Molecular Coupling between Organic Molecules and Metal - The

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Molecular Coupling between Organic Molecules and Metal Pengcheng Hu, Xu Li, Bolin Li, Xiaofeng Han, Furong Zhang, Keng C. Chou, Zhan Chen, and Xiaolin Lu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01765 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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The Journal of Physical Chemistry Letters

Molecular Coupling between Organic Molecules and Metal Pengcheng Hu † Xu Li,† Bolin Li,† Xiaofeng Han,† Furong Zhang,† Keng C. Chou,& Zhan Chen,*,‡ and Xiaolin Lu*,†

†

State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering,

Southeast University, Nanjing 210096, China &

Department of Chemistry, University of British Columbia, Vancouver, BC Canada V6T 1Z1

‡

Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor,

Michigan 48109, United States

AUTHOR INFORMATION Corresponding Author * E-mails: [email protected] (X. L); [email protected] (Z.C.)

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ABSTRACT. Molecular couplings at interfaces play important roles in determining performance of nanophotonics and molecular electronics. In this letter, using femtosecond sum frequency generation (fs-SFG) to trace free-induction decay (FID) of vibrationally excited aromatic thiol molecules immobilized on metal with and without the bridged methylene group(s), metal surface free electroncoupled and uncoupled phenyl C-H stretching vibrational modes were identified, with dephasing times to be ~0.28 ps and ~0.60 ps, respectively. For thiols on Au with the bridged methylene group(s) (benzyl mercaptan and phenylethanethiol), both the coupled and uncoupled modes were observed; for thiol on Au without the bridged methylene group (thiophenol), only the coupled mode was observed. This indicates that the bridged methylene group(s) serving as a spacer can be used to adjust the molecular coupling between the phenyl vibration and surface free electrons. The experimental approach can be used to tune molecular couplings in advanced nanophotonics and molecular electronics.

KEYWORDS: : fs-SFG; free-induction decay (FID); surface free electrons; molecular coupling; phenyl.

TOC GRAPHICS


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Molecular couplings at interfaces play important roles in determining performance of nanophotonics and molecular electronics for remote sensing, optical control, light harvesting, and biomacromolecular immobilization (DNA and protein molecules), etc.1-6 Understanding such couplings at interfaces could greatly facilitate the development of new optical and biomedical devices. In the past few decades, timeresolved surface vibrational spectroscopic techniques have been greatly advanced, providing exciting new opportunities to study ultrafast surface dynamics7-13 and impacting many areas like energy transfer, surface reactivity, and photosynthesis process.14-18 Sum frequency generation (SFG) vibrational spectroscopy, a second-order nonlinear optical technique with surface/interface sensitivity, has been developed into a powerful tool to probe molecular-level surface and interfacial structures. Also, when applying two femtosecond input laser pulses, ultrafast surface and interfacial dynamics can be probed. Many surface sensitive techniques (for example, surface plasmon resonance (SPR)19-21 and surface enhanced Raman scattering (SERS)22,23) have been applied to investigate various surface processes (for example, adsorption, desorption and catalysis) associated with the metal substrates. Albeit, the ultrafast dynamics of molecules on metal surfaces cannot be probed with the above traditional techniques. The fs-SFG vibrational spectroscopy is an ideal tool to examine such ultrafast dynamics because of its surface/interface specificity and time-resolved capability. For example, Bonn et al. applied the fs-SFG to study the transient structure of carbon monoxide (CO) upon desorption from Ru (001), where change of the band frequency and bandwidth revealed the coupling between the C-O stretch with the frustrated rotational mode (νrot) or the substrate (νRu-CO).24 Wang et al. measured the thermal conductivity for selfassembled monolayers (SAMs) of long-chain thiol molecules upon ultrafast laser flash heating, in which the heat burst rate and molecular conductance could be extracted.25-28 These studies demonstrated the power of fs-SFG to probe ultrafast behaviors of molecules on the metal substrates. Recently, quantum beating was observed when the free induction decay (FID) of the SFG signals from SAMs of siloxane or surfactant was measured, which allows for differentiating adjacent vibrational modes in the frequency domain.29-31



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On the metal substrates, free electrons can be excited by the incident laser, leading to surface enhanced vibrational modes for improving the detection limit of trace molecules (For example, SERS).32,33 It is thus fundamentally important to study the couplings between molecular vibrations and free electrons on the metal surfaces. For example, molecules with phenyl groups have extensively been employed in molecular junctions,34-36 exhibiting a highly electron-transferred property. Vibrational modes of phenyl groups on metals could be significantly influenced by surface electrons but have not been probed previously using nonlinear vibrational spectroscopy. Here we measured the FID of excited phenyl vibrational modes of organic molecules on the Au substrates using the fs-SFG, where the molecular vibrations of the phenyl groups were coupled to the excited surface free electrons. Analysis on the damping process of the SFG signals allows us to differentiate the electron-coupled and uncoupled phenyl vibrational modes on the Au substrates.

Figure 1. a. Illustration of the SFG data collection geometry in our study. SAMs of thiophenol (TP), benzyl mercaptan (BMP), 2-phenylethanethiol (PET), and trichloro(phenyl)silane (TPS) are shown in Panels b, c, d and e, respectively. Schematic diagrams for a FID measurement are shown in Panel f (energy level diagram) and Panel g (time-dependent process).


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In this study, we chose three aromatic thiol molecules, including thiophenol (TP), benzyl mercaptan (BMP) and 2-phenylethanethiol (PET), which have zero, one and two bridged methylene group(s) between the phenyl ring and the hydrosulfuryl group, respectively (Figure 1). Their self-assembled monolayers (SAMs) were prepared on the Au substrates with a sulfur atom and zero, one and two methylene groups between the phenyl ring and the metal surface. As a control, SAM of trichloro(phenyl)silane (TPS) on a silica substrate was also prepared for comparison. The description of the fs-SFG setup has well been documented in the literatures.37-48 In the current study, both the time- and frequency-domain measurements were performed, as shown in Figure 1. First, a macroscopic 1st order polarization was created by a fs-infrared pulse. In the time domain, the 1st order infrared polarization was up-converted using a ~70 fs (spectrally broad) pulse centered at ~800 nm, and the SFG signal was measured as a function of the delay time (τ) between the infrared and visible pulses.49 In this case, the time evolution of the nonlinear polarization can be traced. In the frequency domain, the 1st order infrared polarization was up-converted using a temporally long (spectrally narrow) 800 nm pulse, the vibrational mode(s) can be directly probed in a collected SFG spectrum. Figure 2 shows characteristics of the frequency-domain and time-domain measured results. In Panels g, h, i and j of Figure 2, upon delay, damping of the up-converted 1st order polarization can be traced. Even in the frequency domain, the delay leads to useful information in the collected SFG spectra. As shown in the inserted graphs in Panels a, b, c, d and f, when the delay time is sufficiently large (~2 ps), the Au signal is absent due to its fast decay, and only the vibrational band denoting the phenyl vibrational mode is observed for all of the SAMs. When the delay time was 0, only one phenyl C-H stretching vibrational mode was observed for TP on Au and TPS on silica. But for BMP and PET on Au, besides a strong mode (~3062 cm-1 for BMP and ~3072 cm-1 for PET), a shoulder band in the lower frequency range was also observed (~3036 cm-1 for BMP and ~3046 cm-1 for PET, respectively), as shown in Panel e of Figure 2. For convenience, we refer the mode contributing the stronger signal as the strong mode; while the mode contributing to the shoulder signal as the weak mode. All of the resonant bands detected here were located in the phenyl C-H stretching vibrational frequency region. ACS Paragon Plus Environment


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Figure 2. The frequency-domain SFG spectra collected from the SAMs of TP on Au, BMP on Au, PET on Au, and TPS on silica are shown in Panels a, b, c, d, e, and f; the inserted graphs show the spectra collected at certain delay times. The time-resolved spectra are shown in Panels g, h, i, and j to track the damping process. The rescaled and offset frequency-domain spectra for SAMs of TP, BMP and PET denoting the two phenyl C-H stretching vibrational modes (3000 cm-1 to 3100 cm-1) are shown in Panel e (τ=0). Since the frequency-domain polarization is the inverse Fourier transform of the time-domain one, the above frequency-domain spectra contain the information detected in the time domain. In the timedomain measurement, the spectrally broad infrared and visible pulses rendered an un-deconvoluted SFG spectral lineshape, as shown in Panels g, h, i, and j of Figure 2, where the integrated intensity over the delay time can be used to trace the damping process. When we examine the results from the timeACS Paragon Plus Environment

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resolved measurements (Figure 3, a, b, c and d), SFG signal FIDs for BMP and PET show an apparent three-segmental damping and FID for TP shows an apparent two-segmental damping. Such FID processes look different from the previously reported results with multiple interfered vibrational modes of self-assembled monolayers.29-31 Phenomenologically, the three-segmental decay for BMP or PET corresponds to damping of three surface oscillations, i.e. surface free electrons, weak shoulder phenyl C-H stretching mode and strong phenyl C-H stretching mode; while the two-segmental decay for TP corresponds to damping of two surface oscillations, i.e. surface free electrons and phenyl C-H stretching vibrational mode. It should be noted that the weak interfered FID patterns between the two phenyl vibrational modes still existed due to the quantum beating effect for BMP and PET (Panels b and c in Figure 3). In the frequency domain, when the delay time was adjusted, the shoulder band disappeared ahead of the strong phenyl mode, for both BMP and PET (see Supporting Information). This proves the faster damping of the shoulder phenyl mode in comparison to that the strong mode. In order to extract more dynamic information, the slope for each segment in the logarithmic plot was obtained by fitting. As a comparison, FID of SAM of TPS was also fitted, which shows a two-segmental decay behavior. (See Supporting Information) Besides the segmental fitting for the FIDs, we applied the method including the quantum beating to fit our data, as demonstrated by Benderskii and Borguet et al.29,30 The results were shown in Panels I, II, III and IV of Figure 3. The fitted parameters for the frequency-domain spectra are listed in Table 1, which were used to simulate the time-domain spectra. It can be seen, the fitted curves for TP, PET, BMP and TPS can well simulate the FID data. The dephasing times of the phenyl vibrational mode(s) from the quantum beating fitting (consistent with frequency domain) are similar to those obtained from the segmental fitting (Supporting Information). For example, for Au-BMP, the two dephasing times from the quantum beating fitting were ~0.25 ps and ~0.67 ps, respectively; while they were ~0.24 ps and ~0.66 ps respectively from the segmental fitting. It should be noted that, fitting with the quantum beating provides a more reliable approach with respect to the FID data. The following analysis will be based on parameters from the quantum beating ACS Paragon Plus Environment


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fitting. As shown in Figure 3 and Table 1, for TPS, the dephasing time of the phenyl C-H stretching vibrational mode is ~0.60 ps; while for TP, the dephasing time is ~0.28 ps. For BMP and PET, there exist a fast damping mode and a slow damping mode. The two dephasing times for BMP are ~0.25 ps and ~0.67 ps, respectively and the two dephasing times for PET are ~0.21 ps and 0.61 ps, respectively. TPS has no coupling with surface free electrons, therefore the measured ~0.60 ps represents a pure phenyl C-H stretching dephasing process. It is obvious that the measured dephasing time of ~0.67 ps or 0.61 ps (for BMP or PET) also indicates a pure phenyl C-H vibration dephasing process. It is interesting to see that there is no such a pure dephasing step for TP on Au. This is because the phenyl group of TP is directly connected to Au via the sulfur atom and coupled with surface free electrons. As we know, in SERS, both the charge-transfer and local-field enhanced mechanisms apply to molecules with conjugated structures.32,50 Here the relatively fast decay of phenyl C-H vibration (~0.28 ps) for TP on Au was due to the strong electron-transfer coupling effect between the phenyl vibration and surface free electrons. The same argument can be applied to BMP and PET on Au, since the dephasing times of the fast modes (~0.25 ps and ~0.21 ps) are similar to that of TP. In summary, absence of a pure dephasing decay for TP indicates direct coupling between the phenyl C-H vibration and surface free electrons, and therefore only the coupled mode was observed. For BMP and PET on Au, because the bridged methylene group(s) can impact the electron-transfer coupling between the phenyl vibration and excited surface free electrons, there exist one coupled mode and one uncoupled mode. For TPS on silica, since there was no Au substrate and no metal free electrons, only the uncoupled mode was detected.


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Figure 3. SFG signal FID behaviors of SAMs of TP, BMP, PET and TPS with the segmental fitting results are shown in Panels a, b, c and d. The corresponding quantum beating fitting results in consistence with the frequency domain spectra are shown in Panels I, II, III and IV. Dots are original data and curves are fitted results. If we examine the frequency-domain spectra again, it is understandable that for TP there is only one phenyl vibrational mode; and for BMP and PET, there exist a strong mode and a weak mode. Likely the weak mode is a phenyl C-H stretching mode strongly coupled with surface free electrons. Therefore, in the frequency domain, the decay of the weak mode signal is much faster than that of the strong mode. Also, its dephasing time (~0.25 ps for BMP or ~0.21 ps for PET) was similar to that of the coupled mode for TP (~0.28 ps). Therefore, the bridged methylene group(s) has a substantial effect on the coupling between the phenyl vibration and excited free electrons. For BMP and PET, where there is (are) bridged methylene group(s), surface free electrons can directly interact with the phenyl C-H stretching ACS Paragon Plus Environment


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vibration to form the coupled mode; meanwhile, the uncoupled mode still exists due to the “blocking” effect from the bridged methylene group(s) (as a barrier). For TP, where there is no bridged methylene group, the direct charge transfer allows for the efficient coupling between the phenyl C-H vibration and excited surface free electrons, where there is no uncoupled vibrational mode. In consideration of the fast development of the nanophotonics and molecular electronics, where the molecular couplings between the free electrons and molecular vibrations play an important role in determining the device performance, we thus believe it is necessary to introduce the fs-SFG into this field, which will pay off in the long run. Table 1. Parameters from fitting the frequency-domain spectra, which were used to simulate the SFG FIDs (Figure 3, Panels I-IV). Parameters

Self-assembled samples Au − TP

Au− BMP

Au− PET

Silica window − TPS

ω1

3065 ± 1

3062 ± 2

3072 ± 3

3048 ± 1

Α1 (a.u.)

1225± 25

1800 ± 100

1240 ± 10

105± 5

ε1

-1.46 ± 0.36

-1.24 ± 0.09

-1.30± 0.02

−

Γ1 (cm-1)

19.0± 2.0

8.0 ± 0.5

8.8 ± 0.8

9.0 ± 0.5

Τ2 (ps)

0.28 ± 0.03

0.67 ± 0.04

0.61 ± 0.06

0.60 ± 0.03

ΑNR (a.u.)

7175 ± 825

9500 ± 1500

9775 ± 275

120 ± 10

ω2

/

3036 ± 2

3046± 4

/

Α2

/

575± 25

850± 150

/

ε2

/

0.29 ± 0.01

0.18 ± 0.02

/

Γ2 (cm-1)

/

21.0 ± 1.0

26.0 ± 1.0

/

Τ2 (ps)

/

0.25 ± 0.02

0.21± 0.01

/

Note: An, εn, and Γn represent the amplitude of surface vibration with frequency ωn, phase between resonant and nonresonant contributions, and damping constant, respectively. Τ2 is the dephasing time. ANR is nonresonant contribution.

In summary, using the fs-SFG, we performed both the frequency-domain and time-domain SFG measurements on SAMs of TP, BMP, and PET on Au and SAM of TPS on silica. The FID results indicate there were surface free electron coupled and uncoupled phenyl C-H stretching vibrational modes for BMP and PET; while only coupled phenyl C-H vibrational mode was detected for TP. This suggests that the bridged methylene group(s) can substantially affect the coupling between the phenyl C-H vibration and excited surface free electrons. The uncoupled phenyl C-H vibrational mode shows a dephasing time of ~0.60 ps (Taking TPS on silica as an example) while the surface free electron coupled phenyl C-H vibrational mode shows a ACS Paragon Plus Environment

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dephasing time of ~0.28 ps (Taking TP on Au as example). To the best of our knowledge, this is the first time to differentiate the surface free electron coupled and uncoupled phenyl vibrational modes for molecules on metal using the fs-SFG. This study demonstrates the power of the fsSFG to reveal the interaction between the molecular vibration and the excited surface free electrons. We believe that the fs-SFG will be developed into a powerful tool to study many important coupling effects in important applications in the future, including those in molecular electronics and nanophotonics,34-36,51 where the molecular couplings need to be tuned on demand. Materials. Thiophenol (TP, C6H6S, 99.9%) was purchased from Xiya reagent company. Benzyl mercaptan (BMP, C7H8S, 99%) and trichloro(phenyl)silane (TPS, C6H5Cl3Si, 97%) were purchased from Sigma-Aldrich. 2-Phenylethanethiol (PET, C8H10S, 97%) was purchased from Tokyo Chemical Industry (TCI). All the chemicals were of the analytical reagent grade and used without further purification. Silica windows were purchased from Chengdu YaSi Optoelectronics Co., Ltd. Glass substrates were purchased from Qingdao Joyjun Medical Products Co., Ltd. Sample Preparation. Sulfuric acid (H2SO4, 96%) and hydrogen peroxide (H2O2, 30%) were mixed with a volume ratio of 3:1 to prepare the piranha solution. The silica windows were soaked in the piranha solution for one day to remove the possible surface organic contaminants, then rinsed with Milli-Q water (18 MΩ·cm, Millipore, France) for three times, dried with nitrogen gas, and finally cleaned with oxygen plasma.52 A Gold (Au) layer with a thickness of ~200 nm was sputtered onto the cleaned glass substrates to prepare the Au substrates for the FID experiments. Before that, a layer of nickel (Ni) was deposited first in order to improve the adhesion between the substrate and Au. The Au substrates were immersed in anhydrous toluene solutions of TP, BMP and PET respectively overnight to prepare the self-assembled monolayers (SAMs). All the solution concentrations used in this study were ~20 µM. SAM of TPS was


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prepared by immersing a clean silica window into a toluene solution (~20 µM).53All the SAM samples were washed with toluene for several times to remove the unreacted molecules on the Au or silica surfaces. To ensure a compact SAM layer on the substrate, the immersion times for all the SAM samples were controlled to be ~12 h. The curve of the contact angle versus the immersion time was recorded for each SAM sample, (see Supporting Information). By transferring the time-dependent contact angle into the apparent surface coverage54, the apparent surface coverages for all the SAMs were calculated, all of which are more than ~99%. SFG Experiment. The SFG experiment was carried out by using ~100 fs FWHM Gaussian pulse duration infrared pulse (~10 µJ energy for both frequency-domain and time-domain measurements; FWHM spectral bandwith of ~188 cm-1) centered at ~3047 cm-1 for investigating the C-H stretching modes of the phenyl ring. A pulse-shaper was used to change the temporal/spectral profile of the 800 nm visible pulse (repetition rate of 1 kHz). The pulse-shaper was composed of an optical grating and a slit.49 The frequency-domain SFG spectra were recorded with a ~10 cm-1 FWHM spectral bandwidth upconversion pulse. The time-domain spectra were measured with ~70 fs visible pulses (~0.6 µJ). The incident angles for IR beam and visible beam with respect to the surface normal were 56° and 65° respectively. The overlap beam spot diameter at the sample surface was ~0.6 mm. In this study, we collected SFG spectra using the ppp polarization combination (p-polarized SFG beam, p-polarized 800 nm beam, p-polarized IR beam). In the frequency-domain and time-domain measurements, the time delay τ was defined as positive when the IR pulse arrives at the sample prior to the visible pulse and negative verse versa.37 We adjusted the delay time (τ) by moving a delay line for the visible beam. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available on the ACS Publications website. Materials, time-domain and frequency-domain spectra, and fitting details, such information is available free of charge on the ACS Publications website at DOI:.

AUTHOR INFORMATION Corresponding Author * E-mails: [email protected] (X. L); [email protected] (Z.C.) Notes The authors declare no competing financial interests

ACKNOWLEDGMENT This study was supported by the State Key Development Program for Basic Research of China (2017YFA0700500, 2017YFA0700503), the National Natural Science Foundation of China (Grant No. 21574020), the Fundamental Research Funds for the Central Universities, and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the National Demonstration Center for Experimental Biomedical Engineering Education (Southeast University). REFERENCES


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