Single-Molecule Transport of Fullerene-Based Curcuminoids

We present experimental and theoretical studies of single-molecule conductance through nonplanar fullerocurcuminoid molecular dyads in ambient conditions using the mechanically controllable break j...


INTRODUCTION
Due to their unique electronic properties, C60 and its derivatives represent ideal candidates for molecular-based devices. 1,2 Thus far, the synthesis of a wide variety of fullerene derivatives has been reported and their potential applications as electronic, magnetic, catalytic, biological and optical materials have been investigated. [3][4][5][6][7][8][9] At the single-molecule level, establishing robust electronic functionalities in fullerene derivatives connected to gold electrodes has, however, proven to be a challenging task. Pristine C60 has been studied using both scanning tunneling microscope break-junctions 10 and mechanically controllable break junction (MCBJ) techniques. 11,12,13 Conductance values around 0.1 G0 have been reported, where G0 is the quantum of conductance equaling 2e 2 /h = 77.48 µS, where e is the elementary charge and h is Planck's constant, in agreement with theoretical calculations. 14,15 Taking advantage of their affinity to metals, various groups have studied 'dumbbell' benzenedifullerene derivatives in which C60 ' s are placed at both ends of the molecule and act as the anchoring groups to the metal leads. 16,17,18 A conductance value around 10 −4 G0 was reported in these cases. The lower conductance value compared to that of pristine C60 was attributed to a charge transport limiting barrier created by the nitrogen atoms of the pyrrolidine bridging the benzene backbone to the C60 end groups. A more recent study of a dumbbell fullerene derivative shows two different electronic transport configurations, one assigned to transport through the molecular bridge and the second, at higher conductance values, ascribed to a single C60 anchoring group trapped between the two adjacent electrodes. 19 Additional experimental studies of amino 20 and diazofluorene 21 C60 terminated derivatives also show multiple conductance configurations.
Here, we present experimental and theoretical charge transport studies of the single-molecule conductance through fullerocurcuminoid (CCM-C70/C60) molecular dyads (see Fig. 1). Me-S terminated CCM's were chosen because they form stable and well-defined molecular junctions. 22,23 Three systems with the CCM skeleton connected to a C70, C60 and to a C60 fullerene all-equatorial tetramalonate derivative (CCM-C70, CCM-C60 and CCM-C60P, respectively) have been investigated. In the latter, four equatorial diethyl malonate groups were added to the C60. The ethyl esters in these groups are expected to have a high contact resistance and low affinity for gold, thus modifying the interaction between the fullerene moiety and the electrodes in comparison with the non-equatorially modified molecule (CCM-C60). We find that the presence of the fullerenes attached to the CCM backbone lowers the conductance only slightly with respect to the single CCM's without fullerenes attached to them. A priori the results were unexpected, due to the dramatic conformational changes that occur to the curcuminoid 4 backbone in CCM-C70, CCM-C60 and CCM-C60P, where the CCM structure changes from almost planar to one with a distinct V-shape in the fullerocurcuminoid systems, with a corresponding change in charge transfer pathway (see Fig. 1). In addition, the two unprotected fullerene-based derivatives show multiple junction configurations at lower conductance values, attributed to junction formation between one of the Me-S groups and the respective fullerene moiety; this second charge transport pathway is not found for CCM-C60P, showing that the diethyl malonate groups effectively disrupt the direct interaction of the fullerene to the gold electrodes. subsequently, thousands of consecutive breaking traces are measured in an automated way.

COMPUTATIONAL DETAILS
Full consistent non-equilibrium Green function (NEGF) calculations 28,29 were performed using the ATK program (2016.3 version). 30 The PBE exchange-correlation functional 31 was used together with numerical wave function of double-ζ quality with polarization for all the atoms except the gold atoms using a single-ζ quality with polarization. To calculate the conductance values assuming a linear regime, the experimental bias value used in the break-junction measurements was 0.10 V. We have thus corroborated the linear regime between current and bias, and therefore the conductance can be easily obtained as one calculated value for a nonzero bias. The electrode structure employed in the calculations was a 2D 4x4 gold superlattice including a tip of 10 gold atoms on each electrode while a 4x8 superlattice was employed for the fullerene systems (see Fig. S16). A total number of 138 k-points was used to calculate the energy and wave function properties, while a 23x15 2D grid was used for transport properties.
The distances between the anchoring S atoms to the gold atom of the tip was fixed to a value of 2.49 Å for all the molecules. The molecules were optimized with Gaussian09 code 32 using B3LYP functional 33 with a 6-311G* basis set. The position of the methyl of the anchoring group is coplanar for the geometry optimization of the isolated molecules but to reduce the repulsion with the gold electrodes a perpendicular disposition to the molecular plane was adopted. To functional/def2-SVP basis set. 37,38,39, This transport code allows calculations with more accurate hybrid functionals improving the orbital energies but using non-periodic models with a rather simpler wide-band limit approximation. 40 The fullerene-electrode contacts for the sliding process were optimized by using the tight-binding approach proposed by Grimme et al.. 41

RESULTS
To facilitate the identification of molecular traces, we applied two types of statistical analyses: a filtering method (method I), based on conductance plateau recognition, and an unsupervised clustering 42 method (method II), both described in the SI.  Table 1. 3.9 x 10 -5 - The 2D histograms in Fig. 2a,c,e,g show high-counts regions between 10 -4 and 10 -5 G0, which extend up to lengths of 1-1.5 nm. For CCM-C70, the counts are mostly concentrated around two values and the log-normal fit to its one-dimensional histogram (Fig. 2b) indicates that the corresponding most probable conductance values (using method I, see Table 1)  In contrast, for CCM-C60P, the 1D histogram of Fig. 2f only shows one clear peak, with a conductance value of 2.4 x 10 -5 G0. The analysis of conductance histograms shows that the most prominent and highest conductance peak found for all three molecular junctions is the one close to 2 x 10 -5 G0. We attribute this value to the conductance through the Me-S terminated flat CCM backbone which equals 3.9 x 10 -5 G0 (Fig. 2h). 22

DISCUSSION
Initially, the close resemblance of the conductance values to the original CCM, without fullerene substituents, was unexpected considering the different conformation of the fullerenebased CCM skeleton (see Fig. 1 and optimized structures in Figs. S16). Specifically, in the CCM flat molecule with an enol structure, the two oxygen atoms are close due to the intramolecular hydrogen bonding (Fig. 1, top). Instead, the CCM-C60 system presents a di-keto moiety (same for CCM-C70 and CCM-C60P) displaying a V-shape. The central carbon of this CCM skeleton is the apex and in opposite direction the two remaining halves of the molecule contain each a keto group (Figs. 1 and S16). It is also worth to note that the central carbon of the isolated CCM is involved in a delocalized π bond while it has sp 3 hybridization in the fullerocurcuminoid systems. Analysis of conductance traces (Fig. S12) corresponding to the fullerocurcuminoid systems are slightly shorter than those of the CCM molecule, in agreement with their modified V-shape structures. The additional lower-conductance peaks in CCM-C70 9 and CCM-C60 are longer and more slanted therefore they may be attributed to molecular junctions formed between one side of the Me-S backbone and the fullerene moieties. CCM-C60P traces, on the other hand, show more straight plateaus (see Fig. S12).
Additional insight from the conductance histograms can be obtained by performing a clustering analysis on the breaking-trace data sets (method II, see Table S1). For the three cases, the highest yield class (class 1) for each molecule corresponds to the conductance values close to those extracted using method I in the region of 1-4 x 10 -5 G0. It is worth noting that the highest yield class, considering the three systems, is class 1 of CCM-C60P, probably helped by the noncoordinative nature of bulky substituent leading to a higher number of measurements with the electrodes anchored to the two Me-S groups. In CCM-C60 (Fig. S10c)   The total transmission is the sum of the eigenvalues of all transmission eigenfunctions. 43 As expected, the calculated transmission curves using non-periodic models and hybrid B3LYP functional (see Fig. 3) show a larger HOMO-LUMO gap, however, the LUMO is still mainly responsible for the transport properties. The position of the LUMO orbital with respect to the Fermi level for all the systems is relatively similar (Fig. 3), and consequently, they lead to comparable conductance values (see Table S2 and details in SI). In the case of the CCM molecule, the analysis of the transmission eigenfunctions corresponding to the LUMO shows a delocalized π distribution along the whole molecule. For the CCM-fullerene systems, delocalization is interrupted by the central sp 3 carbon. The transmission eigenfunction appears delocalized along the whole backbone consistent with the conductance values found for such types of organic systems.
In addition, a detailed analysis of the calculated conductance values in Table S2 reveals   The isovalue employed for the isosurface was 0.3.
We have also analyzed the transmission pathways (Fig. 4) to gain insight into the transport mechanism considering the structural differences observed among the systems. Transmission pathways are calculated by the projection of the total transmission in pair contributions between atoms employing localized orbitals. This decomposition can be represented in real space by arrows indicating forward and backward current flow. 44 We find that the transport mechanisms are different for the flat CCM in comparison to the non-planar fullerocurcuminoid systems. In the CCM system, transport is directly through the neighboring carbon atoms but with some loss of current due to the two oxygens coordinated to the carbon backbone (destructive interference).
However, in the fullerocurcuminoid systems transport through each keto unit is maintained as in the flat molecules, but with a direct tunneling pathway in the central region between the two external carbons of the C3O2 diketo unit (Fig. 4). This pathway, assisted by the shorter S···S distance due to the bending of the CCM system compensates for the non-flat structure of the CCM and results in a similar conductance value to that of the flat CCM system. Furthermore, the calculations indicate the same conductance for systems with the fullerene or two hydrogen atoms replacing carbon atoms of the cyclopropane ring (Table S2). This observation indicates that there is neither significant transport through this ring nor that there are (destructive) quantum interference effects as can also be concluded from inspection of the transmission pathways in Figure 4. In short, in the conjugated regions the electrons are travelling through the bonds, but in the non-conjugated parts of the molecule, i.e., the central carbon of the fullerocurcuminoid, the electrons are flowing around the bonds. 45 Finally, the lowest conductance region has been explored by analyzing the sliding process of the gold electrode on the fullerene surface. For that, a total of three optimized structures taking into account the displacement of electrodes with respect to the molecule have been calculated (see Fig. S18). The decrease found in the calculated conductance values reproduces the experimental trend for CCM-C70 and CCM-C60; such values decrease while the electrodemolecule distance increases, with the fullerene and one of the branches of the V-shaped CCM attached to the gold electrodes (Fig. S18). Logically, the sliding process is not found in the CCM-C60P system as it is blocked by the malonate addents in agreement with the presence of horizontal plateaus in the experimental breaking traces plotted in Fig. S12.