SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Efficient Electronic Coupling and Improved Stability with Dithiocarbamate-Based Molecular Junctions Supplementary Information Florian von Wrochem*, Deqing Gao, Frank Scholz, Heinz-Georg Nothofer, Gabriele Nelles, Jurina M. Wessels* Sony Deutschland GmbH, Materials Science Laboratory, Hedelfinger Strasse 61, Stuttgart, Germany nature nanotechnology 1

2 supplementary information Synthesis SH 1 N SNa S 2 Figure 1. Chemical structures used in this work for electrical characterization. The following are descriptions of the synthesis of compounds 1 and 2. Compound 1 was already reported (Reference to Himmel et al. J. Am. Chem. Soc. 1998, 120, 12069). In contrast, our procedure gives a higher yield due to the use of more efficient reagents. Compound 2 has not previously been synthesized and reported. Br 1.5% Pd 2 (dba) 3 + B(OH) 2 4.5% P(t-Bu) equiv KF 1,4-dioxane/reflux 3 NBS/AIBN CCl 4 /reflux 4 Br KSCOCH 3 DMF/r.t. O NaSCH 3 S MeOH/r.t. 5 1 SH 4''-Methyl-[1,1';4',1''] terphenyl (3): To a solution of 1-bromo-4-methyl-benzene (0.6 g, 0.43 ml, 3.5 mmol) in 1,4-dioxane (6 ml) were added [1,1'-biphenyl]-4-ylboronic acid (1.04 g, 5.25 mmol), KF (0.68 g, mmol), Pd 2 (dba) 3 (49 mg, mmol), P(t- Bu) 3 (32 mg, 0.16 mmol) and the mixture was refluxed overnight. When the system cooled down, the solid portion was filtered out. The solvents were removed by evaporation. After flash chromatography on silica gel (ethyl acetate/hexane: 1:49; TLC, R f = 0.5) and recrystallzation with ethyl acetate, pure 4''-methyl-[1,1';4',1''] terphenyl (3) was obtained as a white solid (65 % yield). 1 H NMR (400 MHz, CDCl 3 ): δ (ppm) 7.67 (s, 4H); 7.65 (m, 2H); 7.55 (m, 2H); 7.46 (t, 2H); 7.36 (m, 1H); 7.27 (m, 2H); 2.41 (s, 3H). 13 C NMR (400 MHz, CDCl 3 ): δ (ppm) (s); (s); (s); 2 nature nanotechnology

3 supplementary information (s); (s); (s); (s); (s); (s); (s); (s); (s). 4''-Bromomethyl-[1,1';4',1''] terphenyl (4): To a solution of 3 (74 mg, 0.3 mmol) in CCl 4 (5 ml) were added N-bromosuccinimide (NBS) (59 mg, 0.33 mmol) and 2,2 - azobis(2-methylpropionitrile) (AIBN) (0.4 mg, mmol). After the mixture was refluxed for 0.5 h, AIBN (0.15 mg, mmol) was added once again. The mixture was refluxed for two more hours to complete the reaction. The collected solid was purified with flash chromatography on silica gel (ethyl acetate/hexane: 1:49; TLC, R f = 0.4) and recrystallization with THF. Pure 4''-bromomethyl-[1,1';4',1''] terphenyl (4) was obtained as a white flaky solid (70 % yield). 1 H NMR (400 MHz, CDCl 3 ): δ (ppm) 7.67 (m, 4H); 7.63 (m, 4H); 7.47 (m, 4H); 7.36 (m, 1H); 4.56 (s, 2H). 13 C NMR (400 MHz, CDCl 3 ): δ (ppm) (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s). 4''-Acetylsulfanylmethyl-[1,1';4',1''] terphenyl (5): To a solution of 4 (0.32 g, 1 mmol) in DMF (30 ml) was added potassium thioacetate (0.12g, 1.05 mmol). After stirring 3 h under argon at room temperature, DMF was removed by evaporation. The remaining portion was treated with a little water and extracted with chloroform. The obtained solid from the organic portion was recrystallized with ethyl acetate. Pure 4''- acetylsulfanylmethyl-[1,1';4',1''] terphenyl (5) was collected as white solid (quantitatively). 1 H NMR (400 MHz, CDCl 3 ): δ (ppm) 7.65 (m, 4H); 7.63 (m, 2H); 7.57 (m, 2H); 7.45 (m, 2H); 7.37 (m, 3H); 4.17 (s, 2H); 2.37 (s, 3H). 13 C NMR (400 MHz, CDCl 3 ): δ (ppm) (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s). GC-MS (EI) m/z: calcd. for C 21 H 18 OS, ; found, (M + ). [1,1';4',1'']Terphenyl-4''-yl-methanethiol (1): To a solution of 5 (0.32g, 1 mmol) in methanol (10 ml) was added methanol solution (1 M) of sodium thiomethoxide (91 mg, nature nanotechnology 3

4 supplementary information 1.3 mmol). The mixture was stirred under argon at room temperature for 2 h and the reaction was complete (monitored by TLC). Aqueous HCl solution (0.1 M) was added until the ph value was about 6. The solution was extracted with chloroform. The collected solid was recrystallized with n-butyl alcohol. Pure [1,1';4',1'']terphenyl-4''-ylmethanethiol (1) was obtained as a white solid (90 % yield). 1 H NMR (400 MHz, CDCl 3 ): δ (ppm) 7.66 (m, 4H); 7.63 (m, 2H); 7.59 (m, 2H); 7.45 (m, 2H); 7.41 (m, 2H); 7.35 (m, 1H); 3.80 (d, 2H); 1.81 (t, 1H). GC-MS (EI) m/z: calcd. for C 19 H 16 S, ; found, (M + ). H N Br 1.5% Pd 2 (dba) 3 + B(OH) 2 4.5% P(t-Bu) equiv KF 1,4-dioxane/reflux 6 N H CS 2 /NaOH/THF r.t. 2 N SNa S N-Methyl-[1,1';4',1'']terphenyl-4''-ylmethyl-amine (6): To a solution of (4-bromobenzyl)-methyl-amine (1.4 g, 1.4 ml, 7 mmol) in 1,4-dioxane (20 ml) were added [1,1'- biphenyl]-4-ylboronic acid (2.08 g, 10.5 mmol), KF (1.36 g, 23.5 mmol), Pd 2 (dba) 3 (98 mg, mmol), P(t-Bu) 3 (63 mg, 0.31 mmol) and the mixture was refluxed overnight. When the system cooled down, the solid portion was filtered out. The solvents were removed by evaporation. After flash chromatography on silica gel (methanol: TLC, R f = 0.2) and recrystalization with THF, pure N-methyl-[1,1';4',1'']terphenyl-4''-ylmethylamine (6) was obtained as a white solid (92 % yield). 1 H NMR (400 MHz, CD 3 OD): δ (ppm) 7.69 (s, 4H); 7.65 (m, 4H); 7.42 (m, 4H); 7.33 (m, 1H); 3.74 (s, 2H); 2.39 (s, 3H). 13 C NMR (400 MHz, CD 3 OD): δ (ppm) (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s). 4 nature nanotechnology

5 supplementary information Sodium N-methyl-[1,1';4',1'']terphenyl-4''-ylmethyl dithiocarbamate (2): To a solution of 6 (68 mg, 0.25 mmol) in THF (2.5 ml) was added CS 2 (2 equiv., 0.03 ml, 0.5 mmol) and aqueous 1N NaOH (2 equiv., 0.5 ml, 0.5 mmol). The mixture was stirred for 2 h at room temperature, yielding a white precipitate came out. The solid collected by filtration was purified by recrystallization with THF. Pure sodium N-methyl- [1,1';4',1'']terphenyl-4''-ylmethyl dithiocarbamate (2) was obtained as a white crystalline (86 % yield). 1 H NMR (400 MHz, CD 3 OD): δ (ppm) 7.70 (m, 4H); 7.65 (m, 4H); 7.44 (m, 4H); 7.33 (m, 1H); 5.56 (s, 2H); 3.45 (s, 3H). 13 C NMR (400 MHz, CD 3 OD): δ (ppm) (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s); (s). GC-MS (EI) m/z: calcd. for C 21 H 18 NaNS 2, ; found, ([M]- SCS-Na + ). N H CS 2 /NaOH/THF r.t. N SNa S Sodium N-methyl benzyl dithiocarbamate (MBDTC): To a solution of N-methyl benzylamine (0.12 g, 0.13 ml, 1 mmol) in THF (2 ml), CS 2 (0.12 ml, 2 mmol) and aqueous 1N NaOH (2 ml, 2 mmol) were added. The mixture was stirred overnight at room temperature. After the solvents were removed by evaporation, acetone was added into the remaining solid. The liquid portion with the dissolved product was collected by filtration. Pure sodium N-methyl benzyl dithiocarbamate was obtained as a white solid (90 % yield) by the removal of the solvent. 1 H NMR (400 MHz, (CD 3 ) 2 CO): δ (ppm) 7.37 (m, 2H); 7.30 (m, 2H); 7.21 (m, 1H); 5.52 (s, 2H); 3.38 (s, 3H). 13 C NMR (400 MHz, (CD 3 ) 2 CO): δ (ppm) (s); (s); (s); (s); (s); (s); (s). GC-MS (EI) m/z: calcd. for C 9 H 10 NaNS 2, ; found, ([M]- SCS-Na + ). nature nanotechnology 5

6 supplementary information Valence band structure of DMDTC and butanethiol monolayers Figure S1 shows UPS spectra of butanethiol (a) and DMDTC (b) monolayers on Au(111). The projected density of states (DOS) is obtained from electronic structure calculations based on plane wave DFT. A pronounced difference in the spectra of butanethiol and DMDTC is observed in the low energy band at 1-2 ev binding energy. Butanethiol on Au shows a weak signal at ~ 1.3 ev related to the HOMO (sulfur-au antibonding hybrids), as already reported by Frisbie 1 and Armstrong. 2 However, DMDTC on Au shows a more distinct resonance in this range. It is centered at ~1.5 ev, with low energy onset at ~0.9 ev (Figure 1b). For assignment of the electronic states contributing to this resonance, dialkyl-dithiocarbamates with different alkyl chainlenghts were compared (Figure S2). The intensity of the resonance is found to decrease with increasing chainlength. The attenuation is a consequence of the inelastic scattering of photoelectrons by the alkyl chains, indicating that the resonance is related to states localized at the molecule-au interface. Monolayer structure from theoretical modelling The structure and DOS of the molecular layers BM and MBDTC on Au(111) has been analyzed by theoretical calculations on the basis of density functional theory (DFT) using a Au slab model (4 layers of Au) and a plane wave basis set (see Materials). At the given coverage (deduced from XPS, see Table 1), the resulting structures show evidence that the molecules form close packed layers assuming a standing-up orientation on the Au surface (Figure S3). Even though this analysis is performed for BM and MBDTC only, the conclusions are applicable for TPT and TPDTC as well. 6 nature nanotechnology

7 supplementary information Atomic orbital based DFT calculations. The molecule-au 1 complex is a simplified model system to understand the electronic structure of dithiocarbamates on Au. It allows us to visualize the overlap of sulfur p orbitals with the metal states on Au (Figure S4). This overlap is a relevant criterion for the estimation of the coupling strength Γ, since this enters into the equation for the calculation of charge transport across a metal-molecule-metal junction. From the geometry of the anchor groups and the orientation of the p orbitals of sulfur with respect to the Au surface, it becomes clear that DMDTC couples more efficiently than butanethiol to Au. This effect is also disclosed in density difference plots that reveal the high degree of hybridization (and thus of bonding strength) occurring between the dithiocarbamate adsorbate and the metal surface (see Figure 4). The orbitals in Figure S4 are obtained from DFT calculations at the B3LYP theory level using a LanL2DZ basis set, performed using the Gaussian 98 program suite. 3 The simulations are done on molecules coupled by the anchor groups to a single Au atom. Energy levels and molecular orbital isosurfaces are obtained upon full relaxation of the target structures. nature nanotechnology 7

8 supplementary information a Butanethiol Intensity (arb. units) SAM DOS But DOS p - S SAM Intensity (arb. units) Au Au Binding energy (ev) Binding energy (ev) b DMDTC Intensity (arb. units) DOS DMDTC SAM DOS p - S SAM Intensity (arb. units) Au Au Binding energy (ev) Binding energy (ev) Figure S1. UPS spectra of butanethiol (a) and DMDTC (b) monolayers (black) on Au(111). The spectrum of a clean Au surface is shown for comparison (blue). The intensity of the spectra is normalized at the Fermi level and an arbitrary offset between the plots is introduced for clarity. The DOS projected on the adlayer (red) and on sulfur p states (green) is obtained from plane wave DFT calculations with an Au surface slab 8 nature nanotechnology

9 supplementary information model. In the right panel, the UPS spectrum in the low binding energy region (Fermi edge) is shown. An important difference in the spectra is observed at ~1.5 ev, where (DOS) DMDTC shows a pronounced resonance (from 0.9 ev to 2 ev). Intensity (arb. units) x 4 x 2 DDDTC DHDTC DMDTC Binding energy (ev) Figure S2: UPS spectra of Dialkyl-dithiocarbamate SAMs on Au(111) with different chainlengths: Dimethyl-dithiocarbamate (DMDTC) (black), N,N-dihexyldithiocarbamate (DHDTC) (blue) and N,N-didecyl-dithiocarbamate (DDDTC) (red). An arbitrary offset between the plots and scaling factors for DHDTC (x 2) and DDDTC (x 4) were introduced for comparability. The resonance at 1.5 ev binding energy decreases in intensity with increasing thickness of the hydrocarbon layer due to the electron attenuation (attenuation length ~0.6 nm) of UPS photoelectrons. nature nanotechnology 9

10 supplementary information a 10 nature nanotechnology

11 supplementary information b Figure S3: Relaxed structure of BM (a) and MBDTC (b) monolayers on a periodic Au(111) slab-model (4 layers Au). (a) The number of BM molecules/unit cell are found to be close to that of hexagonally close packed alkanethiol monolayers on Au(111) (( 3 x 3)R30 structure), since XPS results show comparable signal intensities for BM and dense dodecanethiol monolayers (Table 1 and Materials). In the relaxed structure (equilibration of BM and of the first Au layer only), the thiolate group of BM adopts the bridge adsorption site on the Au(111) lattice. The next-neighbor S-Au bond distance is 2.46 Å and the tilt angle of the phenyl ring towards the surface normal is 11. (b) The same number of molecules/unit cell as for BM is employed for MBDTC, as the nature nanotechnology 11

12 supplementary information experimental packing density of the two compounds is very close (Table 1). In the equilibrium structure, MBDTC is chemisorbed to Au(111) with both sulfur atoms in the top adsorption site, resulting in an orientation of the S-S axis along the [ 101 ] direction. The S-S distance is 2.96 Å, almost matching the distance between the NN Au atoms (2.86 Å). The orientation of the DMDTC backbone is almost perpendicular to the surface. The presence of a number of local minima in the potential energy surface of DMDTC adsorbed to Au (not investigated) implies that other equilibrium structures cannot be ruled out. Note that the intermolecular dispersion forces are not fully accounted for within the DFT model. In the figure, orange, yellow and grey spheres are Au, sulfur and carbon atoms, respectively. For clarity, the upper Au layer is drawn using large spheres. DMDTC Butanethiol E HOMO - 4 ev E HOMO ev Figure S4: Selected bonding sulfur-au molecular orbitals of the DMDTC-Au 1 and the butanethiol-au 1 complex. The orbitals are derived from DFT calculations based on 12 nature nanotechnology

13 supplementary information the B3LYP/LanL2DZ theory level. The isosurfaces show that, as a result of the geometry of the dithiocarbamate anchor group, both sulfur 3p lobes of DMDTC are oriented towards the Au atom, increasing the overlap between metal s and d states and the molecular states on the anchor group. Reference List 1. Kim, Beebe, J.M., Jun,Y., Zhu, X.Y. & Frisbie, C. D. Correlation between HOMO Alignment and Contact Resistance in Molecular Junctions: Aromatic Thiols versus Aromatic Isocyanides. Journal of the American Chemical Society 128, (2006). 2. Alloway, D.M. et al. J. Phys. Chem. B 107, (2003). 3. Frisch, M. et. al. Gaussian 98. Gaussian Inc., Pittsburgh PA (1998). nature nanotechnology 13