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Dynamics of Electron Transfer in SelfAssembled Monolayers with Acene Backbone Tobias Wächter, Alix Tröster, Sven Hock, Andreas Terfort, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Dynamics of Electron Transfer in Self-Assembled Monolayers with Acene Backbone

Tobias Wächter,1 Alix Tröster,2 Sven Hock,2 Andreas Terfort,2* and Michael Zharnikov1*

1

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

2

Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-LaueStraße 7, 60438 Frankfurt, Germany

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Taking the specifically designed self-assembled monolayers (SAMs) of nitrile-substituted anthracenethiolates (NC-AntS) and -selenolates (NC-AntSe) on Au(111) as target systems and combining the results with literature data for the analogous films with the naphthalene backbone, we studied electron transfer (ET) dynamics across the molecular acene "wire". As the experimental tool we used resonant Auger electron spectroscopy, processing the data within the core hole clock framework. These experiments were accompanied by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy measurements. Both NC-AntS and NC-AntSe films were found to be well defined and having similar packing density, orientational order, and molecular inclination. The characteristic ET time for NC-AntS/Au and NC-AntSe/Au was estimated at 42±5 and 40±5 fs, respectively. The ET dynamics attenuation factor for the acene backbone (βET) and time associated with ET across the S−Au anchor were estimated at ~0.25 Å-1 and ~1.5 fs, respectively. The same value (~1.5 fs) was obtained for the Se−Au anchor, indicating a similar efficiency of both docking groups in terms of ET dynamics and, presumably, in terms of the static electric properties as well. Comparison of the derived βET value for the anthracene and naphthalene SAMs with the analogous static values found for the anthracene dithiols gives a good agreement, verifying the static attenuation factor corresponding to a good coupling to the electrodes.


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1. Introduction Charge transport at the nanoscale and in individual molecules and molecular assemblies in particular has recently aroused considerable interest of scientific and industrial communities. The respective processes are both of fundamental importance and direct relevance to such frontier areas of research as organic and molecular electronics.1-4 In this context, electric transport properties of a variety of small functional molecules and their assemblies, fulfilling potentially a specific function in a particular test system or a device, have been studied.2-5 The respective measurements are usually performed in two-terminal (molecular) junction geometry where a molecule or molecular assembly is contacted by two electrodes, with one being frequently a conducting substrate. Available experimental strategies involve both small (including molecular) and large area contacts, utilizing conducting probe atomic force microscopy, scanning tunneling microscopy, break junction setups, in-wire junctions, mercury drop approach, and eutectic Ga−In electrode (see refs 4 and 6 for more details). In most cases, transport properties of molecules in such metal-molecule-metal junctions could be described well by the simplified Simmons equation,7 J = J0 exp(-ßd), where J is the current through the junction, J0 is a parameter closely related to the molecular contact resistance, ß is an attenuation factor, and d is the width of the junction corresponding to the molecular length,1 as far as tunneling through the molecular framework (“through-bond”) is preferable as compared to the tunneling through the matter-filled medium (“through-space”).8 The attenuation factor is generally considered to be characteristic of a molecular framework or, if this framework is represented by an oligomer, of a particular "molecular wire". The standard approach to derive the attenuation factor for such a wire is a variation of its length, i.e. the number of individual repeating units within the oligomer. Using this procedure and selfassembled monolayers (SAMs) of suitable molecules as the target systems, the values of attenuation factor for a variety of molecular wires were determined, including alkanes (0.6-1.0 Å-1),9-11 alkenes (β = 0.27 Å-1),12 acenes (0.2 and 0.5 Å-1),13,14 oligo(phenylene ethynylenes) (0.23-0.3 Å-1),10-12 and oligophenyls (0.41-0.7 Å-1).1,10,15,16 A particular interesting observation was made for acene SAMs, for which an attenuation factor of 0.5 Å-1 was recorded for acene thiols and isocyanides13,14 but a significantly lower value of 0.2 Å-1 was measured for acene dithiols,14 having the same molecular backbone. Such a behavior is quite surprising since the attenuation factor should be a property of the molecular chain only and should not depend on the quality of the contact to the top electrode, which is certainly better in the dithiol case (the bottom electrode is the substrate, with the proper contact mediated by the docking group).


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To get a new insight into this behavior and to clarify what value of the attenuation factor is more representative of the acene backbone, we used here an alternative approach to study electric transport properties of molecules containing this moiety, viz. applied the so-called core hole clock (CHC) method in the framework of resonant Auger electron spectroscopy (RAES).17-22 In contrast to the experimental techniques dealing with the static properties (see above), this approach allows to investigate dynamics of electron transfer (ET) in monomolecular assemblies relying 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 only relying on "core hole clock", viz. the known lifetime of inner shell vacancy, appearing in the course of the Auger process, as an internal time reference. The most suitable (in our opinion) adaptation of this approach utilizes SAMs consisting of specifically designed molecules,23,24 which carry a specific tail group (terminal group at the SAM-ambient interface), that can be addressed by narrow-band X-rays (see refs 25-28 for alternative strategies). Following the resonant excitation of this group, the ET pathway, through the molecular framework and the docking group to the conductive substrate, will then be unambiguously defined.23,24 The most suitable tail group turned out to be nitrile which can be synthetically integrated into a variety of molecules and which has suitable molecular orbitals for resonant excitation within the RAES-CHC approach.23,24,29-34 Following the above design concept, in the present work we studied SAMs of nitrilesubstituted anthracenethiolates (NC-AntS) and -selenolates (NC-AntSe) on Au(111) (Figure 1). The properties of these monolayers, which have not been studied so far and were specifically designed for the given project, are of interest on their own, but also in context of the comparative performance of the thiolate and selenolate as docking groups for molecular self-assembly on coinage metal substrates (see ref 32 for a detailed discussion on this issue). In addition, the ET dynamics results for these films can be combined with the analogous data for nitrile-substituted naphthalenethiolate (NC-NapS)32 and -selenolate (NC-NapSe)32 SAMs as well as with the data for nitrile-substituted benzenethiolate (NC-PT)30, providing insight in the ET properties of the acene backbone. To verify the suitability of the NC-AntS and NCAntSe films for the RAES-CHC experiments and to clarify general parameters of these novel monolayers, the RAES measurements were accompanied by extensive characterization of these films by suitable combination of spectroscopic techniques. Note

that

the

SAMs

of

non-substituted

anthracenethiolates

(AntS)

and

anthraceneselenolates (AntSe), which can be considered as references to their nitrile-


4

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substituted analogues, have been extensively studied by a variety of experimental techniques,35-38 supported by theoretical simulations.38 Both types of SAMs were found to be well defined, densely packed, and characterized by upright orientation of their constituents, but the orientational and structural order turned out to be higher in the Se case, which was explained by the better ability of selenolates to adapt to the structural template provided by the substrate,37 in agreement with the original model proposed in ref 39. In addition, as stated in some publications,36 the quality of the AntS SAMs depends critically on the choice of solvents and rising conditions. N

N

C

C

SH

NC-AntS

SeAc

NC-AntSe

Figure 1. The SAM precursors used in this study. The H atom and acetyl (Ac) protection group are cleaved off upon the SAM formation and the thiolate and selenolate docking groups are generated. The adsorbed molecules can then be assigned as NC-AntS and NC-AntSe which will be used as abbreviations for the molecules in the respective SAMs.

2. Experimental The precursors for the NC-AntS and NC-AntSe SAMs, viz. NC-AntSH und NC-AntSeAc (Figure 1), were prepared by a multi-step synthesis, in which the anthracene backbone was build up by a cyclization. A central molecule in this approach is 6-bromoanthracene-2carbonitrile, into which the sulfur atom and the selenium atom, respectively, were introduced by published methods.40-42 The procedure is described in detail in the Supporting Information, where the relevant spectroscopic data for these compounds and their intermediates are also presented. All other chemicals and solvents were purchased from Sigma-Aldrich and used as received. The gold substrates for the SAM preparation were purchased from Georg Albert PVD, Silz, Germany. They were fabricated by thermal evaporation of ~30 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had


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been precoated with a 9 nm titanium adhesion layer. The films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The NC-AntS und NC-AntSe SAMs were formed by immersion of freshly prepared gold substrates into 1 mmol solutions of the respective precursors in absolute ethanol for 24 h at 60 °C. The choice of ethanol as the solvent was based on our previous positive experience with AntS SAMs on Au(111) and Ag(111).35 The elevated temperature was selected in view of our previous experience with the NC-NapS und NC-NapSe monolayers; it resulted in a better quality of these SAMs,32 which was also the case for the anthracene films of the present study, as confirmed by comparison of the samples prepared at room temperature and 60 °C (the data for the room temperature samples are not shown). After immersion, the samples were rinsed with the solvent, blown dry with argon, and put into argon-filled glass containers for the transport to the synchrotron radiation facility (see below). In addition, reference SAMs of hexadecanethiol (HDT), anthracenethiol (AntS), nitrile-substituted biphenylthiol (NCBPT), and nitrile-substituted terphenylthiol (NC-TPT) were prepared on the same kind of gold substrates using the established procedures.30,35,43 The NC-AntS and NC-AntSe monolayers were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and RAES. The experiments were performed at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, Germany, using a custom-made experimental station.44 The measurements were carried out in UHV and at room temperature. The XPS spectra were acquired in normal emission geometry using a Scienta R3000 electron energy analyzer. The synchrotron light served as the primary X-ray source. The photon energy (PE) was set to either 350 eV or 580 eV, dependent on the binding energy (BE) range. The energy resolution was ~0.3 eV at a PE of 350 eV and somewhat lower at a PE of 580 eV. The BE scale of every spectrum was individually calibrated to the Au 4f7/2 line of the underlying Au substrate at 84.0 eV.45 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 and Se 3d5/2,3/2 doublets, we used a pair of such peaks with the same full width on half maximum (fwhm), branching ratios of 2:1 (S 2p3/2/2p1/2) and 3:2 (Se 3d5/2/3d3/2), and spin-orbit splittings of ∼1.18 eV (S 2p3/2/2p1/2) and ∼0.86 eV (Se 3d5/2/3d3/2),45 which were additionally verified by fit.


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The effective thickness of the monolayers was calculated using a standard procedure,46 based on the C 1s/Au 4f intensity ratio. A standard expression for the attenuation of the photoemission signal was assumed47 and the literature values for attenuation lengths were used.48 The spectrometer-specific coefficients were determined by using HDT/Au as a reference, relying on the well-known thickness of this monolayer. The NEXAFS spectra were acquired at the C and N K-edges using a custom-made partial electron yield detector; the retarding voltages were set to −150 and −300 V, respectively. Linearly polarized synchrotron light with a polarization factor of ~91% was used. The energy resolution was ~0.3 eV at the C K-edge and somewhat lower at the N K-edge. 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 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).49 Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The PE scale at the C K-edge was calibrated by means of the most intense π* resonance of highly oriented pyrolytic graphite at 285.38 eV.50 The energy calibration at the N K-edge was performed with a help of reference measurements performed at the D1011 beamline of the MAX IV facility in Lund, Sweden. This beamline is equipped with a plane grating monochromator exhibiting the well-known ∆hν ∝ (hν)3/2 behavior.51 The resulting energy positions are expected to be accurate and reproducible within ±0.05 eV. The RAES spectra were acquired at the N K-edge using the same electron energy analyzer as in the XPS case. The incidence angle of the primary X-ray beam was set to 55° to suppress orientational effects;49,52 the take off angle of the electrons was close to normal emission. The PEs for the resonant excitation were determined in the NEXAFS spectroscopy experiments (see above). Non-resonant Auger electron spectrum, serving as a reference for the ET contribution in the RAES spectra, was recorded at an excitation energy of 5-6 eV above the absorption edge. In addition, a spectrum for the pre-edge excitation was recorded. This spectrum was subtracted from the RAES and non-resonant AES spectra to correct them for a contribution from the photoemission appearing in the relevant energy range. The purely resonant spectra, necessary for the analysis and decomposition of the RAES spectra of NCAntS/Au and NC-AntSe/Au were measured using a reference NC-TPT SAM on Au(111).24


7


The ET process in this sample, having the same, RAES addressable tail group (nitrile), is too slow to be recorded within the given RAES-CHC schema with the O 1s "core hole clock", so that the ET contribution in the RAES spectra is not perceptible and these spectra represent, thus, the purely resonant (autoionization) ones.24

3. Results and Discussion 3.1. XPS C 1s, N 1s, S 2p, and Se 3d XPS spectra of NC-AntS/Au and NC-AntSe/Au are presented in Figure 2. The S 2p spectrum of NC-AntS/Au in Figure 2a is dominated by a S 2p3/2,1/2 doublet (1) at ~162.0 eV (S 2p3/2), which is characteristic of the thiolate species bound to noble metal substrates.53 This means that basically all molecules in the NC-AntS films were anchored to the substrate via a thiolate-gold bond, forming well defined SAMs. There is also a minor trace of atomically adsorbed sulfur (2) at 161.0 eV (S 2p3/2), perceptible frequently in the spectra of thiolate SAMs, as far as they are acquired with sufficiently high energy resolution.53 The identification of this species has been most clearly demonstrated in the thermal stability experiments on alkanethiolate SAMs on Au(111) where it was the only species left on the surface at 490 K.54 This minor contamination is only present at the SAM-substrate interface and should not affect noticeably the molecular packing in the NC-AntS monolayer since this interface can accumulate much more S atoms as compared to the docking groups of the densely packed monodentate SAMs, such as alkanethiolates or oligophenylthiolates on Au(111).55

a C 1s

XPS: S 2p Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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hν ν = 350 eV NC-AntS

1

c N 1s

hν ν = 350 eV 2 NC-AntS

d

hν ν = 580 eV

1

2

NC-AntS

166 164 162 160

Se 3d hν ν = 350 eV

b

NC-AntSe

NC-AntSe

NC-AntSe

62 60 58 56 54 52 50

291

288

285

282

402

399

396

Binding energy (eV)

Figure 2. S 2p (a), Se 3d (b), C 1s (c), and N 1s (d) XPS spectra of NC-AntS/Au and NCAntSe/Au. The S 2p and C 1s spectra are decomposed into individual component peaks and doublets drawn in different colors and marked by numbers (see text for details). The primary photon energies are given in the panels.


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The Se 3d spectrum of NC-AntSe/Au in Figure 2b exhibits a strongly dominant Se 3d5/2,3/2 doublet at a BE position of ~54.25 eV (Se 3d5/2) characteristic of the selenolate species bound to noble metal surfaces.32,39,53,56 This means that basically all molecules in the NC-AntSe films were anchored to the substrate via a selenolate-gold bond, forming well defined SAMs. A very weak shoulder which can be traced at the low BE side of the selenolate doublet corresponds presumably to a minor trace of atomic Se. We believe that this tiny contamination did not affect the molecular packing in the NC-AntSe monolayer to a noticeable extent. The C 1s spectra of both NC-AntS/Au and NC-AntSe/Au in Figure 2c are dominated by an intense component peak at a BE of ~284.4 eV (1), assigned to the anthracene backbone. This peak is accompanied by a less intense feature at a BE of ~285.9 eV (2), appearing as a shoulder of the main peak. This shoulder can be assigned to the carbon atom in the nitrile group, similar to the analogous, nitrile-substituted SAMs with the naphthalene and oligophenylene backbone.32,57 The N 1s spectra of both NC-AntS/Au and NC-AntSe/Au in Figure 2d exhibit a single N 1s peak at ~398.65 eV, assigned, similar to the analogous systems,24,32,57 to the nitrogen atom in the terminal nitrile group. This suggests a structural homogeneity of the NC-AntS and NCAntSe monolayers, with all nitrile groups located exclusively at the SAM-ambient interface. Along with the above analysis of the XPS spectra, effective thickness of the NC-AntS and NC-AntSe SAMs was evaluated, based on the C1s/Au4f intensity ratio (see Section 2). This evaluation gave 12 Å and 12.5 Å for NC-AntS/Au and NC-AntSe/Au, respectively. These values are close to the molecular length, even though somewhat smaller, which suggest upright molecular orientation with a certain inclination. In addition, the effective thickness can be considered as a measure of the relative packing density in the given case, in view of the similar molecular structure of the NC-AntS and NC-AntSe adsorbates. Consequently, the similarity of the effective thickness values for the NC-AntS and NC-AntSe films indicates that the packing densities of these films are similar as well.

3.2. NEXAFS Spectroscopy NEXAFS data for NC-AntS/Au and NC-AntSe/Au are compiled in Figure 3. Two kinds of spectra are presented for both C and N K-edge. First, there are the spectra acquired at a "magic" X-ray incidence angle of 55°, which are not affected by any effects associated with


9


molecular orientation and are exclusively representative of the electronic structure of the SAMs.49 Second, there are differences between the spectra acquired at the normal (90°) and grazing (20°) incidence angles of the primary X-rays (the original 90° and 20° spectra are presented in Figures S1-S4 in the Supporting Information). They represent useful fingerprints of molecular orientation,49 relying on the linear dichroism effects in X-ray absorption (see Section 2).

a

NEXAFS: C K-edge

b

C K-edge: π* region 1b 1a 1c

55°

55°

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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π 3* π1*

AntS 3

NC-AntS

c

N K-edge

π4*

NC-AntS

55°

2

90°-20°

NC-AntS

55°

NC-AntSe

NC-AntSe

290

300

310

320

NC-AntSe

55°

NC-BPT

90°-20°

280

90°-20°

4 5

285

288

1a-1c - π*ant 4 - π1* 5 - π3* 291

294

297

90°-20° 400

410

420

430

440

Photon energy (eV)

Figure 3. C K-edge (a, b) and N K-edge (c) NEXAFS spectra of NC-AntS/Au and NCAntSe/Au. 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 55° C K-edge are presented for both the entire photon energy range (a) and the range of the π* resonances (b); in the latter case the spectra of AntS/Au and NC-BPT/Au are presented as well - for comparison. The most prominent resonances are assigned or marked by numbers (see text for details). The horizontal dashed lines in panels a and c correspond to zero; the vertical dashed lines in panel b serve as visual guides.

The 55° C K-edge spectra of the NC-AntS and NC-AntSe SAMs in Figure 3a are very similar, which is understandable in view of the same molecular backbone and terminal nitrile group of their constituents. These spectra exhibit a variety of sharp π*-like resonances in the pre-edge and at-edge regions and a series of comparably broader σ*-like resonances at higher PEs. The assignments of the above resonances rely on theoretical simulations for anthracene58,59 and benzonitrlile60,61, with an additional support provided by direct


10

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comparison of the spectra of NC-AntS/Au and NC-AntSe/Au with those of the reference SAMs containing the relevant building blocks, viz. AntS/Au35,36 having the same anthracene backbone and NC-BPT/Au30 having the same terminal benzonitrile moiety. This comparison is presented in Figure 3b where the C K-edge spectra for the most representative PE range are depicted. As shown by the example of AntS/Au and in agreement with theoretical calculations,58,59 the spectra of anthracene-based SAMs are dominated by a pronounced double π∗ resonance structure which is typical for acenes and characteristic of anthracene; this structure originates from the splitting of the characteristic singular π1∗ resonance of poly-pphenylenes due to the chemical shift of the two symmetry independent carbon atoms of anthracene with strong influence of excitonic effects.58,59 The respective double π∗ resonance structure can be found in the spectra of NC-AntS/Au and NC-AntSe/Au, at 284.3 eV (1a) and 285.7 eV (1c). In a similar fashion, the resonances 1b (285.5 eV) and partly 5 (286.7 eV) can be associated with the anthracene moiety as well, appearing at the analogous positions as the “shoulders” of the main π* features in the spectrum of AntS/Au. However the resonance 5 is not exclusively related to anthracene but, as shows the comparison with the spectrum of NCBPT/Au and the analogous spectrum of NC-NapS/Au32, stems predominantly from the π3* orbital of the terminal benzonitrile moiety. Indeed, due to the conjugation between the π* orbitals of the nitrile group and the π* states at the adjacent aromatic system, 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.60,61 Significantly, the π1* resonance has a lower intensity since the respective orbital is delocalized over the entire aromatic π∗ system, whereas π3* is exclusively localized at the nitrile group, resulting in a higher intensity of the respective resonance.24,60,61 Consequently, in the given case, the π1* resonance of benzonitrile is practically non-perceptible in the C K-edge spectra of NC-AntS/Au and NC-AntSe/Au, whereas the contribution of the π3* resonance into the joint resonance 5 is presumably strong enough. Finally, there are a variety of the σ*-like resonances, most prominent of which are marked as 2 (285.05 eV) and 3 (293.7 eV) in Figure 3b. The splitting of the π∗(C≡N) orbital of the terminal benzonitrile moiety is exhibited even more clear in the N K-edge spectra in Figure 3c. These spectra are dominated by the distinct π1*/π3* feature at 398.65 eV and 399.7 eV, respectively, with the lower intensity of the π1* resonance, stemming, as mentioned above, from strong conjugation of the π1* orbital of the


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nitrile group with the π* system of the adjacent phenyl ring, accompanied by delocalization of the resulting joint orbital over the entire benzonitrile moiety. In addition, there is a weaker π4* resonance and several low intense σ*-like resonances at higher photon energies. Such spectra are typical of both benzonitrile60,61 and SAMs containing this group.24,30,32,34 The energies of the π1*/π3* resonances are ~398.8 eV and ~399.75 eV for the nitrile-substituted oligophenyl SAMs24 and ~398.8 eV and ~399.7 eV for the nitrile-substituted naphthalene SAMs32, which are quite close to the PE positions of the π1*/π3* resonances for NC-AntS/Au and NCAntSe/Au. The π1* and π3* orbitals of the nitrile group have not only different energies and degrees of localization but also different orientations: whereas the former orbital is oriented perpendicular to the plane of the adjacent ring and, consequently, to the plane of the entire anthracene moiety, the latter orbital is oriented parallel to it.60,61 The resulting system of two mutually perpendicular orbitals with defined orientations with respect to the adjacent anthracene backbone provides the possibility to derive the orientation of this backbone given by the molecular tilt angle with respect to the surface normal, β, and the twist angle, γ, describing the rotation of the backbone along the molecular axis in relation to the tilt direction (see Figure 4).32,62 According to the difference spectra for both C and N K-edge, which show positive and negative peaks for the π*-like and σ*-like resonances, respectively (Figures 3a and 3c), the molecules in the NC-AntS and NC-AntSe SAMs have an upright orientation. The numerical values of β and γ were calculated within the established evaluation procedure32,62 according to the following equations cos(α1) = sin(β) cos(γ)

(1)

cos(α3) = sin(β) cos(π/2−γ),

(2)

where α1 and α3 are the average tilt angles of the π1* and π3* orbitals of the nitrile group (see Figure 4). The latter angles were calculated on the basis of the entire set of the N K-edge spectra, using the standard theoretical framework for a vector-type orbital,49 which is well applicable to the π1* and π3* states. All relevant values are compiled it Table 1.


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Figure 4. A schematic drawing of the orientation of the NC-AntS molecules in the SAM, valid also for the NC-AntSe case. The πarom* orbitals of the anthracene backbone and the π1* orbital of the nitrile group are parallel to each other and perpendicular to the molecular plane; the respective transition dipole moment TDMπ* is shown as a gray arrow; its orientation is given by the angle α, equal to α1. The π3* orbital of the nitrile group is parallel to the molecular plane; the respective TDM has the angle α3 to the z-axis (substrate normal). The tilt angle β and twist angle γ describe the molecular orientation. The tilt occurs within the z-y plane, with the molecular plane being perpendicular to this plane at γ = 0.

Table 1. Average tilt angles of the π1,3*(NC) orbitals derived from the numerical evaluation of the NEXAFS spectra of the NC-AntS and NC-AntSe SAMs as well as the average molecular tilt and twist angles of the SAM constituents in these monolayers (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-AntS

NC-AntSe

π1* orbital (NC) - α1

67°

71°

π3* orbital (NC) - α3

70°

65°

Twist angle (γ) from α1 and α3

42°

53°

Molecular tilt (β) from α1 and α3

31°

32°


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According to these data and in full agreement with the qualitative considerations (see above), both NC-AntS and NC-AntSe SAMs are characterized by upright molecular orientation, with the similar inclination (31-32°) but somewhat different twist angles, within the 42-53° range. Note that analogous, quite high twist angle values were observed in the nitrile-substituted oligophenyl and naphthalene SAMs,32,62 which indicates that they are typical for aromatic monomolecular films. The average molecular inclination is smaller in the NC-AntS and NCAntSe SAMs as compared to the analogous films with the naphthalene backbone (42-43°).32 This is understandable since a longer aromatic backbone results usually in a smaller molecular inclination and a higher orientational order.35 Significantly, due to the averaging character of NEXAFS spectroscopy,49 the molecular inclination represents also a fingerprint of the orientational order which seems to be similar and sufficiently high in the NC-AntS and NC-AntSe SAMs.

3.3. ET dynamics The measurements of the ET dynamics within the RAES-CHC approach involve resonant excitation of an electron into a specific molecular state at a specific functional group, with the subsequent monitoring of the de-excitation process, including ET along a predefined pathway. In our case, the suitable molecular states are represented by the π1* and π3* orbitals of the nitrile group, which are either partly delocalized due to the conjugation with the π* system of the adjacent anthracene moiety (π1*) or localized at the nitrile group (π3*). The pathway from the nitrile group to the substrate, across the molecular framework and the docking group, is well defined. A scheme of core de-excitation routes for the nitrile group in NC-AntS/Au and NC-AntSe/Au upon the resonant and non-resonant excitation is shown in Figure 5, along with all relevant abbreviations. Note that shallow core holes as the N 1s decay nearly exclusively by an Auger process, i.e. an electron from OV fills the hole (2) and a second electron carrying the excess energy is emitted (3). Following the resonant excitation of the N 1s electron (1) into a bound state (π*), the excited electron can either take part in this decay process (P) or "watch" it as a spectator (SP). Both P and SP processes lead to characteristic final states with effectively one hole in the valence states. Alternatively, if the excited group is weakly coupled to a continuum (conduction band of the Au substrate in the given case), transfer of the excited electron to Au can occur (ET; 2'), leading to the same final state with two holes in OV as the non-resonant Auger process (A), viz. the excitation of the N 1s electron into a low lying


14

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continuum state (1) followed by the interband transition (2) and the emission of an OV electron (3). Accordingly, since the spectral shape is defined by final state in the given case, the ET contribution in a RAES spectrum is identical to the non-resonant AES spectrum in terms of the spectral shape.

Figure 5. A scheme of excitation and de-excitation routes for the terminal nitrile group in the NC-AntS and NC-AntSe SAMs upon the resonant (P, SP, and ET) and non-resonant (A) excitation of an electron from the N 1s core level into unoccupied 2s/2p-derived valence states (UV). P, SP, and ET denote the participant, spectator, and electron transfer scenarios, respectively. ET occurs into conduction band of the substrate (Au). Filled and hollow circles represent electrons and holes, respectively, with red and blue color-code corresponding to the N1s/UV and occupied valence (OV) states, respectively. Individual excitation/de-excitation steps are assigned by numbers (see text for details) for convenience but should be considered as parts of a one-step process with the excitation and de-excitation taking place simultaneously under the given experimental conditions.52

An efficient ET from the excited state at the tail group to the substrate is generally possible if the binding energy of the Fermi level of the conductive substrate is lower than the energy of the π* resonance used for the resonant excitation.23 In our case, this condition is almost fulfilled for the [N 1s]π1* excitation and well fulfilled for the [N 1s]π3* process. At the same time, apart from the above energetics relations, much more efficient ET can be expected in the [N 1s]π1* case, because of the conjugation of the π1* orbital with the adjacent aromatic system.30,31


15


a RAES: [N 1s]ππ1* experiment ET pure resonant reconstruction

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NC-AntS

SP1

SP2 P

b SP1

NC-AntSe SP2 P

360

370

380

390

Kinetic energy (eV)

Figure 6. [N 1s]π1* RAES spectra (open circles) of NC-AntS/Au (a) and NC-AntSe/Au (b) along with their reproduction (red solid line) by the linear combination of the purely resonant (blue dashed line) and ET (black dotted line) contributions. The relative spectral weight (intensity) of the ET contribution gives probability of the ET process, PET. A legend is given; individual features (P, SP1, and SP2) are marked.

The [N 1s]π1* RAES spectra of NC-AntS/Au and NC-AntSe/Au are presented in Figures 6a and 6b, respectively. They exhibit typical features characteristic of benzonitrile-terminated monolayers,24,32 with a dominance of strong spectator contributions (SP1 and SP2), accompanied by a weak participant (P) feature, strongly suppressed in the [N 1s]π1* case.57 The overall spectral shape and the relative intensities of the individual contributions are very similar for both systems studied. A comparison of these spectra with the purely resonant one, measured for the sample where ET is too slow to be recorded within the given RAES-CHC scheme (see Section 2), shows some characteristic differences (see Figure 6), pointing to the existence of a certain ET contribution (see Figure S5 in the Supporting Information). The exact spectral shape of this contribution can be determined by the measurement of the nonresonant AES spectra of NC-AntS/Au and NC-AntSe/Au, since the respective final state (A route in Figure 5) is identical to the final state of the ET process in the case of resonant excitation (ET route in Figure 5). In contrast to the purely resonant spectrum only, a linear combination of this spectrum and ET contribution, with the spectral weights determined


16

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within the fitting procedure, resulted in a reasonable reconstruction of the [N 1s]π1* spectra of NC-AntS/Au and NC-AntSe/Au, as shown in Figure 6. The spectral weight of the ET contributions, PET, was estimated at 13 ± 2% and 14 ± 2% for NC-AntS/Au and NCAntSe/Au, respectively. Based on these values, we calculated the characteristic time for ET from the π1* orbital of the nitrile group to the substrate, through the molecular framework and the docking group, τET. For this purpose, we applied the established equation17 τET = τcore (1-PET)/PET,

(3)

where τcore is the lifetime of inner shell vacancy (~6.4 fs for N 1s)63, known from photoionization experiments. The derived values of τET are 42±8 fs and 40±8 fs for NCAntS/Au and NC-AntSe/Au, respectively. They are almost identical within the accuracy of our experiment and data evaluation procedure (see also Section 4). This accuracy is governed by the uncertainty of τcore and the error of determining PET within the fitting procedure. The preliminary spectra processing, including the background subtraction, correction for photoemission contributions, etc. is well established and verified by previous experiments on a broad variety of other systems.23,24,29-34 In addition, all the procedures were applied to all spectra, both experimental and reference ones, in the same fashion, which minimizes a systematic error. In contrast to the [N 1s]π1* case and in accordance with the expectations, based on the previous results for NC-NapS/Au and NC-NapSe/Au,32 the [N 1s]π3* RAES spectra of NCAntS/Au and NC-AntSe/Au are very similar to the respective pure resonant spectra (see Figure S6 in the Supporting Information), which suggests that τET for the transport from the π3* orbital, localized at the nitrile group, to the substrate is beyond the range resolvable within the applied CHC scheme for the N 1s core hole clock (~ 120 fs)24.

4. Discussion According to the XPS and NEXAFS spectroscopy data, both NC-AntS and NC-AntSe SAMs are well defined and contamination free, apart from a small portion of atomic sulfur in NCAntS/Au and a tiny portion of atomic selenium in NC-AntSe/Au. The parameters of these films are quite similar, as highlighted by the similar values of the effective thickness (a measure of the packing density in the given case), average molecular inclination (a fingerprint of the orientational order), and the values of the average twist angle differing by just 11°. The nitrile groups were found to be located exclusively at the SAM-ambient interface.


17


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Consequently, both NC-AntS and NC-AntSe SAMs are well suitable for RAES experiments in framework of the CHC approach, targeting the ET dynamics across the anthracene backbone and the docking groups, and providing general information regarding electric transport properties of acene-based SAMs. For both NC-AntS/Au and NC-AntSe/Au, ET to the substrate upon the [N 1s]π1* excitation was found to be more efficient than that upon the [N 1s]π3* excitation. This behaviour agrees well with all previous experimental data for benzonitrile-terminated monolayers24,30,32,34 and supports the statement that the efficiency of ET depends strongly on the character of the molecular orbital which mediates the ET process.30,31 The transport starting from the π1* orbital of nitrile, conjugated with the adjacent phenyl ring, is much more efficient than that from the π3* orbital, localized at the nitrile group, so that an additional injection barrier for ET can be assumed. The characteristic ET times for NC-AntS/Au and NC-AntSe/Au were found to be almost identical within the accuracy of our experiments, indicating, in view of the similar structural properties of these monolayers, that electric transport properties of the thiolate and selenolate anchors do not differ significantly. Similar behavior was previously observed for the analogous SAMs with the naphthalene backbone.32 Of course, the contribution of the anchoring to the substrate is small as compared to that of the entire molecular backbone in terms of the ET dynamics.29,30 However, it is nevertheless obvious that the substitution of the thiolate anchor by the selenolate one gives neither noticeable gain nor loss in terms of ET efficiency of the entire monomolecular film, which is presumably also true for the electric transport properties of such a film, in contrast to some previous reports.64-69 The derived τET value for NC-AntS/Au can be combined with the analogous values for nitrilesubstituted thiolate SAMs having the naphthalene32 and phenyl30 backbone, obtained within our previous studies. This combination is reliable due to the persistent and established experimental and evaluation procedures; its validity as well as the accuracy of these procedures have also been verified by the measurements performed at the reference samples, above all NC-(CH2)2-SH SAMs on Au(111), with similar (within 1-2 fs) τET values obtained at three different beamlines at two different synchrotron radiation facilities.23,29,31 The data for the anthracene, naphthalene32 and phenyl30 SAMs are presented in Figure 7, in semilogarithmic fashion. They can be analyzed within the simplified Simmons equation


18

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which is not only applicable to the static electric conductance but, in a modified fashion, to the ET dynamics as well,29 expressed as

τET = τ0·exp(−βETl), (4) where τ0 is characteristic time for ET through the docking moiety and βET is a dynamical attenuation factor. Significantly, the latter parameter for a particular molecular chain is quite close to its static analogue, being e.g. 0.72 Å-1 for alkyl chain29 and 0.27 Å-1 for oligophenyl chain30 in particular (see Section 1 for the respective static attenuation factors). In our opinion, such a similarity cannot be just a coincidence but, most likely, a reflection of the intrinsic properties of molecular wires, probed by the different techniques. It represents a basis for comparison of βET in the given case to its static analogues. 5 NC-NapS & NC-AntS NC-PhS fit NC-NapS & NC-AntS fit incl. NC-PhS

4

lnτET

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


β=0.25 Å-1

3

-1

β=0.35 Å

2 8

9

10

11

12

13

14

Molecular length (Å)

Figure 7. Natural logarithm of the [N 1s]π1* ET time (bottom panel) versus molecular length for nitrile-substituted SAMs with the anthracene (this work; red circle), naphthalene (ref 32; red circle), and phenyl (ref 30; blue circle) backbone. The blue dashed line represents the best linear fit for all experimental data; the red dashed line represents the best fit for the anthracene and naphthalene values only. The derived values of β are shown; they are color-coded in accordance with the respective fits.

Performing a linear fit over all available experimental points in Figure 7, one gets a βET of -1

0.35 Å and a τS-Au of 0.45 fs. These values describe the ET properties of the acene backbone and the efficiency of the thiolate anchors in terms of ET, respectively. A questionable point of this analysis is, however, the inclusion of the phenyl moiety in the acene series which is done frequently13,14 but is not entirely strict in our opinion because of the differences in the electronic structure. Excluding the experimental point for the phenyl backbone, a linear fit -1

over the τET values for NC-AntS/Au and NC-NapS/Au, gives a βET of 0.25 Å and a τS-Au of 1.5 fs (Figure 7). Note that the obtained τS-Au is close to the analogous values obtained


19


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previously in RAES-CHC experiments on alkanethiolate and oligophenylthiolate SAMs on Au(111), viz. 2.3 fs29 and 2.8 fs30. An even more interesting aspect is the similarity of the derived βET to the analogous value obtained in the static conductance experiments on acene dithiol SAMs (0.2 Å-1)14. This value differs significantly from the attenuation factor obtained for the analogous monothiol series (0.5 Å-1).14 Obviously, the efficient coupling of the acene molecules to the top electrode over the S−Au linker, as this occurs for the acene dithiol SAMs, affects not only the contact resistance but the attenuation factor as well, lowering it from 0.5 Å-1 to 0.2 Å-1. Such an efficient coupling takes also place in our RAES experiment, in which the terminal nitrile group is directly addressed by the narrow-band X-ray photons. Consequently, the similarity of the βET derived in the present work for the nitrile-substituted acenethiolate monolayers and the static attenuation factor for the acene dithiol SAMs is understandable. On the other hand, this similarity can be considered as an indirect verification that the coupling of the terminal group has a significant effect on the determined attenuation factor, which is a non-trivial point since the latter parameter should be the property of a molecular wire only and should not depend on the quality of the contact to the electrodes. Obviously, description of molecular junctions by the simplified Simmons equation has its limits and the attenuation factor cannot always be considered as a fully representative parameter for a particular type of a molecular wire (see also a discussion on this issue in ref 70). Considering that both βET and τS-Au values for the analysis based on naphthalenethiolate and anthracenethiolate only look quite reliable, we performed an analogous analysis for the naphthaleneselenolate32 and anthraceneselenolate as well (regretfully, there are no ET dynamics data for benzeneselenolate). The resulting values of βET and τSe-Au are practically identical to those for the thiolate case, viz. 0.25 Å

-1

and 1.5 fs, respectively, additionally

validating βET for acene and directly indicating similar efficiency of the thiolate and selenolate anchors in terms of ET.

5. Conclusions Here we studied the basic structural properties and ET dynamics of the nitrile-substituted anthracenethiolate and anthraceneselenolate monolayers on Au(111). These films were found to be well defined SAMs. The packing density, orientational order, and molecular inclination in these SAMs turned out to be quite similar, exhibiting no distinct dependence on the identity


20

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of the docking group. The only difference was observed in the average molecular twist angle, indicative of somewhat different structural organization at the molecular level. The [N 1s]π1* RAES spectra of NC-AntS/Au and NC-AntSe/Au exhibited a distinct contribution of the ET processes, corresponding to the transfer of the resonantly excited electron at the nitrile group to the substrate, over the molecular framework and the S/Se-Au anchor. The evaluation of these spectra within the CHC approach resulted in τET values of 42±5 fs and 40±5 fs for NC-AntS/Au and NC-AntSe/Au, respectively. Combining these values with the literature data for NC-NapS/Au32 and NC-PT/Au30 and processing them within the simplified Simmons equation, adapted to the ET case, we got a βET of ~0.35 Å-1 for the acene backbone and a τS-Au of ~0.45 fs for the S−Au anchoring. Excluding the data for NC-PT/Au, in view of the different electronic structure, we got βET and τS-Au of ~0.25 Å-1 and ~1.5 fs, respectively. Practically the same values were obtained upon the combination of the τET data for NC-AntSe/Au with NC-NapSe/Au32, with a τSe-Au of ~1.5 fs, directly indicating a similar efficiency of the S−Au and S−Au anchoring in terms of the ET dynamics and, presumably, static electric properties as well. Comparison of the derived βET value for the anthracene and naphthalene SAMs with the analogous static values14 gives a good agreement for the case of anthracene dithiols, verifying the static attenuation factor corresponding to a good coupling to the electrodes and suggesting that it is better representative of the acene moiety. On the other hand, the verification of the static value for a good coupling to the electrodes is, in view of the much larger value for a weak coupling,14 an indication for the importance of coupling to the electrodes for the molecular conductance. This issue has been addressed recently in several publications.71,72 A side result of the given work is the applicability of NC-AntS/Au and NC-AntSe/Au for studies targeting the comparison of the S−Au and S−Au anchoring in SAMs, similar to a recent study on the relative thermal stability of thiolate- and selenolate-bonded aromatic monolayers performed on the basis of NC-NapS/Au and NC-NapSe/Au.73 ■ Associated content: Supporting Information: Synthesis of NC-AntSH, NC-AntSeAc and their intermediates; additional NEXAFS spectroscopy data, additional RAES data. 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];

(A.T.)

E-mail:

[email protected].

Notes The authors declare no competing financial interest.

Acknowledgements We thank Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at Bessy II and A. Nefedov and Ch. Wöll (KIT) for the technical cooperation during the experiments at this facility. We appreciate financial support of the German Research Society (Deutsche Forschungsgemeinschaft; DFG) within the grants ZH 63/14-2 and ZH 63/22-1.


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