Experimental Reorganization Energies of Pentacene and

Article pubs.acs.org/JPCC

Experimental Reorganization Energies of Pentacene and Perfluoropentacene: Effects of Perfluorination Satoshi Kera,*,† Shunsuke Hosoumi,† Kazushi Sato,† Hirohiko Fukagawa,†,# Shin-ichi Nagamatsu,†,¶ Youichi Sakamoto,‡ Toshiyasu Suzuki,‡ Han Huang,§ Wei Chen,∥ Andrew Thye Shen Wee,§ Veaceslav Coropceanu,⊥ and Nobuo Ueno† †

Graduate School of Advanced Integration Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan § Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore ∥ Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore ⊥ School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡

ABSTRACT: Electron−phonon coupling of the highest occupied molecular orbital (HOMO) state is studied by high-resolution ultraviolet photoelectron spectroscopy (UPS) for pentacene (PEN) and perfluoropentacene (PFP) monolayers on graphite. The reorganization energy and related coupling constants associated with the interaction between holes and molecular vibrations are obtained experimentally using a single mode analysis (SMA) of the observed vibronicsatellite intensities of the monolayers. The results are compared with those estimated by multimode analyses of UPS spectra and those derived by means of theoretical approaches, indicating that the purely experimental method with SMA is useful for studying the reorganization energy and the hopping mobility of organic systems. Furthermore, we found that the reorganization energy of PFP is significantly greater than that of PEN, which is ascribed to the extended HOMO distribution of PFP by perfluorination of PEN. The comparison with the results derived from gas-phase UPS measurements is also discussed.

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(Epol).13,14 In general both molecular and lattice vibrations contribute to Epol. However, the results of recent theoretical investigations indicate that in systems such as oligoacenes the main contribution to the polaron binding energy is due to the molecular vibrations.12 Theoretical studies on intramolecular contribution to the polaron binding energy were performed and compared with the ultraviolet photoelectron spectroscopy (UPS) spectra of gasphase molecules measured at an elevated temperature. These studies provided vital information about electron−phonon interaction and contributed to the understanding of charge transport in organic solids.2−12,15−24 It is important to note that most theoretical and experimental studies on the local electron−phonon interaction have been performed on isolated molecules. Therefore, it is important to understand how these results are altered by intermolecular interactions that take place

INTRODUCTION Organic semiconductors are molecular solids with specific charge transport properties, due to weak intermolecular interaction,1 and have been increasingly studied for various device applications. However, the transport properties of organic single crystals and organic thin films are far from being adequately understood.2−7 Important subjects still to be understood are related to molecular and lattice vibrations (phonons) and their coupling to charge carrier.8−12 The electron−phonon interaction depends on the molecular structure and their packing motif, and therefore it can impact both molecular site energies (HOMO energies, for instance) and transfer integrals. The first coupling mechanism is commonly referred to as the local electron−phonon coupling mechanism and the second as the nonlocal electron−phonon coupling mechanism.3−11 Here we focus on the local electron− phonon coupling mechanism by molecular and lattice vibrations. The overall strength of this interaction is given by the relaxation energy of neutral and ionized states and the reorganization energy (λ) associated.3 The relaxation energy of the ionized state in the context of small (molecular) polaron theory is also referred to as the polaron binding energy © 2013 American Chemical Society

Special Issue: Ron Naaman Festschrift Received: April 1, 2013 Revised: June 6, 2013 Published: June 10, 2013 22428

dx.doi.org/10.1021/jp4032089 | J. Phys. Chem. C 2013, 117, 22428−22437

The Journal of Physical Chemistry C

Article

Figure 1. (a) Molecularly resolved 20 × 20 nm2 STM image of a well-ordered PFP monolayer (ML) on HOPG (Vtip = 2.0 V, I = 100 pA) at 77 K and (b) its corresponding Fourier transform of the image. (c) The schematic showing the proposed molecular packing structure for the PFP ML on HOPG.

type organic semiconductors.29 Moreover, the lattice constant of the PFP monolayer is not significantly different from that of PEN.30,31 Here, we show the impacts of perfluorination on the electronic structure of the PEN monolayer on graphite and hole−vibration coupling, which is specified by the reorganization energy λ and the binding energy of molecular polaron (small polaron). We demonstrate that the electron-withdrawing property of the F atom mediates an increase in spatial spread of HOMO, which contributes significantly to an increase in the vibronic-satellite intensity and then leads to a significant increase in λ. This contribution is much larger than an opposite contribution by lowering of vibration energies by perfluorination of PEN.

in the solid state. However, direct experimental study on this issue has been neglected until recently because of experimental difficulties in resolving vibronic satellites of the highest occupied molecular orbital (HOMO) state in UPS spectra of organic thin films.9 The UPS line width of the thin film is broadened by intermolecular interaction, molecule−substrate interaction, and insufficient uniformity of the film structure, which mask the vibronic satellites. After successful high-resolution UPS measurements of the vibronic satellites for Cu-phthalocyanine ultrathin film by Kera et al.,25 Yamane et al. studied the HOMO hole−vibration (molecular vibration as a local phonon) coupling with UPS for the pentacene (PEN) monolayer on graphite and determined the reorganization energy (λ).8,9 They pointed out that λ in the film is slightly larger than that in a free molecule.2 Afterward, Paramonov et al. discussed the difference of λ’s in gas and monolayer phases theoretically by considering intermolecular and molecule−substrate interactions.16 In this work, we report an experimental study of the electronic structure and electron−phonon coupling of the perfluoropentacene (PFP) monolayer (ML) on graphite. We compare the hole−vibration couplings in PFP and PEN monolayers and in the gas phase to study the effect of perfluorination on these parameters. We observed that the UPS vibrational structure is largely different between PEN and PFP in the monolayer phase and in the gas phase. Furthermore, although the vibration energy is decreased by perfluorination as expected from a much larger mass of fluorine atom than hydrogen, the vibronic-satellite intensity due to hole−vibration coupling is significantly enhanced in PFP. We found that the latter contribution to λ is much greater than the former one, resulting in a serious increase in λ of PFP also for the monolayer. PFP was designed and synthesized to realize an n-type semiconductor with high electron mobility,26−28 since due to the strong electron-withdrawing property of the F atom perfluorination is effective to modify the electronic structure to increase the electron affinity with respect to PEN that had been known as a prototype high hole mobility system among p-

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EXPERIMENTS AND THEORETICAL CALCULATIONS High-resolution UPS spectra were obtained using a VGCLAM4 analyzer with a 9-channel detector and p-polarized He I radiation from an Omicron-HIS13 VUV source with a rotatable linear polarizer. The angle between the incident photon and the emitted photoelectron direction was fixed at 45°, and the acceptance angle of the photoelectron was ±12°. The total energy resolution was less than 20 meV, as determined from the Fermi edge of an evaporated Au film. To observe the vacuum level (VL) cutoff thus to determine the work function (ϕ), UPS spectra were measured with −5.00 V bias applied to the sample. A highly oriented pyrolytic graphite (HOPG:ZYA grade) substrate was cleaved in air just before loading into the UHV chamber and cleaned by heating in situ at ∼673 K for 15 h. The cleanliness of the substrate surface was confirmed by measuring the ϕ using UPS and by metastableatom electron spectroscopy (MAES). PFP was purified by three-cycle sublimation and deposited onto the HOPG substrate at a rate of 0.02 nm/min. The well-ordered monolayer with flat-lie molecular orientation was prepared by heating a 0.3 nm thick film (nominal thickness) at ∼373 K for 2 h and confirmed by a narrowing of the HOMO feature with vibration satellites at 295 K.9 The LT-STM experiments were 22429



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carried out in a custom-built multichamber ultrahigh-vacuum (UHV) system housing an Omicron LT-STM interfaced to a Nanonis controller (Nanonis, Switzerland).32 All STM images were taken at 77 K. PFP was deposited from a Knudsen cell at 400 K with HOPG at room temperature (300 K) in a separate growth chamber. Theoretical calculations were conducted to estimate the electronic density of -states (DOS), molecular orbital (MO) energies, and spatial distribution of MOs by using a Gaussian 03W package33 at the B3LYP/6-311++G** density functional theory (DFT) level. The vibration frequencies, molecular geometries of the neutral and cation states, and the Huang− Rhys factors (S) of the ionized molecule were calculated by DFT (B3LYP/6-31G**). The S factors are also experimentally obtained by the least-squares fitting of HOMO spectra.

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RESULTS AND DISCUSSION Energy Levels and Their Temperature Dependence. We first show a representative molecularly resolved 20 × 20 nm2 STM image of a well-ordered PFP monolayer (ML) on HOPG, in which each rod-like bright feature represents a single PFP molecule. Perfectly ordered PFP MLs with a lying-down configuration on HOPG are found. The Fourier transforms of the image and unit cell are highlighted in Figure 1(b) and 1(c) with a = 2.04 ± 0.05 nm, b = 0.96 ± 0.05 nm, and θ = 60° ± 2°. The schematic in Figure 1(c) shows the proposed molecular packing structure for the PFP ML on HOPG. The ML structure resembles the PEN ML on HOPG.30 Figure 2 shows HeI UPS spectra of the PFP (ML)/HOPG measured at 295 and 53 K, compared with the gas-phase UPS spectrum reported by Delgado et al.18 The binding energy (Eb) is measured from the Fermi level of the substrate (EF), and the ionization energy (Ei) is from the vacuum level (VL). The 53 K spectrum is shown after shifting Eb by 0.18 eV to align the HOMO peak to that of the 295 K spectrum. The valence band features in the ML spectra correspond well with those in the gas phase, indicating the electronic structure is not largely modified by intermolecular and molecule−substrate interactions. One clearly sees that the HOMO band represents an asymmetric shape caused by a vibrational progression toward the higher Eb side as in other previously reported monolayer systems.8,9,25,34−43 The vertical Ei of the main HOMO peak is determined to be 7.50 eV (gas phase), 6.18 eV (ML at 295 K), and 6.02 eV (ML at 53 K). Adiabatic Ei (0−0 vibronic transition) is estimated to be 5.97 eV for the ML at 53 K with the multimode analysis as described later. The energy diagram is summarized in Figure 2(c) with the PEN/HOPG system.8 The lowering of Ei in the monolayers compared with the gaseous is due to polarization screening (relaxation of hole) in the ML. The hole injection barrier of the PFP/HOPG is larger by 0.49 eV than that of the PEN/HOPG at room temperature. Considering the calculated HOMO−LUMO gap of the gas molecule (EHOMO−LUMO = 2.02 eV (PFP) and 2.20 eV (PEN)26) and also the optical band gap in solution (Eopt = 1.95 eV (PFP) and 2.07 eV (PEN)26), we roughly estimate that the EF is located closer to LUMO for the PFP/HOPG, while it is located near the center of the HOMO−LUMO gap for PEN. The effect of exciton binding energy (0.6 eV for PEN)44,45 on the band gap and transport gap energies needs to be studied further. The work function (ϕ) of the HOPG is increased by several tens of meV upon deposition of the PFP monolayer at 295 K. In general, the work function change upon molecular

Figure 2. (a) UPS spectra (θ = 0° and 45°) for the PFP/HOPG measured at 295 and 53 K with p-polarized HeI radiation. The 53 K spectrum is shown after shifting binding energy by 0.18 eV to align the HOMO peak. (b) Gas-phase UPS spectrum [from ref 18] compared with the calculated DOS curve (with Gaussian broadening of 0.2 eV). Calculated energy is shifted by 2.3 eV to align the HOMO with the observed one. Vertical bars represent the calculated orbital energies (pink bars, π MOs; and black bars, σ MOs). (c) Energy diagram of PFP/HOPG and PEN/HOPG [ref 8]. UPS HOMO peak position is used. Band gap energy is obtained from the DFT calculation of a gas molecule [ref 26].

deposition on an inert surface is caused by (i) relevant ordering of molecular dipole moments,34−43 (ii) the adsorption-induced interfacial dipole such as by Pauli repulsion, and/or (iii) interfacial intramolecular dipole by charge rearrangement.46,47 Contribution (i) is excluded for PFP since it has no electric dipole moment. The contribution of Pauli repulsion in an ideal physisorption system was estimated using a Xenon/HOPG system to increase the VL by 10 meV (data not shown). Therefore, the observed increase in ϕ in the present PFP/ HOPG involves contributions of (ii) and (iii). These contributions are small enough; therefore PFP−HOPG interaction does not impact the electronic states of PFP seriously. This is consistent with the excellent agreement between the gas-phase and the monolayer UPS spectra [Figure 2(a) and (b)]. Upon cooling the PFP film down to 53 K, the UPS spectrum changes slightly. In particular, the HOMO is shifted to the lowEb side by 0.18 eV, and the ϕ increases by 0.02 eV. When the 22430



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Figure 3. Calculated orbital energy and typical MO patterns of PFP.

and is delocalized over fluorine and carbon atoms, peaks in B to E to π-orbitals (π10−π5), those in F to a π orbital (π4) and a σ orbital, G to a π orbital (π3) and σ orbitals, and H to two π orbitals (π2, π1) and two σ orbitals. Of course, there are some problems in the assignments for deeper-lying states in DFTMO calculation,53 but it is feasible to determine the character of each state for lower-lying states. These MOs are listed in Figure 3. Vibration Satellites and Their Characteristics. We discuss UPS fine features of the HOMO band in Figure 2, where the band is observed as a sharp peak with several fine satellites in the high Eb side. Note that similar fine features are also observed for deeper-lying bands F, G, and H. These fine features also come from hole−vibration coupling as for the HOMO (band A) that will be discussed later. Note that (i) the vibronic satellites in bands F, G, and H are much sharper in the monolayer phase than those in the gas phase and that (ii) the vibrational progression depends largely on the MOs, indicating that hole−vibration coupling depends significantly on the nature of electronic states of the cation.54 In the gas phase the molecular structure at the initial state of the photoionization process is dynamically deformed by thermal excitation of molecular vibrations to result in broadening of UPS bands,55 whereas the corresponding thermal excitation is quenched in the ML film especially at 53 K to give sharper UPS features. Before discussing the analysis methods for obtaining λ and related parameters, we briefly introduce hidden effects of other dynamic phenomena on the UPS band shape. Figure 4(a) shows photoelectron-takeoff angle (θ) dependence of the HOMO band spectra of the PFP/HOPG at 53 K, where the intensity is normalized to the main peak a0 after subtracting the background. θ dependences of the vibronic satellite intensities

film is heated to 295 K, the spectrum recovers to the initial 295 K spectrum, indicating a fully reversible temperature dependence of the electronic structure as observed on the PEN/ HOPG8,9 (see Figure 2c). Although we cannot specify origin of the temperature dependence of spectra, we assume that it originates from a tiny change in the film structure or the molecule−substrate distance that is affected by thermal excitation of phonons.34,43 Interestingly, the temperaturedependent shift in the HOMO Eb is opposite between PFP/ HOPG and PEN/HOPG. Upon cooling Ei decreases by 0.16 eV for PFP and increases by 0.14 eV for PEN, while the increase in ϕ is the same. Intuitively the decrease in the PFP Ei may be dominated by the relaxation of the hole (final-state effect), while the increase in the PEN Ei may be ascribed to stabilization of the initial states due to the temperaturedependent molecule−substrate distance and/or film structure because we suppose the molecule−substrate distance is larger for PFP than PEN as observed on Cu(111) (2.98 Å for PFP and 2.34 Å for PEN)48 and on Ag(111) (3.16 Å for PFP49 and 3.12 Å for 0.75 ML-PEN film50). It might be expected that electronegativity of the fluorine atom impacts the molecule−substrate interaction through a change in the intramolecular charge distribution. However, it is reasonable to assume from reports on above-described metal substrates48−52 that the perfluorination of PEN significantly reduces the adsorbate−substrate interaction. We therefore consider tentatively that the interaction between PFP and graphite is weaker than the case of PEN. We classified spectral features of the PFP/HOPG into 10 energy regions (A−J) (see Figure 2). Among these, the features in regions A−H are assigned as follows: the peak in region A is to a π-orbital (π11, HOMO) where the orbital consists of F 2p and C 2p orbitals 22431



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λ(+) =

∑ Sihνi , i

Epol = λ(+) , λ = λ(0) + λ(+) ≅ 2λ(+) = 2 ∑ Sihνi i

(1)

where λ(0) and λ(+) are the relaxation energies of a neutral and an ionized molecule, respectively. Si corresponds to the Huang−Rhys factor and hνi to the vibrational energy of the vibrational mode i of the ionized molecule.2,9 It is useful to point out that S is directly related to the linear electron− phonon coupling constant of the small polaron model3 and can be experimentally determined by measuring UPS intensity of the nth vibrational satellite (In) of the vibrational mode i. In is given by following the Poisson distribution for each vibration mode, when the neutral state (initial state of the photoionization) is in the vibrational ground state

In =

S n −S e n!

(2)

Thus, once the energy of the vibration mode i (hνi) and corresponding In are measured with high-resolution UPS of HOMO, Si, λ(+), Epol, and λ can be experimentally obtained. Single-Mode Analysis: Direct Experimental Estimation of Reorganization Energy. Prior to the multimode analysis, we describe purely experimental estimation of λ (λ(+) = Epol = λ/2), where we use only experimental values of the vibration energy and satellite intensity within the available experimental energy resolution. Here we call this method single-mode analysis (SMA). Figure 5(a) shows angleintegrated UPS spectra (θ = 0−60° after subtracting the background) of the PFP(ML)/HOPG system (53 K) with the gas-phase (vaporized at 503−563 K) UPS results by Delgado et al.18 For the PFP(ML)/HOPG system the photoemission intensity is integrated also for the azimuthal angle around the surface normal due to the azimuthal disorder of single-crystal domains in HOPG. The HOMO consists of four fine features, namely, a main peak (0−0 vibronic transition) with three satellites (0−1, 0−2, 0−3 vibronic transitions) for the gas phase and the 53 K spectra. These can be reasonably deconvoluted with four Voigt functions (the fwhm’s of WG and WL are fixed to 110 and 111 meV, respectively) with an energy separation of 173 meV and using eq 2 with S = 0.727. The intensity follows Poisson distribution for the vibrational progression in this evaluation both on gas and ML spectra. There is a marked difference between the gas phase and the 53 K spectra in both the satellite intensities and the line width. The results give hν = 173 meV, λ = 252 meV, λ(+) = Epol = 126 meV for the ML at 53 K and hν = 173 meV, λ = 224 meV, and λ(+) = Epol = 112 meV for the gas phase. The reorganization energy in the film is evaluated to be ∼12% larger than that in the gas phase. In the SMA, an error into λ(+) is less than 5 meV. The previously reported angle-integrated UPS spectra (θ = 0° to 60°) of the PEN(ML)/HOPG8 and the gas-phase spectrum of PEN2 are also shown in Figure 5(b) for comparison. We reevaluated the reorganization energies of the PEN(ML)/HOPG and the gas-phase PEN with SMA. The direct experimental values are hν = 167 meV, λ = 100 meV, and λ(+) = Epol = 50 meV for the ML at 49 K and hν = 167 meV, λ = 90 meV, and λ(+) = Epol = 45 meV for the gas phase. Since there is a strange large tail in the low-Eb side of the gas-phase HOMO

Figure 4. (a) Photoelectron-takeoff angle (θ) dependence of UPS spectra for the PFP(ML)/HOPG at 53 K after background subtraction. The angle between the incidence photon and photoelectron is fixed at 45°. The spectra are recorded with Δθ ± 12° integration. Circles represent the observed spectra, and thin curves are convoluted profiles of four Voigt functions, where the fwhm of WG is fixed to be 110 meV and WL = 111 meV. The vibration energy, 173 meV, of the strongest coupling mode in the calculation is selected for these fittings. The angle-resolved intensities are obtained by the leastsquares method where the intensity is not required to follow Poisson distribution. (b) The θ dependence of the main peak a0 (top panel) and intensity ratio a1/a0, S factor (bottom panel), are also shown.

are also seen for deeper lying states [see Figure 2(a)]. In Figure 4(a), the HOMO bands at θ = 0−60° shown are deconvoluted using Voigt functions, which consist of Gaussian and Lorentzian components, by considering coupling with a specific single vibrational mode of hν(film) = 173 meV to determine the intensity of each satellite. In this fitting, the fwhm of Gaussian components (WG) and Lorentzian components (WL) are fixed to 110 and 111 meV, respectively, and their intensities are obtained by the least-squares method. The θ dependence of a0 as obtained by the fitting is plotted in Figure 4(b) (upper panel). Note that the θ dependence of a0 has a maximum at θ ∼ 45°. The θ dependence of the intensity ratio a1/a0, that gives a similar value of so-called S factor, is shown in Figure 4(b) (lower panel), which shows a maximum at θ = 0°−20° and a minimum around θ = 20°−40°. Such a satellite-dependent θdependence suggests that the Franck−Condon (FC) principle (Born−Oppenheimer approximation) is not strictly satisfied in the low-energy photoionization experiment,56,57 and dynamical position of the atoms affects the satellite intensities for the PFP/HOPG system as in other materials.9,38 Reorganization Energy. The reorganization energy (λ) can be evaluated from UPS spectra.2,8,9,38 Briefly, λ and molecular (small) polaron binding energy (Epol) are given by 22432



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Figure 5. Comparison between gaseous (yellow traiangles: from refs 15 and 18) and angle-integrated monolayer (ML) UPS spectra (blue circles) for PFP (a) and PEN (b), compared with convoluted curves by the single-mode analysis (SMA) (solid curves). The fwhm’s of WG and WL are fixed to 110 and 111 meV for PFP and 70 and 83 meV for PEN, respectively. For the gas phase, the Gaussian function is used for the analysis, and WG is fixed to be 110 meV for PFP and 70 meV for PEN. The vibration energy is 173 meV for PFP and 167 meV for PEN, and the same for the ML and gas phase. Poisson distribution is used in the analysis with S factor as an adjustable parameter. HOMO distributions (B3LYP/6-311++G**) of PFP and PEN are also shown.

Figure 6. Comparison between gaseous (yellow triangles: from refs 15 and 18) and angle-integrated monolayer (ML) UPS spectra (blue circles) for PFP (a) and PEN (b), compared with convoluted curves by the multimode analysis (MMA) for the ML (solid curves). The fwhm’s of WG and WL are fixed to 50 and 111 meV for PFP and 50 and 83 meV for PEN as an example, respectively. The WG is fixed at the instrumental resolution, and WL is determined by fitting the lower binding energy tailing. The 0−0, 0−1, 0−2, and 0−3 transition intensities are indicated by vertical bars. In this analysis each Sfilm is determined by the least-squares fitting, and calculated hν is used (see Table 1). The simulated HOMO curves by the theoretical calculation (B3LYP/6-31G**) are also shown convoluted with WG = 110 meV, WL = 20 meV (PFP) and WG = 70 meV, WL = 20 meV (PEN). The calculated hν and S factors are used from refs 15 and 18. The arbitrary property in the MMA fitting introduces the maximum error about ±10 meV for PFP and ±15 meV for PEN to λ(+). The energy positions of gas-phase and calculated spectra are shifted arbitrarily for the comparison.

spectrum,15,18 we used Gaussian function for the SMA. The reorganization energy of PEN in the film is ∼10% larger than that in the gas phase, which shows a similar trend as PFP. Multimode Analysis. Brédas and co-workers reported that UPS vibrational fine structures of the gas-phase PEN and PFP were well reproduced by using all 18 totally symmetric (Ag) vibrational modes whose intensities satisfy the FC principle (linear coupling model) [multimode analysis (MMA)].2 The MMA analysis of the PFP(ML)/HOPG has been performed by using the DFT vibration energies of the ionized molecule, while the S factors were obtained by the least-squares method in which the intensity of the vibration satellites, 0−0, 0−1, and 0− 2 transitions, is given by Poisson distribution as in the case of the SMA.2,9 The convoluted result is shown in Figure 6, where the fully theoretical simulation curve is also shown at the bottom with Voigt functions (WG = 110 meV, WL = 20 meV); here we added Lorentzian function to Gaussian function as a trial of a better agreement with the experiment. The obtained parameters are listed in Table 1. This analysis yields λ(film) = 334 meV and λ(+)(film) = Epol(film) = 167 meV (WG = 50 meV and WL = 111 meV) at 53 K as an example. In this case the WG was fixed to the instrumental resolution, and WL was determined by fitting the lower-lying spectral tailing. The values depend largely on selected Voigt functions (mainly Lorentzian functions) and S factors in the fitting, introducing an error of ±10 meV to λ(+). However, the experimental λ(+) is larger than the theoretical values [111 meV in ref 18 and 91− 117 meV in ref 21] (see Table 2).

An example of parameters determined for PEN/HOPG by a similar MMA analysis is listed in Table 1. Consequently, λ for the PEN/HOPG is obtained to be λ(film) = 118 meV and λ(+)(film) = Epol(film) = 59 meV (WG = 50 meV and WL = 83 meV) at 49 K. The theoretical simulation curve is also shown at the bottom with Voigt functions (WG = 70 meV, WL = 20 meV). By considering error from the various fitting procedures, the MMA analysis gives λ(film) = 314 ± 20 meV for the PFP/ HOPG and λ(film) = 136 ± 30 meV for the PEN/HOPG. Note that the λ value of PFP is 1.8 to 3.2 times larger than those of PEN which is in agreement with the results of recent DFT calculations,18 indicating that there is a large impact of perfluorination on the HOMO hole−vibration coupling and thus on the transport properties as described in the later section of Impact on the Hopping Mobility. Furthermore, the λ(film) values are obviously larger than those estimated from gas-phase UPS and by theoretical calculation for both PFP and PEN probably due to electronic coupling to the substrate and neighbor molecules. This suggests that solid-state effects must be considered for the determination of the electron (hole)− vibration couplings, which is supported by the experimental 22433



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Table 1. Calculated 18 Vibration Modes, Their Assignment, and Huang−Rhys (S(th)) Factors for an Ionized Perfluoropentacene (PFP) and Pentacene (PEN) from Refs 15 and 18 and Experimental Huang−Rhys (S(ex)) Factors and Relaxation Energies (λ(+)) by the Least-Squares Fitting of HOMO Spectra of Monolayers Displayed in Figure 6a perfluoropentacene (C22F14)

a

mode

freq/meV

S(th)

S(ex)

λ /meV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

202.9 194.9 191.1 181.8 176.7 172.6 161.3 157.0 145.1 128.6 88.8 61.5 56.2 54.6 43.2 42.8 34.0 22.0

0.197 0.050 0.019 0.016 0.015 0.107 0.118 0.001 0.006 0.013 0.037 0.134 0.000 0.000 0.003 0.000 0.020 0.020

0.203 0.000 0.000 0.003 0.001 0.326 0.111 0.000 0.000 0.027 0.000 0.246 0.055 0.150 0.156 0.134 0.157 0.131

41.2 0.0 0.0 0.5 0.2 56.3 17.9 0.0 0.0 3.5 0.0 15.1 3.1 8.2 6.7 5.7 5.4 2.9 b 166.7

(+)

pentacene (C22H14) assignment

mode

freq/meV

S(th)

S(ex)

λ(+)/meV

assignment

ν(CC) ν(CC) ν(CC,CF) ν(CC,CF) ν(CC) ν(CC) ν(CC,CF) ν(CC,CF) ν(CF),δ(CC) ν(CF),δ(CC) ν(ring) ν(ring) δ(CC) δ(CC) δ(CF) δ(CF) δ(CF) ν(ring)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

399.7 397.0 396.3 395.8 197.1 193.4 187.8 178.7 176.7 165.9 152.1 148.8 129.7 100.1 94.9 78.9 76.0 32.6

0.001 0.000 0.000 0.000 0.033 0.059 0.000 0.097 0.003 0.004 0.043 0.015 0.004 0.002 0.000 0.000 0.000 0.030

0.000 0.000 0.000 0.004 0.000 0.003 0.059 0.053 0.053 0.019 0.053 0.038 0.015 0.000 0.030 0.038 0.030 0.013

0.0 0.0 0.0 1.6 0.0 0.6 11.0 9.4 9.3 3.2 8.0 5.6 2.0 0.0 2.8 3.0 2.3 0.4 b 59.2

ν(CH) ν(CH) ν(CH) ν(CH) ν(CC),ν(CH) ν(CC),δ(CH) ν(CC),δ(CH) ν(CC),δ(CH) ν(CC),δ(CH) ν(CC),δ(CH) δ(CH) δ(CH) δ(CH) δ(CC) ν(ring) δ(CC) ν(ring) ν(ring)

ν(CC,CH,CF): stretching. ν(ring): ring breathing. δ(CC,CH,CF): in-plane bending. bTotal relaxation energy (meV).

Table 2. Physical Parameters of Charge-Local Phonon Coupling Obtained for Perfluoropentacene (PFP) and Pentacene (PEN) by Fully Theoretical Calculation and for Gas-Phase and Monolayer PFP and PENa PFP

Theory

k

Theoryj Gas ML/Graphite@53 K PEN

Theoryk

Theoryh Theoryi Gas ML/Graphite@49 K

6-31G 6-31G* 6-31+G** 6-31G** UPSj UPS UPS 6-31G 6-31G* 6-31G** 6-31+G** 6-31G** 6-31G** UPSi UPS UPS

hν/meVb

λ(+)/meVc

Sd

λ/meVe

MM MM MM MM 173 (SM) 173 (SM) MMf MM MM MM MM MM 59.9/167.1 (2 modes) 167 (SM) 167 (SM) MMf

91 111 117 111 112 126 157 ± 10 46 47 48 46 49 58 45 50 68 ± 15

MM MM MM MM 0.645 0.727 ADJg MM MM MM MM MM 0.279/0.251 (2 modes) 0.269 0.302 ADJg

181 222 232 223 224 252 314 ± 20 90 93 94 90 98 117 90 100 136 ± 30

a SM: single-mode vibration account for SMA. MM: multimode vibration or S factor for MMA. bVibration energy (MM: each table must be referred to for theoretical values). cRelaxation energy in the ionized state, corresponding to small/molecular polaron binding energy (Epol). dHuang−Rhys factor. eReorganization energy fSee Table 1 by the present calculation. gADJ: Adjustable parameter for MMA; see Table 1 as an example. h Computed values from ref 15. iComputed values from ref 17. jComputed values from ref 18. kComputed values from ref 21.

HOMO distribution over the molecule. Such a small change in the HOMO yields a significant increase in the HOMO hole− vibration coupling. This again indicates that electron−phonon coupling is more sensitive to the wave function of the responsible electronic state than the phonon energy. Comparison between Single and Multimode Analyses. The results obtained by single and multimode analyses can be seen in Table 2. When we compare λ(+) (= Epol = λ/2) values obtained by the SMA and MMA analyses for gas-phase PFP and PFP(ML)/graphite, where the SMA uses only experimental results, while the MMA uses theoretical vibration

results that weak intermolecular and molecule−substrate interaction influence the vibrational spectra.58,59 The present results also clearly demonstrate that change in the wave function by perfluorination is more critical for hole− vibration couplings and related energy parameters than change in vibration energies, although the substitution of H atoms with heavier F atoms can diminish the vibration energies and thereby contributes to reduce values of these parameters. The difference in the HOMO distributions of PEN and PFP is not very large (see insets in Figure 5), but there is contribution of the F 2p orbital to the PFP HOMO to result in slightly larger 22434



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energies as well as the spectrum, the difference between the λ(+) values obtained by the SMA and the MMA is large (∼30%). The values by the SMA agree fairly well with the theoretical values (MMA) due to self-convolution of vibration modes with weak coupling intensity. The theoretical spectral simulation by MMA reproduces the gas-phase UPS relatively well if we assume a Lorentzian contribution. The λ(+) by SMA for gas and monolayer shows a significant difference, suggesting the theoretical MMA would be useful to describe the λ for an isolated molecule; however, solid-state effects must be considered to describe the electrical properties. The simplest analysis of SMA with UPS implies that purely experimental values of λ(+), Epol, and λ can be used in discussing the hopping charge mobility without using any theoretical computation. A similar agreement between the SMA and the theoretical values was recently demonstrated for rubrene by Duhm et al.60 Impact on the Hopping Mobility. To demonstrate how the increase in λ upon fluorination affects the charge transport properties we evaluate the hole mobility (μh) of the PFP and PFP monolayer. We assume that the transfer integral (t) between lying molecules is about t = 3 meV as was previously derived for PEN.16 Since t ≪ λ, we evaluate the hole mobility in the framework of the Marcus semiclassical model61 μh =

1/2 2πea 2 2⎛ 1 ⎞ −λ /4kBT t ⎜ ⎟ e ℏkBT ⎝ 4πλkBT ⎠

effects must be considered for accurate estimation of the electron (hole)−vibration couplings.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Phone: +81(0)43 2902958. Fax: +81(0)43 2073896. Present Addresses #

NHK Science and Technical Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan. ¶ The University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by global-COE program (G03 by MEXT), Grant-in-Aid for Scientific Research (A) (Grant No. 24245034), Grant-in-Aid for Scientific Research (B) (Grant No. 23360005), Grant-in-Aid for Young Scientists (A) (Grant No. 20685014), and Singapore MOE FRC grants R143000-505-112, R143-000-530-112, and NUS-YIA grant R143000-452-101. We thank Dr. F. Bussolotti, Dr. S. Duhm, Dr. H. Yamane, Dr. K. Sakamoto, and Dr. K. K. Okudaira for stimulating discussions. We also acknowledge students of Chiba University, T. Kataoka, R. Nakagawa, and K. Nebashi, for their support for the measurements and sample preparations.

(3)

where e is the electron charge, and a is the intermolecular distance. Using a typical intermolecular distance from the monolayer structure (a = 1 nm) as seen in Figure 1 and t = 3 meV for PEN, an upper limit of μh in a lying PFP and PEN monolayer can be evaluated with the experimental SMA values of λ(PFP) = 252 meV and λ(PEN) = 100 meV (assuming temperature-independent λ), as 0.01 cm2/V−1s−1 and 0.07 cm2/ V−1s−1 at 295 K, respectively. These results show that even a relatively moderate increase in λ leads to a significant lowering of μh.

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REFERENCES

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CONCLUSION We studied electron-local phonon coupling of pentacene (PEN) and perfluoropentacene (PEF) to understand the impact of perfluorination of PEN on charge hopping transport in these molecular solids. Electron-local phonon coupling of the highest occupied molecular orbital (HOMO) state was measured by high-resolution UPS for PEN and PFP monolayers on graphite. The relaxation energy, reorganization energy, and small polaron binding energy, associated with coupling between the HOMO hole and molecular vibrations, are evaluated purely experimentally, and the results are compared with those from gas-phase measurements and those derived from DFT calculations. The purely experimental values of these parameters (single-mode analysis) are useful and agree reasonably well with those determined by multimode analysis using computed vibration energies and those obtained by using DFT values of both vibration energies and S factors. We also found that polaron binding energies and related energies become significantly greater upon perfluorination of PEN, which demonstrates that these parameters are more sensitive to the spatial distribution of relevant wave function than to the vibration energy. Finally, the present results show that the reorganization energy of PFP and PEN films is to some extent (∼10%) larger than those obtained from gas-phase ultraviolet photoelectron spectra indicating thus that solid-state 22435



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Experimental Reorganization Energies of Pentacene and

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