It is demonstrated that halogen bonding can be used to construct low-molecular-weight supramolecular complexes with unique light-responsive properties. In particular, halogen bonding drives the formation of a photoresponsive liquid-crystalline complex between a non-mesogenic halogen bond-donor molecule incorporating an azo group, and a non-mesogenic alkoxystilbazole moiety, acting as a halogen bond-acceptor. Upon irradiation with polarized light, the complex exhibits a high degree of photoinduced anisotropy (order parameter of molecular alignment > 0.5). Moreover, efficient photoinduced surface-relief-grating (SRG) formation occurs upon irradiation with a light interference pattern, with a surface-modulation depth 2.4 times the initial film thickness. This is the first report on a halogen-bonded photoresponsive low-molecular-weight complex, which furthermore combines a high degree of photoalignment and extremely efficient SRG formation in a unique way. This study highlights the potential of halogen bonding as a new tool for the rational design of high-performance photoresponsive suprastructures.
The design of photocontrollable and switchable suprastructures is an important topic in modern materials science.1 A non-exhaustive list of achievements in the field includes, for example, optical alignment and patterning of block copolymer nanodomains,2 photoinduced isothermal phase transitions in macrocyclic compounds,3 and light-induced control over the self-assembly of supramolecular nanostructures.4 Central to the practical realization of light-responsive supramolecular complexes5 is the incorporation of photoactive moieties, which can be obtained by means of self-assembly processes driven by complementary hydrogen bonding or ionic interactions between the constituent compounds.6 Recently, we have introduced a novel design tool, halogen bonding (XB), for realizing high-performance photoactive azobenzene-containing supramolecular polymers.7 Halogen bonding refers to any non-covalent interaction involving halogen atoms as an electrophilic species.8 What makes halogen bonding particularly attractive as a design tool for photoresponsive materials is its high directionality,9 which can enhance the optical performance,7 and the ability to tune the interaction strength between the building blocks via single halogen atom mutation at the binding site. Herein, we report on the first example of a photoresponsive liquid-crystalline (LC) complex assembled by means of halogen bonding. The complex exhibits both efficient photoalignment and exceptional photoinduced mass migration, which is a unique combination among the low-molecular weight (low-Mw) photoactive materials reported to date.
Azobenzene derivatives, due to their large structural change upon photoisomerization, are versatile photochromic units that are considered as the ultimate photoswitches for many applications.1a, 10 Indeed, azobenzenes can act as “master” molecules that control, through collaborative movements, the orientation of neighboring “slave” molecules.11 This property makes azobenzenes highly attractive for photoalignment control (either when doped into a material system of interest or through so called “command surfaces”11d also see Gelbaor et al.11e as an example of inorganic photoalignment layer) and for the design of rewritable photoresponsive optical elements.11, 12 The potential of azobenzene-based materials is further boosted by their unique photomechanical response,13 the most-pronounced examples of which are photoinduced three-dimensional motion and mass transport. The latter takes place upon irradiating azobenzene thin films with light bearing an intensity/polarization gradient: for instance, a sinusoidal interference pattern at the sample surface can lead to a sinusoidal surface corrugation, often referred to as a surface-relief grating (SRG).14 Due to simple, one-step fabrication process, and pronounced surface deformation, the surface mass-transport phenomenon has high potential for applications in, for example, photochemical imaging, nano- and microfabrication, and as diffractive optical elements.15 It is noteworthy that efficient photoalignment and SRG formation usually occur in very distinct molecular environments, the former being boosted by collaborative effects and the existence of LC phases.16 Conversely, SRG formation can be severely suppressed by LC behavior, particularly at high spatial frequencies,17 even if detailed structure–performance relations of the process are not completely understood.
Majority of azobenzene-based photoalignment and SRG materials studied to date are polymeric, due to their desirable film-forming and mechanical properties. However, SRG formation is a highly complex process involving the motions of both light-responsive units and polymer chains. Hence, issues related to polydispersity and polymer-chain entanglement may compromise the efficiency and reproducibility of the mass-migration process, rendering the grating formation efficiency strongly dependent on, for example, the molecular weight of the polymer.18 In 2002, Nakano et al. demonstrated that amorphous low-Mw photochromic materials can also undergo photoinduced mass transport, and soon after they showed that the SRG formation in such molecular glasses can, in fact, be more efficient than in corresponding polymeric materials.19 Later on, the SRG formation in amorphous low-Mw glasses has been studied by several other research groups, with the particular aim of understanding the structure–performance relationships governing this complicated photomechanical process,20 and photoinduced SRG formation on a single crystal has also been reported.21 At the same time, photoactive low-Mw glasses were investigated in a number of other applications,22 but being amorphous, they are not suitable candidates for efficient photoalignment. The best low-Mw materials for photoalignment have been reported by Zakrevskyy et al. who have shown that ionic self-assembled LC complexes can exhibit exceptionally high values of photoinduced anisotropy with dichroic ratios as high as 50.23 However, no SRG formation occurred in their complexes. On a somewhat different approach, also azo-containing ionic LC dendrimers have been reported to yield a high photoinduced anisotropy.24
Herein, we show that halogen-bonded low-Mw LC complexes can exhibit both high degree of photoinduced anisotropy upon irradiation with polarized light (promoted by liquid crystallinity)16, 23 and efficient photoinduced mass transport upon irradiation with an interference pattern (promoted by the directionality of halogen bonding).7 The strategy we adopted was to synthesize a halogen-bond-donor molecule incorporating an azo group and assemble it with an alkoxystilbazole, which is a known promesogenic molecule25 and has proven particularly reliable for the construction of new halogen-bonded, supramolecular mesogens.26 This design strategy allowed us to obtain an order parameter of molecular alignment exceeding 0.5 upon excitation with linearly polarized light, as well as to inscribe an SRG with surface-modulation depth exceeding the initial sample thickness by a factor of 2.4.
2. Results and Discussion
2.1. Supramolecular Complex Design and Structural Characterization
The halogen-bonded complex 3 was prepared by self-assembly of the N,N-dimethyl-4-[(2,3,5,6-tetrafluoro-4-iodophenyl)diazenyl]aniline (1) and 4-[(E)-2-(4-methoxyphenyl)-ethenyl]-pyridine (2), as shown in Scheme1. Generally, the number of carbon atoms (n) in the flexible alkoxy chain of the stilbazole module affects the type of the given mesophase: Nematic phases are seen for complexes with n = 1–5, while longer alkoxy chains (n = 6–l0) promote smectic phases.25a Since long alkyl chains have been reported to suppress the formation of SRGs in LC polymers,17b, 17c we decided to use the methoxy-substituted stilbazole (2). The structure of the azobenzene compound is optimized for efficient SRG formation, due to the presence of the dimethylamino group that promotes efficient trans–cis–trans cycling upon photoirradiation.17d, 27 Moreover, the iodine atom on the tetrafluorobenzene ring is a very good halogen bond-donor site and it is possible to foresee the formation of a quite strong interaction with the pyridyl moiety of the stilbazole module.7 Therefore, the rod-like structure of the dimeric complex, driven by the highly directional halogen bond, is anticipated to promote liquid crystallinity, and as a result photoalignment,16, 23 while the lack of long flexible chains and the presence of the dimethylamino group would promote SRG formation.
To investigate the supramolecular organization of the halogen-bonded complex in the crystal lattice, single crystals of 3 were grown by slow isothermal evaporation of an equimolar solution of the starting components in tetrahydrofuran (THF). As expected, halogen bonding between the pyridyl nitrogen atom and the iodine atom of the azobenzene module is the structure-determining factor that drives the self-assembly of the dimeric complex between 1 and 2 (Figure1). The N···I distance is 2.860(2) Å, corresponding to a ca. 19% contraction of the sum of the van der Waals radii for N and I.28 The C–I···N angle is 173.64(7)°, which is perfectly consistent with the expected high directionality of halogen bonding. The halogen-bonded dimers self-assemble head-to-tail into highly undulated polar chains as a consequence of a much weaker hydrogen bond involving the methoxyl oxygen atom and a methyl hydrogen of the dimethylanilino group (C21met···O1 distance: 3.316(3) Å). In the crystal lattice these polar chains stack in an antiparallel manner through π–π interactions between the tetrafluorophenyl ring and the methoxybenzene ring (the distance between benzene centroids (C1-C6)···(C22-C27) is about 3.57 Å (Supporting Information, Figure S1)) and residual crystal-packing interactions (mainly H···F contacts), resulting in an overall centrosymmetric structure.
In order to verify that the 1:1 ratio of the starting compounds observed in the single crystal was also representative of the entire bulk sample, we performed 1H NMR spectroscopy experiments. By calculating the ratio between the integral of the –N(CH3)2 signal of the azobenzene compound at 3.15 ppm and the –OCH3 signal of the stilbazole at 3.85 ppm, we were able to confirm the 1:1 bulk stoichiometry of the azobenzene-stilbazole complex. Moreover, powder X-ray diffraction analysis (Supporting Information, Figure S2) confirmed that the crystalline bulk sample had the same crystal structure as determined on one single crystal of 3.
The halogen-bond-driven complex formation was also verified using IR spectroscopy. It is well-established that halogen bonding between pyridine and iodo-perfluoroarene moieties leads to a blue-shift and intensity decrease in the pyridine bands in the region 3000–3100 cm−1 and a red-shift of the bands associated with the tetrafluorophenyl ring.29 This is clearly evident in the infrared spectrum of the complex 3 (see Table1). The νC–H absorption of the pure stilbazole at 3021 cm−1 becomes less intense upon complexation and shifted to 3025 cm−1 as a result of a higher positive charge on the pyridyl hydrogens in the complex. A blue-shift is also observed for the four bands associated with the pyridine ring breathing vibrations at 1585, 1572, 1507, and 1415 cm−1. Similar shifts have been observed in hydrogen-bonded adducts and attributed to increased stiffness of the pyridine ring when it behaves as a hydrogen bond-acceptor.30 Similar increased stiffness can also be anticipated for pyridine rings working as halogen-bond acceptors. On the other hand, due to an increased electron density of the fluorinated ring upon XB formation, the vibrations related to the fluorophenyl moiety at 1476, 940, and 806 cm−1 for the p-iodo-tetrafluorophenyl ring in pure 1 are red-shifted to 1472, 933, and 799 cm−1, respectively, upon complex formation.
Table 1. Selected FTIR spectroscopy absorptions (cm−1) of 1, 2, and their halogen-bonded complex 3.
Pyridine νC–H stretching
Pyridyl ring breathing
Pyridyl ring breathing
Pyridyl ring breathing
Pyridyl ring breathing
Fluorophenyl νC–F stretching
Fluorophenyl νC–F bending
Fluorophenyl νC–F bending
The liquid-crystalline properties of the complex 3 were examined by hot-stage polarized optical microscopy (POM). Despite the non-mesogenic nature of the starting materials, the halogen-bonded complex 3 showed a nematic LC phase. On heating, the complex melted directly to the isotropic liquid at 423 K. Upon cooling from the isotropic phase, a monotropic nematic phase was observed (Figure2a) with an isotropic-to-nematic (I–N) phase-transition temperature of 404.8 K (cooling rate: 5 K min−1; mesophase temperature range: 18.2 K). The thermal behavior was reproducible, even after several excursions into the isotropic phase.
2.2. Thin-Film Characterization and Phoresponsive Behavior
In order to study the photoresponsive behavior of the halogen-bonded complex 3, we prepared non-annealed spin-coated thin films (thickness ca. 250 nm) on glass substrates. The films were crystalline at room temperature, and exhibited relatively high optical scattering. Their average surface roughness was approximately 30 nm, as determined by atomic-force microscopy (AFM). Figure 2b displays a POM image of the spin-coated crystalline thin film. The texture shown is reproducible, and the films are stable at ambient conditions for several months. The occurrence of halogen bonding in spin-cast thin films of 3 was verified by X-ray photoelectron (XPS) and IR spectroscopy. XPS is a powerful tool for studying halogen bonding in supramolecular complexes as the I3d binding energies are sensitive to non-covalent interactions and shift to slightly lower energies upon complexation.31 The spectra of the I3d doublet are shown in Figure 2d. For the starting azobenzene molecule, the doublet binding energies are 620.94 eV/632.33 eV; for the spin-coated thin film of the azobenzene–stilbazole complex these values are 620.81 eV/632.26 eV. These shifts are small but repeatable, indicating that halogen-bonded species exist also in the spin-coated films. This was further confirmed by IR analysis: The thin films exhibited blue-shifts for several bands of the stilbazole moiety and corresponding red-shifts for the bands attributable to the fluorinated aromatic ring, identical to the bulk samples, as reported in the Table 1.
Even a short irradiation with a 488 nm laser beam (30 s, 100 mW cm−2, circular polarization; the wavelength was chosen to induce both trans–cis and cis–trans photoisomerization of the azobenzene) transforms the pristine crystalline film (Figure 2b) into an amorphous film. As a consequence, the film appears as opaque when imaged between crossed polarizers (Figure 2c), that is to say, it becomes macroscopically isotropic. Such photoinduced isotropization results in various changes in the absorption spectrum of the complex (Figure 2e). The gradual decrease in absorbance is an indication of the trans–cis photoisomerization of the azobenzene moiety. The low-energy shoulder in the absorption spectrum becomes less pronounced, most likely due to changes in the molecular packing upon photoirradiation, and the optical scattering, seen as a relatively large extinction in the NIR-region, decreases significantly. Equivalently, the reduced scattering can be seen as increased transmittance of a non-resonant (633 nm) He–Ne probe beam through the sample (Figure 2e, inset), based on which we can observe that 30 s irradiation suffices to reach saturation, that is to say, to break the crystal structure. Due to the reduced scattering, the irradiated isotropic films were used as a basis for further optical measurements, although similar results were also obtained using non-irradiated pristine crystalline films. Importantly, the irradiated isotropic film displayed I3d binding energies identical to those seen in the pristine crystalline sample, confirming that irradiation does not disrupt the halogen bonding within the film (see Figure 2d).
Upon irradiating the spin-coated thin films with linearly polarized light (488 nm) at room temperature, they became highly anisotropic (Figure3). Based on the example spectra shown in Figure 3a, 2 min irradiation at moderate intensity (100 mW cm−2) leads to a 49% decrease in the absorbance parallel to the polarization direction of the excitation beam, and a 38% increase in the absorbance in the perpendicular direction. This result is a clear evidence of a photoinduced reorientation of the supramolecular complex 3. Prior to irradiation, the molecular alignment is random, but excitation with linearly polarized light causes the molecules to align preferentially in the direction perpendicular to the polarization plane, which is seen as an increase in A⟂.23, 32 Based on the polarized absorption spectra, the order parameter of molecular alignment can be determined as S = (A⟂ - A//)/(A⟂ + 2A//), which is plotted at different time instances in Figure 3b. If the irradiation intensity is increased, the reorientation dynamics is speeded up, and for 300 mW cm−2 irradiation intensity the order parameter of molecular alignment exceeds 0.5 already after 2 min of irradiation. An order parameter of 0.5 is quite typical for nematic liquid crystals and sufficiently high for many purposes: for instance, typical nematic liquid-crystal displays have an order parameter of 0.4.33 The anisotropic alignment order remains stable over at least a period of several months at ambient conditions. However, unlike for many azobenzene-containing polymers,11a,d the alignment order could not be satisfactorily randomized with circularly polarized light even if slow decrease in order parameter upon circularly polarized irradiation was observed.
The efficient photoalignment of the low-Mw supramolecular complex 3 can be attributed to its high-temperature liquid crystallinity,16c,d even if further investigations are required to comprehensively understand the connection between the molecular-level properties, structural properties, and the optical response. In spite of the fact that the photoalignment experiments are carried out at room temperature, the LC nature, which enhances the molecular cooperative motions, allows both the acceleration and stabilization of the photoinduced anisotropic molecular alignment. This was recently demonstrated by Schmidt and co-workers using low-Mw tris-azobenzene derivatives.16c,d Moreover, even if the dichroic ratio of 50 reported by Faul and co-workers in their self-assembled ionic LCs23 is beyond our reach, the present complex possesses an important and unique advantage compared to previously reported low-Mw photochromic materials, which is combining efficient photoalignment and SRG inscription.
The essence of the SRG inscription investigations is summarized in the Figure4. Upon irradiating the thin film (Figure 4a) with a polarization interference pattern generated using a two-beam interferometer setup with counter-circularly polarized beams and a period of 2 μm, it underwent pronounced photoinduced surface deformation (Figure 4b). The SRG formation is driven by polarization modulation (in the case of two counter-circularly polarized beams intersecting at small angle, the intensity profile within the interference pattern is essentially flat) rather than intensity modulation, which was verified by the observation that when s-polarized input beams were used (producing intensity-modulated interference pattern with no polarization modulation), no gratings were formed. The troughs of the surface pattern appear as flat, which indicates that all material is removed from the troughs of the grating. This is confirmed by the AFM surface-profile (Figure 4c): the surface-modulation depth is 600 nm, which is 2.4 times higher than the initial sample thickness of 250 nm. We used first-order diffraction to monitor the surface-deformation dynamics, as shown in Figure 4d. Being driven by subsequent trans–cis–trans photoisomerization cycles, SRG formation dynamics is highly intensity dependent: 300 mW cm−2 irradiation (for both beams) results in 40% diffraction efficiency within 5 min exposure whereas the dynamics are significantly slower when irradiation intensity of 100 mW cm−2 is used. By increasing the inscription intensity to 500 mW cm−2, 40% diffraction efficiency is obtained within 2 min exposure.
The most-sensitive SRG materials reported to date are based on the so-called “phototriggered mass-migration” phenomenon occurring in LC polymers, as introduced by Seki and co-workers.34 In such materials, however, the mechanism of the grating formation is completely different than in amorphous molecular glasses or in the supramolecular complex 3, as attested by their distinct polarization dependence,34b as well as their limitation to relatively thin films and low spatial frequencies. Hence, the overall diffraction efficiency and modulation depth of the “phototriggered LC materials” are limited, although the surface-modulation depth can be as high as twice the film thickness. In the case of photochromic molecular glasses, a modulation depth exceeding 500 nm has been reported in several instances, but only for thicker films (>500 nm) than in the present case (250 nm).20b,c,f In fact, to the best of our knowledge we report here on the first example of photoinduced SRG with modulation depth significantly higher than twice the film thickness, enabled by the unique halogen-bond-based materials design.
The results shown in Figure 3,4 imply that halogen bonding may provide unique possibilities in the design of photoresponsive and multifunctional supramolecular complexes. The halogen-bonded complex 3 reported in this paper has the capability of combining efficient photoalignment (order parameter of molecular alignment > 0.5) with exceptional surface-relief-grating formation efficiency (modulation depth of at least 2.4 times the initial film thickness). This combination may be attributed to the following features: i) the presence of a high-temperature nematic LC phase (presumably enabled by the high directionality of halogen bonding), which enhances and stabilizes the photoalignment; ii) the lack of flexible alkyl spacers and the presence of the dimethylamino group, which enhances the SRG formation efficiency. The films are crystalline, which compromises their optical quality; however, even short irradiation with circularly polarized light destroys the crystal structure and makes the samples amorphous. We also note here that the relatively high surface roughness of the fluorine-containing thin films may be beneficial for designing photocontrollable superhydrophobic surfaces. This latter topic, in combination with: i) detailed structure-performance characterization and ii) studying the feasibility of halogen-bonded complexes as photoalignment layers for liquid crystals, will be investigated in detail in the near future.
This study highlights the potential of halogen bonding in the design of photoresponsive supramolecular complexes, even if detailed understanding on the connection between the molecular-level properties, structural properties, and the optical response requires further detailed studies. Like no other non-covalent interaction, halogen bonding combines high directionality and controllable interaction strength, hence providing a facile platform for the design of photocontrollable suprastructures, with potential use in both fundamental studies as well as in applications ranging from liquid-crystal photoalignment to the design of diffractive optical elements and novel classes of smart functional materials.
4. Experimental Section
Materials and Methods: The starting materials were purchased from Sigma–Aldrich, Acros Organics, and Apollo Scientific. Commercial HPLC-grade solvents were used without further purification, except for acetonitrile, used as solvent for the synthesis of 1, which was dried over CaH2 and distilled prior use. 1H and 19F NMR spectra were recorded at room temperature on a Bruker AV500 spectrometer, using CDCl3 as solvent. 1H NMR spectroscopy chemical shifts were referenced to tetramethylsilane (TMS) using the residual proton impurities of the deuterated solvents as standard reference, while 19F NMR spectroscopy chemical shifts were referenced to an internal CFCl3 standard.
The LC textures were studied with an Olympus BX51 polarized light optical microscope equipped with a Linkam Scientific LTS 350 heating stage and a Sony CCD-IRIS/RGB color video camera connected to a Sony video monitor CMA-D2. The melting points were also determined on a Reichert instrument by observing the melting process through an optical microscope. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained using a Nicolet Nexus FTIR spectrometer. The values, given in wave numbers, were rounded to 1 cm−1 using automatic peak assignment. X-ray powder diffraction experiments were carried out on a Bruker D8 Advance diffractometer operating in reflection mode with Ge-monochromated Cu Kα1 radiation (λ = 1.5406 Å) and a linear position-sensitive detector. Powder X-ray diffraction data were recorded at ambient temperature, with a 2θ range of 5−40°, a step size 0.016°, and exposure time of 1.5 s per step. The single-crystal X-ray structure was determined using a Bruker Kappa Apex II diffractometer. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication no. 883070 and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. Experimental details about the crystal structure can be found in the Supporting Information.
Synthetic Procedures: N,N-dimethyl-4-[(2,3,5,6-tetrafluoro-4-iodophenyl)diazenyl]aniline, 1,7 and 4-[(E)-2-(4-methoxyphenyl)-ethenyl]-pyridine, 2,26a were prepared as previously reported.
Co-crystallization Experiments: The azobenzene derivative 1 and the stilbazole 2 were separately dissolved in THF at room temperature in a 1:1 ratio, under saturated conditions. The two saturated solutions containing the XB-donor and the XB-acceptor were then mixed in a clear borosilicate glass vial, which was left open in a closed cylindrical wide-mouth bottle containing paraffin oil. The solvents were allowed to slowly evaporate at room temperature for three days until the formation of good-quality single crystals of 3 occurred.
The POM images of the thin films were taken using an Olympus BH-2 optical microscope equipped with a Mettler–Toledo hot stage. The XPS measurements were performed using an ULVAC-PHI Inc. 1700R ESCA spectrometer equipped with a Mg Kα X-ray source (1253.6 eV) and a hemispherical analyzer. The resolution used was 0.05 eV. The spectra of 1 and 2 were measured from powders mounted on double-sided carbon tape. The spectra of 3 were measured from the spin-coated thin film. All of the spectra were referenced to the C1s neutral carbon peak at 284.6 eV.
The photoalignment and SRG inscription were performed using a spatially filtered and collimated Ar+-laser beam at wavelength of 488 nm. The photoalignment was carried out using a vertically polarized laser beam. In order to determine the order parameters at different stages of irradiation, the sample was mounted to a holder containing a 1 mm aperture through which it was irradiated. The aperture size was smaller than the size of the irradiation beam, which ensured uniform photoalignment throughout the whole aperture area. The irradiation was ceased at time instances of 10 s, 30 s, 60 s, 90 s, 120 s, 180 s, 240 s, and 360 s, and the sample was inserted into a polarizer-equipped spectrophotometer (Jasco V-650); also the polarized absorption spectra were measured through the 1 mm aperture. Since the photoinduced anisotropy is temporally stable, such a two-step procedure was justified. The order parameter was determined at different time instances from the polarized absorption spectra using the equation S = (A⟂ - A//)/(A⟂ + 2A//), where A// and A⟂ are the absorbances parallel and perpendicular (the uniaxial direction) to the writing beam polarization, respectively. The absorbance values were obtained in each case by averaging over a wavelength range of 440–460 nm. The gratings were inscribed by intersecting two counter-circularly polarized laser beams on the film surface. The incidence angle of the beams was 7°, which yielded a polarization-modulated interference pattern with a 2 μm period. The time evolution of the resulting diffraction gratings was monitored by collecting the transmitted first-order diffraction of a normally incident 633 nm He-Ne laser beam using an oscilloscope. The diffraction efficiency of the gratings was defined as η = I1/I0, where I1 and I0 are the intensities of the first-order diffracted and the transmitted beam, respectively. AFM images used for surface profile characterization were taken with a Veeco Dimension 3000 scanning force microscope in tapping mode.
Supporting Information is available from the Wiley Online Library or from the author.
A.P. acknowledges the financial support provided by the Japanese Society for the Promotion of Science and the Foundations' Post Doc Pool in Finland. P.M. and G.R. acknowledge the Cariplo Foundation (projects 2009-2550 and 2010-1351) for financial support. Prof. Baba and Prof. Motokura from Tokyo Tech. are greatly acknowledged for their assistance with XPS spectroscopy, and Prof. Iyoda and Prof. Komura for their assistance with atomic-force microscopy.