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Impact of Orbital Hybridization at Molecule−Metal Interface on Carrier Dynamics Masanori Sakamoto,*,† Kim Hyeon-Deuk,*,‡ Daichi Eguchi,† I.-Y. Chang,‡ Daisuke Tanaka,§ Hirokazu Tahara,† Akihiro Furube,∥ Yoshihiro Minagawa,⊥ Yutaka Majima,⊥ Yoshihiko Kanemitsu,† and Toshiharu Teranishi*,† †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Chemistry, Kyoto University, Kyoto, Kyoto 606-8502, Japan § Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan ∥ Department of Optical Science, Tokushima University, Tokushima, Tokushima 770-8506, Japan ⊥ Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama 226-8503, Japan

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‡

S Supporting Information *

ABSTRACT: The orientation of a molecule on a metal surface can impact the performance of electronic devices fabricated from organic materials. This orientation effect of physiosorbed or weakly chemisorbed molecules has been widely debated, and its origin remains unknown because methods to investigate the weak interaction at the molecule/ inert-metal interface have been limited. Here, it is shown via spectroscopy and density functional calculations that molecule/metal orbital hybridization, which is determined by the molecular orientation, is an identity of the orientation effect dictating the carrier dynamics at the interface. Nanoscale model interfaces, where molecules were weakly chemisorbed on a metal, made it possible to visualize the orientationdependent shift of the electronic state. The transient absorption spectroscopy and scanning tunneling spectroscopy of porphyrin derivative coordinated on gold nanoparticle demonstrated that the orbital hybridization dictates the interfacial carrier dynamics. This new understanding of the molecule/metal interfaces will enable functional-molecule designs based on the cross-materials orbital hybridization for various devices.

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INTRODUCTION The composite materials combining organic molecules and nanoparticles have proven fruitful in various research fields including photovoltaics, catalysts, electronics, etc. owing to the synergy of advantages of organic molecules and inorganic nanoparticles (NPs).1−7 The carrier dynamics at the crossmaterial interfaces dominate the properties of the above hybrid materials and devices, while the factors holding the key to controlling the dynamics have not yet been well-understood. The orientation of an organic molecule on a metal surface impacts the interfacial carrier dynamics, which determines the performance of organic electronics such as transistor, lightemitting devices, and photovoltaics.8−17 While the electronic interaction at a molecule/metal or semiconductor interface is generally weak, the molecular orientation can substantially affect device performance.8−11 A favorable molecular geometry depends on the applications. In molecular electronics, the orientation that promotes an overlap between the molecular and metallic orbitals is favorable,12−15 while such an orientation is often unfavorable for photovoltaics.16,17 Hence, the orientation of adsorbed molecules has been implicit design © XXXX American Chemical Society

guides for organic devices as well as hybrid nanomaterials. However, the origin of the molecular orientation effect has been unclear because of the difficulty in creating uniquely oriented molecules on metal substrates without an intermolecular interaction and in investigating the small interaction at the bulk interface. The transition of the interface from the bulk to the nanoregime is a key to solving this problem. By shifting from a heterogeneous interface to a homogeneous solution makes it possible to utilize spectroscopic techniques such as ultraviolet−visible−near-infrared (UV−vis−NIR) absorption and transient absorption (TA) measurement. Molecules attached to metal nanoparticles (NPs) are an ideal homogeneous system with a large molecule/metal interface avoiding intermolecular interaction. However, because the NPs require a strong surface passivation, studies on molecule/metal interactions has been limited to chemisorbed molecules. New Received: May 4, 2019 Revised: June 25, 2019 Published: June 25, 2019 A

DOI: 10.1021/acs.jpcc.9b04231 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Chemical structures and schematics of porphyrin derivative coordination on AuNPs. (a) Chemical structure of SC1P. The inset is a schematic illustration of the coordination fashion of SC1P on a NP. (b) Transmission electron microscopy (TEM) image and size distribution of SC1P−AuNPs. Further characterizations are shown in the SI. (c) Optimized structure of SC1P−Au55 calculated by DFT. (d) Chemical structure of m4SC1P. The inset is a schematic illustration of the coordination fashion of SC1P on a NP. (e) TEM image and size distribution of m4SC1P− AuNPs. Further characterizations are shown in the SI. (f) Optimized structures of m4SC1P−Au55 (at 0, 45, and 90°) calculated by DFT.

acetate (SC1) protected AuNPs with the diameter of 1.2 nm was synthesized and then m4SC1P was attached on the AuNPs via ligand exchange (see the Supporting Information (SI) for details). To avoid π−π interactions between the m4SC1P molecules, they were sparsely inserted on the SC1-coordinated AuNPs via ligand exchange (see the SI). Theoretical calculations on the molecule/NP configurations were performed with DFT. For edge-attached m4SC1P on an Au55 cluster (m4SC1P−Au55) with 0, 45, and 90° attachment angles, and for the face-attached SC1P on an Au55 (SC1P− Au55), geometry optimization taking electrostatic potentials into account produced four stable geometries shown in Figure 1c,f (see also the SI and Figures S12−S18). The total energies of the 45°-attached and 90°-attached m4SC1P−Au55 were, respectively, 0.075 and 0.366 eV higher than that of the 0°attached m4SC1P−Au55. Therefore, the 0°-attached m4SC1P− Au55 more strongly and stably interacted with the Au55 cluster and stabilized the entire m4SC1P−Au55 complex. The strongest interaction with Au55 was also reflected by the largest root-mean-square displacement of the 0°-attached m4SC1P molecule from the isolated m4SC1P structure (Figure S13). Thus, the 0°-attached m4SC1P−Au55 was the predominant configuration. For face-attached SC1P−Au55, the nonflat corner attaching complex, in which a SC1P molecule attached to one corner edge of the Au55 surface, was more stable than the flatattaching SC1P−Au55 complex, in which a SC1P molecule was attached to the flat Au55 surface and was more likely formed in a solution (see Figure S17). The Au55 crystal structure in faceattached SC1P−Au55 was more preserved than that in the edgeattached m4SC1P−Au55 complexes because the m4SC1P molecules had pointlike interactions with the Au55 surface. In contrast, the structure of the face-attached SC1P molecule, which interacted with Au55 through large porphyrin face and four legs, deviated the most from an isolated porphyrin structure among all of the porphyrin−Au55 complexes (Figure S13). UV−vis−NIR absorption spectra of the porphyrin-coordinated AuNPs in N,N′-dimethylformamide are shown in Figure

experimental systems are required to investigate the dependence of interfacial carrier dynamics on molecular orientation. Here, the synthesized π-conjugated macrocycles had uniquely controlled orientations on gold NPs (AuNPs). A well-designed system enabled a direct observation of spectral shifts caused by the weak molecule/metal interactions, which were reproduced by time-dependent density functional theory (TDDFT). Spectroscopy and TDDFT calculations were used to visualize the orientation-dependent orbital hybridization between the molecule and the AuNP surface that determines the carrier dynamics. By controlling the interfacial structure, the close relationship between the molecular orientation and the interfacial electronic state was clarified.

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RESULTS AND DISCUSSION Precise control over the configuration of π-conjugated macrocycles on a metal NP was the key to this work. The structures of porphyrin derivatives were designed to tune the orientation of the porphyrin rings on an AuNP. Two porphyrin derivatives with differently aligned coordination arms with respect to the porphyrin plane determined the contact geometry (Figure 1). For face-attached porphyrin coordination on an AuNP, quadridentate macrocyclic porphyrin thioester derivatives (tetrakis-5α,10α,15α,20α-(2-acetylthiomethylphenyl)porphyrin (SC1P)) were synthesized. SC1P contained four acethylthio groups facing the same direction with respect to the porphyrin ring (Figure 1a).18−21 The face-attached SC1P−AuNP was synthesized by the reduction of HAuCl4 in the presence of SC1P according to previous works.18,20,21 The face coordination of the SC1P on a Au cluster encased the Au cluster with the diameter of 1.2 nm in a hexahedron with six porphyrin faces.18 In contrast, a monodentate porphyrin thioester derivative (meso-triphenyl-(4acetylthiomethylphenyl)porphyrin (m4SC1P)) was synthesized such that the porphyrin ring was in a free-standing, edge-attached coordination to AuNPs (Figure 1c). The m4SC1P had a single coordination site in the porphyrin plane, which was favorable for the edge coordination. For the fabrication of edge-attached m4SC1P−AuNPs, benzyl thioB

DOI: 10.1021/acs.jpcc.9b04231 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 2a,b. The broad absorption from the UV to the NIR derived from AuNP inter- and/or intraband transitions.22 When the

were localized only on Au55, or exhibited delocalization between SC1P and Au55 (see Figure S17). This difference in orbitals between the edge-attached m4SC1P−Au55 and the face-attached SC1P−Au55 appeared as shifts in the recorded absorption spectra (Figure 2d,e). To analyze the spectra, reduced structures shown in Figure S18 were used that retained the most critical aspects of the porphyrin−Au interface. The spectra were then obtained by TDDFT calculations of the excitation energies and oscillator strengths of the reduced structures and by thermal broadening each oscillator strength by a Gaussian function having a full width at half-maximum (FWHM) = 15 (see Figure S19 for details). Figures 2d,e and S19 broadened oscillator strengths corresponding to the absorption spectra observed at room temperature. The porphyrin−Au interaction split and weakened the oscillator strength of the isolated porphyrin. Stronger porphyrin−Au interactions resulted in further splitting and weaker oscillator strengths. Hence, the 0°attached m4SC1P−Au20 and the face-attached SC1P−Au18 exhibited more splitting and weaker oscillator strengths than that observed for other edge-attached m4SC1P−Au20 cases, which was consistent with the UV−vis−NIR absorption spectra in Figure 2a,b. The porphyrin−Au interaction mixed and deformed the original orbitals of the isolated porphyrin with several AuNP orbitals. The resulting complex orbitals led to increased splitting and weakened oscillator strength. However, only face-attached SC1P−Au18, which had the strongest porphyrin−Au interaction, exhibited an oscillator strength red-shift to 410 nm, in good agreement with the absorption spectra in Figure 2a. Therefore, only the strongest face-on interaction shifted the absorption peak to lower energy. In Figure S20, the peak intensities of all the edge-attached m4SC1P−Au20, which had oscillator strengths before Gaussian broadening that depended on attachment angles, almost coincided with each other after Gaussian broadening. Thus, the absorption spectra observed at low temperatures could specify, via the spectral intensity, the attachment angle for the edge-attached m4SC1P−AuNP, as also demonstrated in Figure S20. To clarify the relationship between interfacial interaction and carrier transportation, transient absorption (TA) measurements were performed.23−27 They provided information on how orbital hybridization affected the carrier dynamics (Figure 3). For free m4SC1P, typical TA spectra of porphyrin derivatives in the singlet-excited state (S1) with Q-band bleaching were observed upon excitation (Figure 3a).17 Excitation of the Q bands in the free porphyrin derivatives with a 520 nm laser populated the upper vibrational state (0.4 ps lifetime), which decayed in the porphyrin derivatives (S1) (Figure 3b). The TA spectra changed in appearance for the edge-attached coordination of m4SC1P−AuNP (Figure 3c). A transient species assigned to a radical cation was observed upon excitation.28,29 A peak assigned to radical cations appeared within the instrument response function (FWHM 260 fs). This assignment was further confirmed by a quenching experiment (see Figure S8), where ultrafast electron transfer from excited m4SC1P to AuNP proceeded during the laser pulse (FWHM 260 fs). The decay profile of m4SC1P−AuNP radical cations was fit by a double-exponential decay function with 1.1 ns and >1.5 ns fast and slow components, respectively (Figure 3d). Consequently, the excitation of porphyrin in m4SC1P−AuNP

Figure 2. UV−vis−NIR absorption spectra of porphyrin derivatives and porphyrin-derivative-coordinated AuNPs. Orbitals of edgeattached m4SC1P−Au55 and face-attached SC1P−Au55 calculated with DFT. (a) UV−vis−NIR absorption spectra of m4SC1P (black) and m4SC1P−AuNP (blue). (b) UV−vis−NIR absorption spectra of SC1P (black) and SC1P−AuNP (red). (c) Typical orbitals of 0°attached m4SC1P−Au55 (upper images) and face-attached SC1P− Au55 (lower images) calculated with DFT. Corresponding results for 45- and 90°-attached m4SC1P−Au55 are shown in Figures S14−S16. (d) TDDFT-calculated absorption spectra at room temperature of 0°attached m4SC1P−Au18 (blue) and isolated m4SC1P molecule (black). (e) TDDFT-calculated absorption spectra at room temperature of face-attached SC1P−Au18 (orange) and isolated SC1P molecule (black).

porphyrins were attached to the AuNPs, their Soret bands were broadened and red-shifted and the molar absorption coefficients ε were greatly reduced. The Soret band red-shift for face-attached SC1P-coordinated AuNPs (SC1P−AuNPs) was larger than that for the edge-attached m4SC 1 Pcoordinated AuNPs (m4SC1P−AuNPs). Furthermore, the ε damping ratio for the SC1P−AuNPs (−91% of the free porphyrin) was larger than that for m4SC1P−AuNPs (−67%). Hence, the porphyrin orientation-dependent interaction triggered a perturbation of the porphyrin electronic state. The DFT/TDDFT calculations explained the large shift in the absorption spectra in terms of orbital hybridization at the porphyrin−Au interface. Edge-attached m4SC1P−Au55 and face-attached SC1P−Au55 exhibited different orbitals, especially near the band gap edge, as shown in Figures 2c and S14−S17. Effective charge separation could be seen in all of the orbitals of the 0-, 45-, and 90°-attached m4SC1P−Au55; the orbitals were localized on only one m4SC1P or Au55. No qualitative differences were observed in the orbitals as a function of attachment angles (see Figures S14−S16 for details). In contrast, most of the orbitals of the face-attached SC1P−Au55 C

DOI: 10.1021/acs.jpcc.9b04231 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. TA spectra of porphyrin derivatives and derivative-coordinated AuNPs. (a) TA spectra of free m4SC1P after 520 nm laser excitation. (b) Kinetic trace of free m4SC1P at 800 nm. (c) TA spectra of m4SC1P−AuNPs after 520 nm laser excitation. (d) Kinetic trace of edge-attached m4SC1P−AuNPs at 620 nm after 560 nm laser excitation. (e) TA spectra of SC1P−AuNP after 520 nm laser excitation. The blank at around 1050 nm is masking the noise. (f) Kinetic trace of SC1P−AuNP at 800 nm upon 520 nm laser irradiation. Red lines are the best fits.

predicted by theoretical calculation. In addition, it is obvious that the edge-attached configuration, which realizes the linglived charge separation, is favorable for photoelectric conversion systems. To understand the effect of molecular orientation on carrier transport, we investigated the electron transport of single 5α,10α-bis(2-ethylthiophenyl)-15α,20α-bis(2′ethylthiophenyl)porphyrinatozinc(II) (ZnSC2P-SS) face-attached on AuNP using scanning tunneling spectroscopy (STS) at 300 K (see the SI for details).30,31 In the current− voltage characteristic, the Coulombic blockade can be observed in the range of −0.3 to 0.3 V (Figure 4). The average charging energy was estimated to be 396 ± 113 meV from the theoretical fitting. The average tunneling resistance R2 (87 ± 18 MΩ) was estimated from the set point current dependence of the tunneling resistance R2 of the face-attached ZnSC2P−AuNP (1 nm) chemisorbed on mercaptopyridineself-assembled monolayer (SAM) Au(111) (Figure 4). Interestingly, the R2 of the face-attached ZnSC2P−AuNP (1 nm) on mercaptopyridine-SAM Au(111) was decreased compared to that of the face-attached ZnSC2P−AuNP (1 nm) on Au(111). The single molecule tunneling resistance of the face-attached ZnSC2P-SS on AuNP set on the selfassembled monolayer on Au(111) was significantly small (87 ± 18 MΩ) in comparison with the previously reported edgeattached systems between porphyrin derivatives and Au substrate (Figure 4).32,33 This fact indicates that the resistance of ZnSC2P-SS face-attached on AuNP was modified by the

induced ultrafast electron transfer and a long-lived charge separation (>1.5 ns). The face-attached SC1P−AuNPs exhibited broad and structureless TA spectra (Figure 3e). The spectra, coupled with bleaching of the ground-state SC1P absorption, were attributed to excited SC1P. When the Au cores were selectively excited with a 475 nm laser, these transient species were not observed, indicating that the TA spectrum did not come from isolated AuNPs. The DFT calculation of the face-attached SC1P−Au55 indicates that some SC1P and AuNP orbitals effectively mix to deform, resulting in the delocalized orbital over the entire SC1P−AuNP structure. Thus, the observed TA spectrum corresponded to the formation of a hybridized orbital state (i.e., an exciplex, (SC1P−AuNP)*). At 3 ps after excitation, the TA spectra changed to a weak peak at 760 nm, which was assignable to porphyrin radical cations (SC1P•+). The kinetic trace of the exciplex is shown in Figure 3f, where a biexponential curve fit provided two decay components. The major component had a short lifetime (0.7 ± 0.2 ps), and the minor component had a longer lifetime (9.1 ± 4.3 ps). The major and minor components corresponded to the exciplex and SC1P•+ lifetimes, respectively. The chargeseparated state generated from the exciplex quickly recombined into the ground state. DFT calculations also indicated a much faster charge recombination in the face-attached SC1P− AuNP, caused by the overlap of electron and hole orbitals, compared with the edge-attached m4SC1P−AuNP. These results experimentally support the orbital hybridization D

DOI: 10.1021/acs.jpcc.9b04231 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Experimental details, synthesis and characterization of porphyrin-protected AuNP, TDDFT calculations, transient absorption measurement, and single-crystal X-ray crystallography (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.S.). *E-mail: [email protected] (K.H.-D.). *E-mail: [email protected] (T.T.). ORCID

Masanori Sakamoto: 0000-0001-5018-5590 Kim Hyeon-Deuk: 0000-0002-5815-8041 Yutaka Majima: 0000-0002-5108-1934 Yoshihiko Kanemitsu: 0000-0002-0788-131X Toshiharu Teranishi: 0000-0002-5818-8865 Notes

The authors declare no competing financial interest.

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Figure 4. (a) Schematic illustration of STS measurement of faceattached ZnSC2P−AuNP (1 nm) on mercaptopyridine-SAM Au(111) and equivalent circuit. (b) Experimental and theoretical current− voltage characteristic and differential conductance of face-attached ZnSC2P−AuNP (1 nm) on mercaptopyridine-SAM Au(111) at 300 K. Broken lines represent theoretical curves. The average charging energy is 396 ± 113 meV. (c) Set point current dependence of tunneling resistance R2 of face-attached ZnSC2P−AuNP (1 nm) on mercaptopyridine-SAM Au(111). The average value of R2 is estimated to be 87 ± 18 MΩ.

ACKNOWLEDGMENTS The authors thank Prof. M. Nakamura and Dr K. Isozaki for single X-ray crystal measurement of m4SC1P and Dr D. Tanaka and Y. Okamoto for the synthesis of materials. All of the data are available in the main text or the Supporting Information. This study was partially supported by a KAKENHI 18H01827 (Grant-in-Aid for Scientific Research (B)) (M.S.), Grant Number JP16H06520 (Coordination Asymmetry) (T.T.), JP17H05257 (Photosynergetics) (M.S.), and 15K05386 (Theoretical calculation) (K.H.-D.). K.H.-D. also thanks the financial supports from JST (PRESTO), Japan Association for Chemical Innovation (JACI), and Cybermedia Center of Osaka University. The authors declare no competing interests.

hybridization of molecular and metal orbitals as indicated by DFT/TDDFT calculation and spectroscopy.

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CONCLUSIONS Spectroscopy, combined with DFT/TDDFT calculations, of systems with uniquely oriented molecules on a metal surface clarified the origin of molecular orientation effects on carrier dynamics at the heterointerface. Orbital hybridization, which was controlled by the orientation of the molecule−metal attachment, was the origin of the orientation-dependent carrier dynamics. A spectroscopic shift in both steady and excited states was observed regardless of the particle size (1.1−3 nm), the porphyrin ring−Au distance (0.24−0.5 nm), and the highest occupied molecular orbital−lowest unoccupied molecular orbital level of porphyrin derivatives (Figures S5−S7), inferring that the present results could be widely applicable to various molecule−metal interfaces. The results were consistent with previous predictions via photoelectron spectroscopy of the interface state of a weakly chemisorbed system.11 Here, the visualization of the interfacial orbital hybridization revealed the origin of the relationship between surface-molecular orientation and carrier dynamics. The observed orientation-dependent carrier dynamics agreed with empirical rules for a favorable molecular orientation for electronic devices. The insight provided an important direction, focusing on the molecular attachment orientation, for the design of high-performance, organic electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04231. E

DOI: 10.1021/acs.jpcc.9b04231 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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