Octadecyl-viologen Photooxidation in Surface Films: Macroscopic

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Octadecil-viologen photooxidation in surface films: Macroscopy contraction of Langmuir monolayer by UV irradiation Jose M. Obrero-Pérez, María T. Martín-Romero, Marta Perez-Morales, Luis Camacho, and Eulogia Muñoz Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02677 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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Octadecyl-viologen photooxidation in surface films: Macroscopy contraction of Langmuir monolayer by UV irradiation

Jose M. Obrero-Pérez, María T. Martín-Romero, Marta Pérez-Morales, Luis Camacho and Eulogia Muñoz*

Department of Physical Chemistry and Applied Thermodynamics, University of Córdoba, Campus Universitario de Rabanales, Edificio Marie Curie, Córdoba, Spain E-14014

Corresponding Author *[email protected]

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Abstract The effects of UV radiation on a viologen derivative, octadecylviologen (OV), in Langmuir monolayers at the airaqueous solution interface and in Langmuir-Blodgett (LB) films, have been investigated. Langmuir monolayers suffer a sharp contraction after UV irradiation, clearly visible by the drop in surface pressure or the loss of surface area observed in the surface pressurearea isotherms. The UV-vis reflection measurements reveal a deep change in the OV monolayer caused by a photochemical reaction, which suggests the pyridones formation as photoreaction products. LB films (Z-type), before and after being irradiated with UV light, have been studied by using UV-Vis absorption, infrared and X-ray photoelectron spectroscopies. The results confirm that after photodegradation of the viologen films, the presence of oxygen results in the appearance of pyridones as reaction products. This paper demonstrates that, in the absence of catalysts, the photooxidation of viologen surface films occurs only under a particular molecular organization imposed by the airwater interface.


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1. INTRODUCTION For a long time, viologens, or 1,1'-dialkyl-4,4'-bipyridinium (see Scheme 1), have been widely studied due to their particular electrochemical and photochemical properties, and have been used in applications as diverse as herbicides, electrochromism, solar conversion energy, molecular electronics or supramolecular chemistry.1

Viologens (V2+) are electron acceptors, resulting in a very stable radical cation (V+•) under inert atmosphere.2–7 This radical cation may be formed by photoreduction in the presence of a wide variety of electron donors, either anions or neutral molecules.8–13 In this way, it has been demonstrated as the excitation of V2+(Cl-)2 may undergo by intercomplex charge transfer and gives rise to the V+• Cl2-• species.14,15 After, back electron transfer and diffusional separation of the radicals must compete. In the presence of oxygen, the V+• species may react with O2, resulting in the formation of superoxide ion (O2-•) and V2+,16 and then O2-• could react with V2+ to form hydrogen peroxide.17–20 It has been shown that superoxide and viologen react irreversibly to form, at least, nine degradation products including some pyridones.21 and -monopyridones have been obtained as major products when the oxidation of methylviologen, in aqueous solution, has been carried out -radiolytically and photocatalytically (in the presence of colloidal titanium dioxide).22 Their relative 3 ACS Paragon Plus Environment

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abundance in the radiolysis experiments is strongly dependent on the absence or presence of secondary oxidants, e.g. dioxygen and ferric ions.22 Recently, it has been demonstrated that the addition of ethylviologen prevents the degradation of lithium-oxygen batteries by the reduction of superoxide by viologen.23 In fact, it has been shown that superoxide reacts irreversibly with most known electrolytes, and is probably also involved in corrosion of carbon electrodes.23 In this paper, octadecylviologen (OV) films have been prepared at the airaqueous solution interface, and the surprising phenomenon that takes place in these monolayers under irradiation with UV light is analyzed. From a macroscopically point of view, it can be observed as the monolayer shrinks, at constant surface pressure, during the irradiation process, and decreasing the surface area in about 50%. Simultaneously, UVvis reflection spectra, recorded at the interface, show a marked decrease in the viologen absorption band, which indicates the photochemical disappearance of this compound. Phenomena of contraction or expansion area of monolayers at the airwater interface after UV radiation application due to 2D photopolymerization processes, have been described previously. An excellent review about this phenomenon has been published by Sakamoto et al.24 Further, films formed at the airwater interface have been transferred onto a solid support by the Langmuir-Blodgett technique (LB films), noting that the photochemical reaction also takes place in these conditions. However, this photochemical reaction does not occur in OV films prepared by solution casting or in OV solutions, at least in the absence of photocatalyst species. This fact suggests that the molecular organization induced by the interface plays a major role in the evolution, or not, of the photochemical reaction.


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2. EXPERIMENTAL SECTION Materials. N,N’-dioctadecyl-4,4’-bipyridinium bromide (OV) was purchased from Sigma-Aldrich. Sodium chloride and potassium nitrate used to prepare the aqueous solutions employed as subphases, were provided by Merck. The pure solvents, chloroform and methanol, were obtained from Sigma-Aldrich. All reagents were used as received. The ultrapure water used as subphase was produced by Millipore Milli-Q unit, pretreated by a Millipore reverse osmosis system (>18 MΩ·cm-1). The subphase temperature was 21ºC. The substrates for LB transfers were cleaned with an alkaline detergent and chloroform and then rinsed several times with ultrapure water. Methods. Monolayers were formed by spreading the OV solution dissolved in a mixture of chloroform/methanol = 3.1 (v/v), on different aqueous subphases. After evaporation of the organic solvent, the monolayer was compressed using a movable barrier on a Nima rectangular trough (NIMA 611D from Nima Technology, Coventry, England) provided with a filter paper Wilhelmy plate, at a compression rate of 0.05 nm2·min−1·molecule−1, facilitating the recording of the surface pressurearea (A) isotherms. Surface potential measurements were performed with the Kelvin Probe (SP1, Nanofilm, Göttingen, Germany) mounted on a NIMA 611 trough. The vibrating plate (frequency 420 Hz and amplitude 0.1 mm) was located ca. 2 mm above the water surface, while the reference electrode made of stainless steel plate was placed in the aqueous subphase. The surface potential measurements were reproducible to ±10 mV. For the UV irradiation a UV lamp (λ = 254 nm, W = 10 W) was mounted on top the trough, keeping a distance of ca. 5 cm from the airwater interface. Simultaneous to the isotherm recording, UV-Vis reflection spectra at normal incidence, as the difference in reflectivity (∆R) of the monolayer-covered aqueous


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surface and the bare surface, were obtained with a nanofilm surface analysis spectrometer (Ref SPEC2, supplied by Accurion GmbH, Göttingen, Germany). The monolayers were transferred onto solid substrates by the Langmuir-Blodgett method, i.e. by vertical dipping at constant surface pressure, at a lifting speed of 5 mm·min−1. The transfer pressure was kept at 20 mN·m-1. In the transfer process, only the monolayer is deposited during withdrawal of the substrate through the interface, giving rise to the so-called LB film type Z. The transfer ratio is close to unity and zero for the withdrawal and immersion processes, respectively. The UV-vis electronic absorption spectra of the films transferred onto quartz substrates were recorded locating the substrate directly in the light path on a Cary 100 Bio UV-Vis spectrophotometer. The FTIR transmission spectra of the films transferred onto CaF2 substrates were measured on a JASCO 6300 FTIR spectrophotometer equipped with a MCT detector. The background spectrum of an uncoated CaF2 substrate was recorded as the reference. In all the cases, the spectrum was the accumulation of 1000 scans. The spectra were recorded with a 4 cm-1 resolution. X-ray photoelectronic spectroscopy (XPS) measurements were performed by employing a SPECS Phoibos 150 MCD (Central Unit for Supporting Research, SCAI, University of Cordoba, Spain) photoelectron spectrometer equipped with an Al Kα Xray source at an energy of 1486.6 eV and 350 W, incident at 90º relative to the axis of a hemispherical energy analyzer. The quartz support coated with a LB film, was mounted on a steel sample holder and introduced directly into the XPS analytical chamber. The working pressure was 110-9 Torr. The spectra were collected using a take-off angle of 45º with respect to the sample surface plane. The spectrometer was operated at high resolution with a pass energy of 23.5 eV and analyzer spot diameter of 1.1 mm. After


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collection of the data, the binding energies were referenced by setting the C 1s binding energy of 284.8 eV. All peaks were fit with respect to spin-orbit splitting. Standard curve-fitting software (Multipak V 5.0 A; Physical Electronics, Inc.) by using Shirley background subtraction and Gaussian and Lorentzian profiles was used to determine the peak intensities. All the experiments were carried out at least two times.

3. RESULTS AND DISCUSSION 3.1. Surface pressurearea (πA) isotherms at the airliquid interface. The surface pressurearea per molecule (πA) isotherms for the OV molecule have been previously studied in the literature by using of different subphases.25–28 Figure 1 shows the πA isotherms of the viologen derivative spread onto pure water (green line), as reference and coincident with those previously published,27-28 0.1 M NaCl (blue line) and 0.1 M KNO3 (red line) subphases. The surface pressure starts raising at areas of ca. 1.5-2.3 nm2·molecule-1, depending on the subphase used. The surface pressure increases monotonically upon compression until an overshoot is reached, around 30 mN·m-1. The overshoot takes place at areas of the order of 0.7-1.0 nm2·molecule-1 as a function of the subphase studied.


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Figure 1. Surface pressurearea per molecule (πA) isotherms for OV at pure water (green line), 0.1 M KNO3 (red line), and 0.1 M NaCl (blue line) subphases.

The precise surface pressure and surface area of the overshoot are highly dependent on the compression rate and on the subphase nature.28 No definite cause of the overshoot in πA isotherms of viologen derivatives has been reported, although some authors25–27,29 have indicated that it could be due to either a bilayer formation or a reorganization of the viologen units from a horizontal position to a more vertical one. In the literature, the stability of the monolayers has been analyzed by performing compression and decompression πA isotherms (hysteresis cycle).26 Thus, when the monolayer is decompressed before the overshoot, the hysteresis is really small. However, when the barrier is reversed after the plateau region, a sharp decrease in surface pressure is observed. Thus, a large hysteresis is registered by means of compression-expansion cycles. This phenomenon is similar for the three aqueous subphases studied in the present work. The surface areatime stability curves have been also performed, keeping constant the surface pressure in different regions of the isotherm. It is found that before the 8 ACS Paragon Plus Environment

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overshoot the surface area barely decreases, which is consistent with the data previously reported.26 Figure S1 (Supporting Information, SI) shows the surface areatime (At) stability curves at 10 and 20 mN·m-1. As observed, on the 0.1 M NaCl and 0.1 M KNO3 subphases, the surface area decreases around 1% after 10 minutes, while at the pure water subphase, the decrease is somewhat higher, in the order of 4%. The effects of ultraviolet radiation on the monolayer are surprisingly visible to the naked eye. It is necessary to indicate it was not possible to irradiate evenly the entire surface. The UV lamp used is placed over the interface at a distance of  5 cm, in the vicinity of the pressure sensor. Therefore, the different regions of the interface are illuminated with different intensity. For this reason it is not possible to perform a rigorous kinetic analysis of the processes occurring at the interface. There are two ways of monitoring the ultraviolet radiation effect at the interface. First, a πA isotherm up to a predetermined surface pressure is made, and then, keeping such surface pressure constant (UV), the interface is irradiated for a specified time tUV, showing the surface area a sharp decrease. As shown in Figure 2, at a surface pressure of 10 mN·m-1 in the presence of NaCl, the decrease of the surface area ranges from 1.65 nm2 to 0.85 nm2, approximately (blue line). The elapsed time at which this phenomenon occurs is approximately 10 minutes.


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Figure 2. πA isotherms on 0.1 M NaCl aqueous solution subphase for OV monolayers irradiated with UV light during 10 minutes at constant surface pressure UV = 10 mN·m-1 (blue line), and at a constant surface area AUV = 1.63 nm2 (red line). As reference, the isotherm of OV monolayer without UV irradiation is also shown (black dashed line).

The second way to analyze the ultraviolet radiation effect on the interface is reaching a predetermined surface pressure as in the previous case and, after irradiating the interface, the surface area is kept constant, AUV. In the example of Figure 2, the surface area was kept constant at a value of AUV = 1.63 nm2 (red line). By this method, a sharp drop in the surface pressure is observed, e.g., starting the irradiation of the interface at a surface pressure of 10 mN·m-1, after tUV = 10 minutes, the surface pressure falls up to ~ 2 mN·m-1. By using this second method, as the surface area is constant, the total surface concentration remains also constant, so this approach is especially useful to compare measurements depending on the reaction conversion degree. However, we must point out that the first method has the advantage that, as the surface area decreases, the surface concentration increases, so the radiation effect is accelerated and hence the conversion degree.


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The UV radiation effect on the OV monolayer is irreversible, as evidenced by the experiment shown in Figure 3. The OV monolayer is compressed up to 10 mN·m-1, and then irradiated for 10 minutes keeping constant the surface area, 1.62 nm2 in this experiment. Once the tUV is finished, the monolayer is again compressed to a surface pressure of 30 mN·m-1, and then decompressed until a surface area of 2.6 nm2. Finally, the monolayer experiments a new compression-decompression cycle. As can be seen, for surface pressures higher than 10 mN·m-1, both compression-decompression cycles are practically coincident, demonstrating the absence of hysteresis and the strong structural change that takes place in the monolayer after the radiation application. This behavior must be related to a photochemical reaction at the interface where the OV molecules are involved. The new species formed after the photochemical reaction must occupy a lower surface area than that occupied by the OV molecule before UV irradiation.

Figure 3. πA isotherm for OV on 0.1 M NaCl aqueous solution subphase for two compressionexpansion cycles: first cycle with UV irradiation at 10 mN·m-1 during 10 minutes at constant surface area (solid line), and second cycle (dashed line).


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Adiditionally, to study in more detail the possible changes of the carges of viologen molecules after UV irradiation, surface potentialarea (VA) isotherms of OV at the different subphases, have been performed. Figure 4 shows the surface potentialarea per molecule (VA) isotherms of the Langmuir monolayer containing a viologen derivative formed at a 0.1 M NaCl aqueous solution subphase without (black line) and under (blue line) UV irradiation. Previously as reference, the VA isotherms on pure water subphase were registered. The results are shown in Figure S2 (Supporting Information). On pure water subphase, and before UV irradiation, the VA isotherm is similar to those published elsewhere.27,28

Figure 4. Surface potentialarea (VA) isotherms at 0.1 M NaCl aqueous solution subphase for OV monolayers without (black line) and under (blue line) during 10 minutes at constant surface pressure UV = 10 mN·m-1.

On salted subphase and without UV irradiation, the evolution of the surface potential is continuous without sudden changes, and a homogeneous monolayer during the compression process is expected to be formed.27,28 However, under UV irradiation a sharp decrease of the surface potential is registered (blue line). In the first moments


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under UV irradiation, the surface potential oscillates slightly because of the monolayer is not irradiated in a homogeneous way, as discussed earlier. After 1 minute under UV irradiation, the surface potential signal decreases continuously up to 390 mV at 0.85 nm2 under tUV = 10 min. This behaviour is an evidence of disturbance of the positively charged viologens during the irradiation process and suggesting a photoreaction which modifies the chemical nature of the monolayer. The effect of the UV radiation application on the OV monolayer is almost independent of the subphase used, as a similar behavior to those described in Figures 2 and 3 is observed with pure water or 0.1 M KNO3 subphases (see Figure S3 in the SI). However, keeping a constant surface area, the surface pressuretime curves under irradiation are subphase dependent. Thus, Figure 5 shows the variation of the surface pressure vs the irradiation time (tUV curves) by using different subphases. The initial surface pressure was 10 mN·m-1, and the constant surface area is indicated for each experiment. Initially, in the presence of chloride (blue line), the surface pressure decreases faster than in the aqueous (green line) and nitrate (red line) subphases. However, at longer times (10 minutes), the final surface pressure in the presence of chlorides or nitrates is approximately the same ( 2mN·m-1), while is somewhat higher in the water subphase (3 mN·m-1), where only bromide counterions are present. Moreover, the little variation observed in the surface areatime curves registered at 10 and 20 mN m-1 and without UV irradiation (see Figure S1) permits to discard the changes shown in Figure 5 are due to the instability of the OV monolayer.


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Figure 5. Variation of surface pressure vs UV irradiation time at constant surface area for OV monolayers. Initial surface pressure of 10 mN·m-1, time of exposure 10 minutes, on different subphases: Pure water (green line), 0.1 M KNO3 (red line), and 0.1 M NaCl (blue line).

From the tUV curve measured in the chloride subphase (Figure 5, blue line), two distinct regions are clearly distinguished: initially the surface pressure drops abruptly, and before the first thirty seconds the surface pressure changes from 10 mN·m-1 to  6 mN·m1. However, for tUV > 30 s, the absolute value of the slope of the πtUV curve decreases considerably. This behavior may be due to an inhomogeneous irradiated interface, as previously commented. Thereby, in those interface regions where the radiation intensity is lower, the kinetics of photochemical process would be slower; also diffusion phenomena may take place in the monolayer from less irradiated to more irradiated regions.

3.2. UV-Visible Reflection spectra at the airliquid interface. The UV-Vis reflection spectroscopy detects only those molecules located at the airwater interface, so this technique may provide information on the changes in the monolayer after the irradiation 14 ACS Paragon Plus Environment

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process. Under light normal incidence, the absorption by a film located on the top of a given interface increases the total reflection of radiation at this interface. In case the absorption is small, i.e., less than 0.05 percent of the incident radiation, as usually found in Langmuir monolayers:30,31 R  RD,S  RS  2.303 103 fo RS 

(1)

where ΔR is the increase of reflection under normal incidence, RS and RD,S are the intensities of reflection of incoming radiation in the absence and presence of a Langmuir monolayer, respectively, ε is the molar absorption coefficient with units M1

·cm-1,  is the surface concentration in mol·cm-2, and fo is the orientation factor. The

orientation factor compares the average orientation of the dipole transition at the airliquid interface with respect to the orientation in bulk solution.31 The influence of surface concentration in the reflection spectrum can be eliminated by normalizing that given spectrum by the surface area:31 Rn  A  R 

2.303  1017 f 0 NA

RS  5.41 108 f 0

(2)

where A is the area occupied per chromophore molecule, NA is Avogadro's number. RS = 0.02 has been used as the reflection value for the bare airliquid interface. ΔRn is expressed in nm2·molecule-1. The reflection spectra of the OV monolayers on pure water subphase have been previously reported in the literature (see Fig. 2a Ref. 28). Figure 6 shows the reflection spectra of the OV monolayer on the 0.1 M NaCl subphase. These spectra are very similar to those obtained on water and 0.1 M KNO3 subphases. As can be seen, a band with absorption maximum at 268-270 nm is observed, this value is approximately coincident to that measured in organic solution (266 nm). Furthermore, ΔRn decreases with increasing the surface pressure applied. This effect is due, as previously described


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(Ref. 28), to the tilt increase of the viologen group as the monolayer is compressed, causing a decrease in the orientation factor (equation 2). However, at a constant surface pressure, ΔRn must be constant.

Figure 6. Normalized UV-Vis reflection spectra, ΔRn = ΔR × A, for the OV monolayer under compression, on 0.1 M NaCl aqueous solution subphase. The legend shows the A values corresponding to each spectrum.

Furthermore, the effect caused by the UV radiation application on the OV absorption band (0.1 M NaCl subphase), by a similar experiment to that described in Figure 3, is shown in Figure 7. Thus, black (π = 0 mN·m-1 and A = 2.72 nm2) and red (10 mN·m-1 and A = 1.62 nm2) lines in Figure 7 depict ΔRn before applying UV radiation. The blue line represents ΔRn after irradiation for tUV = 10 minutes, keeping the surface area constant. During this time, the surface pressure decreases up to a value of 1.8 mN·m1. As can be seen, the normalized reflection (Figure 7, blue line) decreases considerably in the 270 nm region but increases in the 350-400 nm region. Then, the monolayer is compressed upto a surface pressure of 30 mN·m-1, and registering the reflection spectra at 10, 20 and 30 mN·m-1 (Figure 7: cyan, green and orange lines, respectively). It is 16 ACS Paragon Plus Environment

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notorious that during compression, ΔRn decreases again dramatically over the 270 nm region while slightly increases in the 350-400 nm region, although the monolayer is not being irradiated. This behavior must be related to the fact that the photochemical reaction evolution is not homogeneous in the different regions of the interface. Indeed, the reaction should be faster on the surface located directly under the UV lamp, as the radiation flux is more intense. However, due to the design of our experimental set-up, the reflection spectrum must be performed on a close but different interface region, where the reaction degree is consequently lower. The large decrease of ΔRn in the 270 nm region, observed under compression, must be related to the homogenization between regions with different photochemical reaction degrees.

Figure 7. Normalized UV-Vis reflection spectra, ΔRn = ΔR × A, for a OV monolayer on 0.1 M NaCl aqueous solution subphase for different surface pressures before (π = 0 mN/m, black line and π = 10 mN·m1, red line) and after (π = 1.8 mN·m-1, blue line; π = 10 mN·m-1, cyan line; π = 20 mN·m-1, green line; and π = 30 mN·m-1, orange line) being irradiated with UV light during 10 minutes at constant surface area. Normalized reflection spectrum for the same monolayer after decompression (π = 0 mN·m-1, magenta line) is also shown.


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Finally, the monolayer is decompressed to reach the initial situation, i.e., π = 0 mN·m1 and A = 2.72 nm2. The magenta line shows the reflection spectrum obtained. The initial (black line) and final (magenta line) spectra are comparable in the sense that they are measured at the same surface concentration and it gives us insight of the change that the monolayer suffers. The use of pure water or 0.1 M KNO3 subphases leads to similar results to those shown in Figure 7 (see Figure S4 in the SI). However, the changes observed in the reflection spectra do not allow us to clarify the chemical modification affecting the OV molecule during irradiation, although qualitative relevant information has been obtained. For example, from the spectra of Figure 7 the formation of the viologen radical-cation can be discarded, because this species shows absorption bands at 400 nm and 600 nm.17,32 Moreover, the broad bands observed in the 350-400 nm region can be assigned to pyridones formation,22 since the pyridones spectrum comprises mainly two bands in the UV-Vis region: between 230 and 310 nm, and between 320 and 390 nm.33 In any case, as already indicated, we are unable to specify which chemical species are present in the photochemically modified monolayer.

3.3. UV-Visible spectra of Langmuir-Blodgett films. OV monolayers have been transferred to quartz substrates by using the Langmuir-Blodgett (LB) method at a transfer pressure of 20 mN·m-1. Experimentally, it has been found that OV monolayers are not transferred during the immersion of the support, but during the substrate withdrawal the film is successfully deposited with a transfer ratio close to unity (Z type multilayer). Figure 8 shows the UV-Vis absorption spectrum of a LB film formed by six OV monolayers (red line), which has similar characteristics than those reflection spectra shown above for the non-irradiated monolayer (see Figure 6). The LB film was irradiated with UV light for 10 minutes, and then its absorption spectrum was measured 18 ACS Paragon Plus Environment

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(Figure 8, blue line). The obtained spectrum shows a significant decrease in the 270 nm region and a slight increase in the 350-400 nm region, as also observed for the reflection spectra (see Figure 7). Again, these results suggest the pyridones formation as photoreaction products.22

Figure 8. UV-Vis absorption spectra of LB film containing 6-monolayers of OV deposited on a quartz substrate before (red line) and after (blue line) being irradiated with UV light during 10 minutes. As reference, the UV-Vis absorption spectrum of an OV solution in methanol is shown (gray dotted line).

It should be emphasized, that the absorption spectra of OV films prepared by solution casting remain unchanged after prolonged UV irradiation. This fact has been verified starting from solutions in the presence of different counterions. Also, the absorption spectra of OV solutions ( 6·10-4 M) are modified after being illuminated with UV light. Therefore, the photochemical reaction occurs only as a result of a molecular organization imposed by the airwater interface.

3.4. FTIR spectra of Langmuir-Blodgett films. The FTIR transmission spectra of LB films transferred onto CaF2 at a transfer pressure of 20 mN·m-1 have been performed. 19 ACS Paragon Plus Environment

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Figure 9 (red line) shows the FTIR spectrum of a LB film formed by 20 monolayers of OV and that obtained after irradiating the above LB film for 10 minutes with UV light (Figure 9, blue line). The position of the main bands and its assignment are given in Table 1.

Figure 9. FTIR transmission spectra of LB film containing 20-monolayers of OV deposited on a CaF2 substrate, before (red line) and after (blue line) being irradiated with UV light during 10 minutes. Table 1. Assignments and Positions of Major Bands, in cm-1, for FTIR Spectra of LB Films shown in Figure 9. mode

LB-OV film

(LB-OV)UV film

s(CH2), symmetric stretch

2919

2921

as(CH2), asymmetric stretch

2851

2852

s(C=O), symmetric stretch

-

1718

(C-C), ring stretch

1641

1640

 (C-N), ring stretch

1557

weak

(CH2), scissor mode

1467

1466


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As shown in Figure 9, the high wavenumber region (3000-2800 cm-1) includes the bands corresponding to the CH2 asymmetric and symmetric modes at 2920 y 2850 cm1, respectively. The good definition of these bands for compounds with long alkyl chains allows to relate the wavenumbers with the alkyl chain configurations.34–39 The wavenumbers for the CH2 asymmetric and symmetric modes, both for OV++ and its oxided form, are very similar (see Figure 9 and Table 1), and enable us to relate them with the formation of an alkyl chain crystalline phase, where the alkyl chains are located in all-trans configuration. The unique remarkable difference is displayed by the increase of the intensities of these bands after the UV irradiation. This phenomenon may be related to changes in the orientation of the alkyl chains after the photoreaction. In the 1800-1400 cm-1 region, the spectra of Figure 9 show skeletal bands corresponding to C=C and C=N ring stretching (1650-1430 cm-1),40,41 thus, the 1465 cm1 band could be assigned to the CH2 scissoring deformation mode. However, after the UV radiation application, the key feature of this spectral region is the appearance of a new vibrational band at 1718 cm-1. This band can be related to the carbonyl group stretching mode. The position, the intensity, and the relative absence of other interfering bands make this band of the easiest groups to recognize in the IR spectra.41 Therefore, Figure 9 shows the existence of photogenerated species containing carbonyl groups. It is reasonable to think that after the photodegradation of the viologen films, the presence of oxygen results in the appearance of reaction products that could include mono- and/or bi-pyridones. In the case of solution casted films, the FTIR spectra remain unchanged after exposure to UV irradiation, as also happened in the UV-Vis spectra registered. Although the alkyl chains of OV molecules in the cast films show a crystalline packing (see Figure S5), the polar headgroups can be random oriented and therefore less 21 ACS Paragon Plus Environment

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molecular organization of the polar moieties of the viologen prevents the progress of the photooxidation.

3.5. XPS spectra of Langmuir-Blodgett films. X-ray photoelectron spectra of LB films composed by 10 OV monolayers transferred at 20 mN·m-1 onto quartz supports were carried out. The full XPS spectrum is shown in Figure S6 (Supporting Information). For not irradiated films (see Figure 10A), the signal corresponding to the C 1s can be decomposed into two peaks, the first located at 284.8 eV, which may be assigned to the set of C-alkyl (285.1 eV) and C=C (284.4 eV) signals,42 and the second located at 286.3 eV, which may be assigned to the carbon attached to nitrogen (C-N bond).43 When the LB film is irradiated under UV light (see Figure 10B), C 1s signal can be resolved into three peaks. Thus, together with the previous two peaks located at 284.8 eV and 286.3 eV, an additional peak at 287.8 eV is present, which can be related to the C=O group.42,43 The area of this last peak represents about 10% of the total C 1s area, which means approximately four C=O groups per viologen molecule. This would be indicative of the presence of bi-pyridones.


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Figure 10. XPS core-level spectra for C 1s of LB film containing 10-monolayers of OV deposited on a quartz substrate, before (A) and after (B) being irradiated with UV light during 10 minutes. The purple dotted line represents the experimental spectrum; blue, red and green lines represent the fitted curves, and the black line represents the sum of the individual calculated lines.

The ratio between the C atoms and N atoms percentage in the LB films, before and after irradiation, are 21.7 and 22.4, respectively, quite close to the theoretical value of 23 based on the molecular structure values. The XPS core-level spectrum for N 1s of not irradiated OV film is shown in Figure S7 in the SI. However, in the XPS spectrum of irradiated OV film, the signal corresponding to N 1s remains unchanged after irradiation. Thus, in these spectra two peaks may be observed, one at 398.4 eV, which can be related to -N = (398.3 eV),44,45 and another one at 400.4 eV, which can be attributed to –NH-,44,45 (see Figure S4 in the SI). However, it is surprising the absence of peaks around 402 eV, position that should appear as the signal corresponding to N+.43,45–47 According to the literature, that X-ray excitation in the analysis chamber may transform the nitrogen in radical cation.43,47 23 ACS Paragon Plus Environment

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4. CONCLUSIONS The UV irradiation of OV Langmuir or LB films causes the photochemical transformation of this viologen derivative to pyridones. This transformation can be detected at a glance at the airwater interface, through the rapid decrease of the surface area under irradiation at a constant surface pressure. Also, this conversion has been analyzed by using UV_visible reflection (at the airwater interface) and transmission (in LB films) spectroscopies. Moreover, FTIR spectroscopy and XPS have been used to characterize the OV LB films. The behavior of the Langmuir and LB films contrasts with the absence of photochemical reaction in those OV films prepared by solution casting, even in presence of chloride, as well as in OV solutions, at least in the absence of photocatalyst species, suggesting that the interface-induced molecular organization plays a major role in the evolution, or not, of the photochemical reaction. As it is known, the excitation of MV2+(Cl-)2 with UV irradiation may undergo intercomplex charge transfer and gives rise to MV+• Cl2-•.14,15 This charge transfer can take place in the presence of other anions as SCN-.48 In our opinion, this charge transfer must occur also in nitrate presence, although the kinetics of this process seems to be slower than in chloride presence. This can be deduced from the results shown in Figure 5, where the reduction in surface pressure with time is less pronounced in nitrate presence that in chlorides one, at least for short times. In the presence of aqueous subphase (see green line in Figure 5), the bromide anions, present as OV counterions, must remain partially retained at the interface. A low photoyield of OV+• Br2-•, together with its short lifetime, may be the reason why the kinetics of the process is intermediate, at least initially, to that described above.


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After, the formation of OV+• and Cl2-•, back electron transfer and diffusional separation of the radicals must compete. However, the back electron transfer seems to have little influence on the photochemical process, possibly due to spatial distribution of the monolayer, allowing efficient separation of the two radicals. Thus, on the one hand, the strong aggregation OV molecules could allow the rapid migration and delocalization of the cation radical along the interface plane, while the radical anion, Cl2-•, can only diffuse through the aqueous subphase, which makes the two radicals away from each other. Furthermore, the oxygen presence enables its rapid reaction with the OV+• species to form superoxide ion (O2-•) and OV++ dication.16 Finally, superoxide ion and viologen react irreversibly to form different degradation products (pyridones).17–22 The most noteworthy aspect of this process is that the viologen photodegradation occurs in the absence of a photocatalyst. This process takes place thanks to the special organization of the viologen in the interface.

Supporting Information Available: Figure S1, Surface areatime curves for OV monolayers at 10 and 20 mN·m-1 in different subphases. Figure S2, VA isotherms on pure water subphase for OV monolayers without UV irradiation and irradiated with UV light during 10 minutes at constant surface pressure. Figure S3, πA isotherms for OV monolayers irradiated with UV light on different subphases. Figure S4, reflection spectra for the OV monolayer in 0.1 M KNO3 aqueous solution subphase for different surface pressures before and after being irradiated with UV light. Figure S5, FTIR transmission spectra of casted film of OV before and after being irradiated with UV light during 10 minutes. Figure S6, XPS wide scan spectra of LB film of OV before and after being irradiated with UV light. Figure S7, XPS core-level spectrum for N 1s of LB


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film of OV. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors thank the Junta de Andalucía (Consejería de Innovación, Ciencia y Empresa) for financial support of this research in the framework of Project P10-FQM6703, and also thank the Spanish MINECO for special financial support (CTQ201457515).

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Octadecyl-viologen Photooxidation in Surface Films: Macroscopic

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