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. 2024 Oct 18;146(43):29977–29986. doi: 10.1021/jacs.4c13551

Single-Molecule Conductance of Neutral Closed-Shell and Open-Shell Diradical Indenofluorenes

Raquel Casares †, Sandra Rodríguez-González ‡,*, Álvaro Martínez-Pinel †, Irene R Márquez †,§, M Teresa González ∥, Cristina Díaz ⊥, Fernando Martín ∥,#, Juan M Cuerva †,*, Edmund Leary ∥,*, Alba Millán †,*
PMCID: PMC11528439  PMID: 39423204

Abstract

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Organic diradicals are highly promising candidates as future components in molecular electronic and spintronic devices because of their low spin–orbit coupling. To advance toward final circuit realizations, a thorough knowledge of the behavior of diradicals within a single-molecule junction framework is imperative. In this work, we have measured for the first time the single-molecule conductance of a neutral open-shell diradical compound, a [2,1-b] isomer of indenofluorene (IF). Our results reveal that the conductance of the [2,1-b] isomer is about 1 order of magnitude higher than that of the corresponding closed-shell regioisomer [1,2-b] IF. This is significant, as it fundamentally demonstrates the possibility of forming stable single-molecule junctions using neutral diradical compounds which are also highly conducting. This opens up a new approach to the development of externally addressable spintronic devices operable at room temperature.

Introduction

Organic radicals are highly appealing building blocks for creating novel molecular spintronic devices due to the typically low spin–orbit coupling and hyperfine interactions.1 To reach this ultimate goal, one has first to understand their behavior within the testbed environment of single-molecule junctions, which allow for a reliable characterization of the transport properties of individual molecules.2 Various compounds that display stable and localized monoradical character have been studied by means of either scanning tunneling microscope-break junction (STM-BJ) or mechanically controllable break-junction (MCBJ) techniques.3−8 Recently, several cationic compounds, with more than one radical center, have also been explored.9 A key question in these studies is whether the molecules retain their particular electronic structure inside the junction, which cannot be easily determined from room-temperature measurements. Assuming that the electronic structure indeed persists in the junction, does it actually have a significant contribution to the electronic transport?

Another important point for everyday applications in molecular spintronics is that devices should ideally operate at close to room temperature and, if possible, under ambient conditions.10 Monoradicals have limited scope in this regard since a switchable magnetic behavior, such as spin flips, is only possible at low temperatures.11 In contrast, diradicals can be designed with magnetically active triplet states, thermally accessible close to room temperature.12 Switching processes between singlet and triplet states can then be induced by external stimuli (e.g., via temperature or applied magnetic fields).13 This possibility allows for new spintronic devices, such as bistable molecular-based memories, which are closer to those required in actual devices.14 Due to their inherent instability, however, and challenging synthesis and measurement, single-molecule conductance studies of neutral open-shell organic diradicals at room temperature have not been reported so far.15

In this context, indenofluorene (IF) scaffolds have been extensively studied over the past decade, not only to unravel their fundamental properties, but also to evaluate possible application in organic optoelectronics.16 IF is formally considered the Hückel antiaromatic analogue of pentacene, bearing alternating five- and six-membered rings and, thus, 20 π-electrons in its polycyclic conjugated core. Its structure has a pro-aromatic central quinodimethane unit (QDM), which can provide diradical character (Figure 1A), and its electronic structure in the ground state is described as a mixture of the Kekulé closed-shell (CS) and non-Kekulé open-shell (OS) configurations. The contribution of the OS configuration to the overall structure is given by the diradical character index, y (0 (CS) ≤ y ≤ 1 (OS)).17 Depending on the relative fusion pattern of the rings, different regioisomers are possible, covering a wide range of diradical character indexes.18 High y values can be rationalized by considering the gain in the number of Clar sextets from the CS configuration (1 sextet) to the OS forms (3 sextets) (Figure 1B, bottom) and correlates with the thermal accessibility of the triplet (T) state. The synthesis of the five possible IF structural isomers has previously been accomplished in solution by inclusion of either bulky units or spin-delocalizing groups on the reactive bridging carbons of the five-membered rings.19−23 Thus, their structural, optoelectronic and magnetic properties have been studied in detail through kinetic or thermodynamic stabilization of the molecule.24 In contrast, no studies on the single-molecule conductance across any type of IF core have been reported.20e,25 We have, therefore, investigated the behavior of formally antiaromatic IFs in a molecular junction environment operating under ambient conditions.

Figure 1.

Figure 1

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(A) p-Quinodimethane resonance form of indeno[1,2-b]fluorene. (B) Examples of two of the five possible structural isomers of the indenofluorene core and their diradical character indexes (y). Clar sextets are highlighted in blue. Diradical character index values were taken from reference18. (C) Our target molecules for single-molecule conductance studies. y values obtained in this work (see Results and Discussion).

Aiming to shed some light on the relationship between the diradical index and the conductance, we have synthesized two IF regioisomers: indeno[1,2-b]fluorene (IFA) and indeno[2,1-b]fluorene isomer (IFS) (Figure 1C, where A and S stand for the relative orientation of the apical carbon of the five-membered ring, anti and syn respectively), which display opposite diradical character, zero for the former while the latter has a significant nonzero value. Each molecule bears two thiomethyl groups (−SMe) as anchor groups located meta to the bridging carbon atoms of the five-membered rings. Also, 2,4,6-trimethylphenyl (mesityl, Mes) groups were introduced at the reactive positions of the five-membered rings to enhance molecular stability. Additionally, we prepared two structurally related analogues to IFS, but with no diradical character: the indolocarbazole IFSN and the dihydroindenofluorene DH-IFS (Figure 2). We then used the scanning tunneling break junction (STM-BJ) technique,26 operating under ambient conditions, to form single-molecule junctions and evaluate the molecular conductance. We were able to detect clear and well-defined single-molecule signals from all the studied compounds, and we observed different conductance values for each IF regioisomer. Our results show that the conductance of the diradical IFS is much higher than that found for the purely CS IFA and the other analogues (IFNS and DH-IFS). In order to corroborate the experimental results and gain a more fundamental understanding of why this occurs, we have performed gas-phase and single-molecule junction transmission simulations based on density functional theory (DFT), and nonequilibrium Green’s function (NEGF) formalism coupled to DFT, respectively.

Figure 2.

Figure 2

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Structures of other molecules studied in this work. R = 2,4,6-trimethylphenyl; R’= n-hexyl.

Results and Discussion

We first focus on the preparation and the study of IFA and IFS. To synthesize both compounds, we followed the reaction sequences shown in the Scheme 1. Starting from the corresponding bromoarenes 1a/1b, we obtained the dialdehydes 2a/2b by selective benzylic oxidation of the methyl groups. Subsequently palladium-catalyzed Suzuki reactions were carried out to give compounds 3a/3b. The corresponding boronic acid bears the SMe-anchor groups necessary for the single-molecule studies. Following this, nucleophilic addition of MesMgBr to the corresponding aldehydes 3a/3b, followed by Friedel–Crafts reactions gave the dihydroindenofluorenes DH-IFA/DH-IFS. Each compound was obtained as a mixture of syn-anti diastereoisomers (referred to the relative orientation of the Mes groups), which were not separated as it is inconsequential for the outcome of the next reaction. Finally, after oxidative dehydrogenation, using 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ), the corresponding compounds IFA and IFS were obtained.

Scheme 1. (A) Synthesis of IFA (Inset: Aromatic Region of the 1H NMR Spectrum of IFA in CD2Cl2); (B) Synthesis of IFS (Inset: Aromatic Region of the 1H NMR Spectrum of IFS in CDCl3).

Scheme 1

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Reaction conditions: (a) (i) CrO3, Ac2O, AcOH, H2SO4, 10–15°C, 18 h; (ii) EtOH, H2SO4, H2O, 85°C, 3 h; (b) 4-thiomethylphenyl boronic acid, Pd(OAc)2, Bu4NBr, K2CO3, toluene, H2O, 70°C, 18 h; (c) (i) MesMgBr, THF, 25°C, 1 h; (ii) BF3·OEt2, DCM, 25°C, 30 min; (d) DDQ, toluene, 70°C, 3–6 h. See SectionS2 for further details.

IFA showed well-defined 1H- and 13C NMR spectra, confirming its closed-shell configuration (Scheme 1A, inset). In addition, IFA was crystallized from hexanes/DCM mixtures, and its structure analyzed by X-ray diffraction (Section S5). As expected, this molecule features the distinct p-QDM unit within the core, with alternating long (1.436 and 1.470 Å) and short (1.380 and 1.361 Å) C–C bond lengths, in good agreement with the reported structure without -SMe groups.20c On the other hand, IFS was NMR silent at room temperature (Scheme 1B, inset), which is consistent with an open-shell configuration in which the paramagnetic triplet state is thermally accessible. This was corroborated by electron paramagnetic resonance (EPR) in which we could observe the signal for the Δms= ± 2 transition characteristic of the triplet state (Figure S27). These results are in line with the observations reported by Shimizu et al.22a for the structure without anchoring groups. Finally, for IFS, the exact mass and isotopic distribution pattern of the peak, corresponding to the [M]+ species in the high-resolution mass spectrum, fits very well with the theoretical data (Figure S25). We also checked the general stability of each compound. IFA was stable for months under ambient conditions. In the case of the open-shell diradical IFS, the compound was reasonably stable up to 1 week when kept under an argon atmosphere at −20 °C. In terms of its stability in solution under ambient conditions, we found its half-life to be ∼38 h. This lifetime decreases down to ∼1 h if the sample is further manipulated (i.e., evaporation/redissolution cycles). Remarkably, and despite its inherent instability, this lifetime proved to be enough to perform BJ experiments. In agreement with these experimental results, our (U)DFT calculations predict that the electronic ground state of IFA corresponds to a singlet closed-shell (CS) structure, whereas a singlet open-shell (OS) structure is the most energetically stable configuration for IFS, with a low-lying triplet state at only 1.10 kcal/mol (0.048 eV) (Table S2). For a comprehensive description about the computational methods employed and related results, please refer to the Supporting Information.

As the diradicaloid character is related to the singlet–triplet energy gap (ΔES-T) and, concomitantly, to the HOMO–LUMO gap (Egap), optoelectronic properties of both compounds have also been examined (see Section S7 and S8). The UV–Vis absorption for IFA (Figure S28) showed a low-energy band with an absorption maximum at 521 nm, which corresponds to an optical gap (Egap) of 2.23 eV, similar to that observed for the molecule without -SMe groups previously reported (Egap = 2.29 eV).20c In the case of IFS (Figure S29), we observed a band centered at λmax = 653 nm and a very weak band starting at ∼930 nm spreading until the detection limit of our spectrophotometer (∼1100 nm) (Figure S30).27 From the cyclic voltammetry we estimated an electrochemical HOMO–LUMO gap of 1.89 eV for IFA and 1.15 eV for IFS. Likewise, DFT calculations predict a HOMO–LUMO gap of 2.34 eV in IFA, and a much lower gap of 1.23 eV for IFS. The same trend is observed for ΔES-T, the value for IFS being more than 15 kcal/mol (0.76 eV) lower than for IFA (see Section S10.1). This open shell diradicaloid behavior of IFS is related to the electronic density distribution within their singly occupied molecular orbitals (SOMOs), which exhibits distinct α and β distributions, characteristic of the disjointed nature (Figure S45), and to a spin density distribution mainly located on the bridging carbon atoms of the five membered rings. The density coincides with the protected radical centers, spreading over the external phenyl rings in the OS, and on the central phenyl ring in the T state (Figure S46). This spin distribution explains the larger instability observed for this isomer due to spin delocalization onto unprotected positions. Finally, we have quantitatively evaluated the open-shell character by using the single-determinant (U)DFT scheme. For that we calculated the diradical character index (y), a theoretical parameter that gives an idea of the contribution of the OS configuration to the overall structure (see Figure 1a).28 In the case of IFA, a value of y = 0 was determined, while IFS shows a considerable amount of diradical character (y = 0.669), confirming the fact that, in the ground state, the CS structure of IFA is the most stable one, whereas a mixture of CS and OS configurations best describes the electronic structure of IFS.

Single-molecule conductance experiments were carried out using a home-built STM following the STM-BJ methodology, in air under ambient conditions.26 The experiment consists of driving a gold tip in and out of a gold surface covered with target molecules while monitoring the conductance (G) as a function of distance (z). During retraction, a nanoscale gold bridge forms between the two electrodes, which breaks leaving a small gap into which molecules can bind. If no molecule binds, the tunneling current between the separating electrodes decreases exponentially. When a molecule binds, however, a plateau in G is seen as the electrodes are separated (examples given in Figure S34). Further details on the procedure can be found in Section S9. We recorded thousands of G-z traces (at least 3 independent runs with between 8000 and 20 000 traces per run) which were analyzed with an automated algorithm to separate the plateau-containing traces from those with only the tunneling background. 1D and 2D histograms were built accordingly, in which peaks (1D) or clouds (2D) form in the regions where plateaus repeatedly appear, allowing us to extract the characteristic molecular conductance values and stretching lengths.

The single-molecule conductance results of IFA and IFS are shown in Figure 3. For IFA, we identified plateaus in 19% of the G-z traces, which were therefore used in the analysis. The 2D histogram in Figure 3a displays a pronounced and clear plateau region between approximately log(G/G0)= −3 and −4.5. By fitting a Gaussian to the conductance peak in the 1D histogram (green trace, Figure 3c), we determined the most-probable conductance of IFA to be log(G/G0)= −3.8, with a half width at half-maximum (HWHM) of 0.4. IFA is structurally similar to a dihydroindenofluorene bearing SMe groups (DHIF-SMe) which has been previously studied in single-molecule experiments.29DHIF-SMe is very similar to the IFA precursor DH-IFA, but having H instead of Mes groups in its structure. The reported conductance of DHIF-SMe is log(G/G0)= −3.1,29 which is higher than that of IFA (see also Table S1). For DHIF-SMe, it is possible to draw resonance structures connecting the two S atoms using curly arrows (Figure S35b), which, according to the methodology described by O’Driscoll and Bryce (curly arrow rules, CARs) suggests that transport takes place via constructive quantum interference (CQI).30 The p-quinodimethane at the core of IFA, on the other hand, leads to a lack of direct conjugation (Figure S35a, top), which should give rise to destructive quantum interference (DQI)31,−33 and provides a reason for the observed lower conductance. This also confirms the negligible presence of the diradical resonance form of IFA (Figure S35a, down), since in OS IFA it would, in principle, be possible to have direct conjugation between S atoms due to the presence of three Clar sextets rather than two. It is interesting to note, however, that despite this conjugation break, the conductance of IFA is not dramatically lower than DHIF-SMe.

Figure 3.

Figure 3

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(a) 2D-histogram of IFA built from all traces presenting plateaus; (b) 2D-histogram of IFS built from all traces presenting plateaus; (c) 1D-histograms of IFA and IFS. Inset: plateau length distributions, vertical dashed lines indicate the theoretical maximum plateau length expected for IFA (green) and IFS (blue). For both compounds, the histograms were built with above 3300 traces.

We next discuss the single-molecule conductance results of IFS. The experiments were challenging due to the inherent instability of this diradical under ambient conditions. In order to avoid significant decomposition during the measurements, we performed the break-junction experiments using substrates exposed to freshly synthesized IFS. In this way, we found a highly reproducible signal, with plateaus in 25–45% of the recorded traces in different experimental runs. The 2D histogram (Figure 3b) showed a prominent peak centered at log(G/G0)= −3.0 with a HWHM of 0.4. In the Supporting Information we give details about the minority low conductance signals contributing to the histogram below log(G/G0)= −4, which most probably results from partial decomposition of IFS. The evolution of the STM-BJ plateaus of IFS over time is shown in addition in Figure S42. The most probable conductance of IFS can be compared to that of CS IFA, the former being about 1 order of magnitude higher than the latter. More precisely, we observe an increase in conductance of approximately 800% with respect to IFA. Meanwhile, the plateau length distributions for both compounds overlap, as shown in the inset of Figure 3c. As is typical for -SMe terminated compounds,34 the tail of their distributions extends up to the theoretical molecular length (corrected for the initial jump-out-of-contact, JOOC) shown by vertical dashed lines in the figure. This length was obtained from the S–S length of each molecule subtracting a JOOC distance of 0.4 nm, typical of gold contacts. Therefore, these results confirmed the S–S anchoring in both compounds.

In addition, we can compare IFS with DHIF-SMe, previously studied in the literature.29 Both IFS and DHIF-SMe display similarly high conductances around log(G/G0)= −3. Both are planar compounds with three six-membered rings. However, in DHIF-SMe each ring is formally an aromatic phenyl and the SMe groups are connected para to one another. On the other hand, IFS has a diradical character of y = 0.669, meaning that it can be thought of a mixture of resonance isomers (as depicted in Figure 1B, bottom). Neither resonance form leads to direct formal conjugation between the SMe anchor groups (Figure S36a). Crucially, however, IFS exhibits two key differences: one, a very small HOMO–LUMO gap, and two, the existence of resonance structures connecting the radicals located on the bridging carbon atoms with the SMe groups (Figure S36b). It has been shown that compounds with anchor groups formally out of conjugation with each other can, nevertheless, lead to a high conductance when side groups or heteroatoms are placed at specific positions along the backbone.31a,32c,35,36 O’Driscoll and Bryce showed that, if it is possible to draw resonance structures connecting these groups with the anchor groups (or more specifically two acceptor groups placed at the sites of the original anchor groups), then the energetic position of the destructive interference, which would otherwise dominate at the center of the HOMO–LUMO gap, can be shifted to higher/lower energies (shifted-DQI, s-DQI).30 Typically, this has a positive influence on the conductance.37 Here, we suggest that the radicals in IFS may play a similar role in alleviating DQI, which, based on its main resonance isomer, may otherwise be expected to dominate the transport behavior.

To further check if the diradical nature of IFS is involved in the high conductance of the molecule, we prepared a structurally related closed-shell compound, IFSN (the synthesis of IFSN is shown in Scheme S3). This compound has the same ring-fusion mode as IFS, with the anchor groups located in the same relative positions, but the apical carbon atoms of the five-membered rings have been replaced by nitrogen atoms (Figure 2, left). In this way we have a purely aromatic compound (22 π-electrons), that formally could be described as a reduced version of IFS. Additionally, we also measured compound DH-IFS, the synthetic precursor of IFS, which contains an sp3 saturated carbon at the bridge locations (Figure 2, right).

The break-junction results of IFSN and DH-IFS are shown in Figure 4. Plateaus were identified in 30% and 27% of the G-z traces, respectively. The shape of the 2D histograms for IFSN (Figure 4a) and DH-IFS (Figure 4b) are very similar to that of IFS, with the distributions tapering to a point. This can be interpreted as an indication that similar binding geometries occur for the three compounds, which reinforces the idea that the conductance differences arise from the core of the molecules and not from different binding configurations. By fitting a Gaussian to the conductance peak in the 1D histograms (Figure 4c), the most probable conductance of IFSN is found to be log(G/G0)= −3.7, with a HWHM of 0.6. This value is in the range of that found for IFA but, more importantly, is close to 1 order of magnitude lower than that for the structurally related IFS (also shown in Figure 4c). In IFSN there is no path that directly connects both anchor groups according to CARs (see Figure S37a) which means, a priori, DQI should dominate around the Fermi level. Although, the presence of heteroatoms can, as previously mentioned, alleviate DQI (Figure S37b), the nitrogen lone pair in IFSN is not very well conjugated into the π-system, and so we expect this effect to be somewhat diminished. The more delocalized radicals in IFS better couple to the π-system which, we suggest, leads to a stronger shifted-DQI effect and thus higher conductance. The results for DH-IFS, a molecule with no heteroatoms or radicals in the polycyclic structure, further support our hypothesis. As can be seen in Figure 4b, DH-IFS displays a single conductance group at log(G/G0)= −4.3 with a HWHM of 0.4. This value is much lower than that for IFNS and IFS (approximately half an order of magnitude and 1 order of magnitude, respectively), due to the lack of a conjugation pathway connecting both anchor groups (Figure S38). Therefore, the trend in conductance values observed for this set of molecules (log(G/G0): IFS > IFSN > DH-IFS) is consistent with both lone pairs and unpaired electrons being able to alleviate the DQI that would otherwise dominate the low-bias conductance. Further, it suggests the higher efficiency of unpaired electrons in doing so for this structural case, leading to higher conductance values.

Figure 4.

Figure 4

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(a) 2D-histogram of IFSN built from all traces presenting plateaus; (b) 2D-histogram of DH-IFS built from all traces presenting plateaus; (c) 1D-histograms of IFSN, DH-IFS, and IFS. Inset: plateau length distributions, vertical dashed lines indicate the theoretically expected maximum plateau length.

To shed additional light on the experimental results, first-principles elastic transport simulations based on NEGF-DFT were carried out within the Landauer formalism,38 according to which, in the linear regime, the conductance is approximately given by the transmission at the Fermi level: G= G0 T(EF), with G0 being the quantum conductance. First, gold-molecule-gold junction geometries were optimized using the SIESTA code.39 To ensure an accurate comparison, the molecules were connected to gold with the same end-to-end arrangement in all cases (Figure S47). Then, the TranSIESTA package and the postprocessing TBTrans tool40 were employed to obtain the zero-bias electron transmission probability function, T(E). Due to the diradical nature of IFS, spin-polarized calculations were performed on the different spin states, whereas spin-unpolarized were carried out on IFA, IFSN and DH-IFS. See Section S10, for a comprehensive description of the whole computational procedure.

Figure 5 presents a comparison of the total transmission spectra for the electronic ground state of IFA, IFS, IFSN and DH-IFS-syn, as estimated by DFT. These refer, respectively, to the transmission of CS IFA, IFSN and DH-IFS-syn, and to the average spin-up and spin-down transmissions (Tσ(E)) of OS IFS. Given its proximity in energy to the OS ground electronic state (as estimated theoretically), the transmission function for IFS in the T state was also computed (see Table S2). Spin-dependent and total-transmission coefficients for OS, and T states are shown in Figure S48. It should be noted that the total transmission curves for open-shell configurations are associated with a distribution of spin densities analogous to those found for molecules in the gas phase, as can be confirmed by comparison between Figures S46 and S49. Furthermore, the transmission curves of the two diastereoisomers (syn/anti) of DH-IFS are nearly indistinguishable, as detailed in Section S10.2.2.

Figure 5.

Figure 5

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Zero-bias transmission spectrum for IFA (green), IFSN (orange), DH-IFS-syn (olive), and the averaged spin-up and spin-down transmission coefficient of IFS OS (blue).

As shown in Figure 5, the transmission is characterized by interference phenomena in the vicinity of the Fermi level (EF) in both IFS and IFA (see Section S10.2.1 for more details), leading to near-resonant transport via the tail of the LUMO. In contrast, the wider energy gap in IFSN and DH-IFS, results in a nonresonant transport mechanism, even though the tunneling distance is practically the same. The computed conductance values, in units of log (G/G0), are −2.94 for IFA, –1.35 for IFS-OS, –1.18 for IFS-T, –2.83 for IFSN, and −3.20 for DH-IFS. Therefore, the higher conductance of IFS compared to IFA, IFSN and DH-IFS, is independent of IFS being in the OS or T states, and is likely related to its smaller energy gap as a consequence of its open-shell character.

To better understand the role of the unpaired electrons in conductance, a description of the interference signals is required. The final transmission in IFA is influenced by a destructive antiresonance dip close to EF (−0.12 eV), associated with the lack of direct conjugation between anchors, as detailed in the experimental section (see Section S10.2.1 for a detailed description). Conversely, IFS-OS exhibits a Fano-resonance adjacent to EF with a width of only 0.1 eV, along with a sharp antiresonance located too far from EF to efficiently contribute to the conductance. Additionally, IFSN displays a DQI feature situated close to the HOMO energy which, considering its structure, is consistent with the idea of s-DQI based on the CARs. This supports the experimental conclusion that unpaired electrons from radicals are more effective in alleviating DQI within the HOMO–LUMO gap than the lone pairs of nitrogen. Finally, no strong DQI feature was found in the transmission spectrum of DH-IFS. Although, based on curly arrow principles, we would expect destructive quantum interference to dominate inside the HOMO–LUMO gap, the absence of such interference may be explained by the existence of a lower-lying sigma channel that dominates the conductance, as previously reported for shorter molecules.41

Overall, our theoretical results qualitatively align with the experimentally observed trends, providing a solid basis for their interpretation. Given, however, the low first oxidation wave of IFS (Figure S33), one cannot completely discard that the diradical was oxidized during the break-junction experiment, giving rise to other open-shell species. We have explored this possibility by further calculating the transmission spectrum of the IFS radical cation (details in Section S10.2.3). This structure, as for the neutral diradical, shows a similarly high transmission (log (G/G0)= −1.71) with respect to IFA, IFSN and DH-IFS. However, the radical cation would itself be a reactive species due to its positive charge which, in the absence of a polarizable medium, would most likely lead to further reactions with, for example, the ambient water present on the surface. This would thus lead to species similar to DH-IFS, which, as mentioned above, may account for low-conductance plateaus in the signal of IFS (see Section S9.4). Other compounds with low oxidation potentials, such as porphyrins, also tend to form charged states that are stable for at most a few seconds, and typically a lot less, resulting in strong conductance fluctuations on the time scale of the plateau measurement.42 For IFS, the plateaus do not exhibit this type of behavior, thus suggesting that any oxidation occurring in the break junction results in the formation of other neutral species with much lower conductance than the diradical. This reinforces our interpretation that the neutral diradical (in resonance with the quinoid configuration) is responsible for the high conductance observed for IFS.

Conclusions

We report the first single-molecule conductance measurements of a neutral open-shell diradical based on an indeno[2,1-b]fluorene core (IFS). Using a combination of the STM-BJ technique along with NEGF-DFT theoretical simulations, we show that the presence of unpaired electrons facilitates conductance across IFS compared to that of the closed-shell indeno[1,2-b]fluorene isomer IFA. For IFA, the measured conductance of log G/Go = −3.8 is related to the widening of the energy gap and to destructive interference resulting from the lack of direct conjugation between the thiomethyl anchor groups. On the other hand, the open-shell IFS displays a significantly higher conductance of log G/Go = −3.0, despite none of the formal resonance structures having direct conjugation between the anchor groups. The high conductance can be, at least partially, attributed to its small HOMO–LUMO gap. Further, we suggest that the lack of DQI in the HOMO–LUMO gap, which would be predicted based on simple “curly arrow” rules, can be understood in terms of the efficient coupling of the radicals with the π-system, which, as we have shown for compounds containing heteroatoms, alleviates the potential appearance of DQI. Comparison with the structurally related aromatic IFSN along with the CS dihydro precursor to IFS (DH-IFS) supports this idea. DH-IFS has the lowest conductance, probably dominated by transport via the σ bond channel. IFSN, on the other hand, sits between DH-IFS and IFS. The nitrogen lone pairs lie out of plane with the π-system and the DQI is alleviated, but less efficiently than in IFS.

Our calculations support the break-junction results by matching the trend in conductance found at the Fermi level. An antiresonance occurs within the HOMO–LUMO gap for IFA, in line with the lack of direct S–S conjugation. On the contrary, no antiresonance is found for IFS within its HOMO–LUMO gap. Instead, we find a Fano resonance at the position of the LUMO. Due to the much smaller overall energy gap, the calculated transmission is, therefore, larger than for IFA.

This work showcases the behavior of an open-shell diradical when incorporated to single-molecule junctions and the remarkable difference in conductance with respect to other compounds in the IF family. We hope that these results, which demonstrate that open-shell species can form stable molecular junctions under ambient conditions, may encourage the investigation of single-molecule magnetism at room temperature, particularly, via singlet–triplet excitation.

Acknowledgments

This work received financial support from grant PID2021-127964NB-C22 funded by MICIU/AEI/10.13039/5011000-11033 and by’ERDF A way of making Europe’. We also acknowledge grants PID2021-127964NB-C21, PID2022-138288NB-C31, PID2022-138288NB-C32, PID2022-138288NB-C33, and PID2020-113059GB-C21 funded by MICIU/AEI/10.13039/5011000-11033 and by’ERDF A way of making Europe’. R.C. acknowledges grant PRE2018-083406 funded by MICIU/AEI/10.13039/501100011033 and by’ESF Investing in your future’. Á.M.-P. thanks Ministerio de Universidades for an FPU contract (FPU19/03751). I.R.M. acknowledges grant B-CIC-034-UGR23 funded by Consejería de Universidad, Investigación e Innovación and by ERDF Andalusia Program 2021-2027. M.T.G. and J.M.C. acknowledge the EIG Concert Japan project DECOSMOL (PCI2023-143389). E.L. thanks the Comunidad de Madrid Atracción de Talento grant 2019-T1/IND-16384 and grant CNS2023-145464 funded by MICIU/AEI/10.13039/501100011033 and by EU NextGenerationEU/PRTR. IMDEA Nanociencia acknowledges support from the’Severo Ochoa’ Programme for Center of Excellence in R&D (CEX2020-001039-S). We acknowledge the generous allocation of computer time at the Barcelona Supercomputer Center and the Centro de Computación Científica at the Universidad Autónoma de Madrid (CCC-UAM). EPR experiments were carried out at the Instrumental Technical Services of the Estación Experimental del Zaidín (CSIC), Granada, Spain, by Beatriz Sánchez Romera, who acknowledges PTA2020-018871-I funded by MICIU/AEI/10.13039/501100011033 for her contract. Finally, we thank Dr. Víctor Blanco and Dr. Luis Lezama for assistance with crystallography and EPR analyses, respectively. Funding for open access charge: Universidad de Granada / CBUA.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c13551.

  • Synthetic procedures, NMR spectra, high-resolution mass spectra, UV–vis spectra, voltammograms, EPR spectrum, STM-BJ experimental details and results, theoretical calculation methods and results (PDF)

Author Contributions

∇ R.C. and S.R.-G. contributed equally as first authors.

Author Contributions

□ Á.M.-P., I.R.M., and M.T.G. contributed equally. All authors have given approval to the final version of the manuscript

The authors declare no competing financial interest.

Supplementary Material

ja4c13551_si_001.pdf (4.8MB, pdf)

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