Single-molecule junctions of multinuclear organometallic wires: long-range carrier transport brought about by metal–metal interaction†‡

Here, we report multinuclear organometallic molecular wires having (2,5-diethynylthiophene)diyl-Ru(dppe)2 repeating units. Despite the molecular dimensions of 2–4 nm the multinuclear wires show high conductance (up to 10−2 to 10−3G0) at the single-molecule level with small attenuation factors (β) as revealed by STM-break junction measurements. The high performance can be attributed to the efficient energy alignment between the Fermi level of the metal electrodes and the HOMO levels of the multinuclear molecular wires as revealed by DFT–NEGF calculations. Electrochemical and DFT studies reveal that the strong Ru–Ru interaction through the bridging ligands raises the HOMO levels to access the Fermi level, leading to high conductance and small β values.


S20
Synthesis of 2 H . To a THF solution (10 mL) of 2 TMS (104 mg, 0.048 mmol, 1.0 eq) was added TBAF (138 µL, 1 M in THF, 0.138 mmol, 2.9 eq), and the resultant mixture was sttirred overnight at room temperature. The reaction mixture was evaporated in vacuo, and the residue was dissolved in CH2Cl2, and MeOH was added to the solution to give precipitates, which was washed with MeOH and dried under vacuum to give a yellow solid (51.0 mg, 0.025 mmol, 53%).

S29
Synthesis of 3 H . To a THF solution (7 mL) of 3 TMS (98.5 mg, 0.030 mmol, 1.0 eq) was added TBAF (0.1 mL, 1M in THF, 0.1 mmol, 3.3 eq), and the resultant mixture was sttirred for 2 hours at room temperature. The reaction mixture was concentrated, and diethyl ether was added to the solution to give precipitates, which was washed with MeOH to give a yellowish green solid (66.0 mg, 0.022 mmol, 70%).

S31
Synthesis of 3 Au . To a benzene solution (5 mL) of 3 H (51.5 mg, 0.017 mmol, 1.0 eq) and AuNHC(Mes)Cl (22.0 mg, 0.041 mmol, 2.4 eq) was added CsOH H2O (60.0 mg, 0.36 mmol, 21.0 eq) and the resultant mixture was sttirred for 2 days at 70 . The reaction mixture was filtered through via cannula and was recrystallized in benzene / hexane and to give a yellow crystalline solid (32.5 mg, 0.008 mmol, 48%).   ',a'' c,c',c'',d,d',d (111) was formed on surface of gold beads, which were prepared as follows. Au wires (0.90 mm diameter, ca. 5.0 cm long, and 99,99% purity, obtained from The Nilaco Corporation) were boiled in a concentrated HCl solution for more than 10 min, and then rinsed with ultrapure water.

IR (KBr
Then the Au wires were flame annealed until melted into a sphere to form Au beads. The Au beads were, then, gently remelted until they showed single-crystalline Au (111) surface. Solutions of molecular wires in tetraglyme (~1.0 mM) were used for the measurements. Conductance was measured during the breaking process under an applied bias of 100 mV.

Conductance histogram
Each break-junction measurement was repeated 100 times (1 set) and each set of the measurement was repeated 20 times (in total 2000 break-junction measurements for an individual experiment). The 20 sets of the measurements were repeated at least 3 times with different gold tips and substrates, and were acquired on different days. 1D linear histogram were made by 1 set of the measurements (100 traces), and 1D log and 2D histograms were constructed from successive 20 set of data. No preference or data selection was performed in making the histograms. S35 Figure S11. 1D log histograms of Au-n Py -Au (n = 2 and 3) constructed from 2000 traces without any data selection.  Conductance (G 0 )

V. Discussion on metal-metal interactions in solution
Cyclic voltammograms of 2 R and 3 R (R = Py and TMS) show successive reversible redox waves, indicating strong metal-metal interactions through the diethynylthiophene bridging ligands ( Figure   S13). Then we performed spectroelectrochemical measurements. Because the results obtained for 2 R and 3 R (R = Py and TMS) were virtually the same ( Figure S14-17), here, we focus on the TMS derivatives 2 TMS and 3 TMS . Upon applied bias voltages on a CH2Cl2 solution containing 1 mM of 2 TMS and 0.1 M of [NBu4][PF6] as electrolytes, intense NIR absorption bands appeared around 1000-2000 nm, which can be ascribed to the monocationic species [2 TMS ] + ( Figure S16). Further oxidation led to spectral changes, and the NIR bands of [2 TMS ] + disappeared due to generation of dicationic [2 TMS ] 2+ species. Similarly, we performed spectroelectrochemical measurements for the trinuclear analogue 3 TMS . In this case, spectral changes due to generation of mono-([3 TMS ] + ), di-([3 TMS ] 2+ ) and tricationic species ([3 TMS ] 3+ ) were observed ( Figure S17). These NIR bands appeared to be a sum of overlapped multiple bands and their origin are due to presence of rotamers. [S12] Therefore, deconvolution analysis was performed for the NIR bands of [2 TMS ] + and [3 TMS ] + , and three gaussian fitting curves were obtained (Band A-C).
To get some insight into the NIR bands origin, we performed DFT and TD-DFT study for the monocationic species ( Figure S18,20-22 with the BLYP35/DefSVP, CPCM(CH2Cl2) levels of theory, which is known to reproduce NIR bands of mixed-valence species. [S13] We calculated one of the most highly symmetrical conformers for each molecule. The spin density of [2 H ] + is distributed over the Ru-CC-C4H2S-CC-Ru moiety. A large contribution on the thiophene part suggests the oxidation also occurs in the bridging ligand. Judging from the fact that the metal centers' spin density is nearly equal (Ru: 0.12-0.13), [2 H ] + is classified as a Robin-day class III or a borderline of class II and class III. TD-DFT study suggests the lowest energy band of [2 H ] + is derived from the transition from b-HOSO to b -LUSO, which can be ascribed to the p-p* transition of the delocalized orbital containing ruthenium centers. This character is typical for a fully delocalized class III compound with a significant bridging ligand oxidation state. Because of the large oscillator strength and the energy, band A in Figure S16 can be attributed to this rotamer.
Similarly, the trinuclear analogue [3 H ] + was calculated. Spin density is localized on the central ruthenium centers together with the -CC-C4H2S-CC-linkers. On the other hand, there is little contribution to the terminal ruthenium centers, suggesting the class II system. A computed NIR band with large oscillator strength was assignable to band B in Figure S17. This NIR transition band is ascribed to the transition from b-HOSO to b-LUSO, which involves charge transfer character from terminal ruthenium centers to the central ruthenium center and p-p* character of the CC-C4H2S-CClinkers. Thus, according to the theoretical study, there is no evidence for the intramolecular interaction between terminal ruthenium centers, whereas strong electronic interactions between adjacent ruthenium centers are supported.        VI. Theoretical study DFT study DFT and TD-DFT calculations were performed by using the Gaussian 16 program package. [S14] Neutral complexes are optimized with the B3LYP/LanL2DZ (for Ru) and 6-31G(d) (for C, H, N, P) levels of theory, while open shell monocationic complexes are calculated at the UBLYP35/Def2SVP,CPCM(CH2Cl2) level of theory. [S13] Single point calculation and TD-DFT study were performed at the same level of theory.

DFT-NEGF study
DFT calculations for electron transport were performed by using the Gaussian 16 ad 09 program package [S14,15] and the non-equilibrium Green's function (NEGF) method in a level-broadening approach [S15] . For the molecular junction models, B3LYP/LanL2DZ (for Ru) and 6-31G(d) (for C, H, N, P) levels of theory were adopted for the transport calculations. The details of computational procedure in the level-broadening approach are described in elsewhere. [S16]