Photoinduced Modulation of the Oxidation State 
of Dibenzothiophene S-Oxide Molecules 
on an Insulating Substrate

doi:10.21203/rs.3.rs-4761213/v1

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Photoinduced Modulation of the Oxidation State of Dibenzothiophene S-Oxide Molecules on an Insulating Substrate

https://doi.org/10.21203/rs.3.rs-4761213/v1

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On-surface chemistry aims to overcome the limitations of conventional in-solution synthesis by taking advantage of the confinement in two dimensions to master highly ordered covalent structures with tailored properties. So far, most of the reported work was conducted on metal substrates and relied on unconventional mechanisms, thereby precluding a direct transposition of well-established organic reactions from solutions to surfaces. In addition, the intrinsic properties and reactivity of metal substrates often limit the activation methods available to trigger on-surface reactions, and photoinduced processes are especially difficult to handle due to quenching of the adsorbed precursor molecules. Herein, the photoinduced deoxygenation of dibenzothiophene S-oxide (DBTO) derivatives was transposed from solutions to insulating alkali halide surfaces in ultra-high vacuum. By combining in-solution and on-surface investigations by means of scanning tunneling microscopy, non-contact atomic force microscopy, as well as Bias spectroscopy measurements, we provide evidence of the successful on-surface deoxygenation of individual DBTO derivatives under UV irradiation. The photoinduced deoxygenation is conducted at low temperature (< 25 K) on a NaCl thin film formed on a Au(111) substrate to yield the reduced dibenzothiophene (DBT) product with excellent chemoselectivity. This work thus opens the way to in-situ photocontrolled charge state manipulation in purely organic compounds.

Physical sciences/Chemistry/Surface chemistry/Scanning probe microscopy

Physical sciences/Chemistry/Photochemistry

Physical sciences/Chemistry/Surface chemistry/Surface spectroscopy

The synthetic chemists’ toolbox includes nowadays a large array of chemical reactions, allowing for virtually any structural or functional modification of organic and metal-organic scaffolds, with high levels of efficiency and selectivity. Highly elaborated molecular structures displaying well-defined properties can thus be prepared from elementary building-blocks by multistep synthesis. However, in-solution chemistry techniques are not suited for the synthesis of poorly soluble π-conjugated extended molecular structures and for the controlled growth of 1D- and 2D-covalent networks, which are key to progress in nanotechnology. In this context, on-surface synthesis has witnessed tremendous development over the last two decades as an alternative bottom-up approach to produce new (metal-)organic nanomaterials and extended molecular structures exhibiting tailored structural, chemical, optical, electronic and magnetic properties.^1,2

Following 2007’s Grill, Hecht et al. pioneering work on the formation of covalently bound porphyrinic networks through thermal activation of halogenated tectons,³ numerous surface-confined reactions have been studied, mostly on well-defined crystalline metallic substrates. To date, knowledge has thus been gathered to master covalent assemblies and intramolecular scaffold modulations leading to well-defined nanoobjects of various dimensions and structures.¹ Thermal annealing is typically used to trigger these processes, but electron beam irradiation,⁴ scanning tunneling microscopy (STM) tip manipulations,^4,5 or light irradiation can also be used occasionally.^1,6,7 The choice of the activation method is intimately related to the nature of the underlying surface, which acts as a non-inert support for the 2D-confined reactions, with a major impact on the diffusion, reactivity and optoelectronic properties of the adsorbed species. In particular, in the case of metallic surfaces, the overlap of the precursors’ molecular orbitals with the electronic structure of the metal underneath leads to an active role of the substrate, e.g. as a catalyst in thermally induced covalent couplings and as a provider of photoexcited hot electrons in most photoinduced reactions. Consequently, most of on-surface reactions have been reported to proceed via unconventional mechanisms, and they are thus not a straightforward transposition of well-known in-solution organic chemistry processes from a 3D- to a 2D-environment.

In this work, in-solution and on-surface investigations were combined to demonstrate that conventional chemical transformations of small molecules can indeed be directly transposed from diluted solutions to surfaces through a careful combination of precursor design, activation method and, most importantly, substrate nature. Insulating substrates were selected here as supports for the 2D-confined reaction, since the electronic decoupling of overlying adsorbates preserves their intrinsic electronic and optical molecular characteristics.^8,9 It was thus hypothesized that, on insulators, the chemical reactivity of molecular precursors could be anticipated, based on their behavior in diluted solutions. Moreover, it also becomes possible to probe in-situ the native properties of the newly formed products and to stabilize charged states on such surfaces, in contrast with metallic ones.

In spite of these major assets, synthesis on insulating substrates is still in its infancy, ¹⁰ with only scarce reports to date of successful reactions on bulk insulators such as calcite or alkali halides.^11–13 The key challenges on such substrates lie in the absence of inherent catalytic activity, while possibilities for thermal activation are limited by weak molecule-substrate interactions, resulting in enhanced dewetting and desorption of molecular species. These issues may be overcome by combining a strategic molecular design,¹⁴ in particular with polar functional groups, and light as an energy source for chemical reactions.⁶ In this regard, biarylsulfoxides appear particularly promising, owing to the significant polarity of their S = O sulfinyl group coupled with their well-established photoreactivity in solution. For instance, dibenzothiophene S-oxides (DBTO) are known to undergo a cleavage of the S = O bond under UV-irradiation, thereby releasing atomic triplet oxygen O(³P) and dibenzothiophene (DBT).^15–18 During this reaction, the sulfur atom oxidation state varies from (0) to (-II) and the photodeoxygenation of sulfoxides thus represents a powerful and reagent-free synthetic tool formally allowing to perform the reduction of sulfur.

Herein, we report the direct transposition of an organic reaction, namely the photodeoxygenation, from solutions to insulating alkali halide surfaces under Ultra-High Vacuum (UHV) conditions, to allow for the photocontrolled modulation of the oxidation state of sulfur atoms in-situ. A sulfoxide precursor displaying finely tuned molecular properties was thus designed, synthesized and its photoreactivity in solution was assessed in terms of efficiency and selectivity. In parallel, its adsorption configuration on an alkali halide surface was unveiled by means of Scanning Probe Microscopy (SPM) techniques, as sulfoxides have never been reported so far on such substrates. Finally, deoxygenation of this model compound was successfully achieved on surface upon irradiation, giving rise to the corresponding sulfide product, and the formal in-situ reduction of sulfur was evidenced by bias spectroscopy measurements.

Design, synthesis and characterization of the sulfoxide precursor

The photodeoxygenation of dibenzothiophene S-oxides, leading to the formal reduction of sulfur(0) to sulfur(-II), has been reported in solution on diversely substituted DBTO derivatives.^15–18 In these systems, the dibenzothiophene core exhibits a fully planar geometry, but the pyramidal character of the sulfoxide moiety induces an out-of-plane location of the oxygen atom, thus leading to an overall bent geometry of the molecule. In anticipation of in-depth structural characterization and photoreactions on surface, the design of the DBTO precursor was adjusted to favor its face-on adsorption on weakly interacting insulating substrates.

Compared to metal surfaces on which the adsorption energy of planarly adsorbed aromatic molecules is relatively high due to the interaction between the molecule’s π-electrons and the electrons of the metal substrate, on insulating substrates, the fabrication of molecular layers in a planar adsorption geometry is rather challenging. Prior research on insulating surfaces, both theoretical and experimental, has emphasized the role of polar anchoring groups in the development of such highly ordered self-assemblies.^19–22 Although DBTO molecules natively display a S = O sulfinyl group with significant polarity, it was envisioned to insert two additional electron-withdrawing bromine atoms on symmetrical ortho positions (with respect to the bisarylsulfoxide moiety), with the aim to optimize the adsorption energy on alkali halide substrates. 4,6-Dibromodibenzo[b,d]thiophene 5-oxide ((o-Br)₂-DBTO, Fig. 1 left) was thus synthesized and fully characterized (see Methods section and Supplementray Information file). Its structure was unambiguously confirmed by single-crystal X-ray diffraction, revealing an angle of 117° between the plane of the polycyclic scaffold and the S = O bond (Fig. 1 and S18). In addition, thermogravimetric analysis (TGA) was carried out in anticipation of on-surface physical vapor deposition of this model compound, which highlighted its thermal stability up to ca. 200°C (Fig. S7).

In-solution photoreactivity of (o-Br)-DBTO

The absorption spectrum of (o-Br)₂-DBTO in dichloromethane (DCM) displays a wide band peaking at λ_max = 280 nm (Fig. S8a) and photoreactivity studies were thus performed under UVA-irradiation (280–365 nm). Noteworthy, wavelengths shorter than 280 nm were excluded as this leads to the cleavage of the bromine anchoring groups as a side reaction. On the first hand, the photoreactivity of (o-Br)₂-DBTO was investigated in a dichloromethane solution at room temperature (RT). After 48h of UVA-irradiation, the expected deoxygenated and fully planar compound (o-Br)₂-DBT was obtained as the major product in 53% isolated yield (Fig. 1). Besides, two byproducts incorporating one additional oxygen atom were isolated: the thiophene S,S-dioxide derivative (o-Br)₂-DBTO₂ (8%) and the oxathiine S-oxide derivative Br₂-OTO (21%). The structure of the latter was unambiguously assigned thanks to an X-ray crystal structure, revealing the presence of the oxygen atom embedded in the central six-membered ring (Fig. S19).

From a mechanistic point of view, upon UVA-irradiation, the sulfur(0) precursor (o-Br)₂-DBTO undergoes photodeoxygenation to give rise to the reduced sulfur(-II) product (o-Br)₂-DBT and releases atomic triplet oxygen O(³P) in the reaction medium.¹⁵ The latter is then trapped by remaining (o-Br)₂-DBTO molecules, leading to the formation of both oxidized sulfur(+ II) byproducts (o-Br)₂-DBTO₂ and Br₂-OTO. Consequently, in solution, the efficiency of the photodeoxygenation reaction is hampered by intermolecular oxygen transfers, which inherently limit the selectivity of this process and the control of the resulting sulfur oxidation state.

Once the model compound designed, synthesized and its in-solution photoreactivity assessed, the next step was the investigation of (o-Br)₂-DBTO sulfoxides on surface. Surprisingly, SPM studies involving sulfinyl derivatives on surface are scarce, mainly involving dimethylsulfoxide (DMSO) and tetramethylenesulfoxide on metallic substrates.^24,25 A recent investigation also examined the impact of the molecular configurations of sulfonyl chloride compounds on desulfonylation reactions occurring on Au(111) and Ag(111) surfaces.²⁶ To the best of our knowledge, sulfoxides have never been investigated by SPM on insulating substrates.

NaCl(100) thin films/ Au(111) substrate

Beyond the molecule's structure and the presence of polar anchoring groups, the adsorption of precursors on alkali halide substrates is critically influenced by the crystal's ionic structure.^{20,22,27–29} Indeed, matching the molecular network, i.e. the position of its electronegative anchoring groups, with the cationic lattice is crucial to control the self-assembly on ionic substrates. Therefore, the alkali halide surface NaCl(100) was chosen, since it features a cation distance (nearest neighbor: d_Na−Na = 0.4 nm) matching quite well the Br − O distance (d_Br−O = 0.36 nm) and, to a less extent, the Br − Br distance (d_Br−Br = 0.66 nm) of the functional groups in the (o-Br)₂-DBTO molecular tecton. Satisfyingly, upon deposition on bulk NaCl(100) at room temperature, (o-Br)₂-DBTO molecules adsorbed in a planar geometry and formed stable monolayers (ML), with each polar functional group potentially located on a cationic site (Fig. 3c).

In this work, the main challenge lies in establishing the proof of a photocontrolled modulation of the oxidation state of sulfur-based molecules by high-resolution Scanning Probe Microscopy, using functionalized CO-tips to image the planarly adsorbed molecules.³⁰ Since our aim is to characterize both structural (oxygen loss) and electronic modifications (change in sulfur oxidation state) upon irradiation, ultrathin insulating layers were deposited on a metal substrate to allow us to conduct STM measurements in combination with non-contact-Atomic Force Microscopy (nc-AFM) measurements (hereafter referred to as “dynamic STM”, see Methods section). Indeed, whereas STM cannot be performed on bulk insulating crystals, electron tunneling is transparent through ultrathin layers of wide-bandgap insulators (one to three monolayers), such as oxides or alkali halides, deposited on metal surfaces.^31,32 Additionally, it has been demonstrated that in the case of ultrathin NaCl films, one double layer already exhibits the distinctive electronic structure of its bulk form,^33,34 meaning that the NaCl layers on metals are thin enough to allow electrons to tunnel while preserving the optical properties of the adsorbed molecules.

NaCl islands have been successfully grown on a wide range of crystalline metal surfaces, including Ge(100),³¹ Al(111),^32,35 Al(100),^32,35 Cu(111),³⁴ Au(111),^36,37 Ag(100),³⁸ Ag(001)³⁹ and Ag(110).⁴⁰ According to these experimental investigations and numerous theoretical studies, NaCl forms (100) oriented layers on these substrates. In the frame of this study, NaCl thin layers were deposited on gold surface Au(111) (see Methods section). As expected, STM images indicate that NaCl grows as (100) layers on the reconstructed (111) gold terraces. A high-resolution STM image of the NaCl unit cell (Fig. S2c) shows that its lattice constant of 5.65 Å is consistent with that of the NaCl bulk crystal. Larger-scale STM images (Fig. S2a) reveal a carpetlike growth of the initial NaCl(100) layer, with the formation of the second and third NaCl layers on the first one before the latter entirely covers the Au surface.

The NaCl layers on Au(111) form an ideal substrate for the present study, since they display an atomically clean surface on which i) the adsorption of the (o-Br)₂-DBTO precursor molecules is expected to be in a planar geometry, ii) such molecules will be electronically decoupled from the metal substrate, thus preserving their intrinsic photophysical properties and allowing UV-induced deoxygenation, and iii) high-resolution SPM imaging with functionalized CO-tips is possible to observe the expected chemical modification of the molecules and characterize the change in the sulfur atom oxidation state.

Description of the supramolecular phase

A typical STM image following the deposition of 0.1 ML of (o-Br)₂-DBTO onto a thin layer of NaCl(100) is displayed in Fig. 2a. It appears that the molecular islands, denoted by the white arrows, are substantially longer than they are wide, suggesting that in-line interactions are stronger than lateral ones. Additionally, the image shows that the molecules preferentially nucleate in the NaCl's polar directions [110] and [\(\:\stackrel{-}{1}\)10]. A close inspection on a zoomed STM image (Fig. 2b) revealed that the molecules self-assemble into parallel stripes, consisting of oppositely oriented molecular rows. The inter-rows distance is ∼12 Å, whereas the distance between two consecutive molecules within the same row is ∼8 Å. These values are coherent with a (6a × 2a) rectangular unit cell consisting of 2 molecules, where parameter a = 4 Å represents the distance between the nearest cations in a NaCl crystal. Figure 3b presents a structural model for this self-assembly, which illustrates a point-on-point adsorption, highlighting the match between d_Na−Na and d_Br−Br, as anticipated. Within a molecular row, (o-Br)₂-DBTO molecules adsorb every second line of cations, while the electronegative atoms (both bromine atoms and the oxygen of the sulfinyl group, see below) match the cationic sites. We may therefore conclude that the growth of these lines is driven by the adsorption of the anchoring groups matching the periodicity of the substrate and stabilized by van der Waals (vdW) interactions. Residual electrostatic interactions are optimized by alternating the molecular orientation in neighboring lines while maintaining a rectangular unit cell. As evidenced by the work of Neff et al. for one dimensional lines of vertically adsorbed molecules,⁴¹ the parallel aligned dipole moments are sufficiently large to produce a remarkable long-range repulsion between molecular lines on an insulating substrate. In the present case, the molecules are planarly adsorbed with their dipole moment essentially aligned parallel to the surface (Fig. S5). We assume that the oppositely oriented lines of molecules are a result of maximized (attractive) electrostatic interactions between the intrinsic dipoles of the molecules.

Finally, the adsorption configuration of each single molecule within the supramolecular assembly was investigated, as the pyramidal shape of the sulfinyl group (Fig. 1, bottom left) leads to two scenarios for (o-Br)₂-DBTO on the surface. Indeed, the molecule may adsorb with its polycyclic scaffold parallel to the surface and the out-of-plane oxygen atom pointing upwards, or towards the surface. To unambiguously determine the configuration of (o-Br)₂-DBTO on NaCl thin layers, the Probe Particle Model (PPM), a numerical AFM model that simulates high-resolution non-contact AFM images with CO-functionalized tips,^42,43 was used. Simple probe particle approaches combined with experimental measurements including dibenzothiophene have already been shown to be effective in providing rapid structural assignment.⁴⁴

High-resolution nc-AFM images of (o-Br)₂-DBTO issued from experimental observations with a CO-functionalized tip (Fig. S3b) were compared with those obtained from PPM simulations (Fig. S1b,c). The calculated images show that an oxygen-up configuration (Fig. S1c) would result in a very distinct contrast compared to a molecule with its oxygen facing the surface (Fig. S1b). Based on experimental images (Fig. S3b), which display a weak contrast in the central part of the molecules, it was thus inferred that (o-Br)₂-DBTO is adsorbed with the oxygen pointing towards the substrate. This can occur through the favorable interaction between the oxygen and the cations of the NaCl thin layer (Fig. 3c).¹¹ Importantly, in anticipation of photoreactivity studies, PPM-simulated images show very limited differences in the imaging contrast between a (o-Br)₂-DBTO molecule with the oxygen pointing down (Fig. S1b) and a deoxygenated one, i.e. (o-Br)₂-DBT (Fig. S1a). It is also worth noting that in these simulations, the carbon scaffold of all molecules was kept parallel to the substrate surface. In experimental conditions, this is expected for the planar C_2v symmetric (o-Br)₂-DBT molecule, whereas the adsorption of the bent (o-Br)₂-DBTO sulfoxide derivative with the oxygen facing the surface is likely to be slightly tilted with respect to the surface. This induces a modification of the molecular contrast, allowing a clear distinction between the pristine (o-Br)₂-DBTO and the deoxygenated (o-Br)₂-DBT molecules (see below).

On-surface photoinduced deoxygenation

Once the self-assembly of (o-Br)₂-DBTO molecules thoroughly characterized, their on-surface photoreactivity was explored by both, STM imaging and Local Contact Potential Difference (LCPD) measurements, as deduced from Bias spectroscopy (see below). The surface was thus exposed to a 280 nm wavelength irradiation, in line with in-solution photochemistry experiments. This was performed by using a light emitting diode (LED) and irradiating the sample inside the SPM head (see Methods section).

Following illumination, STM imaging in gentle conditions indicate that the molecules now exhibit two distinct contrasts, which is indicative of on-surface deoxygenation. Figure 3a displays a molecular island after 190 minutes of light exposure. The sulfur atoms of (o-Br)₂-DBTO molecules are spot on the bright circle-shaped protrusions. These were less visible in Fig. 2b owing to smoother tip imaging conditions. Here, the molecules exhibit a “croissant-like” shape and a “butterfly-like” shape (see corresponding grey forms in the Fig. 3a.). To assign a particular contrast to the pristine sulfoxide, or to the deoxygenated compound, we performed high-resolution dynamic STM imaging of the molecular assembly using a CO-functionalized tip (Fig. S3a). In that figure, the deoxygenated (o-Br)₂-DBT molecules are identified with white frames. The associated frequency shift map (Fig. S3b) shows only a minor change between (o-Br)₂-DBTO and (o-Br)₂-DBT molecules, as already expected from the series of PPM simulations (Fig. S1a,b). A slight modification of the contrast is noticeable: the tip-molecule interaction is no longer as repulsive above the sulfur atom. As explained earlier, this is rather an effect of a modified adsorption geometry of the now planar, deoxygenated (o-Br)₂-DBT molecule compared to its tilted precursor, than a direct visualization of the missing oxygen atom. Therefore, these observations allow us to unambiguously assign in Fig. 3a the “croissant-like” shaped molecules to the pristine (o-Br)₂-DBTO sulfoxides and the “butterfly-like” shaped ones to the (o-Br)₂-DBT deoxygenated sulfoxides.

A statistical analysis of that sample presented in Fig. S4, involves 260 molecules and reveals that approximately 36% of them underwent the deoxygenation.

To complete this structural characterization and analyze the charge state of sulfur atoms before and after deoxygenation, individual molecules were selected among a statistical set of pristine/deoxygenated species. A series of Δf vs. bias voltage curves (∆f(V), so-called Bias spectroscopy curves) were performed on top of the sulfur atom of each of these molecules. Bias spectroscopy curves are known to exhibit a typical parabolic shape, whose maximum (Δf*, V*) is a measurement of the Local Contact Potential Difference (LCPD), a quantity that can be connected to the local work function difference between the tip and the surface. When the bias voltage matches V*, the local electrostatic forces occurring between the tip and the surface are canceled out. Any charge state manipulation of an atom on the surface (tip manipulation, bias pulses, light…) induces a change of its LCPD, which can therefore be tracked by bias spectroscopy. This was demonstrated in 2009 by L. Gross et al.⁴⁵ using a qPlus operated at low temperature, which is like our experimental conditions. They have shown that a negatively charged single Au atom adsorbed on an ultrathin NaCl film on Cu(111) featured a larger LCPD than its neutral counterpart.

Before drawing any conclusions from our bias spectroscopy study, the charge distributions of the pristine and deoxygenated molecules must be compared. To this end, DFT calculations with Natural Bond Orbital (NBO) analysis were undertaken (see Methods section and Supplementary Information file), showing that the sulfur atom in the deoxygenated (o-Br)₂-DBT molecule displays a higher density of negative charge than its pristine counterpart (Fig. S5). Therefore, as stated by Gross et al., we anticipate that, upon further measurement, the experimental spectra corresponding to the deoxygenated molecules will shift to more positive values when compared to those of the pristine (o-Br)₂-DBTO molecules.

Figure 4a shows one of the molecular matrices on which the bias spectroscopy was performed. Three horizontal rows, each containing four molecules, are visible. Two bias spectroscopy curves were conducted on each molecule, with a total of eight measurements per line. The spectroscopy curves corresponding to the four molecules in the white frame are presented in Fig. 4b (red • and blue x correspond to the croissant-like pristine (o-Br)₂-DBTO and butterfly-like deoxygenated (o-Br)₂-DBT molecules, respectively). Experimentally, the tip was placed above a sulfur atom and a ∆f(V) curve was measured for each molecule. STM images were taken before and after each series of four curves to confirm that the molecules did not change position. Next, a parabolic fit was performed on the gathered data. In light of this, a global examination reveals that the bias spectra of the deoxygenated (o-Br)₂-DBT molecules (blue x) are consistently shifted to the right with respect to those of the pristine (o-Br)₂-DBTO molecules (red •). The average LCPD shift of S with respect to S = O is found to be (0.119 ± 0.045) V (see Fig. S6 for detailed explanations and a full bias spectroscopy study of 24 molecules). Bias spectroscopy experiments then confirm that the charge state of the sulfur atom is directly impacted by its chemical environment, and more particularly by the presence of the electronegative oxygen atom, with the sulfur displaying a higher density of negative charge in the deoxygenated (o-Br)₂-DBT molecules. Most importantly, owing to the decoupling character of the NaCl thin layer, such difference in the charge state of sulfur atoms in parent dibenzothiophene derivatives such as (o-Br)₂-DBT and (o-Br)₂-DBTO can be directly probed on surface, which opens the way to in-situ charge state manipulation in purely organic compounds. In the present case, the photoinduced modulation of the charge state of a sulfur atom was successfully achieved on surface.

Finally, it is important to note that the photoreactivity of (o-Br)₂-DBTO sulfoxide on surface features excellent chemoselectivity, since oxidized byproducts (o-Br)₂-DBTO₂ and Br₂-OTO, resulting from intermolecular oxygen transfers, were never observed. This is a striking difference with the in-solution reaction, which furnishes large amounts of byproducts, and encourages the transposition of further photochemical reactions from solutions to surfaces.

We have presented a comprehensive on-surface study of a specially designed dibenzothiophene S-oxide (DBTO) molecule adsorbed on an ionic substrate, including the unprecedented characterization of this sulfoxide’s adsorption configuration and photoreactivity on such surfaces. Polar anchoring groups were strategically added to the molecular scaffold to match up to the substrate’s cationic lattice, resulting in planar adsorption and the formation of an ordered molecular self-assembly arrangement with oppositely oriented rows. Since the adsorbed molecules interact very weakly with both, the underlying ionic substrate and the neighboring species, the (o-Br)₂-DBTO model compound appeared as the ideal candidate to directly transpose the photoinduced deoxygenation process from solutions to surfaces. Using UV-light with similar energy, both in-solution and on-surface reactions induced a modulation of the oxidation state of the sulfur atom in (o-Br)₂-DBTO, although with different levels of efficiency and chemoselectivity. Indeed, the on-surface reaction resulted in a successful photoinduced deoxygenation of (o-Br)₂-DBTO on crystalline NaCl thin films on Au(111) at low temperature (< 25 K), yielding selectively the reduced (o-Br)₂-DBT sulfide. The first indication of the deoxygenated molecules was a slight, noticeable structural alteration seen in the topographical imaging. Local charges were then measured above the sulfur atom to further prove the deoxygenation, and these measurements depicted a shift towards a higher density of negative charge, in agreement with theoretical calculations.

Our work thus shows that by properly choosing both, the substrate and the precursor molecules, conventional photochemical reactions can directly be transposed from solutions to surfaces. In addition, the successful modulation of sulfur oxidation state upon on-surface photodeoxygenation opens the way to in-situ photocontrolled charge state manipulation in purely organic compounds.

Design of the molecules

All required information related to the chemical properties of the (o-Br)₂-DBTO molecules, including molecule synthesis, photoreactivity in solution, NMR spectra and crystallographic data are gathered in the Supplementary Information file.

Sample preparation & UV-irradiation conditions

The samples were prepared in-situ in a preparation chamber with a base pressure p \(\:\le\:\) 10⁻⁹ mbar. Au(111) sample was cleaned through several sputtering (Ar⁺, 1 kV, 1–2 µA) and annealing cycles (∼800 K). Molecules were deposited on the surface using a commercial Kentax UHV-evaporator⁴⁶ that features three independent and separately controllable evaporation cells (quartz crucibles) and comes with an integrated shutter and a water-cooling system. (o-Br)₂-DBTO adsorption on the surface was performed by sublimation during 20 min at 394 K while maintaining the sample at RT. A quartz balance was used to calibrate the flux of molecules before deposition on the surface. Before the deposition of the molecules, fragments of a bulk NaCl(111) sample were loaded in a second crucible of the Kentax and sublimed at a rate of ∼0.03 ML.min⁻¹ and a temperature of 730 K onto the Au(111) sample held at 373 K yielding a coverage with large NaCl islands showing a thickness up to three monolayers on the Au(111) surface.

( o -Br) ₂ -DBTO molecules adsorbed on the as-prepared sample were exposed to a 280 nm wavelength irradiation using a LED of 15 W/m² intensity. This was performed by irradiation of the sample inside the SPM head. The light spot is estimated to have a diameter of ∼7mm. During the entire process, the sample was maintained in scanning position at low temperature (∼25 K). Three irradiation durations have been used: 10 minutes, 70 minutes, and 190 minutes. Surface investigations and scanning probe measurements were performed after each irradiation process, along with a statistical analysis to support the qualitative findings derived from STM and nc-AFM images.

STM and nc-AFM experiments

The studies were conducted using a commercial nc-AFM/STM (LT-SPM Infinity, from Scienta Omicron) operated by a Nanonis Control System (SPECS MIMEA control unit). The measurements were performed under ultrahigh vacuum conditions (p \(\:\le\:\) 10⁻¹⁰ mbar) at low temperature (9 K), using a qPlus sensor (oscillation amplitude A_min ~ 50 pm, resonance frequency ~ 23 kHz, quality factor Q ~ 50 000). Experimental images are either acquired in regular STM mode, i.e. while the nc-AFM mode is disengaged (no tip oscillation), or with the nc-AFM mode engaged. In the latter case, we talk about “dynamic STM” and the tunneling current regulation setpoint is then indicated as a mean tunneling current (due to the tip oscillation cycles) <I_t>. When the nc-AFM mode is engaged, topography (< I_t> regulated) and frequency shift (Δf) images are acquired simultaneously.

Experimental images were processed with the WSxM software⁴⁷.

Computational methods

Density functional theory (DFT) calculations at the gas phase were carried out using B3LYP⁴⁸ hybrid functional and 6–311 + + G(2d,p) basis set implemented in Gaussian 16 software package.⁴⁹ The charge distribution was analyzed using the natural bond orbital method (NBO 7.0).⁵⁰

Acknowledgements

This work was supported by the Centre National de la Recherche Scientifique (CNRS), Aix-Marseille Université and the Université de Toulouse (UPS). We thank the ANR funding agency for financial support of the CROSS project (ANR-21-CE09-002) and the Light4Net project (ANR-21-CE09-0004). We thank Ms Iulia Cretoiu for her contribution to the synthesis of a sample of (o-Br)₂-DBTO, Franck Para (IM2NP) for experimental support and the services of the “Institut de Chimie de Toulouse” (ICT-UAR 2599) for their technical assistance.

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Photoinduced Modulation of the Oxidation State of Dibenzothiophene S-Oxide Molecules on an Insulating Substrate

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Abstract

Figures

Introduction

Results & Discussion

Design, synthesis and characterization of the sulfoxide precursor

In-solution photoreactivity of (o-Br)-DBTO

NaCl(100) thin films/ Au(111) substrate

Description of the supramolecular phase

On-surface photoinduced deoxygenation

Conclusion

Methods

Design of the molecules

Sample preparation & UV-irradiation conditions

STM and nc-AFM experiments

Computational methods

Declarations

Acknowledgements

References

Additional Declarations

Supplementary Files

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