Substrate-Directed Growth of N-Heteropolycyclic Molecules on a Metal

Article pubs.acs.org/JPCC

Substrate-Directed Growth of N‑Heteropolycyclic Molecules on a Metal Surface Friedrich Maass,† Arnulf Stein,† Bernd Kohl,‡ Lena Hahn,¶ Lutz H. Gade,¶ Michael Mastalerz,‡ and Petra Tegeder*,† †

Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 253, Heidelberg, 69120, Germany Organisch-Chemisches Institut, Im Neuenheimer Feld 270, Heidelberg, 69120, Germany ¶ Anorganisch-Chemisches Institut, Im Neuenheimer Feld 270, Heidelberg, 69120, Germany ‡

ABSTRACT: N-Heteropolycyclic compounds are promising organic n-channel semiconductors for applications in field effect transistors. The adsorption behavior of these molecules on inorganic substrates is of great interest, since it affects the transport properties. Utilizing high-resolution electron energyloss spectroscopy (HREELS) and density functional theory (DFT), we determined the adsorption geometry of three different N-heteropolycyclic molecules as a function of coverage on Au(111). All three π-conjugated aromatic molecules adopt a planar geometry with respect to the substrate in both the monolayer (ML) and thin films (up to 10 ML). Contrary, in their crystal structure the molecules are tilted up to 82° between the molecular planes in neighboring stacks. Electronic HREELS and DFT calculations allowed the determination of the optical gaps of the molecules which are unaffected by the nitrogen substitution of the polycyclic aromatic hydrocarbons, while the frontier orbitals of the N-heteropolycyclic compounds are stabilized. The present study provides important aspects such as adsorption and electronic properties which are essential for designing organic-molecules-based electronic devices.

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INTRODUCTION Organic semiconductors based on small molecules are potential candidates for many applications in (opto)electronic devices such as solar cells, light emitting diodes, or field effect transistors.1,2 For device performance the properties of organic/ inorganic interfaces as well as between active organic layers (electron donor/acceptor interfaces) are of great importance. For instance, the orientation of molecules plays a crucial role for the light absorption/emission and charge transport properties. Intermolecular (lateral) and molecule/substrate interactions influence the adsorption geometry and the growth of molecular films, and accordingly, the electronic structure of the system.3−9 The latter crucially affects the functional performance of organic films.9−12 A large number of organic hole-transporting (p-channel) semiconductors, the most prominent being pentacene and its derivatives,13 have been reported in contrast to the less developed organic electron-transporting (n-channel) semiconductors, which are of great interest especially for field effect transistors, for example, as part of complementary circuits.14 Promising candidates for n-channel semiconductors are N-polyheterocyclic aromatic compounds.15−18 The introduction of nitrogen atoms into the π-backbone of the polycyclic aromatic hydrocarbons (PAHs) stabilizes the frontier orbitals and increases the electron affinity, while the size of the HOMO− LUMO (optical) gap is nearly unaffected. Thus, specific implementation of nitrogen atoms can cause a change of the electronic properties from a p-channel to n-channel semiconducting © XXXX American Chemical Society

behavior. In addition, particular substituents and functional groups may open the opportunity to modify the molecular packing and thus the film morphology in a controlled manner (structure−property relationship). In the present contribution we study the adsorption and electronic properties of three N-polyheterocyclic aromatic compounds on Au(111), namely quinoxalino[2′,3′:9,10]phenan-thro[4,5-abc]-phenazine (QPP),19 the 2,11-di-tertbutyl-substituted QPP (tBu-QPP)19 and 1,3,8,10-tetraazaperopyrene (TAPP)20,21 (see Figure 1). Derivatives of the latter have already been proven to be promising organic semiconductors for the fabrication of high-performance n-channel transistors.22 Recently QPP has been tested in p-channel transistors which exhibit rather low field-effect mobilities.23 So far, only the TAPP parent compound has been studied on a metallic substrate by means of scanning tunneling microscopy, giving results related to adsorbate phases and temperatureinduced on-surface reactions.21 Here we use high-resolution electron energy-loss spectroscopy (HREELS), which provides insights into both the adsorption geometry via vibrational excitations and electronic properties of the molecules by utilizing electrons with sufficient energy to excite electronic transitions. Vibrational HREELS has been used to study, for example, the adsorption behavior of organic semiconductors,24−27 the Received: December 10, 2015 Revised: January 12, 2016

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DOI: 10.1021/acs.jpcc.5b12080 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. N-Heteropolycyclic molecules investigated in the present study: (a) quinoxalino[2′,3′:9,10]-phenanthro[4,5-abc]-phenazine (QPP), (b) 2,11-di-tert-butyl-substituted QPP (tBu-QPP), and (c) 1,3,8,10-tetraazaperopyrene (TAPP).

Figure 2. (a) Formation of an electric field due to the approach of an electron in front of a metallic substrate. (b) Qualitative explanation of the orientation selection rule for dipole surface scattering: the image dipole within the substrate compensates the effect of the adsorbed dipole for parallel orientation but enhances the effect of dipoles with a normal orientation.

Figure 3. HREEL spectra in specular (black) and off-specular (red) scattering geometry of (a) 1 ML and (b) 12 ML QPP on Au(111). E0 is the primary energy of the incident electrons. (c) DFT calculated intensities and frequencies of vibrational modes with a dynamic dipole moment perpendicular to the molecular plane (z-direction, representation B3u).

on-surface synthesis of graphene nanoribbons,28,29 electroninduced reactions in self-assembled monolayers and thin solid films,30−33 isomerization processes in molecular switches at surfaces,34−38 and surface-plasmons or surface-phonon dispersions.39,40 By exciting intramolecular electronic transitions from a ground state (S0, HOMO) to an excited state (e.g., S1, LUMO), this technique gives information about the electronic structure such as optical gaps28,29,41 or even normally spinforbidden direct singlet to triplet excitations.42,43 Here, we demonstrate that all three molecules adopt an adsorption geometry in thin films very different from the crystal structure. In the monolayer (ML) as well as in multilayers (up to 10 ML) the molecules adsorb on the Au(111) metal surface in a ”lying-down” geometry, that is, with the molecular plane parallel to the surface. In contrast, in the molecular crystals a tilting between the molecular planes of two neighboring stacks up to 82° has been observed. Using electronic HREELS and density functional theory (DFT) calculations we determined the optical gaps, which possess the same values as for the undoped (non N-substituted) corresponding PAHs. A stabilization of the highest occupied molecular orbital (HOMO)

and lowest unoccupied molecular orbital (LUMO) due to isosteric N-for-CH substitution is confirmed by DFT.

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EXPERIMENTAL SECTION All HREEL spectra were recorded with a commercial spectrometer (SPECS Delta 0.5). The experiments were performed in an ultrahigh vacuum chamber at a base pressure of 5.0 × 10−11 mbar. A clean Au(111) surface was prepared by repeated Ar+ sputtering (at room temperature) and subsequent annealing (at 750 K) cycles. The molecules were evaporated from a home-built effusion cell held at 450 K and deposited at a substrate temperature of 300 K. The dosing was monitored with a quadrupole mass spectrometer. For the determination of the adsorbate coverage temperature-programmed desorption was used. In HREELS low-energy electrons emitted from a cathode are monochromatized by a series of electrostatic lenses, hit the sample at an angle of incidence Φi (see Figure 2a), and are scattered at an angle Φs. The scattered electrons are analyzed B


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Table 1. Assignments of the QPP and tBu-QPP Vibrational Frequencies (in cm−1) for 1 ML and a Multilayer QPP and tBu-QPP, Respectivelya QPP 1 ML 109 166 434 506 711 747 799 950

da da da da da

DFT

tBu-QPP 1 ML

tBu-QPP 10 ML

138 180 469 da

142 190 468 da

720 (s) 756 (s) 812 (s)

97 173 466 542 755 da 798 853

730 da 763

744

728 (s) 760 (s)

758 798

905

900

897 (m)

966

954

975 (m)

995

1096

1099 (s)

1136

1354 1477

1356 (s) 1477 (m)

1413 1555

1111 1198 1359 1467 2951

3060

3072 (w)

3215

1017 1119 1221 1358 1468 2952 3069

950 (w) 1115 (m) 1225 (m) 1360 (m) 1476 (m) 2862 (w) 2961 (m)

1030 1159 1238 1415 1553 3089 3208

QPP 12 ML 96 190 434 503 722 751 808

da da da da da

FT-IR (ATR)

FT-IR (ATR)

DFT

mode

130 187 487

buckl. long buckl. short γ(C−C−C) γ(C−C−C)P γ(C−H)P γ(C−H)Q γ(C−H)P γ(C−H)P δ(C−C−C) ν (C−C)P ν(C−C) ν (C−C)tBu sym. ν(C−C) ν(C−N) ν(C−H)al ν(C−H)ar

repr. B3u (z) B3u (z) B3u (z) B3u (z) B3u (z) B3u (z) B3u (z) B3u (z) B2u (y) (x) B1u (x) Ag B3g B1u/B2u (x/y)

a

In addition, DFT-calculated frequencies of the free molecules and attenuated total reflectance (ATR) FT-IR19 data are shown. da refers to dipole active modes, (s) to strong, (m) to medium, and (w) to weak modes. ν, stretch; δ, in-plane bending; γ, out-of-plane bending; P, pyrene; Q, quinoxaline; tBu, located at the tert-butyl groups; al, alcylic; ar, aromatic. In brackets, representation and corresponding orientation of the calculated dipole derivative vector with respect to the molecular geometry: x, long axis; y, short axis; z, perpendicular to the molecular plane.

and angle- and energy-resolved, and the resulting intensity is plotted against the electron energy-loss. For vibrational HREELS the scattering mechanisms are dipole, impact, and resonant scattering. The latter involves the creation of temporary negative ions, which can by generated at particular incident electron energies. The long-range dipole scattering occurs when the electric field of the incoming electrons (which is parallel to the surface normal, z-direction, between the electron and its image charge in conducting surfaces, see Figure 2b) couples to the z-component of a dynamic dipole moment of a molecular vibration. The counterpart is the short-ranged impact scattering, in which the electrons interact with the electron density distribution of the adsorbed molecules. On a metal surface, the parts of dynamic dipole moments of molecular vibrations of adsorbed molecules parallel to the surface are diminished, the parts perpendicular are increased due to image dipole effects (see Figure 2b; ”surface selection rule”). Owing to the lack of momentum transfer parallel to the surface, dipole scattered electrons occur only under a small angle around the specular direction (Φi = Φs). In contrast, impact scattering is more isotropic and therefore an angle-resolved measurement (Φi ≠ Φs) recording the intensity of the scattered electrons contains information about the orientation of the corresponding dynamic dipole moments.44 Together with results from DFT calculations it is possible to determine the orientation of the molecules on the surface. The vibrational HREEL measurements were performed at a primary electron energy of 3.5 eV, while for the electronic HREELS the energy was set to 15.5 eV. The energy resolution measured as fwhm of the elastically scattered electrons (elastic peak) was around 28 cm−1. The assignment of vibrational modes is based on DFT calculations of the free molecules done with the Gaussian 09 program package.45 Optimizations of the molecular geometries, subsequent frequency calculations, and calculation of HOMO and LUMO energies for the ground state were carried out with the B3LYP functionals and the 6-311G basis set. The same methods were applied to calculate excitation energies for the first 20 excitations including potentially observable triplet excitations

Figure 4. Visualization of calculated (B3LYP/6-311G) atomic displacements (red arrows) for the γ(C−C−C)-1, γ(C−C−C)-2, ν(C−H)-1, and ν(C−H)-2 (a−d) vibrations of QPP at the declared values (HREELS monolayer/HREELS multilayer/DFT). The calculated dipole derivative unit vectors μ are shown as black arrows (calculations with Gaussian 09, visualizations with the Frencio).

with time-dependent DFT. Visualization of vibrational modes was achieved with the use of the Facio 19.1.4 program.

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RESULTS AND DISCUSSION The properties of an organic molecular film depend on the adsorption structure at the inorganic substrate. Hence we begin by analyzing the growth of the molecules with increasing coverage. The results will be compared with the corresponding molecular crystal structures.19,21 In the second part the electronic properties in particular the optical gaps are investigated and compared with the calculated values for the nondoped PAHs. Adsorption Properties of the N-Heteropolycyclic Aromatic Molecules on Au(111). In Figure 3a the HREEL C


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Figure 5. HREEL spectra in specular (black) and off-specular (red) scattering geometry for (a) 1 ML and (b) 10 ML tBu-QPP on Au(111). E0 is the primary energy of the incident electrons.

spectra of 1 ML QPP measured in specular and 5° off-specular scattering geometry are displayed, while the respective data for a coverage of 12 ML are presented in Figure 3b. In addition the calculated intensities and frequencies of vibrational modes possessing a dynamic dipole moment perpendicular to the molecular plane are shown in Figure 3c. In the HREEL spectra of the mono- and the multilayer five clearly dipole-active vibrations (high specular to off-specular ratio) are visible. They can be assigned to the γ(C−C−C) (at 434 cm−1 and around 506 cm−1) and γ(C−H) out-of-plane wagging modes (around 711, 747, and 799 cm−1). All vibrational modes belong to the representation B3u and exhibit a dynamic dipole moment perpendicular to the molecular plane (z-direction). For the assignment of all observed vibrations see Table 1. To determine the adsorption geometry, Figure 4 visualizes some important modes with their respective dipole derivative unit vectors as calculated by DFT. The γ(C−C−C) and γ(C−H) out-of-plane wagging modes show dynamic dipole moments perpendicular to the molecular plane (see Figure 4a and b). The corresponding measured vibrational modes are dipoleactive. Thus, dipole moment changes due to these vibrations have a large component perpendicular to the Au(111) surface, which is only possible if the molecules lie flat on the surface. This is observed for the monolayer as well as for the multilayer. The conclusion is supported by the observation that intensity of the ν(C−H) stretching mode does not change between the specular and off-specular scattering geometry in the multilayer (Figure 3b). Therefore, this vibration is clearly not dipole-active

Figure 6. HREEL spectra in specular (black) and off-specular (red) scattering geometry for (a) 1 ML and (b) 10 ML TAPP on Au(111). E0 is the primary energy of the incident electrons. (c) DFT calculated intensities and frequencies of vibrational modes with a dynamic dipole moment perpendicular to the molecular plane (z-direction, representation B3u).

(for the corresponding dipole derivative unit vectors see Figure 4c,d). Note that, the broad feature around 2000 cm−1 (Figure 3a) can be assigned to an artifact, since its energetic position depends on the setting of distinct lenses of the spectrometer as well as the primary energy (see Figure 9 below). We note that for this type of spectrometer the appearance of this artifact has been reported previously.46 For the tBu-substituted QPP the corresponding vibrational spectra of a mono- and multilayer are shown in Figure 5. Again the out-of-plane wagging modes γ(C−C−C) at 469 cm−1 and γ(C−H) at 730 and 763 cm−1 (see Table 1 for the assignments) with their dynamic dipole moments orientated perpendicular to the molecular plane are strong and display a large dipole activity. In contrast, in-plane modes such as the stretch vibrations ν(C−C) (1111 cm−1), ν(C−N) (1468 cm−1), and ν(C−H)ar (3069 cm−1) in the multilayer (too weak to observe in the monolayer) are not dipole active, as the comparison between specular and off-specular intensities clearly demonstrates. D


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The Journal of Physical Chemistry C Table 2. Assignments of the TAPP Vibrational Frequencies (in cm−1) for 1 and 10 MLa TAPP 1 ML 178 229 449 da 488 da

778 da 827 da 927

TAPP 10 ML

FT-IR

DFT

mode

repr.

459 (w) 511 (s) 565 (m) 690 (m) 806 (s) 852 (s) 962 (m) 1060 (m) 1151 (w) 1215 (m) 1330 (s) 1485 (m) 1518 (s) 1618 (m) 2931 (w) 3043 (w)

176 239 465 504/529 572 692 814 880 990 1080 1163 1242 1350 1510 1558 1663/1650 3159 3219

buckl. long buckl. short δ(C−C−C) γ(N−C−N)/γ(C−C−C) δ(C−C−C) δ(C−C−C) γ(C−H)C γ(C−H)/γ(C−C−C) γ(C−H)T δ(C−C−C) δ(C−C−H) δ(N−C−N) ν(C−C) δ(N−C−H) ν(C−N) ν(C−C) ν(C−H)T ν(C−H)C

B3u (z) B3u (z) B2u (y) B3u (z) B1u (x) B2u (y) B3u (z) B3u (z) B3u (z) B1u (x) B2u (y) B1u (x) B1u (x) B2u (y) B1u (x) B1u/Ag (x/−) B1u (x) B2u (y)

199 237 510 da 565 687 809 da 855 da 963 1058 1142 1221 1331 1475 1520 1619 2929 3065

a In addition, the FT-IR (KBr pellet) data and DFT calculated frequencies for the free molecules are shown. da refers to dipole active modes. ν, stretch; δ, in-plane bending; γ, out-of-plane bending; C, core; T, tips. In brackets, representation and corresponding orientation of the calculated dipole derivative vector with respect to the molecular geometry: x, long axis; y, short axis; z, perpendicular to the molecular plane.

This behavior does not change up to 10 ML. Thus, for the tBu-QPP we also suggest a planar adsorption at least up to a coverage of 10 ML. For even thicker films, the dipole selection rule at metal surfaces gets less relevant44,47 and accordingly results from angle-resolved HREELS cannot be used for assignments to particular adsorption geometries. For the third adsorbate system studied in this work, the TAPP/Au(111) the results of the vibrational HREEL measurements are displayed in Figure 6 and Table 2. The out-of-plane wagging modes γ(N−C−N)/γ(C−C−C) at 488 cm −1 (monolayer, visualization in Figure 7a) and γ(C−H) at 778, 827, and 927 cm−1 dominate the spectra. These modes are dipole active. The in-plane modes are too weak to be observed

Figure 8. Crystal structures of (a) QPP, (b) tBu-QPP, and (c) TAPP obtained from X-ray structure analysis (data from refs 19 and 21).

Figure 7. Visualization of calculated (DFT, B3LYP/6-311G) atomic displacements (red arrows) for the γ(C−C−C) and γ(N−C−N), γ(C−H) ν(C−N), and ν(C−H) (a−d) vibrations of TAPP at the declared values (HREELS monolayer/HREELS multilayer/DFT). The calculated dipole derivative unit vectors μ are shown as black arrows.

in the monolayer. In the multilayer, the in-plane vibrations such as the ν(C−C), the ν(C−N), and the ν(C−H) stretching modes (1331, 1520, and 2929/3065 cm−1, respectively) exhibit different intensities between specular and off-specular scattered electrons (Figure 6b). However, the dipole active modes (γ(C−H), γ(C−C−C), γ(N−C−N)) show the same relative intensities as in the monolayer. Our previous study on substituted TAPPs27 revealed significant intensity differences of the dipole active modes between the mono- and multilayer, which we attributed to a tilted adsorption geometry in the multilayer while the molecules in the monolayer adsorb planar. For the multilayer, the splitting of the ν(C−H) vibration into a signal of the hydrogen at the tips of the molecule (2929 cm−1) and one of the hydrogens at the core (3065 cm−1) as expected from DFT calculations is readily visible. However, based on the calculated corresponding dynamic dipole moment orientations (Figure 7) we conclude that the molecules adopt a flat adsorption geometry on the surface, like QPP and tBu-QPP in the measured coverage range from 1 to 10 ML. Comparing the energies of the vibrational modes between the molecules in the condensed phase (ATR-FTIR) and the molecules in direct contact with the metal substrate, that is, in the monolayer regime, we observe a red-shift of ca. 10 cm−1 for the intense dipole active vibrational modes in the case of QPP. E


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Table 3. S0−S1 Transition Energies in eV (nm) As Measured with HREELS (Multilayer on Au(111)) and UV−vis Spectroscopic Data (QPP and tBu-QPP in CHCl3, TAPP in Toluene) and Calculated Energies with Time-Dependent DFT (TDDFT, B3LYP/ 6-311G of the Free Molecule). HOMO and LUMO Energies Calculated with DFT (B3LYP/6-311G of the Free Molecule) HREELS QPP tBu-QPP TAPP

2.98 (416) 2.96 (419) 2.84 (436)

UV/vis 19

3.00 (413) 2.99 (415)19 2.86 (434)20

For tBu-QPP no significant shifts are found, indicating a weaker adsorbate/substrate interaction most likely due to the bulky tert-butyl substituents. Pronounced shifts to lower energies are noticed for the TAPP molecules, which are of the order of 25 cm−1. Hence, the degree of electronic mixing between molecular and metal states at the TAPP/Au(111) interface appears to be higher compared to the QPP/Au(111) system. An analysis of the adsorption and electronic properties using the normal-incidence X-ray standing wave technique48,49 and orbital tomography,50,51 respectively, would for instance allow to draw a precise picture about the adsorbate/substrate interactions. To place the adsorption properties of the three π-conjugated aromatic molecular compounds in a more general context we compare our results with the crystal structures of the molecules (see Figure 8). For the QPP crystal a tilting of 82° between the molecular planes of two neighboring stacks of QPP molecules is observed.19 In tBu-QPP the angle is 69°,19 while in TAPP it is 35°.21 Hence the growth behavior of all molecules on a Au(111) substrate up to multilayer coverage differs completely from the growth of the ”free” crystals. In comparison, our recently published results for substituted TAPP derivatives show a planar adsorption geometry only in the monolayer and a more crystal-like growth from the second layer onward.27 In QPP and TAPP as well as pentacene52,53 but also for tBu-QPP with its bulky moieties, it seems that as long as a flat adsorption in a second layer is favored, the film will continue to grow in this fashion. However, our results clearly mirror the delicate balance between adsorbate/substrate and adsorbate/adsorbate interactions which drove the adsorption structure in thin molecular films. The fact that the crystal structure and the adsorption geometry in thin films on an inorganic substrate can differ significantly has to be considered in the analysis of the key processes governing the optoelectronic functionality of the films and designing organic-molecules-based optoelectronic devices. Electronic Properties of the N-Heteropolycyclic Aromatic Molecules on Au(111). Electronic HREEL measurements recorded with E0 = 15.5 eV are utilized to obtain the optical gaps of the N-heteropolycyclic compounds in the multilayer regime (see Figure 9). For all three molecules a dominant electron energy loss feature is observed around 2.9 eV (see also Table 3), which contains vibronic contributions. We assign it to the S0−S1 transition, that is, the HOMO−LUMO transition and accordingly to the optical gap. The vibrations involved are the ν(C−C) and ν(C−N) stretch modes (the so-called breathing modes) around 1500 cm−1 (186 meV). In addition, for the QPP and its derivative loss peaks above 3.5 eV are found. They are attributed to transitions between lower lying occupied molecular states (e.g., HOMO-1) to the LUMO or higher lying unoccupied states (e.g., LUMO+1). These transitions are also observed in the UV−vis spectroscopic data of the molecules in solution as can be seen in Figure 9. The optical gaps determined here are in agreement with the

TDDFT

HOMODFT

LUMODFT

3.18 (390) 3.14 (395) 2.97 (417)

−6.26 −6.13 −6.34

−2.63 −2.55 −3.39

Figure 9. Electronic HREELS spectra of 12 ML QPP, 10 ML tBuQPP, and 10 ML TAPP including fits to the vibronic S0−S1 transition peaks. The primary electron energy is 15.5 eV and the spectra are normalized with respect to the S0−S1 transition peak (dashed line). For comparison the UV−vis spectroscopic data of the molecules in solution are depicted.

Figure 10. Overview of the calculated HOMO and LUMO energies of the QPP and TAPP as well as the energies of the corresponding polycyclic aromatic hydrocarbons, the dibenzo[hi,uv]hexacene (DBH) and peropyrene (PP).

data determined from UV−vis spectroscopy of the free molecules (Figure 9) as well as the time-dependent DFT modeling (see Table 3). This suggests a relatively weak influence of the metal substrate and intermolecular forces due to the packing on F



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the surface compared to the interactions in solution (UV−vis) and the isolated molecules (DFT). Note that the monolayer spectra show no significant energy shift of the dominant features but a very poor signal-to-noise ratio (data not shown). To gain insights into the effect of N-for-CH substitution in PAHs we calculated the HOMO and LUMO energies of QPP and TAPP as well as the corresponding values for the polycyclic aromatic hydrocarbons (Figure 10). As predicted, a stabilization of the frontier orbitals due to N-substitution of around 300 meV is observed, while the size of the HOMO−LUMO gap (optical gap) is nearly unaffected. Thus, at organic/ inorganic interfaces this independence of the size of the band gap on the one hand and the alignment molecular electronic states relative to the Fermi level of the substrate on the other hand may be employed in tailoring the electronic properties of (opto)electronic devices based on N-polyheterocyclic aromatic compounds.

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CONCLUSIONS In summary, we have determined the adsorption structure of three N-polyheterocyclic aromatic molecules on a Au(111) surface as a function of coverage using high-resolution electron energy-loss spectroscopy (HREELS) and density functional theory (DFT). In contrast to the herringbone like orientation of the compounds in the molecular crystals with tilting angles between the molecular planes in neighboring stacks of up to 82° our results show a surface-induced strictly planar growth at least up to a coverage of 10 ML. The plane Au(111) surface adsorbs the planar molecules in a flat adsorption geometry to maximize the substrate/adsorbate interactions and thereby the binding energy. On such a ”flat” monolayer additional molecules also adsorb in a planar fashion. Even the introduction of bulky side chain substituents does not change this growth behavior. The electronic HREEL spectra reveal a HOMO− LUMO (optical) gap of around 2.9 eV for all compounds in the multilayer regime which is in good agreement with theory for the gas phase molecules and results obtained by UV−vis spectroscopy for the species in solution, indicating weak intermolecular interactions. In addition, DFT modeling confirmed the accepted energetic stabilization of the HOMO and LUMO due to the introduction of nitrogen atoms into the π-backbone of the molecules as well as a nearly unaffected size of the optical gap. Detailed insights into the adsorption and electronic structure of π-conjugated N-polyheterocyclic molecules on inorganic substrates are important for improvement and optimization of molecule-based device performance.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Universität Heidelberg, the Fond der Chemischen Industrie as well as the doctoral college ”Verknüpfung molekularer π-Systeme zu Funktionsmaterialien” funded by the Landesgraduiertenfö r derung of BadenWürttemberg is gratefully acknowledged (L. H.). We thank Stephan Stremlau for the experimental support. G


Article

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Substrate-Directed Growth of N-Heteropolycyclic Molecules on a Metal

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