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Dynamics of Electron Transfer in Azulene-Based Self-Assembled Monolayers Tobias Wächter, Kolbe J. Scheetz, Andrew D. Spaeth, Mikhail V. Barybin, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04267 • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

Dynamics of Electron Transfer in Azulene-Based Self-Assembled Monolayers

Tobias Wächter,1 Kolbe J. Scheetz,2 Andrew D. Spaeth,2 Mikhail V. Barybin,2* and Michael Zharnikov1*

1

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

2

Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045-7582, USA

ACS Paragon Plus Environment


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Abstract

Azulene is an attractive building block for molecular electronics design owing to the intrinsic charge separation within its sp2-carbon scaffold. In this context, the structure and molecular organization in self-assembled monolayers (SAMs) of unsubstituted and nitrile-functionalized azulenethiolates on Au(111) substrates was studied by a variety of complementary spectroscopic techniques. The molecule of 2-mercapto-6-cyanoazulene was specifically designed to be addressable within the core hole clock approach in the general framework of resonant Auger electron spectroscopy. The azulenic SAMs described herein were documented to be well-defined and densely packed, with their individual molecular constituents oriented upright with respect to the Au surface, but with considerable tilt and twist. The electron transfer (ET) properties of these azulenic SAMs were found to be comparable to those of analogous napthalene-based monolayers, which suggests that the charge separation and the related dipole moment have a minor effect on dynamics of ET through the molecular framework. The characteristic time for the latter process, triggered by the [N 1s]π1* excitation and occurring along the nitrile-backbone-substrate pathway, was estimated at 23±4 fs.


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1. Introduction Charge transport at the nanoscale is a phenomenon of both fundamental importance and a direct relevance to organic and molecular electronics.1-4 In the context of molecular electronics, the performance of molecular wires and small functional molecules capable of serving as potential components of future electronic circuits has been the subject of extensive research.2,3,5 One of such molecular platforms is based on azulene (C10H8), a nonbenzenoid aromatic hydrocarbon consisting of fused five and seven-membered sp2-carbon rings. The molecule of azulene has a relatively large (for a hydrocarbon) ground state dipole moment of ~ 1 D6,7 due to intramolecular charge transfer between its rings. The frontier orbitals of azulene are mutually complementary with excess orbital densities of the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) localized in the five- and seven-membered rings, respectively.8 Because of this asymmetry and low aromatic stabilization energy, azulene and its derivatives have been considered as potential molecular rectifies,7-9 platforms for effective molecular wires,7 coupling moieties for organometallic electron reservoirs,6,10,11 as well as versatile building blocks for design of functional materials.12,13 Within a dedicated theoretical study,7 the electric transport properties of the 2,6-azulene motif were compared to those of the isomeric symmetric 2,6-napthalene framework with the conclusion that the conductance of azulene is higher than that of napthalene and features asymmetry of the I/V curves. Notably, according to the most recent DFT calculations,8 the rectification ratio of azulene itself is not high but can be significantly increased by linking one or two azulenic units together, as well as by substituting the pentagon and heptagon rings with suitable electron withdrawing and donating groups. However, in spite of the intriguing properties discussed above, the electric transport characteristics of azulene have not been addressed experimentally so far with the only exception of ref 14, even though recently the effects of polarity and polar groups in functional molecules and molecular films have come in focus of the respective studies.15-17 A standard prerequisite of such studies is adjustment of the target molecules in a two-terminal junction14 or their assembly on a conductive substrate or metal electrodes,4 most frequently in the form of self-assembled monolayers (SAMs)18. In the case of azulene, the initial examples of azulenic and biazulenic SAMs have been obtained by employing the isocyano anchoring groups.19,20 Later, the formation of analogous films for 2-mercapto-azulene (AzuSH) and several of its derivatives on Au(111) has been reported.21 According to the optical


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ellipsometry and infrared (IR) spectroscopy data, these films exhibit a monolayer nature, with the adsorbed molecules oriented approximately upright with respect to the Au surface.21 Starting from this point and keeping in mind a very limited amount of the experimental data regarding the charge transport properties of azulene and azulene-based SAMs, we present here a dedicated experimental study on this subject taking the monolayers of unsubstituted and nitrile-substituted 2-mercaptoazulene on Au(111) as test systems (Figure 1). In contrast to the most studies addressing the static charge transfer properties of individual molecules and molecular films (e.g., refs 4, 15-17, 22-28), we considered the dynamics of electron transfer (ET) in the azulene-based SAMs by applying the so-called core hole clock (CHC) method in the framework of resonant Auger electron spectroscopy (RAES).29-34 This approach relies on decomposition of suitable RAES spectra into the parts related to the standard decay of the excited state and ET of the resonantly excited electron to the substrate. It is contact-free and uses the known lifetime of inner shell vacancy, appearing in the course of the Auger process, as an internal time reference. In the specific adaptation of this approach to SAMs, it relies on the special design of the target molecules, i.e., the attachment of the RAES-addressable moiety as the tail group to the molecular backbone. Following the resonant excitation of this group, the ET pathway, through the molecular framework and the docking (head) group to the conductive substrate, will then be unambiguously defined.35-40 Such a design is realized in the present study by means of self-assembly of 2-mercapto-6-cyanoazulene (NC-AzuSH, Figure 1) on the Au(111) surface, with nitrile serving as the RAES-addressable tail group and thiolate serving as the docking group. The unsubstituted AzuS monolayer on Au(111) formed from 2-mercaptoazulene (AzuSH, Figure 1) is not only used as a direct reference to the NCAzuS system, but is also of its own importance since its characterization was so far limited to ellipsometry and IR spectroscopy.21

2. Experimental AzuSH was synthesized according to our previous report.21 The synthesis of NC-AzuSH is summarized in Scheme 1 and described in detail in the Supporting Information, where the relevant spectroscopic data for this compound and its intermediates are presented as well. All other chemicals and solvents were purchased from Sigma-Aldrich and used as received. The substrates for the SAM preparation were purchased from Georg Albert PVD, Silz, Germany. They were prepared by thermal evaporation of ~30 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 9 nm


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titanium adhesion layer. The films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The AzuS and NC-AzuS SAMs were formed by immersion of freshly prepared gold substrates into a 1.5 mmol solutions of the above compounds in chloroform (CHCl3) for 24 h at room temperature. After immersion, the SAM samples were carefully rinsed with the solvent and, subsequently, with ethanol. Residues of the solvents were blown dry with argon. Finally, the samples were put into argon-filled glass containers and kept there for several days until they were analyzed at the synchrotron radiation facility (see below). In addition, reference SAMs of octadecanethiolate (ODT) and nitrile-substituted terphenylthiol were prepared on the same kind of gold substrates using standard procedures.39,41 The fabricated films were characterized by high-resolution X-ray photoelectron spectroscopy (HRXPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and RAES. The experiments were performed at the MAX II storage ring at the MAX IV laboratory in Lund, Sweden, using the bending magnet beamline D1011 (plane grating monochromator) and an experimental station equipped with a SCIENTA SES200 electron energy analyzer and a partial electron yield (PEY) detector. The measurements were conducted under UHV conditions at room temperature. Special care was taken to minimize potential damage induced by the primary X-rays.42,43 The HRXPS spectra were acquired in normal emission geometry in all relevant binding energy (BE) ranges. The primary photon energy (PE) was varied. The energy resolution was better than 100 meV allowing a clear separation of individual spectral components. The BE scale of every spectrum was individually calibrated to the Au 4f7/2 line of the underlying Au substrate at 84.0 eV.44 The spectra were fitted by component peaks, mimicked by symmetric Voigt functions, and a Shirley-type background. To fit the S 2p3/2,1/2 doublets, we used a pair of such peaks with the same full width on half maximum (fwhm), a branching ratio of 2:1 (S 2p3/2/2p1/2), and a spin-orbit splitting of ∼1.18 eV,44 which was additionally verified by fit. The NEXAFS spectra were acquired at the C and N K-edges in the PEY acquisition mode with retarding voltages of −150 and −300 V, respectively. Linearly polarized synchrotron light with a polarization factor of ~95% was used. The energy resolution was better than 100 meV. The incidence angle of X-rays was varied from 90° (E vector in the surface plane) to 20° (E-vector near the surface normal) in steps of 10-20° to monitor the molecular orientation in the SAMs. This approach is based on the dependence of the cross-section of the resonant


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photoexcitation process on the orientation of the electric field vector of the synchrotron light with respect to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).45 Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The energy scale was calibrated by means of the most intense π* resonance of highly oriented pyrolytic graphite at 285.38 eV46 in combination with the well-known ∆hν ∝ (hν)3/2 behavior of plane grating monochromators.47 The RAES spectra were acquired at the N K-edge using a SCIENTA SES200 electron energy analyzer. The X-ray incidence angle was set to 55° to suppress orientational effects;45,48 the take off angle of the electrons was close to normal emission. The resonant excitation energies were determined in the NEXAFS experiments (see above). In addition, non-resonant Auger electron spectra were recorded at an excitation energy of 5-6 eV above the absorption edge. This setting turned out to be optimal for nitrile-substituted SAMs on Au(111) in order to maximize the signal to noise ratio and avoid appearance of interfering gold photoemission in the spectra.35 Finally, for every sample, a reference spectrum for the pre-edge excitation was measured. This spectrum was subtracted from the RAES and non-resonant AES spectra to correct them for a contribution from the photoemission appearing in the given energy range.

3. Results and Discussion 3.1. HRXPS C 1s, N 1s, and S 2p HRXPS spectra of the AzuS and NC-AzuS films are presented in Figure 2. The C 1s spectrum of AzuS/Au in Figure 2a is dominated by an intense peak at a BE of ~284.45 eV (1), assigned to the azulene backbone. This peak is accompanied by a very weak shoulder at a BE of ~285.9 eV (2), which can, in part, stem from a shake-up excitation in the azulene matrix43,49 but may also be related to a minor contamination. Significantly, no other features, such as those characteristic of COO or C=O (most frequent contamination), are observed which suggests efficient monomolecular assembly accompanied by self-cleaning of the substrate (see also the O 1s XPS data in the Supporting Information; Figure S8). This is also the case for NC-AzuS/Au. The C 1s spectrum of this film is, again, dominated by the azulene-stemming component peak at a BE of ~284.45 eV (1), accompanied by a quite intense shoulder (3) assigned to the carbon atom in the nitrile group, similar to the previously analyzed SAMs of nitrile-substituted napthalenthiolates (NC-NapS) and oligophenylenes on Au(111).40,50 Interestingly, the dominant component peak for NC-AzuS/Au is slightly


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narrower than that for AzuS/Au, thereby suggesting a better structural homogeneity of the former monolayer. Note also that the comparably high intensity of the shoulder 3 is related to the strong attenuation of the signal from the azulene backbone at the given kinetic energy of the photoelectrons (~60 eV). The N 1s spectrum of NC-AzuS/Au in Figure 2b exhibits a strong N 1s peak at ~398.65 eV (1), assigned to the nitrogen atom in the terminal nitrile group, similar to the analogous systems.39,40,50 In addition, there is weak shoulder at a BE of ~400 eV (2) with a spectral weight of ~14% which can be tentatively assigned to the partial protonation of the nitrile group or its association with water.51,52 The spectrum of NC-AzuS/Au can be compared with that of NC-NapS/Au40 presented in Figure 2b as well. The nitrile-stemming peak for the former film is somewhat shifted to higher BE and is much broader as compared to the latter. Whereas the shift most likely stems from electrostatic effects associated with the molecular dipole,53,54 the larger width of this peak suggests a larger structural inhomogeneity of the NCAzuS film compared to the NC-NapS SAM. The S 2p spectra of AzuS/Au and NC-AzuS/Au in Figure 2c are dominated by a doublet (1) at ~162.0 eV (S 2p3/2), representative of the thiolate species bound to noble metal substrates.43 This underscores the SAM-like character of both films. Unfortunately, traces of unbound thiol (2) and atomic sulfur (3) at ~163.4 eV and 161.0 eV (S 2p3/2), respectively, are perceptible in the spectra as well, suggesting that the monolayers are not completely free of the physisorbed molecules and atomic sulfur contamination. Notably, however, the real weight of the physisorbed molecules is much lower than the spectral weight of the respective component doublet in the spectra, since these species are predominantly located at the SAM-ambient interface, so that the respective signal is hardy affected by attenuation, in contrast to those of the thiolate and atomic sulfur species located at the SAM-substrate interface (see ref 55 to get an impression about the strength of this effect). Accordingly, the portion of the physisorbed molecules with respect to the chemisorbed ones was estimated at 11% and 6% for AzuS/Au and NC-AzuS/Au, respectively. Along with the above qualitative analysis of the HRXPS spectra, their numerical evaluation was performed as well. In particular, from the C1s/Au4f intensity ratio, the effective thickness of the AzuS and NC-AzuS SAMs was calculated using the standard approach40,53 and taken the ODT monolayer with a known thickness as a reference. The derived values are compiled in Table 1. The effective thickness of the AzuS film is in good agreement with the previous ellipsometry-derived value (10.9 Å)21 as well as with the calculated thickness of these


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monolayers (10.4 Å),21 suggesting an upright molecular orientation with respect to the gold substrate. Further, based on the S2p/Au4f intensity ratio with only the thiolate signal being considered, the packing densities of the AzuS and NC-AzuS SAMs were estimated. We followed the approach of our previous publications,53 and used ODT/Au as a reference system with well-known packing density (4.63×1014 molecules/cm2 or 21.6 Å2/molecule)41,56. The derived values of the packing densities are given in Table 1. They are somewhat lower than the analogous value for the reference ODT monolayer, which exhibits the characteristic (√3×√3)R30° and (2√3×3)rect molecular arrangements41,56 typical also for oligophenilthiolate SAMs,57 but comparable with the packing density of NC-NapS/Au estimated at ~4.2×1014 molecules/cm2.40 The slightly lower densities in the present case (Table 1) are, presumably, related to the presence of physisorbed molecules and atomic sulfur contamination. Summarizing, according to the HRXPS data, both AzuSH and NC-AzuSH form well-defined SAMs on Au(111), albeit slightly contaminated with physisorbed molecules and atomic sulfur. The packing density of these SAMs is lower than that of alkanethiolate (AT) monolayers on the same substrate but comparable to that of the NC-NapS films. The molecules in the AzuS and NC-AzuS SAMs are oriented upright with respect to the substrate and bound to it via the thiolate anchors. The nitrile groups are exclusively located at the SAM-ambient interface, with a small portion of them being protonated or interacting with water.

3.2. NEXAFS Spectroscopy Representative NEXAFS data for the AzuS and NC-AzuS SAMs are shown in Figure 3. Two kinds of spectra are presented for both C an N K-edge. First, there are the spectra acquired at a "magic" X-ray incident angle of 55°. These spectra are free of effects associated with molecular orientation and are only influenced by the electronic structure of the films studied.45 Second, there are the difference spectra involving the data collected under normal (90°) and grazing (20°) incidence geometry. Such difference spectra are useful fingerprints of molecular orientation,45 relying on the linear dichroism effects in X-ray absorption (vide infra, Section 2). The 55° C K-edge spectrum of AzuS/Au in Figure 3a is similar to those of napthalene58 and naphthalenethiolate (NapS) SAM on Au(111).40 At the same time, it differs from the spectrum of gas-phase azulene,59 which is, however, questionable since the spectrum of naphthalene, published in the same study,59 seems to be not entirely correct, showing no characteristic


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double resonance structure in the vicinity of 285.0 eV. The spectrum of AzuS/Au exhibits two merged π*-like resonances at ~284.2 and ~284.9 eV (1a and 1b, termed together as πazu*), a weak resonance at ~287.15 eV (2) overlapping with the absorption edge, a π*-like resonance at ~288.3 eV (3), σ*-like resonance at ~292.3 eV (4), and a variety of further σ*-like resonances at higher PEs. The "splitting" of the π1∗ resonance, appearing as a single peak in the spectra of benzene and oligophenyls at ~285.0 eV,45,60,61 in 1a and 1b can be explained, similar to polyacenes,40,58,61 by the chemical shift of the symmetry-independent carbon atoms in the azulene moiety. The π*-like resonance at ~288.3 eV (3) has most likely the π2* character40 but a minor contribution of residual COOH or C=O appearing at the same PE62 cannot be excluded, even though the respective peaks were not observed in the C 1s HRXPS spectrum (Figure 2a). The 55° C K-edge spectrum of NC-AzuS/Au in Figure 3a is similar to that of AzuS/Au, exhibiting, however, a renormalization of the relative intensities of the 1a and 1b resonances within the joint πazu* feature and the appearance of a new resonance at ~286.65 eV (5), akin to that of benzonitrile. The renormalization can be tentatively explained by the conjugation between the π* system of the nitrile moiety and azulene backbone (similar to the NapS and NC-NapS SAMs)40, resulting in a redistribution of the electron density and, subsequently, in changes in the oscillator strengths of the involved electronic transitions. The new resonance 5 can, by analogy with the benzonitrile case,37,39,50 be assigned to the heptagon-nitrile moiety. The 55° N K-edge spectrum of NC-AzuS/Au in Figure 3b is dominated by the double π*-like resonance at ∼398.4 eV and ∼399.8 eV, accompanied by a weak feature at ~401.0 eV (presumably, π4* of nitrile)63,64 and several barely observable σ* resonances at higher PEs. The dominant double resonance mimics an analogous feature of benzonitrile which has been observed in the gas phase,64 molecular solid,63 and benzonitrile-terminated SAMs39. This feature arises from the conjugation between the π* orbitals of the nitrile group and the adjacent phenyl ring of benzonitrile. As a result, the degeneration of the π(C≡N*) orbital is lifted and it splits into two orbitals with distinctly different energies, which are oriented either perpendicular (lower PE; π1*) or parallel (higher PE; π3*) to the plane of the adjacent ring.63,64 The relatively lower intensity of the π1* resonance, observed for benzonitrile,39,63,64 can then be rationalized by delocalization of the respective orbital over the entire benzonitrile moiety, whereas π3* is exclusively localized at the nitrile group. The energies of the


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respective features are ~398.8 eV and ~399.75 eV for the benzonitrile-terminated SAMs39 and ~398.8 eV and ~399.7 eV for the NC-NapS/Au SAMs40. The above considerations are fully applicable to the NC-AzuS/Au case, with the only difference that the nitrile moiety is bound to the seven- rather than six-membered ring. However, conjugation of the π1* orbital of nitrile with the π* system of the azulenyl moiety turned out to be stronger than in the case of phenyl, since the energy of this orbital is shifted downwards (398.35 eV vs 399.8 eV) and the respective resonance exhibits lower intensity than the π1* one of benzonitrile. In addition to the above analysis of the 55° spectra, information on the molecular orientation in the AzuS and NC-AzuS SAMs on Au(111) can be obtained. This information relies on the orientation of specific molecular orbitals which can be derived from the entire set of the NEXAFS spectra acquired at different angles of X-ray incidence for a particular sample. In the present case, the vector-like πazu*, π1*(NC), and π3*(NC) orbitals were selected. The orientation of these orbitals with respect to the molecular backbone is well-defined, viz. either perpendicular, for πazu* and π1*(NC), or parallel, for π3*(NC), to the molecular plane, analogous to benzonitrile63,64. The average tilt angles of these orbitals in the AzuS and NCAzuS SAMs, calculated within the standard theoretical framework,45 are listed in Table 2. These angles agree well with the difference curves in Figure 3 since almost no dichroism was observed for the πazu* orbital for both AzuS/Au and NC-AzuS/Au (except for 1b) and the π1*(NC) orbital for NC-AzuS/Au, whereas a positive difference peak was documented at the position of the π3*(NC) resonance for NC-AzuS/Au. Accordingly, the average tilt angle of the πazu* and π1*(NC) orbitals is close to the magic angle (55°; no dichroism)45, whereas the π3*(NC) orbital is stronger inclined with respect to the substrate. Based on the derived average angles of the molecular orbitals compiled in Table 2, average tilt and twist angles for the molecular backbones in the NC-AzuS SAM were calculated within the established evaluation procedure40,65 according to the following equations: cos(α1) = sin(β)cos(γ)

(1)

cos(α3) = sin(β)cos(π/2−γ)

(2)

cos(αazu) = sin(β)cos(γ),

(3)

and


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where αnap, α1, and α3 are the average tilt angles of the πazu*, π1*(NC), and π3*(NC) orbitals;

β is the molecular tilt angle, and γ is the molecular twist angle, describing the rotation of the backbone along the molecular axis in relation to the tilt direction and defined as 0 if the transition dipole moment of the πazu* orbital lies in the plane spanned by the surface normal and the molecular axis (see a schematic drawing in ref 40). Note that eqs 1 and 2 are fully sufficient to calculate β and γ. Accordingly, eq 3 serves just as an additional proof for the validity of the above evaluation. If the evaluation is correct, the same β should be obtained at the given αazu and γ (determined from eqs 1 and 2). This is, indeed, the case since the α1/α3 and αazu derived values of β are very close (Table 2). Note also that the evaluation of both tilt and twist of the azulene backbones in the NC-AzuS SAM was only possible due to the attachment of the nitrile tail groups to these backbones and alignment of the orthogonal π1* and π3* orbitals of these groups with the π* orbitals of azulene. Without this specific tail group, as e.g. in the case of AzuS/Au, additional information, or at least a reasonable assumption regarding γ, is necessary. Given the similarity of αazu for AzuS/Au and αazu and α1 for NC-AzuS/Au, as well as the similar effective thicknesses and packing densities (Table 1), the average tilt and twist angles calculated for NC-AzuS/Au are, presumably, valid for AzuS/Au as well. Summarizing, according to the NEXAFS data, the molecules in both AzuS and NC-AzuS SAMs are oriented upright with respect to the substrate yet strongly tilted and twisted. Such tilting and twisting are typical for oligophenyl- and polyacene-based SAMs,40,65 including the most similar NC-NapS monolayer, in particular.40 Significantly, in the context of the RAESCHC experiments described in the next section, the N K-edge NEXAFS spectra of NCAzuS/Au exhibit two well-defined resonances, corresponding to the molecular orbitals which are either strongly conjugated with the adjacent heptagon ring (π1*) or localized at the nitrile moiety (π3*). Both these orbitals are well-suitable for resonant excitation by narrow-band Xrays within the RAES-CHC approach, giving the access to molecular-orbital-selective ET dynamics.

3.3. ET Dynamics A scheme of core de-excitation routes for the nitrile group in NC-AzuS/Au upon the resonant and non-resonant excitation is shown in Figure 4. The basic description of these routes along with all relevant abbreviations are provided in the caption of this figure.


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The [N 1s]π1* RAES spectrum of NC-AzuS/Au is presented in Figure 5a; this spectrum, as well as the [N 1s]π3* one, is almost exclusively representative of the chemisorbed molecules in this monolayer since the physisorbed molecules (~6% for NC-AzuS/Au; see Section 3.1) were presumably located at the SAM-ambient interface and electronically decoupled from the substrate. Also, it can be assumed that ET occurred predominately through individual molecular units (after the resonant excitation of the nitrile tail group) with only a small contribution of intermolecular ET processes within the SAM. According to the results of the static conductance experiments on monomolecular films, the tunneling through the chain ("through-bond") is the most generally accepted concept as compared to the tunneling through the SAM matrix ("through-space"), relying on chain-to-chain coupling.66 The spectrum in Figure 5a exhibits pronounced spectator contributions (SP1 and SP2), typical of benzonitrile-terminated monomolecular films,39,40 and a low-intensity participant (P) feature, typical of the [N 1s]π1* case.50 A comparison of this spectrum with the nonresonant one, measured separately, and with the purely resonant spectrum, acquired using a reference SAM of nitrile-substituted terphenylthiols (NC-TPT) on Au(111),39 suggests that it contains a considerable contribution from the ET route, represented as an admixture of the non-resonant spectrum to the pure resonant one. Accordingly, a reconstruction of the [N 1s]π1* spectrum by the linear combination of the purely resonant and non-resonant contributions was performed with the result illustrated in Figure 5. The spectral weight of the ET route, PET, was estimated to be ~22%. Based on this value, we calculated the characteristic time for ET from the strongly conjugated π1* orbital of the nitrile group to the substrate, through the molecular framework, τET. For this purpose, we applied the main formula of the CHC approach29 τET = τcore (1-PET)/PET,

(4)

where τcore is the known lifetime of inner shell vacancy (~6.4 fs for N 1s)67. The derived τET is 23±4 fs. Notably, it is very close to the analogous value for the NC-NapS SAM (24±4 fs)40 but faster than that for the SAM of nitrile-substituted biphenylthiolates (NC-BPT) on Au(111) (29±6 fs)37. This suggests that the annulation of the aromatic rings and, consequently, their better conjugation are of importance for the efficiency of ET across the molecular framework. At the same time, a non-symmetric charge redistribution within this framework, occurring in azulene in contrast to napthalene, seems to be of minor importance.


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In contrast to the [N 1s]π1* spectrum of NC-AzuS/Au, the [N 1s]π3* one, shown in Figure 5b, exhibits only a tiny admixture of the non-resonant contribution characteristic of ET. The respective τET was estimated at ~120 fs, which is at the border of the range resolvable within the applied CHC scheme with the N 1s core hole clock (ca. 0.5 - 120 fs). Thus, the accuracy of the value is quite low, which is additionally complicated by the fact that no perfect fit of the [N 1s]π3* spectrum could be performed, presumably because of the difference between the benzonitrile moiety in the reference film (NC-TPT/Au) and the seven-membered ring bound to nitrile in NC-AzuS/Au. However, several different fits (only one example is shown in Figure 5b) all resulted in similar, large values of τET, suggesting that the ET starting from the π1* orbital is much more efficient than that originated from the π3* one. An analogous situation occurs for the NC-NapS and NC-BPT SAMs as well and is explained by the localization of the π3* orbital at the nitrile group, which can be associated with an additional injection barrier and a less efficient ET as compared to the delocalized and strongly conjugated π1* orbital.37,40 Finally, another special circumstance should be mentioned, viz. the occurrence of efficient ET for the [N 1s]π1* case despite the lower energy of the π1* resonance (∼398.35 eV) as compared to the N 1s BE in XPS (~398.65 eV). Generally, as has been considered before,35,39 the opposite relation is necessary for the efficient ET to the substrate but, as demonstrated in the present study, tail group-to-substrate transfer of the resonantly excited electron can also occur at a small negative bias. We think that the reason for this seeming discrepancy is the strong electrostatic contribution in the N 1s BE in XPS, resulting in its overestimate.

4. Conclusions The focus of the present study is the charge transport properties of azulene, probed as ET dynamics in the framework of the RAES-CHC approach. Accordingly, the NC-AzuSH compound, capable of monomolecular assembly on a conductive substrate and decorated with a group suitable for resonant excitation, was designed and custom-synthesized. The structure and molecular organization of the respective film as well as those of the analogous, unsubstituted azulenthiol on Au(111) were studied by a combination of HRXPS and NEXAFS spectroscopy. Both AzuS and NC-AzuS moieties were found to form well-defined SAMs on metallic gold, albeit slightly contaminated with physisorbed molecules and atomic sulfur. The packing density of these SAMs is slightly lower than that of the most basic AT monolayers on Au(111) but comparable to that of the NC-NapS film, which is the closest


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analogue of the NC-AzuS system. The molecules in the AzuS and NC-AzuS SAMs are oriented upright with respect to the Au surface but are strongly tilted and twisted, which is typical for thioaromatic SAMs including the NC-NapS monolayers. The average tilt and twist angles were determined. The N K-edge NEXAFS spectra of NC-AzuS/Au exhibit two well-defined resonances corresponding to the molecular orbitals which are either strongly conjugated with the adjacent seven-membered ring or localized at the nitrile moiety. These orbitals were used for the resonant excitation by narrow-band X-rays within the RAES-CHC approach, giving access to molecular-orbital-selective ET dynamics. For the strongly conjugated orbital, efficient ET to the substrate, through the molecular framework was found, with a characteristic time of 23±4 fs, close to the analogous value for the NC-NapS SAM but faster than that for the NC-BPT monolayer. This suggests that the annulation of the aromatic rings and, consequently, their better conjugation are of importance for the efficiency of ET across the molecular backbone. At the same time, a non-symmetric charge redistribution within this backbone (dipole moment), occurring in azulene in contrast to napthalene, seems to be of minor importance. In the RAES spectrum corresponding to the orbital localized at the nitrile moiety, only a tiny trace of ET were found, which was explained by higher injection barrier and lower efficiency of transfer associated with the localization. Along with the NC-AzuS system studied in the present work, it would be interesting to investigate the isomeric system featuring inverse orientation of the azulenic moiety with respect to the nitrile and thiol groups (i.e., 2-cyano-6-mercaptoazulene). This would allow for direct comparison and relation of the experimental data to the orientation of the molecular dipole, analogous to the recent study14 where the influence of redox functionality on charge transport in several redox-active azulene derivatives was investigated. Despite extensive efforts, we have not yet succeeded in synthesizing this isomer. We hope, however, to achieve this ambitious goal in the future. ■ Associated content: Supporting Information: Synthesis and characterization of

NC-AzuSH

and its

intermediates. O 1s HRXPS spectra of AzuS/Au and NC-AzuS/Au. This information is available free of charge via the Internet at http://pubs.acs.org.


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■ Author information Corresponding Authors *(M.Z.) E-mail: [email protected]. *(M.V.B.) E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements We thank the Max IV facility for the allocation of the beamtime as well as Max IV staff, and A. Preobrajenski, in particular, for the technical support during the experiments. This work was supported financially by the German Research Society (DFG; grant ZH 63/14-2), funding from the European Community's Seventh Framework Programme (FP7/2007-2013) CALIPSO under grant agreement nº 312284, and US National Science Foundation grant CHE-1214102.


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Figure Captions Figure 1. The 2-mercaptoazulene and 2-mercapto-6-coanoazulene SAM precursors used in this study, along with their abbreviations. Upon monolayer formation, the thiolate anchor is generated, so the adsorbed molecules can be assigned as AzuS and NC-AzuS. The dipole moment of the azulene backbone is directed upwards. Scheme 1. Synthesis of 2-mercapto-6-cyanoazulene. Figure 2. C 1s (a), N 1s (b), and S 2p (c) HRXPS spectra of the AzuS and NC-AzuS SAMs as well as N 1s spectrum of NC-NapS/Au given for comparison (b). The spectra are decomposed into individual component peaks and doublets drawn in different colors and marked by numbers (see text for details). The photon energies are given in the panels. The vertical dashed lines serve as visual guides. The spectrum of NC-NapS/Au is reproduced with permission from ref 40. Copyright 2015 American Chemical Society. Figure 3. C K-edge NEXAFS spectra of the AzuS and NC-AzuS SAMs (a) as well as N Kedge NEXAFS spectra of NC-AzuS SAM (b). Two kinds of spectra are shown: the spectra acquired at an X-ray incident angle of 55° (black solid curves) and the difference between the spectra collected under normal (90°) and grazing (20°) incidence geometry (gray solid curves). The horizontal dashed lines correspond to zero; the vertical dashed lines serve as visual guides. Individual resonances are marked (see text for details). The πazu* orbitals of the azulene backbone (1a+1b) and the π1* orbital of the nitrile group are parallel to each other and perpendicular to the molecular plane. The π3* orbital of the nitrile group is parallel to the molecular plane. Figure 4. A scheme of core de-excitation routes for the nitrile group in NC-AzuS/Au upon the resonant (1) and non-resonant (2) excitation. IS, OV and UV denote inner shell (here N 1s), and occupied and unoccupied 2s/2p-derived valence levels, respectively; CB denotes conduction band of the substrate. Filled and hollow circles represent electrons and holes, respectively, with red and blue color-code corresponding to IS/UV and OV, respectively. Shallow core holes as the N 1s decay nearly exclusively by an Auger process, i.e. an electron from OV fills the hole (gray arrows) and a second electron carrying the excess energy is emitted (green arrows). Following the resonant excitation of an IS electron (black arrow in 1) into a bound state (UV), the excited electron can either take part in this decay process (1a; participator, P) or "watch" it as a spectator (1b; SP). Both P and SP processes lead to characteristic final states with effectively one hole in the valence region. Alternatively, if the


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excited group is weakly coupled to a continuum (e.g. CB), transfer of the excited electron to CB can occur (1c; orange arrow), leading to the same final state with two holes in OV as the non-resonant Auger process (2), viz. the excitation of an IS electron into a continuum state (black arrow in 2) followed by the interband transition (gray arrow in 2) and the emission of an OV electron (green arrow in 2). Figure 5. [N 1s]π1* (a) and [N 1s]π3* (b) RAES spectra of NC-AzuS/Au (open circles) along with their reproduction (red solid line) by the linear combination of the purely resonant (blue dashed line) and non-resonant (black dotted line) contributions. Individual features are marked.


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Table 1. Effective thicknesses and packing densities of the NC-AzuS and AzuS SAMs derived from the HRXPS data. The error bars are estimated at ~5%. Monolayer

Effective thickness (Å)

Packing density (molecules/cm2)

AzuS

10.0

4.1 × 1014

NC-AzuS

10.2

3.9 × 1014

Table 2. Average tilt angles of the πazu* and π1,3*(NC) orbitals derived from the numerical evaluation of the NEXAFS spectra of the NC-AzuS and AzuS SAMs as well as the average molecular tilt and twist angles of the SAM constituents in the NC-AzuS monolayer (see text for details). All angles are given with respect to the surface normal. The error bars can be estimated at ±3°. Average angles /system

NC-AzuS

AzuS

π* orbitals (naphthalene) - αazu

54°

55°

π1* orbital (NC) - α1

55°

π3* orbital (NC) - α3

63°

Twist angle (γ) from α1 and α3

39°

Molecular tilt (β) from α1 and α3

48°

Molecular tilt (β) from αnap and γ(α1,α3)

49°


24

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N C

SH

AzuSH

SH

NC-AzuSH

Figure 1

Scheme 1


25


HRXPS: C 1s

a

b

N 1s

hν ν = 350 eV

c

S 2p

hν ν = 580 eV

hν ν = 350 eV

1

AzuS

1

AzuS

NC-NapS

2

3

2

1

NC-AzuS

NC-AzuS 2

3

NC-AzuS

290 288 286 284 282 404 402 400 398 396

166

164

162

160

Binding energy (eV) Figure 2

a

NEXAFS: C K-edge 3 1b 1a

4

55° Intensity (arb. units)

2

AzuS 90°-20° 5

55° NC-AzuS 90°-20°

280

Intensity (arb. units)


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290

300

310

320

b

N K-edge π3* π1*

NC-AzuS 55° 90°-20°

400

410

420

Photon energy (eV)


430

Figure 3

26

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Figure 4

RAES: [N 1s]π π1*

a

NC-AzuS

experiment non-resonant resonant reconstruction


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SP1

SP2

P

b

[N 1s]π π3*

SP1

SP2 P

360

370

380

390

Kinetic energy (eV)

Figure 5


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TOC Graphic


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Dynamics of Electron Transfer in Azulene-Based ... - ACS Publications

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