Intermolecular Hydrogen Bonding and Molecular Orbital Distortion in 4

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Intermolecular Hydrogen Bonding and Molecular Orbital Distortion in 4‑Hydroxycyanobenzene Investigated by X‑ray Spectroscopy Giorgia Olivieri,†,‡ Albano Cossaro,† Ennio Capria,§ Luca Benevoli,§ Marcello Coreno,⊥ Monica De Simone,† Kevin C. Prince,§,†,@ Gregor Kladnik,#,†,‡ Dean Cvetko,*,#,† Beatrice Fraboni,○ Alberto Morgante,†,‡ Luca Floreano,† and Alessandro Fraleoni-Morgera*,§,∥ †

CNR - Istituto Officina dei Materiali, TASC Laboratory, S.S. 14, km 163.5, 34149 Basovizza (TS), Italy Department of Physics, University of Trieste, I-34123 Trieste, Italy § Elettra - Sincrotrone Trieste SCpA, S.S. 14, km 163.5, 34149 Basovizza (TS), Italy ∥ Dept. of Engineering and Architecture, University of Trieste, v. Valerio 10, 34100 Trieste, Italy ⊥ CNR - Istituto di Struttura della Materia, Trieste, Basovizza Area Science Park, 34149 Trieste, Italy @ Chemistry Laboratory, Faculty of Life and Social Sciences, Swinburne University of Technology, Melbourne, Victoria 3122, Australia # Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia ○ Department of Physics, University of Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy ‡

S Supporting Information *

ABSTRACT: Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation.

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INTRODUCTION Organic semiconductors are receiving growing attention due to their possible applications in several technologically relevant fields, such as light-emitting diodes, solar cells, transistors, and sensors.1−6 However, a number of aspects of organic electronics phenomena must still be elucidated in order to develop the full application potential of this class of materials. In the crystalline solid state, the comprehension of these phenomena can be greatly simplified with respect to more disordered systems like polycrystalline or amorphous ones. Organic semiconducting single crystals (OSSCs), which present structural regularity and a precisely defined lattice structure, are hence regarded as ideal paradigms for understanding organic semiconductors’ properties and behavior,7−9 even though many available data on OSSCs present some variability, mainly due to problems of crystal purity.10,11 This latter problem is being overcome by means of improvements in crystal growth procedures, and, © XXXX American Chemical Society

recently, solution growth has been shown to produce OSSCs of very good electronic quality.12,13 In particular, OSSCs based on the dipolar 4-hydroxycyanobenzene (4HCB, Figure 1a) molecule were shown to have reliable 3D anisotropic transport properties.14−16 The good reproducibility of its electronic features and the ready availability of samples allow us to consider 4HCB as a possible model system for OSSCs. The 4HCB single crystal structure was first elucidated in the 70s, and it has been confirmed recently.16,17 In the crystal, the π-stacking between benzenic rings is present along the crystallographic axes a and b (where the stacking along a occurs at shorter distances than along b), while for the axis c, mainly a hydrogen-bonded, head-to-tail arrangement of the Received: October 6, 2014 Revised: November 26, 2014

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dx.doi.org/10.1021/jp5100878 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION

The gas-phase measurements were performed at the GASPHASE beamline, Elettra Synchrotron in Trieste, Italy.21 The 4HCB powder was inserted into the experimental chamber inside a crucible, and it was introduced into the interaction region via an effusive nozzle. At room temperature, the sample vapor pressure is 6 × 10−7 mbar, which allows the taking of spectra with reasonable statistics and without heating the sample. The C 1s, N 1s, and O 1s core photoemission spectra were taken at 382, 495, and 628 eV photon energy, with an overall energy resolution (photon plus analyzer) of 280 meV for C 1s and N 1s and 370 meV for O 1s. The near edge X-ray absorption fine structure spectroscopy (NEXAFS) spectra were recorded by collecting the total ion yield signal using a channel electron multiplier placed close to the ionization region. The signal was normalized by the photon flux measured by a photodiode. The photon energy resolution for the C, N, and O K-edges was 60, 70, and 100 meV, respectively. Both XPS binding energies and NEXAFS photon energies were calibrated using a mixture of the molecule with CO2 as a calibrant for C 1s and O 1s levels and N2 for the N 1s level.22,23 The solid state measurements were performed at the ALOISA beamline at Elettra. 4HCB molecules (Fluka, purity of 99+ %) were placed in a pyrex cell and pumped down to high vacuum. All the films were grown in situ on Au(110) monocrystalline surfaces. The surface was prepared by cycles of Ar+ sputtering at 1 keV and subsequent annealing up to ∼750 K and checked by XPS to ensure the absence of any contaminants on the surface. The operational pressure for the measurement chamber was maintained at 10−11 mbar. The 4HCB films were then obtained exposing the gold surface to a 4HCB vapor pressure of 5 × 10−7 mbar through a leak valve while keeping the substrate temperature at about 200 K. During the measurements, we kept the sample at the same temperature in order to prevent molecular desorption that has been observed to start at around 240 K for a multilayer film. The sample was continuously displaced during the measurements to minimize the beam induced damage. XPS spectra were taken with the X-ray beam impinging on the sample at a grazing incidence angle (4°), using a photon energy of 650 eV for O 1s and C 1s and 500 eV for N 1s. The photoelectrons were detected in normal emission geometry using a hemispherical electron analyzer with an overall energy resolution (photon + analyzer) of about 300 meV for O 1s and 200 meV for C 1s and N 1s. The binding energies were calibrated using the bulk component of the Au 4f7/2 peak at 84.0 eV.24 NEXAFS spectra at the N and C K-edges were collected in partial electron yield mode by means of a wide acceptance angle channeltron detector. The orientation of the surface with respect to the linear polarization of the photon beam was selected by rotating the sample around the beam axis while keeping a constant grazing angle of 6°.25 The spectra were then normalized to the clean gold signal, and the photon flux using the NEXAFS signal from the bare gold surface, together with the current I0 measured on the last mirror of the beamline. NEXAFS measurements have been taken also on solution grown 4HCB single crystals12 following the same procedure described for the thick film. In this case, samples of a few millimeter size (width and thickness) were mounted by thin Mo clips, and the bottom side was additionally painted with liquid graphite to improve the thermal contact. The sample was cooled down to 150−200 K immediately after insertion in vacuum to prevent

Figure 1. (a) Sketch of the molecular structure of the 4HCB molecule, and (b) a detail of the 4HCB molecular arrangement in the crystalline unit cell, from which it is possible to recognize the nitrogen atom (pictured as blue spheres) sandwiched between two stacked aromatic rings (simplified view, some molecules have been omitted in order to better illustrate the arrangement). The color codes for the cell axes are red for a, green for b, and blue for c.

4HCB molecules is present. In addition, along the axis a, the nitrogen atom is sandwiched between two stacked aromatic rings (Figure 1b). This structural arrangement has been correlated with the 3D anisotropy of observed electronic properties found for solution-grown 4HCB crystals. In particular, the nitrogen atom arrangement between two stacked aromatic rings has been proposed as a possible origin for the increased charge mobility observed along the axis a.16 Photocurrent experiments also showed that the number of electrons excited per incident photon is larger along axis a than along axis b, and space-charge-limited current (SCLC) modelbased I−V measurements confirmed this view delivering a density of states (DOS) distribution larger along a than along b.15 In order to investigate charge transport phenomena occurring in OSSCs, analytical tools able to probe the electronic properties of the basic crystal constituents in conditions of actual current flow have been used. For example, infrared spectroscopy for characterizing 4HCB crystal-based organic field effect transistors (OFETs) has provided interesting insights with respect to the effect of the molecular dipole moment under crystal electrical polarization.18 However, in order to gain a comprehensive and clear view of the electronic behavior of OSSCs, it is necessary to obtain this kind of information down to the single functional group and/or atomic level. Such a precision can be obtained with accurate Xray spectroscopy techniques. In addition, for a correct interpretation of the collected data it is necessary to get a picture of how the transition from the molecular to the crystalline state affects and influences the electronic phenomena occurring in the lattice. While a wealth of information about the behavior of interface states involving organic semiconductors is available,19,20 very little is known about the development of the electronic characteristics of OSSCs on moving from a single molecule to an actual crystalline lattice. To investigate these points, we report here on the use of synchrotron-based spectroscopic techniques to study the detailed electronic structure of 4HCB molecules and crystallike molecular assemblies. Since no X-ray spectroscopic study had been previously reported for this particular molecule, we first examine pristine electronic properties of the isolated 4HCB molecule in the gas phase. We then investigate structural and electronic properties of a thick layer (several nanometers) of the material deposited on a metallic substrate. The properties of such a multilayer are related with those of a bulk single crystal, allowing us to gain information about how the electronic properties of the 4HCB molecule change when the crystal is formed. B



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thermal desorption. The I0 current displays the K-edge absorption features of both N and C, which have been used for the absolute calibration of the photon energy, after simultaneous acquisition of the I0 and the gas phase NEXAFS of N2 and CO with the ALOISA windowless ionization chamber. The photon energy resolution is better than 100 meV for both K-edges. The calculations were performed using the density functional theory (DFT) within a pseudopotential scheme, as implemented in the Quantum ESPRESSO open source suite.26 This method does not take into account explicitly all the electrons, rather it replaces the strong electron−ion potential with a pseudopotential that describes all the salient features of a valence electron moving through the solid. In our system, we treated H(1s), C(2s), C(2p), N(2s), N(2p), O(2s), and O(2p) as valence electrons, while all the inner shells were embedded in a pseudopotential. Following the scheme already used by Bolognesi et al.,27 we chose the norm-conserving MartinsTroullier pseudopotential. We used the BLYP (Becke Lee− Yang−Parr) exchange-correlation functional, which relies on the generalized gradient approximation. To investigate the properties of the isolated 4HCB, we accommodate the molecule in a large cubic supercell (15 Å) and then we make a self-consistent field (SCF) calculation to solve the Kohn− Sham equations. Satisfactory convergence of the total energy, the energy gap, and the single wave functions was achieved using an energy cutoff of 80 Ry for the plane waves and 320 Ry for the electronic density. It has been demonstrated that core level binding energies can be obtained from total energy differences between the ground state and core hole calculations. In the latter case, a core hole is created on the desired atom when the pseudopotential is generated. In our case, the core hole has been created separately on each inequivalent C atom by running a SCF calculation at every step to obtain the whole photoemission C spectrum. Since we do not have a reference value for the calculated binding energy calibration, we cannot directly compare the experimental and theoretical values. However, the relative differences of the calculated binding energies are not affected by the calibration issue, and so this quantity was used for the comparison with the experimental data.

Figure 2. Gas phase C 1s XPS spectrum (diamond blue markers) measured at hν = 382 eV with overall resolution of 280 meV. The continuous line is the best fit obtained by the convolution of a Gaussian overall instrumental resolution with five Lorentzian peaks (shaded curves). The vertical lines show the binding energy of the corresponding carbon atoms, as calculated within the DFT framework. The experimental binding energy scale is referred to the vacuum level, while a rigid energy offset has been applied to the calculated one in order to compare it with the experimental data.

Table 1. Absolute Binding Energy Extracted from the Fitting Procedures and Their Chemical Shifts Relative to Atom C5 binding energy (eV) C1 C2 C3 C4′ C4 C5′ C5

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292.44 291.83 291.32 − 290.90 − 290.69

± 0.04 ± 0.04 ± 0.04 ± 0.04 ± 0.04

chemical shift (experimental) 1.75 1.14 0.64 − 0.21 − 0

± 0.06 ± 0.06 ± 0.06 ± 0.06

chemical shift (DFT) 1.93 1.26 0.75 0.32 0.27 0.15 0

experiment, which allowed us to unambiguously assign the different features of the photoemission spectrum. In particular, the highest binding energy is assigned to the carbon bound to the hydroxyl group, C−OH (C1 peak), while the C−H bonds (C4 and C5 peaks) display the lowest BE. In this regard, the polarity of 4HCB, determined by a charge push−pull mechanism, is put in evidence by the energy shift of ∼1.1 eV between the carbon atom bound to the cyano group, C−CN (C3 peak) and that bound to the hydroxyl one, C−OH (C1 peak). In fact, when the phenol (C6H5OH) and benzonitrile (C6H5CN) molecules are considered separately, the BE shifts between the carbon directly linked to the functional group (C− OH and C−CN), and the other benzene carbon atoms are 1.6 ± 0.1 eV (both solid28−31 and gas32 phase) for the phenol and 0.9−1.0 eV (both solid33 and gas34 phase) for the benzonitrile. This would yield a BE difference between C1 and C3 of only 0.6−0.7 eV. However, when CN and OH are coupled to the opposite sides of a benzene ring, a net charge displacement from the hydroxyl to the more electronegative cyano group takes place, thus yielding a core level shift of the C−OH peak to higher binding energy and vice versa for the C−CN component. The cyano carbon atom, C2 peak, follows a similar

RESULTS AND DISCUSSION 1. Gas State (Electronically Noninteracting Molecules). XPS. The C 1s XPS spectrum of 4HCB in the gas phase is reported in Figure 2. As shown in the inset of the figure, the molecule has seven inequivalent C atoms, the OH group being tilted with respect to the O−N molecular axis (the OH bond angle is 109°). DFT calculations of the ionization energies of the different atoms yield seven different binding energies (BE) for the seven inequivalent C atoms (Table 1, column labeled “Chemical Shifts” and Figure 2, indicated with vertical black lines). As can be inferred from the molecular structure, the binding energies (BEs) of C4 and C5 are very close to those of C4′ and C5′, respectively. Our theoretical prediction is consistent with the experimentally observed splitting of the C 1s photoemission peak into five main peaks, whose relative intensities have the ratio 1:1:1:2:2 for the peaks C1:C2:C3:(C4+C4′):(C5+C5′), respectively (see spectral analysis in Figure 2). In Table 1, the absolute BEs extracted from the fitting procedure are reported together with both the experimental and calculated energy shifts relative to the atom C5. We note an excellent agreement between theory and C



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trend but with a smaller variation if compared with benzonitrile in the gas phase.32 In Figure 3 the photoemission from the valence states is compared with the calculated density of states (DOS) and with

Figure 4. Gas phase NEXAFS on the N-edge (top) and C-edge (bottom). The final states assigned to each peak are reported in the graphs. The details of the assignment are discussed in the text. The corresponding ground state LUMO orbitals are plotted on the right.

multicomponent structure of each peak is associated with the vibrational excitations of the NC−C termination, in particular the broader shape of the π*1 (z) resonance is due to a significant contribution from the out-of-plane bending of the NC−C bond, whereas the asymmetric shape of the π*3 (xy) resonance indicates a prevalent contribution from the in-plane C−N bond stretching.35 The C K-edge presents a rich peak structure reflecting the contribution from the different carbon atoms. Relying on the XPS chemical shifts and on the shape of the calculated LUMOs, it is possible to make the following assignment for the main NEXAFS structures. The first resonance is at the same energy (∼285.1 eV) as the first transition observed in the benzene molecule and in all its derivatives, in particular benzonitrile38 and phenol.28 From a direct comparison with the highresolution gas phase spectrum of benzene,39 one might be tempted to assign the next shoulder peaks to the vibrational structure of the π* benzene backbone (e2u symmetry). However, the intensity ratio between the benzene vibrational peaks is not consistent with the measured one for the main peaks at 285.1 and 285.7 eV. In addition, the LUMO splitting (∼0.2 eV between the peak and its shoulder) is the same as the binding energy difference between C4 and C5; we rather assign the first feature (peak plus shoulder) to the transitions from the atoms C5 (C5′) and C4 (C4′) to the π*1 (z) orbital. On the same basis, the second split structure around 285.7 eV is attributed to the LUMO+1 transitions from the same carbon atoms to the π*2 (z) orbital. The different nature of the molecular orbitals corresponding to the first and second C 1s NEXAFS resonances (285.1 and 285.7 eV) is further confirmed by resonant photoemission spectra of the valence band which displays a different behavior for the two resonances (see the Supporting Information). Then, following the XPS hierarchy, as well as the distribution of the calculated LUMOs, we can assign the structures at 286.2 and 286.9 eV to the C2 → π*1 (z) and the C1 → π*1 (z) transitions, respectively. Finally, the peak at 286.7 eV is tentatively associated with the transitions to the π*3 (y) state. We remark that in the 286−287.5 eV energy range, there is a significant overlap of the contribution from the cyano and hydroxyl carbons to different LUMOs, hampering the reliability

Figure 3. Gas phase valence band measured at hν = 100 eV. The experimental spectrum (green line and markers) is compared with the calculated DOS (black) which, in turn, has been projected on the 2p atomic orbitals of C, O, and N (brown, red, and blue, respectively). The DOS spectra are shown without intensity corrections for the corresponding cross section. On the top part of the graph the charge density of the first five occupied MOs are plotted.

the projected DOS onto the 2p atomic orbitals (PDOS). The peak energies of the calculated DOS match the experimental valence band over all the energy range. The PDOS reveals that all the atoms of every element contribute to the highest occupied molecular orbital (HOMO), while the HOMO−1 is mainly localized on the C atoms. In Figure 3, the first five occupied molecular orbitals (MOs) are also plotted. The HOMO and HOMO−1 correspond to the first two peaks, and their binding energy difference is 1 eV, while the HOMO−2 and HOMO−3 are almost degenerate and give rise together to the structure at 11.7 eV. Concerning the orbital spatial distribution, the HOMO−2 and HOMO−4 extend in the (xy) molecular plane, while all the other occupied MOs are distributed out of the molecular plane (i.e., the nodal plane lies in the (xy) molecular plane). NEXAFS. We measured the electronic structure of the unoccupied MOs by means of near edge X-ray absorption fine structure spectroscopy. The N and C K-edge absorption spectra of 4HCB in the gas phase are shown in Figure 4 (panels a and b, respectively), with a zoom-in on the spectral features present in the energy window corresponding to π*-symmetry orbitals. For convenience, also the first three unoccupied MOs, as obtained by DFT calculations, are plotted (Figure 4c). The first three unoccupied MOs all have π* character, but the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 [labeled π*1 (z) and π*2 (z), respectively] display an out-ofplane orientation, whereas the LUMO+2, labeled π*3 (xy), is oriented in-plane. The N K-edge profile (Figure 4a) is composed of two strong and well-separated resonances that we can confidently assign to the transition from N 1s to π*1 (z) and π*3 (xy), respectively, in full agreement with previous NEXAFS studies of benzonitrile35,36 and acrylonitrile.37 The D



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of a simple building-block analysis.40 On the other hand, this overlap confirms a non-negligible electronic delocalization between side carbons also at the LUMO level. Resonant Photoemission. Next we exploit resonant photoemission spectroscopy across C and N K-edges to measure resonantly enhanced photoemission intensity of the occupied molecular orbitals and, hence, experimentally verify the molecular orbital assignment obtained by the calculated PDOS. In a resonant photoemission experiment valence band spectra are measured for a set of photon energies that are tuned across a chosen elemental absorption edge. Whereas for photon energies below the absorption threshold, only photoemission peaks, corresponding to the nonresonant electron emission from occupied molecular orbitals (HOMO−n, n = 0, 1, 2, 3, etc.), are measured, the VB spectra taken with photon energies that correspond to the main absorption lines in the NEXAFS spectra (e.g., C 1s (N 1s) → LUMO in Figure 4) show resonant intensity increase due to the opening of additional emission channels. There the core-excited electron participates in the core-hole decay, with Auger electron emission from the occupied molecular levels. The cross section for electron emission at the resonance may be written as a Kramers-Heisenberg type equation41,42 2

⎡ ⎤ ⎢ ⟨f |VC|j⟩⟨j|Vr|0⟩ ⎥ wf 0 = ⟨f |Vr|0⟩ + ∑ ⎢ δ(Ef − E0) i Γj ⎥ j ⎣ E0 − Ej + ⎦ 2

where the initial state with energy E0 is equal to |0⟩ = | ϕ1core...ϕ1HOMOϕ0LUMO+j⟩, the intermediate core-excited state with energy Ej and lifetime broadening Γj can be written as |j⟩ = | ϕ0core...ϕ1HOMOϕ1LUMO+j⟩, and the final state with energy Ef is represented by |f⟩ = ϕ1core...ϕ0HOMOϕ0LUMO+j⟩. Vr and VC are respectively the dipole and Coulomb transition operators. The first term represents nonresonant (direct) photoemission from occupied levels, and the second one describes resonant emission in a participator Auger process. The two terms are energetically degenerate, but the second term often dominates in intensity over the direct photoemission term. We note that the second term couples the single-electron wave functions of the initial core electron level ϕcore with those of intermediate (core-excited) level ϕLUMO, as well as of the occupied level(s) ϕHOMO. The intensity of the resonances strongly depends on the spatial overlap among the coupled wave functions, in particular ⟨ϕcore|ϕLUMO+j⟩ and ⟨ϕcore|ϕHOMO−i⟩ or, in other words, on the spatial overlap of the (HOMO−i,LUMO+j) orbital pair (i = 0, 1, and 2; j = 0, 1, and 2) in the narrow region around the initial core site C 1s (N 1s). This may help us to assign the photoemission peaks in VB spectra and resonances in the NEXAFS spectra to specific molecular empty and filled orbitals. Figure 5 (panels a and b) shows the upper valence band photoemission spectra (HOMO−i; i = 0, 1, 2, 3, etc.) of the isolated molecule (gas phase) at four different photon energies, corresponding to the resonant N 1s → LUMO [π*1(z)] and N 1s → LUMO+2 [π*3(xy)] transitions at 399.1 and 399.8 eV and to the resonant C 1s → π*1(z), and C 1s → π*3(xy), at 285.15 and 286.7 eV, respectively. The nonresonant photoemission measured below the absorption edge (hv = 395 eV for N and hv = 282 eV for C) has been subtracted in order to single out the resonant part of the photoemission spectrum. For both C and N spectra, we can distinguish several resonating

Figure 5. Gas phase RESPES on the (a) N-edge and (b) C-edge corresponding to LUMO and LUMO+2 resonances. The spectra corresponding to the excitation to the LUMO+2 orbital (red and orange dots) are displayed with a vertical shift. The 4HCB valence band taken at 100 eV photon energy is reported in (c) to help the comparison with the resonant spectra. The first four HOMOs, which resonate at the different edges and photon energies, are also shown.

states in the valence band in correspondence with both core → LUMO and core → LUMO+2 electronic excitations. From comparison of the RESPES spectrum with the VB spectrum taken at low photon energy (Figure 5c), we observe that the HOMO and HOMO−1 levels in the resonant spectra merge together into a broadened structure at ∼9 eV. This VB structure displays a significant intensity enhancement in correspondence to the C 1s → LUMO resonance at 285.15 eV. We also find an increase of intensity of the VB peaks at the C 1s → LUMO+2 resonance at 286.8 eV, even if this intensity increase is smaller relative to that at LUMO. Moreover, the resonant photoemission in the VB changes its shape in correspondence to the different resonances, being peaked at HOMO−1 for the C 1s → LUMO resonance and at HOMO for the C 1s → LUMO+2 one. This is not surprising since the overlap between ϕHOMO−1 and ϕLUMO+2 is very small. Thus, the HOMO−1 resonates weakly at the LUMO+2 edge. On the other hand, the HOMO−1 resonates much less at energies corresponding to excitations to both the LUMO and LUMO+2 at the N K-edge, while the feature at 12 eV binding energy, which comprises both HOMO−2 and HOMO−3, experiences a strong resonance in correspondence with both C 1s → LUMO+2 and N 1s → LUMO+2 (hv = 399.8 eV) E



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film and the 4HCB bulk crystal, at least with respect to the degree of intermolecular hydrogen bonding existing in the film. This model is confirmed by the close similarity of the N Kedge absorption spectra taken for both a single crystal and a thick film, as evidenced in Figure 7. The NEXAFS signal has

excitations, which is consistent with our DFT-calculated orbital weight of these orbitals over N and C sites. 2. Condensed Phase (Electronically Interacting Molecules). After investigating the electronic structure of 4HCB in the gas phase, several tens of monolayers of the molecule (from now on, a “thick film”) were deposited onto a Au (110) substrate. In Figure 6, the C 1s, N 1s, and O 1s XPS signals in

Figure 6. XPS binding energy shifts between the gas phase (top) and the thick film (bottom) for the three atomic species. The experimental data (markers) are shown together with their best fit (black line). Since the gas phase binding energies are referred to the vacuum level while the solid phase ones to the Au Fermi edge, the two scales are shifted in order to have the C5 peak at the same position. The subsequent shift of the N 1s and O 1s in opposite directions suggests head-to-tail intermolecular hydrogen bonding. Figure 7. Comparison of the N K-edge NEXAFS spactra measured for the single crystal (top graph) and the thick film system (bottom graph). The dichroism between p-pol (red ●) and s-pol (blue ○) is shown for both systems.

the two phases are reported (the gas phase binding energies have been shifted in order to align the position of C5 with the corresponding peak in the condensed phase). Passing from the gas phase to the thick film, C1, C2, and C3 peaks experience an average shift of +0.3 eV (the C2 components presenting a slightly higher change with respect to the other two), while the C4 peak remains at the same position. The shift of C2 is consistent with the reported core level shift for benzonitrile, which is larger in the condensed phase33,43 than in the gas phase.34 More interestingly, the N 1s peak is shifted by 0.5 eV toward higher binding energies, whereas the O 1s peak is shifted by the same amount in the other direction (lower BEs). This finding points to the presence of consistent hydrogen bonding in the thick film. In fact, the hydrogen bond, sometimes considered a “weak electrostatic bond”, can be described as a delocalization of electronic density from an electron donor (in our case the N atom) to the electron acceptor (i.e., the H atom), which in our case is covalently attached to an O atom. This “weak bond” causes the binding energy of the electrons belonging to the donor to shift toward higher values, while the electrons of the atom attached to the proton are shifted toward lower energies.44 In view of the above, and considering that (i) a strong degree of intermolecular O−H···N hydrogen bonding is known to occur in 4HCB single crystals16,18,45 and (ii) intramolecular hydrogen bonding in 4HCB is impossible (see Figure 1a), we interpret the observed N 1s and O 1s binding energies shifts as an indication of a close structural similarity between the thick

been measured for two light polarizations with respect to the substrate, perpendicular (p-polarization, p-pol) and parallel (spolarization, s-pol). In the case of a preferential orientation of the molecules, the p-pol and s-pol signals will be different due to the fact that the probed molecular orbitals will present different cross sections in the two cases. The NEXAFS dichroism is therefore a good tool to measure the order of a film adsorbed onto a substrate. The remarkable similarity between the single crystal and the thick film dichroism supports the hypothesis of a strong structural analogy between the two condensed phases made on the basis of the hydrogen bonding evidence. Resonant Photoemission. We finally turn to resonant photoemission to address the intermolecular coupling in the multilayer phase. Figure 8 shows a set of four VB spectra taken with photon energies corresponding to the C 1s → π1* (π2*) and N 1s → π1* (π3*) absorption resonances. For comparison, we also report the same type of spectra measured in the gas phase, all normalized to the overall Auger intensity, which is a good measure of the overall photoabsorption cross section. Whereas we see no significant differences in the intensity of RESPES resonances (participator Auger lines at the HOMO−i, i = 0, 1, 2, 3, etc.) for the C spectra, the intensity of participator lines in the N spectra are strongly quenched. We note also that the intensity of several HOMO−i lines are quenched, indicating F



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Figure 8. Valence band photoemission spectra measured at the nitrogen (left panels) and carbon (right panels) K-edge in correspondence to core excitations to the LUMO (top panels) and LUMO+2; the gas phase spectra are shown as blue curves. The binding energy of the thick film has been shifted in order to match the valence band structures with that of the gas phase.

that the spatial overlap ⟨ϕcore|ϕLUMO⟩ is reduced. Thus, the LUMO in the multilayer phase is spatially delocalized from the N 1s core site. This spatial delocalization may be expected from the intermolecular coupling that involves hybridization of empty molecular orbitals at the N end-groups. We note that similar observation of reduced participator resonances has been previously observed for the similar nitrile molecular system46 and related to the hybridization of molecular orbitals due to molecular coupling. Moreover, since the N atom is sandwiched between two stacked benzenic rings along the cell axis a (Figure 1b), this delocalization over LUMO orbitals suggests a rationale for the higher charge carrier mobility found along the mentioned crystallographic direction of the crystal. The RESPES analysis thus indicates the establishment of C− OH···N−C intermolecular bonding in the condensed phase, in full agreement with the observed direction of the XPS core level shifts, further contributing to support (together with the discussed NEXAFS dichroism experiments) the view of a close structural similarity between the thick film and the single crystal.

thus opening the possibility to achieve a better understanding and control of the anisotropic charge transport properties of the 4HCB molecular assembly. This is a strong requirement in view of the application of organic single crystals as active materials in innovative optoelectronic electronic devices and sensors. By comparing resonant photoemission spectra of gas phase and multilayer, we were able to identify the nitrogen site of the 4HCB molecules as the site of maximum orbital modification due to intermolecular coupling, which may be related to efficient charge transport over empty molecular orbitals in the condensed phase.

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ASSOCIATED CONTENT

* Supporting Information S

The C 1s resonant photoemission on a thick film of 4HCB and the relative resonant map, together with the corresponding NEXAFS spectrum is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS In summary, the XPS C 1s core level shifts measured for the molecule in the gas phase are quantitatively reproduced by DFT calculations, which demonstrate the characteristic charge redistribution due to a push−pull mechanism taking place between the hydroxyl and cyano molecular terminations. The formation of a condensed 4HBC multilayer gives rise to a large XPS shift of both the N and O 1s peaks but in opposite directions, which is indicative of a significant intermolecular coupling via hydrogen bonding through the nitrogen end groups. The multilayer and single crystal are found to display the same dichroism in the polarization-dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into the same molecular orientation,

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address

(G.O.)Synchrotron-SOLEIL, L’Orme des Merisiers SaintAubin - BP 48, 91192 Gif sur Yvette Cedex, France Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

A.F.M. and B.F. gratefully acknowledge European Union funding for the project “i-FLEXIS” (Grant 611070). G



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Article

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