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Diffuse Unoccupied Molecular Orbital of Rubrene Causing ImagePotential State Mediated Excitation T. Ueba, R. Terawaki, T. Morikawa, Y. Kitagawa, M. Okumura, T. Yamada, H. S. Kato, and T. Munakata* Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan ABSTRACT: We show the significant role of the Rydberg-like unoccupied molecular orbital in the photoexcitation process at the rubrene/graphite interface. Two-photon photoemission (2PPE) spectroscopy revealed a prominently strong resonant excitation of an unoccupied molecular orbital (MO) at 3.26 eV above the Fermi level (J. Phys. Chem. C 2012, 116, 5821). The excitation was attributed to be mediated by the image potential state (IPS) on graphite. We investigate in this paper the nature of the MO by 2PPE spectroscopy and density functional theory (DFT) calculations. The DFT calculation involving diffuse atomic orbitals predicts a nearly cylindrical unoccupied MO extending over four phenyl rings at the energy of the interest. Polarization dependence of 2PPE spectroscopy shows that the unoccupied MO is nearly symmetric to the plane of the light incidence in accordance with the calculated MO. The two-dimensional free electron wave function of the IPS can interact with the MO that has no node within the molecular framework. The bonding interactions of diffuse atomic orbitals are the origin of the cylindrical MO. It can be seen as a Rydberg-like orbital and is similar to the superatom molecular orbital (SAMO) known for C60. molecular orbital (SAMO) of C60,6 plays an important role in the optical excitation process at the interface. The molecular structure of rubrene (5,6,11,12-tetraphenyltetracene) is shown in Figure 1. In addition to the high carrier mobility7 and clear dispersion of the HOMO band8 for a single crystal, the dominant excitonic effects in the optical response have been reported by many works.9−11

1. INTRODUCTION Electronic excitation at the interface between an organic molecular film and a substrate is of general importance for the area of organic electronics, solar cell, and other light conversion processes. As an optical excitation at the interfaces, intramolecular excitation is most frequently discussed. Intramolecular optical excitation occurs between occupied and unoccupied molecular levels which are perturbed by substrate. Though the optical transition between a localized molecular orbital (MO) and a delocalized substrate band is thought to be weak,1 mixing of MO with delocalized substrate band causes significant effects on optical excitaion processes.1−4 Among such transitions, resonant excitation at rubrene/graphite interface provides an interesting case, in which a substratemediated transition overwhelms typical intramolecular transitions.5 Two-photon photoemission (2PPE) spectroscopy for rubrene films formed on the highly ordered pyrolytic graphite (HOPG) surface reveals a prominently enhanced peak due to an unoccupied molecular level (denoted by Ln), which is resonantly excited from the highest occupied molecular orbital (HOMO) derived level. Based on the correlation between the loss of enhancement for this peak and the quenching of the substrate image potential state (IPS), we suggested that the optical excitation of the Ln level is mediated by the IPS on HOPG surface. The excitation process is expected to be useful to highly enhance the efficiency of organic molecular devices and light conversion processes. Though the experimental results indicate the interaction of the IPS on HOPG with the Ln level, the origin of the MO has not been clarified. Here, we reveal the origin of the Ln level from detailed 2PPE experiments and quantum mechanical calculations. We show that a Rydberg-like MO, which is similar to the superatom © 2013 American Chemical Society

2. EXPERIMENTAL METHODS 2.1. 2PPE Spectroscopy. All of the experiments were carried out in an ultrahigh vacuum chamber of the base pressure better than 1 × 10−10 Torr. The light source was the second and third harmonics of a wavelength tunable titanium sapphire laser operated at a repetition rate of 76 MHz and a pulse duration of 100 fs. The light was focused onto the sample with a concave mirror of 40 cm focal length at an incident angle of 60°. The p-polarized light was used unless mentioned. Photoelectrons emitted to the surface normal were detected with a hemispherical energy analyzer of 20 meV resolution and the electron acceptance angle of ±1°. Conventional onephoton photoemission (1PPE) spectroscopy was performed with a helium discharge lamp (hν = 21.22 eV). HOPG was cleaved in air and heated in UHV at 670 K for more than 50 h. Cleanliness of the surface was confirmed by the work function (4.45 eV) and the n = 1 IPS peak, which is narrower than 140 meV. Purified rubrene was deposited on the HOPG surface by sublimation with a rate of 0.1 Å/min as monitored by a quartz microbalance. All of the experiments were performed at room Received: August 8, 2013 Revised: September 3, 2013 Published: September 3, 2013 20098

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Figure 1. 1PPE and 2PPE spectra for the 0.6 ML film (top). Photon energies for 2PPE are shown at the right side. The 1PPE and 2PPE spectra are plotted on the initial energy and the intermediate energy scales, respectively. Both scales refer to the Fermi level (EF). The calculated energy levels and several contours of the orbitals are shown in the bottom for the respective basis sets. The isovalue of the depicted contours is 0.02 atomic units. Green and red colors correspond to the sign of the wave function. Occupied levels are denoted by H0, H1, and so on in the order of the binding energy, and the unoccupied levels, by L0, L1, and so on in the order of the intermediate energy. The calculated energies of HOMO (H0) and LUMO (L0) are adjusted to the experimental H0 and L0 peak energies, respectively. The horizontal arrow shows that the Ln peak is strongly enhanced by the resonant excitation from the HOMO derived level at the photon energy of 4.43 eV. The inset is the molecular structure of rubrene.

that the polarization functions are added to C (3d orbitals), and the superscript ** to both C and H (2p orbitals) atomic orbitals, respectively. The sign + indicates that the diffuse functions are added to C, and ++ to both C and H atomic orbitals. The calculated energy levels and the contours of several orbitals are shown in Figure 1. Optical transition probabilities were calculated by a time-dependent DFT (TDDFT) method at the B3LYP/6-31+G* level. Some of the oscillator strengths are shown in Table 1. We did not carry out band structure calculation for films, because the structure and molecular orientation of the rubrene film on graphite is not known.

temperature. Cooling of the sample to 90 K resulted in no significant change other than slight narrowing of the spectral features. Figure 1 shows 1PPE and 2PPE spectra for the rubrene film of 0.6 monolayer (ML) coverage. In 2PPE spectra two IPS peaks are observed: The IPS1 peak at EF + 3.60 eV is due to the IPS on the HOPG surface, and the IPS2 peak at EF + 3.89 eV is due to the IPS on the rubrene film, respectively. The coverage of 1 ML was defined by the disappearance of the IPS1 peak and also by the change of the work function.5 2.2. Calculation. The ground state electronic levels of rubrene molecule were calculated by Gaussian09 rev B.01 software.12 Rubrene has a planar tetracene backbone in the single crystal phase.13 When rubrene molecules adsorb on surfaces, the tetracene backbone bends.14−16 The bent structure is also known for gas phase molecules. Prior to singlet calculations of rubrene, the geometry was optimized by the ab initio method. To consider an intramolecular dispersion interaction, the second-order Møller−Plesset perturbation theory (MP2) was employed for the geometry optimization. The planar structure was optimized by MP2/4-31G (434 basis functions) calculation in which the X-ray structure13 was taken as the initial structure. The bent structure was, at first, obtained by optimizing at the B3LYP/4-31G level as an initial geometry, and further optimized at the MP2/4-31G level. All optimized structures were confirmed that they did not have any imaginary frequencies by frequency analyses. Density functional theory (DFT) calculations were performed by using the B3LYP functional with the basis sets of 631G*, 6-31+G*, and 6-31++G**. The superscript * indicates

3. RESULTS AND DISCUSSION 3.1. Assignment of the LUMO and LUMO+1 Derived Peaks. Before discussing the nature of the Ln level, we determine the lowest unoccupied molecular orbital (LUMO) derived level. This is necessary to compare the calculated unoccupied levels with experimental results. The photon energy dependence of the 1-color 2PPE spectra is shown in Figure 2. The peaks denoted by L0, F1, F2, L1, and Ln appear at fixed intermediate energies; thus they arise from unoccupied levels of the rubrene film. The peak at EF + 1.0 eV, denoted by L0, is assigned to arise from the LUMO derived level. Figure 1 shows that the HOMO derived peak, labeled by H0, is located at −1.12 eV initial energy. The initial energy is in accordance with other 1PPE results.8,17 The energy difference between the L0 and the H0 peaks of 2.12 eV is in good agreement with the optical absorption results, in which the HOMO−LUMO 20099

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the resonant and is covered with the X peak. This is confirmed by the lifetime measurement, which will be published elsewhere. Figure 1 compares the calculated energy levels for the planar structure with the experimental 1PPE/2PPE results. The calculated HOMO−LUMO energy separation is in the range between 2.51 and 2.54 eV depending on the basis sets. The value is larger than the experimental one of 2.12 eV. The orbital energy of HOMO is in the range between −4.65 and −4.96 eV. To simplify the comparison of the calculated occupied levels with the 1PPE result, the HOMO energy is aligned to the experimental H0 peak. Similarly, the calculated unoccupied levels are shown by aligning the LUMO energy to the experimental L0 peak. Besides the rigid shifts, no adjustment of the energy scale was made. The contours of the calculated MOs with the basis sets of 6-31G*, 6-31+G*, and 6-31++G** are also shown. Though the energy levels for the bent structure are slightly different from those in Figure 1, the differences are typically smaller than 0.1 eV and are not significant when compared with the experimental results. In addition, our experiment shows no clear evidence of coexisting planar and bent molecules.17 The peaks denoted by F1 and F2 are not reproduced by our calculations. Since the peaks are also observed for films as thick as 10 ML, molecule−substrate interaction is not the origin of the peaks. The F1 and F2 levels are absent even in band calculations for single crystals.21,22 A theoretical work reported the significance of many-body effects in the optical spectrum,23 and such an effect may play a role in producing the F1 and F2 peaks. However, at the present stage, determining the origin of these peaks requires more investigation. The occupied energy levels are not very sensitive to the basis sets, whereas the unoccupied levels are dependent on the basis sets. There are several calculated energy levels, LUMO+1 to LUMO+4, in the vicinity of the L1 peak (denoted by Lm in the former paper5). The L1 peak originates from an unoccupied level which is excited from the HOMO derived level. The TDDFT calculation in Table 1 shows that HOMO−LUMO+1 excitation is involved in several transitions (no. 3, 5, 11, and 13), whereas LUMO+2, 3, and 4 levels are scarcely excited from HOMO. This is because HOMO, LUMO, and LUMO+1 are mainly composed of the π-orbital of tetracene backbone, while LUMO+3 and LUMO+4 are mainly composed of the πorbital of the phenyl rings. The transition from HOMO to LUMO+2 is forbidden. Thus the L1 peak is assigned to be due to the LUMO+1 derived level. Though the optical selection rule for adsorbed molecule is frequently different from that for free molecule because of the deformation of the molecular structure,4,24 the feature of the transition probability in rubrene mentioned above is not sensitive to the deformation of the molecular structure. 3.2. Origin of the Ln Peak. Our main interest in this paper is the origin of the Ln peak. The Ln peak is strongly enhanced by the resonant excitation from the HOMO derived level5 as shown by the horizontal arrow in Figure 1. In the calculation with 6-31G* (without diffuse functions), LUMO+5 to LUMO +9 levels are located close to the Ln peak. The MOs contain many nodes within the molecule, reflecting the antibonding nature. It is very difficult to consider that such orbitals effectively hybridize with the two-dimensional free-electron-like IPS wave function. The wavelength of the IPS is infinite at the Γ̅ -point where 2PPE spectra were measured. In addition, the IPS wave function has no node in the surface normal direction.

Table 1. Transition Photon Energies and the Oscillator Strengths Calculated by TD-DFT with the B3LYP/6-31+G* Basis Set for Planar Geometry

a

Only transitions with oscillator strengths higher than 0.0004 are listed. Transitions of high oscillator strengths are highlighted in yellow. b The individual transition between MOs is listed in the order of the contribution decreasing from left to right. cH0 and Hi denote HOMO and HOMO-i orbitals, and L0 and Li, LUMO and LUMO+i orbitals.

Figure 2. 2PPE spectra for the 1.2 ML rubrene film on HOPG, measured with photon energies indicated on the right side. The peaks L0, F1, F2, L1, and Ln arise from respective unoccupied levels. The L0 peak at 1.0 eV energy arises from the LUMO derived level. The intensity of the L0 peak comes to a maximum at the photon energy of 4.13 eV.

transition is reported to occur at 2.32 eV for film18 and at 2.36 eV for solution,19 respectively. The L0 peak in Figure 2 arises from transitions to the LUMO level from occupied levels deeper than −3 eV initial energies. The 2PPE intensity of the L0 peak in Figure 2 comes to a maximum at the photon energy of 4.13 eV. The photon energy is close to the second absorption band of thick rubrene film at 4.2 eV.18,20 According to the TD-DFT calculation shown in Table 1, the second absorption band arises from transitions of no. 11, 13, and 19, which involve excitations to the LUMO level. The resonant enhancement at 4.13 eV photon energy also supports the assignment of the L0 peak to the LUMO derived level. The peak denoted by X shifts to lower intermediate energy at photon energy higher than 4.43 eV. The peak is not due to an intermediate state, but due to scattered electrons or a final state. The L0 peak becomes weak at the photon energies far above 20100

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The quite-significant change by inclusion of the diffuse orbitals in the DFT calculation is the appearance of the spatially extended MOs, L+10 and L+7 obtained by the 6-31+G* and the 6-31++G** basis sets, respectively. The energies of both orbitals are close to the Ln peak. The orbitals are nearly cylindrical with dents in between the 5, 6 and 11, 12 phenyl rings. They diffusely surround the four phenyl rings. The inside part of the cylindrical wave function is shown in Figure 3a. The

Figure 4. Polarization-dependent two-color 2PPE spectra for the rubrene film of 0.7 ML coverage. The pump photon of 4.43 eV is ppolarized. The black spectrum is measured with a p-polarized probe photon of 2.95 eV, and the green, with s-polarized. The green spectrum is multiplied by 2.1 and shown by the blue trace. The L1 and F2 peaks as well as the low energy feature of the blue trace overlap well with the black trace, whereas the Ln peak for the blue trace is weaker by a factor of about 10 than that of the black trace.

Figure 3. Orbitals obtained by the 6-31+G* diffuse basis set. The inside of the doughnut-like MO (L10) is shown in a. The L12 and L18 MOs have p- and d-like shapes as shown in b and c, respectively.

contribution of different sign components is very small. There are essentially no nodes within the molecular frame. We call the MO a “doughnut-like MO”. The doughnut-like MO arises from bonding interactions between high-principal quantum number (n) atomic orbitals. The wave functions between C and H of the phenyl rings are in phase and contribute to forming the doughnut-like MO. Though the shape of the L+7 orbital obtained from the 6-31++G** basis set (see TOC) is slightly different from the L+10 orbital from 6-31+G* basis set due to the contribution of high-n orbitals of H atoms, the difference does not affect the cylindrical nature of the orbitals. It is very probable that such a doughnut-like MO can interact with the IPS wave function. We consider that the Ln peak arises from the doughnut-like MO, which is composed of the diffuse orbitals. 3.3. Polarization-Dependent Two-Color 2PPE Spectroscopy. We employ a polarization dependent two-color 2PPE experiment to confirm the assignment. We discuss the polarization dependence based on the dipole approximation in place of the AP gauge treatment25 which is too difficult to apply to the present system. The two-color 2PPE measurement is performed by focusing both the 3ω (4.43 eV) and 2ω (2.95 eV) lights on the sample with exactly the same beam path lengths. The spectra in Figure 4 are obtained by subtracting one-color 2PPE signals by 3ω and 2ω from the raw spectra. In the two-color experiment (a) 3ω-pump, 2ω-probe and (b) 2ωpump, 3ω-probe processes result in the same final energy. Process b can produce L0, F1, and F2 derived peaks at intermediate energies of 2.5, 3.1, and 3.6 eV in Figure 4, respectively. By comparing with a one-color 2PPE spectrum measured with 3ω (Figure 1), it is confirmed that process a is the major process and the contribution of process b is very small. This is because the 2ω photon energy is far from any resonances. On the other hand, the photon energy of the 3ωpump light is resonant to the HOMO-Ln transition. In Figure 4, the 3ω light is p-polarized, and the 2ω light is p- and spolarized for black and green traces, respectively. The IPS peaks, IPS1 and IPS2, disappear with s-polarized probe light in agreement with the selection rule. Because the wave function of the IPS is symmetric to the plane of the light incidence,

photoemission to the plane-wave photoelectron wave function is prohibited for s-polarized light. The Ln peak is strong with p-polarized probe light, whereas it is very weak with s-polarization. Though weak, the F2, L1, and Ln peaks do not disappear even with s-polarization. The sprobed spectrum is magnified by a factor of 2.1 and is shown by the blue trace. The F2 and L1 peaks as well as the low-energy feature of the magnified trace overlap well with the p-probed spectrum. This suggests that probe p/s ratios from the respective unoccupied levels are similar. On the other hand, the p/s ratio for the Ln peak is very large, about 20. This indicates that the Ln orbital has a character different from typical MOs and rather similar to the IPS wave function. The highly symmetric, doughnut-like orbital shown as L+10 (631+G*) or L+7 (6-31++G**) in Figure 1 is in good accordance with the polarization dependence. Unlike the IPS peaks, the Ln peak does not disappear completely, indicating that the orbital is not perfectly symmetric with respect to the plane of the light incidence. This is very reasonable by taking account of the shape of the doughnut-like MO. As shown in Table 1, the oscillator strength from HOMO to the doughnut-like orbital (transition no. 21) is small but nonzero. The MO derived level can be resonantly excited from the HOMO derived level. Strong enhancement of the Ln peak occurs when the MO is overlapping with the IPS on graphite. The MO mixed with the IPS can be excited from the occupied bands of the substrate resulting in the strong enhancement. In reality, the Ln peak is weaker than the L1 peak for the films thicker than 1 ML.5 The Ln peak in Figure 2 measured for the film of 1.2 ML coverage is not so strongly enhanced as that in Figure 1 for the film of 0.6 ML coverage. The doughnut-like orbital is bound to the phenyl moiety rather than the central potentials of the individal C and H atoms. The orbital can be viewed as a Rydberg state of the molecule because it arises from the diffuse orbitals. It is also similar to SAMOs known for C60.6 SAMOs of C60 are bound to the spherical potential of the molecular shell. The SAMOs were also reproduced by a Gaussian03 calculation.26 SAMOs are also 20101

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known for molecules other than C60.27,28 Though the structure of rubrene molecule is far from spherically symmetric C60, the diffuse orbitals can form p- or d-like MOs as shown in Figure 3b and c, respectively. The nearly free electron (NFE) bands for C6F6 on Cu(111) are under debate1,28,29 and requires comparison with our rubrene case. While Föhlisch assigned the NFE band to the π* orbital under the influence of the IPS in front of the metal surface,29 Dougherty attributed the band to SAMO character of the σ* orbital.28 Although the σ* orbital has a diffuse nature, it has three nodal surfaces normal to the molecular plane. The σ* orbital of the flat lying molecule cannot interact with the IPS.28 In contrast, because the doughnuts-like orbital of rubrene has essentially no nodes, it can interact with the IPS. The NFE band was considered to be populated by the scattering of hot electrons from the metal substrate.29 On the other hand, the Ln peak of rubrene is resonantly excited from the HOMO derived level. Though the diffuse characters are common for C6F6 and rubrene, the detailed natures are different. The role of the Rydberg-like orbitals is not very significant for gas phase molecules because the transition probability from HOMO is small. However, when molecules are adsorbed on surfaces, the diffuse orbitals effectively interact with substrate band orbitals and cause a significant effect on optical excitation processes at surfaces. The role of Rydberg molecular orbitals is also significant in photochemistry at surfaces. For example, the contribution of Rydberg MOs was discussed in the photochemistry of methane adsorbed on metal.30 The interaction of the MO with the substrate causes shifts and broadening in the energy levels of molecule/substrate interface. Not only for the shifts or broadening, it is important that the optical transition probability is strongly affected by the molecule−substrate interaction. The role of the Rydberg MO for rubrene is a typical case of significant effect in the optical transition at the surfaces.

REFERENCES

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4. CONCLUSION In conclusion, the Ln peak, which is prominently enhanced by the resonant excitation from the HOMO derived level, is ascribed to arise from the diffuse unoccupied molecular orbital surrounding the phenyl rings. The polarization dependence of the two-color 2PPE experiment and the DFT calculation consistently support the assignment. The spatially extended, doughnut-like MO derived from diffuse atomic orbitals effectively interacts with the IPS of substrate and significantly contributes to the electronic excitation process. The orbital is a kind of Rydberg orbital of molecule and is not specific to rubrene. This orbital plays an important role in the photoexcitation processes at the rubrene/HOPG interface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], phone: +81(0)6 6850 6082, fax: +81(0)6 6850 5779. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge J. Takeya of Tokyo University for supplying the rubrene sample. T.U. acknowledges support from JSPS research fellow program. This work was partly supported by Grant-in-Aid for Scientific Research from JSPS (25600004, 24656036, and 24685004). 20102

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