂ is somehow counterbalanced by its higher ability of conducting charge carriers with respect to Ru₁. The Ru₁-based organometallic gel was found to be conductive, but it would manifest an insulating character accompanied by a slow yet ultimately large decrease in current, upon drying of the solvent following drop casting (Fig. 3c). By *dry* here we refer to a gel which has lost a significant and undetermined part of the original solvent quantity following its initial deposition by drop casting in between the electrodes (in nitrogen environment). More specifically, right after deposition of a small volume of Ru₁ gel

(~3-4 µl) in between the electrodes, however, the current decreases over time (typically after 20-25 minutes) making the gel more and more *dry* and insulating. Electrical currents measured on as-deposited gel (preformed in a vial) was found to be 2 orders of magnitude larger than those measured in the *dry* state. The as-measured currents scale as a function of the different channel length employed. As expected, larger currents were measured in devices with shorter channel length (2.5 µm) with respect to those measured in wider channels (20 µm) as shown in **Fig. 3c**. As reported in Figure 3c, devices with different channel lengths were employed in our study. More channel lengths are shown in Fig. S8a. Such an experiment further corroborated the evidence that it is actually the material that is being probed in between the electrodes, and the observed current-voltage dependence is not strongly dominated by interfacial effects. Though not ideal, preliminary I<sub>D</sub>-V<sub>G</sub> curves carried out using the gel as the active layer in a 3-terminal device revealed that the holes are the majority charge carriers that lead to a measurable gel conductivity in its pristine *wet* form (**Fig. S8b and S9**) as expected for compounds displaying a low oxidation potential. When *dry* Ru₁-gels are wetted again with a droplet (4 µl) of various organic solvents deposited on top, they swell back (wet), they recover a conductive behavior, and the recovered current intensity depends on the solvent employed. More specifically, as detailed in **Fig. 3d**, we used chlorobenzene, chloroform, toluene, and
<figure><img src="image_5.png" /><figcaption>**Fig. 4** FTIR-ATR spectra of Ru₁-based dry gel re-wetted with different solvents a) chloroform, b) toluene, c) chlorobenzene. N-H vibration frequency decreased (red shift) by 11-13 cm⁻¹ indicating strengthening of the intermolecular interactions compared to the wet form. d) Comparative FTIR spectra of mono- (Ru₁) and bimetallic (Ru₂) complex in toluene.</figcaption></figure>

dichloromethane (DCM), which are poor electrical conductors (between 10⁻¹⁰-10⁻¹¹ ohm⁻¹cm⁻¹) with a polarity index laying between 2.4 to 4.1 (toluene: 2.4, chlorobenzene: 2.7, dichloromethane: 3.1, chloroform: 4.1). Chlorobenzene and chloroform exhibited higher currents following re-wetting, whereas toluene and DCM generally were characterized by lower currents with respect to those measured in the pristine as-deposited gel. A possible explanation for the differences observed in re-wetted gels in Fig 3d can be put forward by considering their rather different boiling point, especially the low boiling point of DCM (39.6 °C). The dry gels were re-wetted with the same amount of solvent, and as such DCM would evaporate at a much faster rate than, for instance, chlorobenzene from the dry gel making the rewetting process more ineffective.

The full I-V traces measured with different solvents on a *dry* Ru₁ gel sample are shown in **Fig. S10**. As an example, when a gel is *re-wetted* with a chlorobenzene droplet, a current boost of ~1,000 times (at -30V) could be observed (**Fig. S10**). According to the data obtained, the gel would typically recover a conductive behaviour till it becomes *dry* again. Also, the time required for the solvent to leave the gel *dry* and non-conductive (currents below 10⁻¹¹ A) would depend on the particular solvent utilized to *re-wet* the gel which is also related to the solvent's boiling point.

As seen for compound Ru₁, whose conductivity could be recovered upon *wetting* of the dry gel with different organic solvents, the bimetallic pristine Ru₂-based gels showed a similar behavior to Ru₁ upon rewetting. In fact, a significant current increase (up to 1 order of magnitude) upon rewetting (with CHCl₃ after 3h) could be observed (**Fig. S13**). However, while in

the case of Ru₁ the recovery of the conductivity upon *wetting* the dry gel (by using the same solvent volume) would start to decrease significantly in the first hour (~50% with respect to its initial value, **Fig. S10**), Ru₂ gels treated with the same experimental parameters i.e. pristine gel deposited from toluene (**Fig. 5a**), and then rewetted only on day 1 with chloroform (**Fig. 5b**) exhibited a more stable device current decreasing of ~50% with respect to its initial value in 2 days and lasting up to 38 days (~10% of its initial value) in a nitrogen environment i.e. in a glovebox (H₂O and O₂ levels < 0.1 ppm). At this point, it is important to emphasize that solvent-gelator interactions are supporting the gel structure and the charge transport properties. Solvent molecules, once immobilized within the gelator network by van der Waals forces and π-π interactions, have a role in determining the structural arrangement of the Ru₁ or Ru₂ aggregates within the gel and ultimately their molecule-to-molecule charge transport ability upon charge (hole) hopping. Hence, as expected, all samples exhibited comparable currents in their pristine gel state (*wet* state) as the pristine gel was always prepared in a toluene solution, but their behaviour upon drying (or not?) with time were found to be different depending on the compound Ru₁ or Ru₂, and upon (*re*-wetting of the gel on the solvent employed to *re-wet* leading to different interactions and thus nanostructuration.

Thus, to better understand the charge transport mechanism in Ru₁- and Ru₂-based organometallic gels and the role of the solvent in the gels, we examined spectral features correlated to the structural supramolecular arrangement and packing, and that are therefore relevant to interpret the measured electrical properties.⁴⁴ First, previous SAXS measurements³⁵ carried out
on the Ru₁ compound nanostructures in gel revealed that Ru₁ self-assembled into lamellae with interlayer distances of 39 Å, which approximatively corresponds to the total length of the molecule. A schematic of such structure is depicted in Fig. S16. As a result, the aliphatic chains most likely extend and segregate from the aromatic cores. In this view, the molecules (probably upright standing), are interacting particularly with H-bonding as observed on FTIR measurements.

<figure><img src="image_6.png" /><figcaption>Fig. 5 a) I-V curves (full cycle) of pristine dry Ru₂-based gel, b) Current values at -15 V of Ru₂ gel rewetted with CHCl₃ in a 38-day timeframe, c) microscope images of the actual gel upon drying over 38 days.</figcaption></figure>

In other words, the supramolecular structure is best described by conductive channels (the organometallic centers) in which intermolecular hopping may happen, insulated by aliphatic sheaths, and trapped solvent molecules. Of importance is the fact that the SAXS pattern of Ru₁ in its solid state mesophase⁴⁰ (dry sample) also was found to have a different supramolecular packing, featuring a sizeable shrinkage of the interlayer distance from 39 Å to 27 Å, thereby forcing the molecules to tilt and therefore disrupting the conductive channels.

Such a significant change in the supramolecular arrangement should induce intermolecular H-bond weakening. We observed this variation by monitoring the slow gel drying phenomenon using Fourier-transform infrared spectroscopy (FTIR) operated in Attenuated total reflectance (ATR). As observed in Fig. S14, the N-H stretching vibration band of the pristine gel indeed experienced a blue shift upon drying of 10 cm⁻¹ confirming H-bonds weakening. Altogether, FTIR and SAXS point toward a sizeable structure change, which most likely accounts for the observed decrease in conductivity upon drying.

Considering now the combination of SAXS and SEM data (vide supra) that we obtained for Ru₂ gels, one may relate the difference of drying / electrical behaviour between the two gels to the formation of 2D layers in Ru₂-gels. Indeed, as the Ru₂ gel loses solvent molecules by slow evaporation, the 2D layers should stack more intimately, resulting in a slower solvent evaporation. Moreover, Ru₂ gel in toluene displays an N-H

stretching vibration peak 5 cm⁻¹ lower than Ru₁, as seen in Fig. 4d, meaning Ru₂ displays slightly stronger H-bonding capability. It is also worth noting that Ru₂ has an increased number of aromatic units (twice as much phenyl groups than in Ru₁) in the center of the molecule. Thus, we tentatively ascribe a better solvent retention in Ru₂ over Ru₁ to the improved ability of Ru₂ to interact with solvents by π-π stacking and a tighter supramolecular packing due to an increased intermolecular H-bonding. As a result, the structural properties and conductivity are preserved for a much longer period (Fig. 4d).

We attempted to investigate the peculiar solvent responsiveness of Ru₁ gels. Again, we monitored the re-wetting steps by FTIR operated in ATR configuration (Fig. 4). For consistency, the samples were prepared by following the very same protocol used for the electrical characterization shown in Fig. 3c. If one compares spectra in Figure 4a, 4b and 4c, the peak corresponding to the amide N-H vibration is deeply affected during the rewetting with red shifts of 13, 11 and 13 cm⁻¹ for chloroform, toluene, and chlorobenzene, respectively, in line with a favorable rearrangement of the supramolecular packing that causes better charge transport as seen with higher currents. In other words, swelling of the gel as seen on FTIR-ATR spectra would most likely lead to better organization of gelator molecules with elongated side chains in larger volume and restoring H-bonds in lamellar organization thanks to amide functions. Regarding Ru₂, our FTIR measurements on pristine gel clearly pointed out to a tighter structure which is less permeable to the solvent. Unfortunately, the same set of FTIR measurements could not further evidence interaction with the solvents method at this stage that are probably weaker. Although if such observations raise the fundamental question of solute-solvent interactions,⁴⁵ a full interpretation of those phenomena does require a deeper investigation which is beyond the scope of this study.

### 3. Conclusions

In conclusion, we have reported the synthesis and the supramolecular behavior of a new organometallic Ru-acetylide compound. The molecule gels aromatic solvents by forming 2D nanosheets that eventually entrap the solvent molecules. The nanosheets growth was characterized as typical isodesmic mechanism of supramolecular polymerization process. We have also reported the electrical properties of Ru₁-based gel in both its dry and re-wetted form, and a novel Ru₂-based gel in whose get form and conductivity were fond to last for as much as 38 days. Current-voltage measurements indicated that the monometallic gel can go from conductive to insulating which could be make it useful for flexible and translucent solvent sensing applications. These observations were tentatively rationalized on the basis of structural considerations, which clearly pointed toward a change of supramolecular packing accompanied by a decrease of the intermolecular interactions strength upon drying. Upon rewetting with other solvents, FTIR spectroscopy indicated a clear recovery of the pristine supramolecular packing toward a tighter packing, favoring the charge transport. On the other hand, pristine Ru₂ gel shows
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