Flipping Behavior of a Porphyrin Derivative Molecule on a Au(111

ARTICLE pubs.acs.org/JPCC

Flipping Behavior of a Porphyrin Derivative Molecule on a Au(111) Reconstructed Surface Hitoshi Suzuki,*,†,‡ Hirofumi Yoshida,†,‡ Hiroyuki Sakaue,† Takayuki Takahagi,† Shukichi Tanaka,‡ Toshiya Kamikado,‡ and Akira Otomo‡ † ‡

Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan ABSTRACT: Porphyrin derivative molecules (MP-tBPP) on a Au(111) reconstructed surface have been investigated using scanning tunneling microscopy. The molecules were found to adsorb with two kinds of orientation at the elbow positions of the reconstructed surface. One orientation had the two di-tertbutylphenyl groups of a molecule aligning along a row of elbow positions, and the other was the same alignment rotated approximately 60°. Flipping behaviors from one orientation to the other were observed in successively obtained scanning tunneling microscopy images. The activation energy for the flipping behaviors was estimated to be approximately 60 meV from the flipping rates. The difference in the molecular populations with each of the orientations represents the potential energy of the adsorbed molecules having asymmetric double minima corresponding to the two orientations. The difference between the potential energies of the molecules in these orientations was approximately 12 meV, which was calculated from the ratio between the populations in the orientations and from the difference in the flipping rates.

1. INTRODUCTION Many rotational behaviors of organic molecules adsorbed on substrates have recently been observed through scanning tunneling microscopy (STM). They have attracted a great deal of interest because of their potential application in molecular machines.1
3 These behaviors reflect the interaction between a molecule and a substrate surface, which is essential knowledge for manipulating molecules to arrange their superstructures. Molecules showing rotational motions can be separated into two groups. Those in the first group are bound to substrate atoms through part of the molecules, such as the thiol group, which works as an axle of the rotation.4
6 In the other group, the molecules are weakly bound to their substrate by van der Waals interaction. They do not have axles for their rotation, and the centers of their rotation are determined by their underlying substrate morphology and the molecular arrangement surrounding the rotating molecule.7
10 In STM images of both groups, the rotating molecules appear as circular images, indicating the trajectories of the rotation, or as superposed images of many snapshots of the molecules during their rotation. This is because STM generally does not have enough temporal resolution to obtain an instantaneous snapshot. Cooling the molecules slows the rotation and enables observation of quasi-stable positions corresponding to those in the snapshots of the rotation, that is, the rotation is a continuous transition among these quasi-stable positions.4,9 The activation energies for the rotation have also been calculated from the temperature dependence of the rotation frequency,6,8,9 which is the barrier height between two neighboring r 2011 American Chemical Society

quasi-stable states. In previous studies, each rotational behavior had a single activation energy value, which meant that the barrier heights between the quasi-stable states did not depend on the direction of the transitions and the molecule rotated in both directions with the same probabilities. In other words, an energy diagram representing the molecular rotation contains minima of the same energy level. Becaue the energy diagram shows the stable positions and orientations, the information in such a diagram is important for assembling structures with molecules on substrates. These minima of the potential energy would also be applicable to switching and memory devices, in which the logical state would be assigned to one of the quasi-stable states showing different molecular orientation.9 We observed single porphyrin derivative molecules that adsorb at the elbow positions of a reconstructed Au(111) surface to characterize their orientation with STM. The molecular orientation with respect to the substrate corrugation reflects the stable states in the potential energy for molecular adsorption. The activation energy to change their orientation was also calculated.

2. EXPERIMENTAL SECTION We investigated a porphyrin derivative molecule (Figure 1a inset), 5,15-bis(3,5-dimethoxyphenyl)-10,20-bis(3,5-di-tert-butylphenyl) porphyrin (MP-tBPP). The molecule has a porphyrin ring in its center with two di-tert-butylphenyl groups and two Received: December 2, 2010 Revised: April 25, 2011 Published: June 07, 2011 12414

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Table 1. Total Numbers of Molecules Showing r and β Orientations and Numbers of Flipping Molecules in STM Images number of molecules

Figure 1. STM image of MP-tBPP molecules on a Au(111) surface (Ugap =
2.0 V; It = 30 pA). (a) A pair of bright spots indicated by a circle corresponds to a single MP-tBPP molecule. Most of the single molecules adsorbed at the elbow positions. The dotted line indicates a row of elbow positions. The molecular axis of the molecule (R) is parallel to the row of elbow positions, which is the R orientation. The axis of the molecule indicated by (β) and the row of elbow positions crosses at approximately 60°, which is the β orientation. The molecule indicated by (γ) is located on a U connection. The image size is 45  45 nm. (b, c) STM images of MP-tBPP molecules showing R orientation and β orientation, respectively. The inset in (a) shows the structure of MP-tBPP.

dimethoxyphenyl groups. The molecule is similar to 5,10,15,20tetrakis-(3,5-di-tert-butylphenyl) porphyrin (TBPP), which has been frequently used in STM measurements.11 In MP-tBPP, two dimethoxyphenyl groups are at the trans position instead of two di-tert-butylphenyl groups. In previous studies, di-tert-butylphenyl groups of TBPP and similar structural molecules have been observed as two bright spots8,12 or a single bright spot that was a fused image of two spots.13 The molecules were deposited from a crucible onto a clean Au(111) surface at room temperature by resistive heating in an ultrahigh vacuum (UHV) chamber. The Au(111) surface was prepared on a cleaved mica substrate and cleaned by cycles of Ar sputtering (1 kV) and annealing (∼800 K). The Au(111) surface showed the well-known reconstructed herringbone structure. All STM measurements were taken in UHV at 77
100 K with a low-temperature STM (Omicron) operated with a scanning probe microscope controller (Nanonis). Each sample was kept at a specific temperature within this range for longer than 1 h before being measured to achieve thermal equilibrium. The images were obtained in constant-current mode with an electrochemically etched W probe. The bias voltage and the tunneling current were kept constant at
2.0 V and 30 pA, respectively, to reduce their influences on the molecular behaviors. The scan area was 60  60 nm, and the scan speed was a constant of 120 nm/s. The pressure in the STM chamber during the measurements was kept below 2  10
8 Pa.

3. RESULTS AND DISCUSSION 3.1. Molecule Orientations. A typical STM image of MPtBPP on Au(111) at 77 K is shown in Figure 1a. A single molecule, indicated by a circle, appears as a pair of bright spots. The distance between these bright spots is consistent with that between two di-tert-butylphenyl groups in the calculated

number of flipping molecules

temperature/K

NR

Nβ

NRfβ

NβfR

77

1578

302

22

15

87

1155

206

33

19

92 100

1957 1396

442 264

143 129

137 134

molecular model. Our molecular image agrees with those in previous reports on similar porphyrin molecules having two ditert-butylphenyl groups.13 Most of the single molecules adsorbed predominantly at the elbow positions of the reconstructed surface ((R) and (β) in Figure 1a). This result also agrees with previously reported results of an adsorbate on a Au(111) surface.4,14,15 Some molecules stayed on the U connections ((γ) in Figure 1a) and on the middle of the corrugation lines of the herringbone structure of the surface (image not shown). The single molecules adsorbed at the elbow positions of the Au(111) surface can be categorized into two groups, depending on their orientation. To clearly distinguish the orientations, we define here the molecular axis. The axis is indicated by the white lines in (R) and (β) of Figure 1a that pass through the pairs of bright spots, that is, the line connecting the centers of the two ditert-butylphenyl groups. The orientation is defined using the angle formed between the molecular axis and the row of elbow positions. The first group consists of molecules whose axes are parallel to the row of elbow positions, which are indicated by (R) in Figure 1a. We call this orientation R (close-up in Figure 1b). The second group consists of molecules whose axes and the row of elbow positions approximately form a 60° angle, indicated by (β) in Figure 1a. In other words, their axes are nearly parallel to the corrugation lines of the herringbone structure of the surface. We call this orientation β (close-up in Figure 1c). Molecules adsorbed at the elbow positions take one of these orientations, which suggests that the R and β orientations are the minimum states in the potential energy determined by the molecular orientation relative to the surface corrugation. After deposition, molecules can generally move and rotate easily on the surface. Cooling the sample makes the molecules settle into stable positions with stable orientations. The STM images at 77 K indicate that the R and β orientations are the stable orientations for MP-tBPP molecules at the elbow positions of a Au(111) reconstructed surface. Because the system is in equilibrium during the cooling process, the number of molecules showing either R or β orientation reflects the difference in potential energy. The numbers of molecules in R (NR) and in β (Nβ) were counted from 78 STM images obtained at 77 K (Table 1). The ratio of the number of molecules in the R orientation to that in the β one is 5.2. This means that R is more stable than β, that is, the potential energy diagram has asymmetric double minima. The lower minimum corresponds to R, and the higher minimum corresponds to β. The Figure 3 inset shows a plausible energy diagram of the system. The difference in the potential energy between the R and β orientations, ΔE, is calculated with the 12415

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Figure 2. Two series of STM images. The first series (a
c) shows flipping behavior of the molecules between orientations. The image size is 34  31 nm. The molecule marked with a circle in (a) shows the R orientation that changed to the β one in (b). In (c), it returned to R again. The molecule indicated by a dashed circle in (b) is a hybrid image of the two orientations. The second series (d
f) shows the transition from R to β with a hybrid image. The image size is 20  21 nm. The molecule with a solid circle in (d) shows R orientation. It changes to a hybrid image, marked by a dashed circle in (e), that can be split into the β orientation in the upper part and the R orientation in the lower part by a straight white line. Finally, it takes the β orientation in (f). All images were taken with Ugap =
2.0 V and It = 30 pA.

following equation
 Nβ ΔE ¼
kT ln NR

ð1Þ

where k is the Boltzmann constant and T is the temperature. The result is 11 meV. ΔE calculated for the number of molecules at 87, 92, and 100 K is 13, 12, and 14 meV, respectively. Hence, the average difference between these minima in the potential energy is approximately 13 meV. 3.2. Spontaneous Flipping between Two Orientations. Three STM images successively obtained at 77 K are shown in Figure 2a
c. The interval between images was approximately 8.5 min. We found that a single molecule at an elbow position flips from the R orientation to the β one and vice versa. The molecule indicated by a circle in Figure 2a is the R orientation. Its orientation flips to β in Figure 2b. The flipping behavior from β to R is also observed in the Figure 2b to c sequence. We could not obtain time-averaged images of the molecules that display the trajectory of the flipping behavior. Some molecules, indicated by the dashed circles in Figure 2b and e, are hybrids of the two orientations. For example, the molecule in Figure 2e is split by the white line into an upper half corresponding to the β orientation and a lower half corresponding to the R orientation. This reflects the transition from one orientation to the other during STM measurement. It is also consistent with the result that the molecule in the preceding Figure 2d is in the R orientation but changes to the β one in the following Figure 2f. The image sequence from Figure 2d to f therefore shows the transition from R to β. The hybrid image does not show a sector shape with a rotational center, which means that the transition finishes in 1 s, the time to scan one line. In the molecule indicated by a dashed circle in Figure 2b, the upper half is R, and the lower half is β (not marked). The corresponding molecule in the images before and after Figure 2b is in the R orientation. This suggests that the flipping

from R to β occurred during the time to obtain Figure 2b and that backward flipping happened before the next image, Figure 2c, was obtained. This indicates that the flipping behavior occurs intermittently and its transition time is short. The flipping behaviors between the orientations correspond to transitions between the two minima in the energy diagram shown in the Figure 3 inset. The numbers of molecules adsorbed with each of the orientations can be expressed with simple rate equations on the numbers of molecules in R and β orientations. N_ R ¼
PRfβ NR þ PβfR Nβ

ð2Þ

N_ β ¼ PRfβ NR
PβfR Nβ

ð3Þ

Here, the rate constants of flipping, PRfβ and PβfR , should be formulated as the Arrhenius expression for the rate constant, that is
 E1 PRfβ ¼ P0 exp
ð4Þ kT PβfR ¼

P00


 E2 exp
kT

ð5Þ

E1 is the activation energy for flipping from R to β, E2 is that from β to R, and P0 and P00 are the respective flipping rates at high temperatures. To estimate the activation energies for the transitions between R and β, the flipping probabilities were calculated from the number of molecules in the STM images obtained at 77
100 K. The rate constant of flipping from R to β, PRfβ, is calculated through comparison of two successive STM images using PRfβ ¼

NRfβ NR

ð6Þ

NR is the number of molecules showing R orientation in the first image, and NRfβ is the number of molecules that flip from R to β 12416

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In an ideal equilibrium, the number of molecules flipping from R to β should correlate with that from β to R, that is, NRfβ = NβfR. The total number of molecules flipping from R to β is, in fact, almost the same as that flipping from β to R (Table 1). The differences in the numbers at 92 and 100 K are less than 4%. At 77 and 87 K, the differences between NRfβ and NβfR are 30 and 40%, which are larger than those at the higher temperatures. Because the numbers of flipping molecules are less than 100, the intrinsic errors from counting flipping molecules in STM observations and analyses, which should be on the order of 1, become relatively larger. This might result in such large differences between NRfβ and NβfR. The relation NRfβ = NβfR can be expressed using NR , PRfβ, Nβ, and PβfR. Figure 3. Arrhenius plot of flipping rate constants and the plausible energy diagram with asymmetric double minima (inset). PRfβ is the rate of flipping from R to β, and PβfR is that from R to β.

in the second image. This formula is the same as those used for calculating the activation energies of translational motion16 and rotational motion.8 The numbers of molecules, NRfβ and NR , were summed for STM images obtained for the same observed area at the same temperature. The rate constant, PRfβ, is the mean value of the rate constants calculated from NRfβ and NR. The rate constant of flipping from β to R is also calculated using the same method. For example, PRfβ at 77 K is 0.012, and PβfR at 77 K is 0.099. The difference between PRfβ and PβfR indicates that the flipping behavior is asymmetric. STM measurement would influence the flipping rates, as reported previously.17 However, the measurement would agitate the flipping behaviors in both directions, with small magnitude for such a large molecule; therefore, it would not significantly affect estimation of the activation energies. An Arrhenius plot of both the rate of flipping from R to β and that from β to R is shown in Figure 3. The activation energies for the flipping behaviors were calculated using the method of least-squares on the logarithms of the rate constants that were the mean values of the probabilities measured from the series of STM images obtained at the same temperature. The obtained activation energy for the transition from R to β (E1) and that from β to R (E2) were 60 ( 7.0 and 49 ( 25 meV, respectively. The results show that these activation energies were approximately 60 meV. The coefficients of correlation for the plots of PRfβ and PβfR are 0.98 and 0.81, respectively, indicating that the lines of best fit describe the data well. This result is consistent with the model based on the difference in the numbers of molecules showing the orientations. The difference between E1 and E2 was 11 meV, which shows good agreement with the value calculated from the difference, ΔE, in the numbers of molecules with R and β orientations. However, the standard errors of the activation energies were of the same order of magnitude as ΔE. Thus, ΔE determined from a comparison between these obtained activation energies is not sufficiently reliable. The activation energies obtained in this study are higher than that for the 12 meV rotational motion of dibutylsulfide, a small molecule, on an Au(111) surface.6 This is logical because the molecular weight of MP-tBPP is approximately 6 times larger than that of dibutylsulfide, which results in larger van der Waals interaction between MP-tBPP and the substrate than that between dibutylsulfide and the substrate.

NR PRfβ ¼ Nβ PβfR

ð7Þ

This leads to PRfβ PβfR

¼ ¼

Nβ NR
 P0 ΔE exp
kT P00

ð8Þ

This indicates that the dependency of PRfβ/PβfR and Nβ/NR on temperature gives ΔE. However, we could not determine ΔE from the Arrhenius plots for PRfβ/PβfR and Nβ/NR because the plotted data were scattered, resulting in coefficients of correlation of 0.24 and 0.20, respectively. This suggests that the ratios PRfβ/PβfR and Nβ/NR involve large errors. E1 and E2 have the same order of magnitude of standard errors as ΔE; therefore, it is understandable that the ratios have large errors that hamper accurate estimation of ΔE using eq 8. The ratios of the flipping rate constants in both directions at 77, 87, 92, and 100 K also give the difference between E1 and E2, ΔE, through the following equation ! PRfβ ΔE ¼
kT ln ð9Þ PβfR The mean difference between E1 and E2 calculated from the flipping rates is 12 meV, and E1 > E2, which supports the model consisting of asymmetric double minima potential shown in the Figure 3 inset. This agrees with ΔE = 13 meV, the value calculated from the ratio between NR and Nβ. From this, we can determine that ΔE is approximately 12 meV. The value of ΔE is of the same order of magnitude as the calculated difference, 9
22 meV, in adsorption energy for stable adsorption sites of a phthalocyanine derivative on Au(111).4

4. CONCLUSION We found that single MP-tBPP molecules adsorb with two different orientations at the elbow positions of a Au(111) surface, R and β. These orientations correspond to the minima in the asymmetric double-minima potential for the adsorption of MP-tBPP on the sites. The molecules flip their orientation from R to β and vice versa. The activation energy for flipping the orientation from R to β calculated from the Arrhenius plot of the flipping rates was approximately 60 meV. The difference between the energy for the transition from R to β and that for the transition from β to R were approximately 12 meV, which were calculated from the ratio of the molecular populations in these 12417

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orientations and that of the flipping rates. The results show that statistics of single molecules observed with STM can elucidate the asymmetric double-minima potential that determines the orientation of the adsorbed molecules and their flipping behaviors as well as the activation energy of rotational motion. Estimation of a corresponding energy diagram can help with the surface chemistry as well as the artificial arrangement of molecules on substrates for application in molecular devices.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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