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Molecule‐Electrode Interfaces Controlled by Bulky Long‐Legged Ligands in Organometallic Molecular Wires

Molecule‐Electrode Interfaces Controlled by Bulky Long‐Legged Ligands in Organometallic Molecular... IntroductionA well‐defined, highly conducting molecule‐electrode interface is essential for development of organic electronics. Interfacial structures of self‐assembled monolayer (SAM) on electrodes are controlled by the dense packing of the molecules and the attracting interactions between molecule and electrodes (Figure 1a).[1,2] On the other hand, single‐molecule junction (SMJ), which is composed of a bridging molecule sandwiched by two electrodes, requires robust connection with electrodes due to lack of intermolecular stabilization. In addition, molecular orientation (molecule‐electrode distance and tilt angle at the surface) and binding modes (σ and π bonds, and number of bonding interactions) in the SMJ significantly vary conductance (Figure 1b).[3,4] Multipodal anchor groups with branched structures have been explored to directly control interfacial structures in SMJ.[5–7] On the other hand, as a new molecular design, we envisioned that rational three‐dimensional structures, which indirectly affect anchor‐electrodes bondings by steric hinderance (Figure 1c), which would be beneficial to control the interfacial structures. Bulky groups on the side chain interact with electrodes when molecular junction forms and may control orientation and binding mode of the anchor groups at the main chain. This strategy gains merit in (i) modifying function of commonly used anchor groups and (ii), unlike multipodal anchor groups, avoiding possible undesired conducting pathways.[8] In this context, coordination complexes with octahedral geometries are suitable building blocks because of their diverse ligand design.1Figurea) Schematic illustrations of molecular junctions in the SAM and SMJ forms. b) Possible structures that may cause conductance variation in SMJ. c) Steric control of interfacial structures by 3D molecular structures. d) Our molecular design: organometallic molecular wires with the long‐legged ligands. e) Molecular structures of 1R‐3R and their molecular junction Au‐n‐Au (n: 1–3).Molecular wires with terminal acetylide anchor groups are known to show small contact resistance and to form robust molecular junctions.[9–13] However, BJ measurements of small organic molecules, e.g., 1,4‐diethynylbenzene‐diyl junction, afforded ill‐defined conductance that hindered determination of accurate single‐molecule conductance.[9,14] Recently, we unveiled single‐molecule conductance properties of the well‐defined metal acetylide molecular wires.[15–18] To account for these results, we hypothesized that relatively bulky bis(diphosphine) ligands affect the junction configurations and spatial arrangements (Figure 1d). In this study, we have designed and synthesized organoruthenium butadiynyl molecular wires with the bulky, long‐legged dppe‐type diphosphine ligands with the biphenyl (2R) and p‐tert‐butylbiphenyl (3R) substituents (Figure 1d,e), and compared their properties with those of the prototypical molecular wire 1R with the parent dppe ligands. We investigated their single‐molecule conductance and surface‐enhanced Raman spectroscopic features when attached to gold surfaces to disclose the steric effect of the ancillary ligands in the molecular wires. The keys of our molecular design are that (i) the charge carriers are transported through the main metallapolyyne‐diyl chain, while (ii) the peripheral long‐legged supporting ligands indirectly control the interfacial structures between the wires and the metal surface.Results and DiscussionSynthesis and CharacterizationTo form a covalently linked organometallic molecular junction, the terminal carbon atoms were functionalized with the gold fragments.[16] Thus, the gold complexes 2Au and 3Au were prepared from the SiMe3 precursors 2TMS and 3TMS, respectively, by treatment with NaOMe and ClAu{P(OMe)3} (Figure 1e). Alternatively, 2Au and 3Au were be prepared in two steps by the desilylation‐auration sequence of nTMS through the terminal acetylene derivatives nH. Their 1H and 31P{1H} NMR spectra contain only one set of signals for the ancillary ligands, indicating the long‐legged ligands are equivalent, i.e., flexible enough in solution at the timescale of NMR spectroscopy. There is no noticeable deshielding for the TMS or P(OMe)3 groups, suggesting negligible spatial proximity between the central alkynyl chain and the long‐legged biphenyl skeletons. Molecular structures of 2TMS and 3TMS are shown in Figure 2a,b. The two alkynyl ligands adopt a trans geometry, and the long‐legged ligands are radially spread. The aromatic rings show no π–π interactions (dC‐C > 3.5 Å), yet some CH–π interaction is observed for the proximal phenyl rings bonded to the phosphorous atoms. The distances between the terminal carbon atoms of the phenyl rings (≈1.3 nm for 2TMS) and the t‐Bu groups (≈1.6 nm for 3TMS) are longer than those between the terminal carbon atoms (Cδ) of the butadiyne moieties (1.17 for 2TMS and 1.16 nm for 3TMS, respectively). Thus, the bulky phosphine ligands may interact with the gold surface when a molecular junction is formed through C–Au covalent bond formation. These structural features resemble those for the optimized structures of 2H and 3H obtained by the density functional theory (DFT) calculations (Figure S22, Supporting Information), being suggestive of negligible packing effect.2FigureThermal ellipsoid plots of: a) 2TMS and b) 3TMS with 50% probability levels. Hydrogen atoms, solvent molecules, and disordered parts are omitted for clarity.SAM StudyTo investigate the spatial arrangement of the molecular wires on the gold surface, we carried out Raman spectroscopic study on their SAMs. The gold substrates prepared by frame annealing of gold wires were soaked in 1 mm CH2Cl2 solutions of nAu for 24 h and rinsed with CH2Cl2. Then, the surface‐enhanced Raman scattering (SERS) spectra were recorded and compared with the parent molecules nAu (Figure 3a). The terminal gold functionalization facilitate formation of the covalent Au–C acetylide bonds through either transmetallation or fusion with the gold complexes.[16] The SAM samples are abbreviated as SAMn (n = 1–3). The peaks around 400 cm−1 in the SERS spectra of SAM1‐SAM3 may be assigned to the vibrations of the formed covalent Au–C bonds (Figure S17, Supporting Information).[12] The vibrations around 2050 cm−1 for neat samples of nAu are assignable to the Ru‐CC‐CC vibrations according to the DFT study (Figure S19, Supporting Information). On the other hand, SAMn showed properties characteristic of each molecule. Raman signals ascribed to the acetylene parts for SAM1 are observed as ill‐defined, broadened signals in the range of 1800–2200 cm−1,[19] whereas well‐defined, sharp signals at 1860, 1898, 1989, and 2040 cm−1 are observed for SAM3. The characteristics of SAM2 are intermediate between SAM1 and SAM3. Because Raman spectra of neat samples of nAu are similar with each other, the differences in SAMn should be caused by different arrangement and connection structures of the molecules on the gold surface.3Figurea) Raman spectra for SAM1‐3 formed on the gold substrates. Bars represent the DFT‐simulated Raman shifts of Au‐1 (black: on‐top, purple: bridge, orange: hollow). The scaling factor was set to 0.93 (B3LYP/LanL2DZ (Ru, Au),6‐31G(d) (C, H, P)). b) Surface model structures for the DFT calculations Au‐1.To consider the interaction modes, we carried out the Raman simulation by the DFT study for the surface‐attached models Au‐1, where 1Au is attached to the 17 and 18 gold atom clusters with the on‐top, bridge, and hollow modes (Figure 3b). The simulated Raman shifts appear within the range of the experimental ones (1900–2100 cm−1, Figure 3a). The simulated vibrational peaks result from a combination of two to four acetylene‐stretching modes. Peak signatures are sensitive to the surface‐bound model and the Au(electrode)‐C bond lengths but relatively insensitive to the Au(electrode)‐CC angles (Figure S21, Supporting Information). DFT simulation suggests that the broadened signals of SAM1 are caused by the variations of the connection modes and orientations. When compared with the Raman spectrum of SAM3 and the simulated signal pattern and energies, the on‐top structure seems plausible among the three fashions. The on‐top structure should be induced by the long‐legged ligands to avoid steric repulsion between the bulky ligands and the electrodes. Molecular modeling of 3H on a flat Au surface suggests that the phosphine ligand is flexible and that 3H can stand on the surface in a way that avoids steric hindrance to the Au surface (Figure S24, Supporting Information). SAM2 showed the spectrum similar to but rather broader than that of SAM3. This result suggests that the medium‐sized biphenyl ligands are also effective to control the binding modes, while biphenyl derivatives have some degree of structural freedom on the Au surfaces compared with the tert‐butyl‐biphenyl derivatives.STM‐BJ StudyWe performed single‐molecule conductance measurements of the molecular junctions of Au‐2‐Au and Au‐3‐Au prepared from 2Au and 3Au, respectively, by the STM‐BJ method,[20] and compared with the conductance of Au‐1‐Au. As we previously reported, 1Au in‐situ formed covalently linked molecular junction Au‐1‐Au, and the single‐molecule conductance of the junction was 2.1 × 10−2 G0 (log GM/G0 = –1.77). The samples were dissolved in tetraglyme (0.25 mm) and bias voltage was set to 100 mV. Typical conductance traces and histograms for Au‐n‐Au are shown in Figure 4. STM break‐junction measurements of Au‐2‐Au and Au‐3‐Au also revealed steps and peaks in the individual traces and log histograms, respectively. These results indicate that molecular junctions Au‐2‐Au and Au‐3‐Au formed despite their bulky long‐legged ligands. Based on deconvolution analysis, the most probable single‐molecule conductance (log GM/G0) is determined to be –1.81 (2Au) and –1.71 (3Au), which are virtually identical to that of 1Au. Thus, it can be concluded that the carrier transport occurred through the Ru(CC‐CC‐C)2 chain regardless of the phosphine ligands. Furthermore, their full widths at the half maximum (FWHM) of the log‐scale conductance histograms (Figure 4a and Table 1) were sensitive to the phosphine ligands. As the length and bulkiness of the phosphine ligands increase, the FWHMs significantly decrease (1.16 (1) → 0.72 (2)→ 0.32 (3)). The individual traces (Figure 4b) reflect the trends observed for the log histograms. In contrast with the relatively flat steps observed for 3, the steps for 1 were fluctuated. Thus, the long‐legged ligands turned out to be able to control the conductance distribution.4Figurea) 1D histograms constructed from 2000 traces and b) typical individual traces for Au‐n‐Au (n = 1–3) obtained by the STM‐BJ study. The arrows indicate conductance peaks. The conductance around log G/G0 = –0.5 is caused by the scattered conductance of Au nanowires.[23,24]1TableSpectroscopic, theoretical, and break‐junction data for 1R‐3RComplexE1/2/mVλmax (λonset)/nmHOMOExpa) (HOMODFTc))/eVLUMOExpb) (LUMODFTc))/eVlog G0FWHMRTMSTMSTMS (H)TMS (H)Au(electrode)Au(electrode)1R150280 (302)–4.95 (–4.63)–0.85 (–1.09)–1.77 ± 0.121.16 ± 0.012R175270 (318)–4.93 (–4.50)–1.03 (–1.12)–1.81 ± 0.060.72 ± 0.173R160282 (325)–4.94 (–4.40)–1.13 (–1.02)–1.71 ± 0.020.32 ± 0.04a)HOMOExp = –[5.1 + E1/2(V vs FeCp2/FeCp2+)]b)LUMOExp = HOMOExp+λonset(eV)c)Calculated by the B3LYP/LanL2DZ,6‐31G(d),CPCM(CH2Cl2) levels of theory.The most probable binding mode of Au‐1‐Au is presumed to be the on‐top structure as reported previously,[16] but we do not exclude the possibility of other binding modes in the BJ process. The binding modes occurring in diethynylbenzene‐diyl junctions are known to impact conductance,[21] and, thus, the presence of a range of binding modes for the acetylide junctions would also result in variations in conductance. As discussed in the SAM study, binding modes on the Au surface can be controlled by the long‐legged ligands, which would also effective to control the binding modes in the SMJ and contribute to the narrow conductance distribution.Furthermore, we are also interested in the effects of molecular orientation on conductance, which have not been examined in the literature. To gain insight into this point, we carried out the DFT‐non equilibrium Green's function (NEGF) calculation. Thus, the plots of the tilt angles ∠CC‐Au (θ) versus conductance are shown in Figure 5, using the molecular junction models with the Au35 clusters. As the angle θ decreases, transmission at the Fermi level is raised by more than one order of magnitude at 140°. Because the main conduction orbitals are not dependent on θ, the enhanced conductance with an increase of θ is probably caused by the changes in the degree of the dπ–pπ orbital interactions of the Au–C bonds, which was previously proposed to affect conductance.[11] In this context, the bulky long‐legged ligand may hinder the formation of the molecular junction with smaller θ values by steric repulsion, leading to suppression of the variation of conductance in the molecular junction.[22] The most probable conductance in Au‐n‐Au is similar regardless of the ligands, suggesting that the molecules are fully stretched with θ close to 180°. Therefore, both the restricted angles and binding modes of the long‐legged ligands should contribute to the narrow conductance distributions.5Figure(Left) Plots of transmission against θ. (Right) Conduction orbitals of Au‐1‐Au.SERS‐MCBJ StudyWe carried out SERS measurements during the mechanically controllable BJ (MCBJ) processes to obtain further information on the electronic structures of the molecular junctions.[25,26] We recorded Raman spectra for Au‐1‐Au and Au‐3‐Au when the junction conductance reached around 10−2 G0, suggesting molecular junction with the on‐top attachment. Both Au‐1‐Au and Au‐3‐Au junctions showed Raman signals around 400 cm−1 assignable to the Au–C covalent bonds (Figure S18, Supporting Information).[12] In contrast with SAM1, the SERS spectrum of the Au‐1‐Au junction showed the simple spectral features with the major peak at 2025 cm−1 accompanying the satellite peak at 1998 cm−1 (Figure 6), and both are ascribed to the ν(CC) vibrations. On the other hand, the Raman spectrum of Au‐3‐Au junction with the bulky substituents showed the single and sharp Raman peak at 1972 cm−1.The Raman spectra contain information on dynamic BJ processes. Though we fixed the junction structure, the motion of the molecule and gold atoms modulate the junction structure. Therefore, the rather broadened and multiple peaks observed for Au‐1‐Au are most probably caused by SMJ with various interfacial structures. On the other hand, Au‐3‐Au adopts limited orientation with the large θ values and uniform Au–C bond length, leading to the sharp, single peak.6FigureRaman spectra of powder samples of 1Au and 3Au, and SERS spectra of molecular junctions Au‐1‐Au and Au‐3‐Au. Conductance values are 1 × 10−2 G0 and 2×10−2 G0 for Au‐1‐Au and Au‐3‐Au, respectively.ConclusionIn summary, we have developed organometallic molecular wires with the long‐legged ligands (2R and 3R), where the metal‐molecule interfacial structures are controlled by the ligands. The bulky long‐legged ligands provide self‐assembled monolayers with sharp SERS CC vibration signals, suggesting formation of well‐defined SAMs. Furthermore, STM break‐junction study reveals that the long‐legged ligands make the conductance distribution narrower. SERS‐MCBJ study supports controlled interfacial structures by the long‐legged ligands at the single‐molecule junction. Thus, our study demonstrates the successful indirect control of molecule‐electrode interfacial structures by long‐legged ligands. This long‐legged strategy will be useful for applications toward various molecular devices.AcknowledgementsThis work was supported by the JSPS KAKENHI [grant Nos. 18K05139 (to Y.T.), 21K05211 (to Y.T.), and 20K05445 (to S.K.)]. Y.T. acknowledges research grants from the ENEOS Tonengeneral Research/Development Encouragement & Scholarship Foundation, The Asahi Glass Foundation, Inamori Foundation, Tokyo Kasei Chemical Promotion foundation, Toyota Physical and Chemical Research Institute, and Tokuyama Science Foundation. The theoretical calculations were performed by using computers in the Research Center for Computational Science, Okazaki, Japan (grant Nos. 21‐IMS‐C071, 22‐IMS‐C071).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the supplementary material of this article.J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides, Chem. Rev. 2005, 105, 1103.F. Ishiwari, G. Nascimbeni, E. Sauter, H. Tago, Y. Shoji, S. Fujii, M. Kiguchi, T. Tada, M. Zharnikov, E. Zojer, T. Fukushima, J. Am. Chem. Soc. 2019, 141, 5995.Y. Komoto, S. Fujii, H. Nakamura, T. Tada, T. Nishino, M. Kiguchi, Sci. Rep. 2016, 6, 26606.M. Valášek, M. Mayor, Chem. ‐ Eur. J. 2017, 23, 13538.Y. Ie, T. Hirose, H. Nakamura, M. Kiguchi, N. Takagi, M. Kawai, Y. Aso, J. Am. Chem. Soc. 2011, 133, 3014.J. Šebera, V. Kolivoška, M. Valášek, J. Gasior, R. Sokolová, G. Mészáros, W. Hong, M. Mayor, M. Hromadová, J. Phys. Chem. C 2017, 121, 12885.T. Ohto, A. Tashiro, T. Seo, N. Kawaguchi, Y. Numai, J. Tokumoto, S. Yamaguchi, R. Yamada, H. Tada, Y. Aso, Y. Ie, Small 2021, 17, 2006709.M. Valášek, M. Lindner, M. Mayor, Beilstein J. Nanotechnol. 2016, 7, 374.J. Ponce, C. R. Arroyo, S. Tatay, R. Frisenda, P. Gaviña, D. Aravena, E. Ruiz, H. S. J. van der Zant, E. Coronado, J. Am. Chem. Soc. 2014, 136, 8314.P. Pla‐Vilanova, A. C. Aragonès, S. Ciampi, F. Sanz, N. Darwish, I. Diez‐Perez, Nanotechnology 2015, 26, 381001.I. J. Olavarria‐Contreras, M. L. Perrin, Z. Chen, S. Klyatskaya, M. Ruben, H. S. J. van der Zant, J. Am. Chem. Soc. 2016, 138, 8465.F. Bejarano, I. J. Olavarria‐Contreras, A. Droghetti, I. Rungger, A. Rudnev, D. Gutiérrez, M. Mas‐Torrent, J. Veciana, H. S. J. van der Zant, C. Rovira, E. Burzurí, N. Crivillers, J. Am. Chem. Soc. 2018, 140, 1691.G. Mitra, V. Delmas, H. A. Sabea, L. Norel, O. Galangau, S. Rigaut, J. Cornil, K. Costuas, E. Scheer, Nanoscale Adv. 2022, 4, 457.D. Millar, L. Venkataraman, L. H. Doerrer, J. Phys. Chem. C 2007, 111, 17635.Y. Tanaka, M. Kiguchi, M. Akita, Chem. ‐ Eur. J. 2017, 23, 4741.Y. Tanaka, Y. Kato, T. Tada, S. Fujii, M. Kiguchi, M. Akita, J. Am. Chem. Soc. 2018, 140, 10080.Y. Tanaka, K. Ohmura, S. Fujii, T. Tada, M. Kiguchi, M. Akita, Inorg. Chem. 2020, 59, 13254.Y. Tanaka, Y. Kato, K. Sugimoto, R. Kawano, T. Tada, S. Fujii, M. Kiguchi, M. Akita, Chem. Sci. 2021, 12, 4338.W. Hong, H. Li, S.‐X. Liu, Y. Fu, J. Li, V. Kaliginedi, S. Decurtins, T. Wandlowski, J. Am. Chem. Soc. 2012, 134, 19425.B. Xu, N. J. Tao, Science 2003, 301, 1221.S. Li, H. Yu, X. Chen, A. A. Gewirth, J. S. Moore, C. M. Schroeder, Nano Lett. 2020, 20, 5490.Our molecular junction models of Au‐3‐Au using ideal trigonal pyramidal Au clusters suggest van Der Waals contact between the molecule and electrodes occurs from 140° (Figure S25, Supporting Information).C. Z. Li, H. X. He, A. Bogozi, J. S. Bunch, N. J. Tao, Appl. Phys. Lett. 2000, 76, 1333.R. Fukuzumi, S. Kaneko, S. Fujii, T. Nishino, M. Kiguchi, ChemPhysChem 2020, 21, 175.S. Kaneko, D. Murai, S. Marqués‐González, H. Nakamura, Y. Komoto, S. Fujii, T. Nishino, K. Ikeda, K. Tsukagoshi, M. Kiguchi, J. Am. Chem. Soc. 2016, 138, 1294.S. Kobayashi, S. Kaneko, S. Fujii, T. Nishino, K. Tsukagoshi, M. Kiguchi, Phys. Chem. Chem. Phys. 2019, 21, 16910. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Molecule‐Electrode Interfaces Controlled by Bulky Long‐Legged Ligands in Organometallic Molecular Wires

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Abstract

IntroductionA well‐defined, highly conducting molecule‐electrode interface is essential for development of organic electronics. Interfacial structures of self‐assembled monolayer (SAM) on electrodes are controlled by the dense packing of the molecules and the attracting interactions between molecule and electrodes (Figure 1a).[1,2] On the other hand, single‐molecule junction (SMJ), which is composed of a bridging molecule sandwiched by two electrodes, requires robust connection with electrodes due to lack of intermolecular stabilization. In addition, molecular orientation (molecule‐electrode distance and tilt angle at the surface) and binding modes (σ and π bonds, and number of bonding interactions) in the SMJ significantly vary conductance (Figure 1b).[3,4] Multipodal anchor groups with branched structures have been explored to directly control interfacial structures in SMJ.[5–7] On the other hand, as a new molecular design, we envisioned that rational three‐dimensional structures, which indirectly affect anchor‐electrodes bondings by steric hinderance (Figure 1c), which would be beneficial to control the interfacial structures. Bulky groups on the side chain interact with electrodes when molecular junction forms and may control orientation and binding mode of the anchor groups at the main chain. This strategy gains merit in (i) modifying function of commonly used anchor groups and (ii), unlike multipodal anchor groups, avoiding possible undesired conducting pathways.[8] In this context, coordination complexes with octahedral geometries are suitable building blocks because of their diverse ligand design.1Figurea) Schematic illustrations of molecular junctions in the SAM and SMJ forms. b) Possible structures that may cause conductance variation in SMJ. c) Steric control of interfacial structures by 3D molecular structures. d) Our molecular design: organometallic molecular wires with the long‐legged ligands. e) Molecular structures of 1R‐3R and their molecular junction Au‐n‐Au (n: 1–3).Molecular wires with terminal acetylide anchor groups are known to show small contact resistance and to form robust molecular junctions.[9–13] However, BJ measurements of small organic molecules, e.g., 1,4‐diethynylbenzene‐diyl junction, afforded ill‐defined conductance that hindered determination of accurate single‐molecule conductance.[9,14] Recently, we unveiled single‐molecule conductance properties of the well‐defined metal acetylide molecular wires.[15–18] To account for these results, we hypothesized that relatively bulky bis(diphosphine) ligands affect the junction configurations and spatial arrangements (Figure 1d). In this study, we have designed and synthesized organoruthenium butadiynyl molecular wires with the bulky, long‐legged dppe‐type diphosphine ligands with the biphenyl (2R) and p‐tert‐butylbiphenyl (3R) substituents (Figure 1d,e), and compared their properties with those of the prototypical molecular wire 1R with the parent dppe ligands. We investigated their single‐molecule conductance and surface‐enhanced Raman spectroscopic features when attached to gold surfaces to disclose the steric effect of the ancillary ligands in the molecular wires. The keys of our molecular design are that (i) the charge carriers are transported through the main metallapolyyne‐diyl chain, while (ii) the peripheral long‐legged supporting ligands indirectly control the interfacial structures between the wires and the metal surface.Results and DiscussionSynthesis and CharacterizationTo form a covalently linked organometallic molecular junction, the terminal carbon atoms were functionalized with the gold fragments.[16] Thus, the gold complexes 2Au and 3Au were prepared from the SiMe3 precursors 2TMS and 3TMS, respectively, by treatment with NaOMe and ClAu{P(OMe)3} (Figure 1e). Alternatively, 2Au and 3Au were be prepared in two steps by the desilylation‐auration sequence of nTMS through the terminal acetylene derivatives nH. Their 1H and 31P{1H} NMR spectra contain only one set of signals for the ancillary ligands, indicating the long‐legged ligands are equivalent, i.e., flexible enough in solution at the timescale of NMR spectroscopy. There is no noticeable deshielding for the TMS or P(OMe)3 groups, suggesting negligible spatial proximity between the central alkynyl chain and the long‐legged biphenyl skeletons. Molecular structures of 2TMS and 3TMS are shown in Figure 2a,b. The two alkynyl ligands adopt a trans geometry, and the long‐legged ligands are radially spread. The aromatic rings show no π–π interactions (dC‐C > 3.5 Å), yet some CH–π interaction is observed for the proximal phenyl rings bonded to the phosphorous atoms. The distances between the terminal carbon atoms of the phenyl rings (≈1.3 nm for 2TMS) and the t‐Bu groups (≈1.6 nm for 3TMS) are longer than those between the terminal carbon atoms (Cδ) of the butadiyne moieties (1.17 for 2TMS and 1.16 nm for 3TMS, respectively). Thus, the bulky phosphine ligands may interact with the gold surface when a molecular junction is formed through C–Au covalent bond formation. These structural features resemble those for the optimized structures of 2H and 3H obtained by the density functional theory (DFT) calculations (Figure S22, Supporting Information), being suggestive of negligible packing effect.2FigureThermal ellipsoid plots of: a) 2TMS and b) 3TMS with 50% probability levels. Hydrogen atoms, solvent molecules, and disordered parts are omitted for clarity.SAM StudyTo investigate the spatial arrangement of the molecular wires on the gold surface, we carried out Raman spectroscopic study on their SAMs. The gold substrates prepared by frame annealing of gold wires were soaked in 1 mm CH2Cl2 solutions of nAu for 24 h and rinsed with CH2Cl2. Then, the surface‐enhanced Raman scattering (SERS) spectra were recorded and compared with the parent molecules nAu (Figure 3a). The terminal gold functionalization facilitate formation of the covalent Au–C acetylide bonds through either transmetallation or fusion with the gold complexes.[16] The SAM samples are abbreviated as SAMn (n = 1–3). The peaks around 400 cm−1 in the SERS spectra of SAM1‐SAM3 may be assigned to the vibrations of the formed covalent Au–C bonds (Figure S17, Supporting Information).[12] The vibrations around 2050 cm−1 for neat samples of nAu are assignable to the Ru‐CC‐CC vibrations according to the DFT study (Figure S19, Supporting Information). On the other hand, SAMn showed properties characteristic of each molecule. Raman signals ascribed to the acetylene parts for SAM1 are observed as ill‐defined, broadened signals in the range of 1800–2200 cm−1,[19] whereas well‐defined, sharp signals at 1860, 1898, 1989, and 2040 cm−1 are observed for SAM3. The characteristics of SAM2 are intermediate between SAM1 and SAM3. Because Raman spectra of neat samples of nAu are similar with each other, the differences in SAMn should be caused by different arrangement and connection structures of the molecules on the gold surface.3Figurea) Raman spectra for SAM1‐3 formed on the gold substrates. Bars represent the DFT‐simulated Raman shifts of Au‐1 (black: on‐top, purple: bridge, orange: hollow). The scaling factor was set to 0.93 (B3LYP/LanL2DZ (Ru, Au),6‐31G(d) (C, H, P)). b) Surface model structures for the DFT calculations Au‐1.To consider the interaction modes, we carried out the Raman simulation by the DFT study for the surface‐attached models Au‐1, where 1Au is attached to the 17 and 18 gold atom clusters with the on‐top, bridge, and hollow modes (Figure 3b). The simulated Raman shifts appear within the range of the experimental ones (1900–2100 cm−1, Figure 3a). The simulated vibrational peaks result from a combination of two to four acetylene‐stretching modes. Peak signatures are sensitive to the surface‐bound model and the Au(electrode)‐C bond lengths but relatively insensitive to the Au(electrode)‐CC angles (Figure S21, Supporting Information). DFT simulation suggests that the broadened signals of SAM1 are caused by the variations of the connection modes and orientations. When compared with the Raman spectrum of SAM3 and the simulated signal pattern and energies, the on‐top structure seems plausible among the three fashions. The on‐top structure should be induced by the long‐legged ligands to avoid steric repulsion between the bulky ligands and the electrodes. Molecular modeling of 3H on a flat Au surface suggests that the phosphine ligand is flexible and that 3H can stand on the surface in a way that avoids steric hindrance to the Au surface (Figure S24, Supporting Information). SAM2 showed the spectrum similar to but rather broader than that of SAM3. This result suggests that the medium‐sized biphenyl ligands are also effective to control the binding modes, while biphenyl derivatives have some degree of structural freedom on the Au surfaces compared with the tert‐butyl‐biphenyl derivatives.STM‐BJ StudyWe performed single‐molecule conductance measurements of the molecular junctions of Au‐2‐Au and Au‐3‐Au prepared from 2Au and 3Au, respectively, by the STM‐BJ method,[20] and compared with the conductance of Au‐1‐Au. As we previously reported, 1Au in‐situ formed covalently linked molecular junction Au‐1‐Au, and the single‐molecule conductance of the junction was 2.1 × 10−2 G0 (log GM/G0 = –1.77). The samples were dissolved in tetraglyme (0.25 mm) and bias voltage was set to 100 mV. Typical conductance traces and histograms for Au‐n‐Au are shown in Figure 4. STM break‐junction measurements of Au‐2‐Au and Au‐3‐Au also revealed steps and peaks in the individual traces and log histograms, respectively. These results indicate that molecular junctions Au‐2‐Au and Au‐3‐Au formed despite their bulky long‐legged ligands. Based on deconvolution analysis, the most probable single‐molecule conductance (log GM/G0) is determined to be –1.81 (2Au) and –1.71 (3Au), which are virtually identical to that of 1Au. Thus, it can be concluded that the carrier transport occurred through the Ru(CC‐CC‐C)2 chain regardless of the phosphine ligands. Furthermore, their full widths at the half maximum (FWHM) of the log‐scale conductance histograms (Figure 4a and Table 1) were sensitive to the phosphine ligands. As the length and bulkiness of the phosphine ligands increase, the FWHMs significantly decrease (1.16 (1) → 0.72 (2)→ 0.32 (3)). The individual traces (Figure 4b) reflect the trends observed for the log histograms. In contrast with the relatively flat steps observed for 3, the steps for 1 were fluctuated. Thus, the long‐legged ligands turned out to be able to control the conductance distribution.4Figurea) 1D histograms constructed from 2000 traces and b) typical individual traces for Au‐n‐Au (n = 1–3) obtained by the STM‐BJ study. The arrows indicate conductance peaks. The conductance around log G/G0 = –0.5 is caused by the scattered conductance of Au nanowires.[23,24]1TableSpectroscopic, theoretical, and break‐junction data for 1R‐3RComplexE1/2/mVλmax (λonset)/nmHOMOExpa) (HOMODFTc))/eVLUMOExpb) (LUMODFTc))/eVlog G0FWHMRTMSTMSTMS (H)TMS (H)Au(electrode)Au(electrode)1R150280 (302)–4.95 (–4.63)–0.85 (–1.09)–1.77 ± 0.121.16 ± 0.012R175270 (318)–4.93 (–4.50)–1.03 (–1.12)–1.81 ± 0.060.72 ± 0.173R160282 (325)–4.94 (–4.40)–1.13 (–1.02)–1.71 ± 0.020.32 ± 0.04a)HOMOExp = –[5.1 + E1/2(V vs FeCp2/FeCp2+)]b)LUMOExp = HOMOExp+λonset(eV)c)Calculated by the B3LYP/LanL2DZ,6‐31G(d),CPCM(CH2Cl2) levels of theory.The most probable binding mode of Au‐1‐Au is presumed to be the on‐top structure as reported previously,[16] but we do not exclude the possibility of other binding modes in the BJ process. The binding modes occurring in diethynylbenzene‐diyl junctions are known to impact conductance,[21] and, thus, the presence of a range of binding modes for the acetylide junctions would also result in variations in conductance. As discussed in the SAM study, binding modes on the Au surface can be controlled by the long‐legged ligands, which would also effective to control the binding modes in the SMJ and contribute to the narrow conductance distribution.Furthermore, we are also interested in the effects of molecular orientation on conductance, which have not been examined in the literature. To gain insight into this point, we carried out the DFT‐non equilibrium Green's function (NEGF) calculation. Thus, the plots of the tilt angles ∠CC‐Au (θ) versus conductance are shown in Figure 5, using the molecular junction models with the Au35 clusters. As the angle θ decreases, transmission at the Fermi level is raised by more than one order of magnitude at 140°. Because the main conduction orbitals are not dependent on θ, the enhanced conductance with an increase of θ is probably caused by the changes in the degree of the dπ–pπ orbital interactions of the Au–C bonds, which was previously proposed to affect conductance.[11] In this context, the bulky long‐legged ligand may hinder the formation of the molecular junction with smaller θ values by steric repulsion, leading to suppression of the variation of conductance in the molecular junction.[22] The most probable conductance in Au‐n‐Au is similar regardless of the ligands, suggesting that the molecules are fully stretched with θ close to 180°. Therefore, both the restricted angles and binding modes of the long‐legged ligands should contribute to the narrow conductance distributions.5Figure(Left) Plots of transmission against θ. (Right) Conduction orbitals of Au‐1‐Au.SERS‐MCBJ StudyWe carried out SERS measurements during the mechanically controllable BJ (MCBJ) processes to obtain further information on the electronic structures of the molecular junctions.[25,26] We recorded Raman spectra for Au‐1‐Au and Au‐3‐Au when the junction conductance reached around 10−2 G0, suggesting molecular junction with the on‐top attachment. Both Au‐1‐Au and Au‐3‐Au junctions showed Raman signals around 400 cm−1 assignable to the Au–C covalent bonds (Figure S18, Supporting Information).[12] In contrast with SAM1, the SERS spectrum of the Au‐1‐Au junction showed the simple spectral features with the major peak at 2025 cm−1 accompanying the satellite peak at 1998 cm−1 (Figure 6), and both are ascribed to the ν(CC) vibrations. On the other hand, the Raman spectrum of Au‐3‐Au junction with the bulky substituents showed the single and sharp Raman peak at 1972 cm−1.The Raman spectra contain information on dynamic BJ processes. Though we fixed the junction structure, the motion of the molecule and gold atoms modulate the junction structure. Therefore, the rather broadened and multiple peaks observed for Au‐1‐Au are most probably caused by SMJ with various interfacial structures. On the other hand, Au‐3‐Au adopts limited orientation with the large θ values and uniform Au–C bond length, leading to the sharp, single peak.6FigureRaman spectra of powder samples of 1Au and 3Au, and SERS spectra of molecular junctions Au‐1‐Au and Au‐3‐Au. Conductance values are 1 × 10−2 G0 and 2×10−2 G0 for Au‐1‐Au and Au‐3‐Au, respectively.ConclusionIn summary, we have developed organometallic molecular wires with the long‐legged ligands (2R and 3R), where the metal‐molecule interfacial structures are controlled by the ligands. The bulky long‐legged ligands provide self‐assembled monolayers with sharp SERS CC vibration signals, suggesting formation of well‐defined SAMs. Furthermore, STM break‐junction study reveals that the long‐legged ligands make the conductance distribution narrower. SERS‐MCBJ study supports controlled interfacial structures by the long‐legged ligands at the single‐molecule junction. Thus, our study demonstrates the successful indirect control of molecule‐electrode interfacial structures by long‐legged ligands. This long‐legged strategy will be useful for applications toward various molecular devices.AcknowledgementsThis work was supported by the JSPS KAKENHI [grant Nos. 18K05139 (to Y.T.), 21K05211 (to Y.T.), and 20K05445 (to S.K.)]. 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Journal

Advanced Materials Interfaces – Wiley

Published: Apr 1, 2023

Keywords: long‐legged ligand; metal acetyide; molecular junction; Raman spectroscopy; SERS

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