Water Confined in Films of Sulphonated Phthalocyanines - American

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Water Confined in Films of Sulphonated Phthalocyanines Arkadiusz Jarota,† Beata Brozek-Pluska,† Wojciech Czajkowski,‡ and Halina Abramczyk*,† †

Institute of Applied Radiation Chemistry, Laboratory of Laser Molecular Spectroscopy, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland ‡ Institute of Polymer and Dye Technology, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland ABSTRACT: The role played by peripheral substituents and the effects of hydration on vibrational and photochemical properties of aluminum phthalocyanine (AlPc) and aluminum tetrasulphonated phthalocyanine (AlPcS4) films have been studied by IR, Raman, emission, and electronic absorption spectroscopy. The O H (O D) stretching modes of water (deuterated water) have been used as a probe of H-bond interactions between water and the phthalocyanine macrocycles. It has been shown that the substituent and humidity have a tremendous influence on water properties confined in restricted environments of the film surface. We have identified the O H stretching vibrations of water involved in the H-bond interactions with the sulphonyl groups (O H 3 3 3 O) and with the central aluminum atom (H O 3 3 3 Al).

1. INTRODUCTION Despite the great diversity of traditional applications as dyes and pigments, the growing number of phthalocyanines play an important role as photosensitizers in photodynamic therapy (PDT) and as materials in low-gap semiconducting electronic devices. The zinc and aluminum complexes of sulphonated phthalocyanines have been developed as PDT agents in clinical use.1 3 The effective medical applications of the photodynamic therapy depend on aggregation because the degree of aggregation plays an important role in the photochemical mechanisms of photooxidation. One of the main features of the photodynamic sensitizers is the ability to generate singlet oxygen that is expected to be different for monomers and dimers since the rate of deactivation by internal conversion to the ground state is suggested to be much greater for dimers.2 A number of metal complexes of phthalocyanines exhibit a pronounced tendency to form stacked aggregates along the axis perpendicular to the plane of the dye macrocycles. The equilibrium constants for dimeric aggregation are large and depend on the complexing metal in the following order Cu > Fe > Vo > Zn > Co > Al. Disaggregation can be achieved by dilution in organic solvents (DMSO, pyridine, and methanol).1 The aqueous solutions of sulphonated metallophthalocyanines play obviously the most important role in medical applications. The growing interest is related to the enhanced awareness that the absorption and emission properties of the sulphonated metallophthalocyanines at biological interface that are crucial in medical applications strongly depend on the degree of hydration. It has been shown3 that the doublet of the Q-band structure evident in the fluorescence emission and absorption spectra of many metallophthalocyanines in various solvents is attributed to splitting into the Qx and Qy components. It has been suggested r 2011 American Chemical Society

that the splitting in absorption and emission spectra of the Q-band in the aluminum tetrasulphonated phthalocyanine is caused by the ligation of water molecules to the aluminum atom which decreases the molecular symmetry. Water molecules are involved also in H-bond interactions with the sulphonyl groups and/or the pyrole nitrogens and the bridging nitrogens of phthalocyanines. Despite the medical applications, the growing numbers of phthalocyanines play an important role as low-gap semiconducting materials. They become relevant in advanced devices like electrochromic displays, optical limiters, light-emitting diodes, recordable digital discs, organic conductors, lasers, nonlinear optical elements, etc. In these applications, the properties such as conductivity, optical absorbance, photoconductivity in the solid state phase are important.5 The effects of aggregation on the absorption and emission spectra of metallophthalocyanines in the liquid solutions are well documented,6 12 but much less information has been accumulated on the solid phases. The characteristic electronic absorption spectra of phthalocyanines consist of two strong Q and Soret bands at about 650 800 and 300 400 nm, respectively. The long-wavelength Q absorption band is a consequence of π-electron transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The absorption and emission spectra of phthalocyanines in the solid phase differ significantly from those observed in solutions. The aggregation effects on the absorption spectra of metallophthalocyanines in the solid state are significantly broadened and red-shifted.13 20 Received: September 5, 2011 Revised: October 19, 2011 Published: November 14, 2011 24920

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The Journal of Physical Chemistry C Spectral effects are expected to depend on macrocycle proximity, overlap between the π electrons of the adjacent macrocycles, tilt angle, and the extinction coefficients of the absorption bands involved.5 The electronic structure of metallophthalocyanines in the solid phase, characterized by the valence band Ev, the conduction band Ec, and charge-transfer (CT) states, plays an important role in charge carrier photogeneration. Because phthalocyanines are very stable functional organic semiconductors, there is a strong interest in synthesizing substituted soluble derivatives that facilitates the technological process of production. The electronic structure of metallophthalocyanines can be strongly modified by substituents. The substitution of metallophthalocyanines with hydrophilic groups like sulphonyl group makes them soluble in aqueous solutions and increases the capability to aggregate. The modifications of their chemical structure open great opportunities in the technology of solid films by increasing good compatibility with a variety of substrates. Since many sulphonated phthalocyanines are salts or acids, it is expected that, due to the dissociation of ionic groups, the electrical conductivity will strongly depend on humidity. Such devices behave as fast humidity sensors with good reproducibility, but from the viewpoint of electronic applications, the humidity instability is a factor of serious disadvantage.21 Therefore, there is a strong interest in understanding mechanisms of interactions between the interfacial water and phthalocyanine macrocycles and its effect on electronic, conduction, and charge transfer states. Interactions, energy transfer, and orientation of water molecules in the restricted environments of the solid films differ markedly from properties of bulk water. Since comprehensive reviews of the studies on water have recently appeared,22,23 we will restrict our discussion to the results on the hydrated phthalocyanine films. There are no papers directly monitoring the H-bond interactions of phthalocyanines in the restricted environments induced by controlled hydration of the interfaces. The IR absorption of the OH stretching modes of water involved in H-bonds is a valuable indicator to gain both qualitative and quantitative information about communication channels between water and phthalocyanines at interfaces. Comprehension of the structure of the bands in the region of the OH stretching vibrations of water is fundamental to explaining the properties of hydrated macrocycles resulting from different chemical and physical environments. This issue is crucial both in medical applications and electronic devices technology. For an isolated water molecule in the gas phase, the double structure in the region of the OH stretching mode has a clear meaning of the symmetric and the asymmetric normal modes. In the condensed phase, the situation becomes much more complicated. The ideal symmetric and asymmetric vibrations decoupled from the other modes and from the environment lose their sense. The intermolecular and intramolecular couplings as well as bath fluctuations lead to marked bandbroadening, spectacular red shift, and complicated band shape structure. The calculations of the vibrational band shapes of the OH stretching mode involved in H-bond interactions have a long history and have captured the attention of a number of papers.24 46 Although vibrational properties of water had been widely studied by Infrared and Raman spectroscopy as well as the timeresolved IR and Raman nonlinear techniques in many papers,47 51 so far there is no complete agreement on interpretation of the results, suggesting the need for further effort in this direction. The essential question, which has engendered particularly intense

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discussion, is whether in liquid water and in water confined in restricted environments the basic idea of vibrational spectroscopy, the localized normal modes, still works. There are at least a few main groups of interpretation of the vibrational properties of water.52 65 There is also a group of papers where the structural approach and the idea of normal modes are combined:66 73 the various subbands are attributed to molecules in different structural environments with further designations of symmetric and asymmetric stretching normal modes. It has been shown from spontaneous Raman spectroscopy in conjunction with quantumchemical calculations73 that the observed broad band in the region 3000 3800 cm 1 of the Raman spectrum of liquid water is actually the result of the superposition of two groups of bands with different magnitudes of the depolarization ratio. These two sets of vibrational bands are formed due to the formation of molecular clusters in the process of gas liquid phase transition. One set of bands in the liquid phase is associated with the symmetric (red side frequency) O H vibration of the water molecule. Another set of vibrational bands is associated with the asymmetric (blue side frequency) vibration. The other group of papers suggests that the idea of collective modes74 79 or anti- or noncooperativity80,81 are better for a proper description of the vibrational properies of water. In this article, we want to ask how much from the idea of symmetric and asymmetric vibrations still remain in the hydrated metallophthalocyanines at interfaces. Answering these questions will help us to learn more about the nature of water in restricted environments. To get deeper insight into the mechanisms responsible for this feature at the molecular level, we have recorded the IR spectra of water at metallophthalocyanine (MP) interfaces as a function of the controlled humidity of the film. The MPs are represented by the aluminum tetrasulphonated phthalocyanine (AlPcS4), one of the main sensitizers in photodynamic therapy and aluminum phthalocyanine (AlPc). In this article, we wish to address the issue related to H-bond interactions in the hydrated films of AlPcS4 and AlPc induced by controlled humidity of the environment. The goal of the article is to understand the nature of vibrational transitions in the region of the O H stretching modes of water at AlPcS4 and AlPc interfaces explored by IR spectroscopy, monitoring the effect of humidity and deuteration.

2. EXPERIMENTAL METHODS 2.1. Synthesis. Aluminum phthalocyanine chloride tetrasulphonic acid was purchased from Frontier Scientific, Inc. (AlPcS834), aluminum phthalocyanines chloride, was purchased from Sigma-Aldrich (362530). They were used without further purification. Water was deionized before preparing the solutions. Aluminum phthalocyanine tetrasulphonic acid, tetrasodium salt, was prepared by a process similar to that described by Griffiths and co-workers for the case of zinc phthalocyanine.82 Thus, the mixture of dry 4-sulphophthalic anhydride (5 g, Sigma Aldrich), urea (4 g), ammonium chloride (0.34 g), ammonium molybdate (0.06 g), boric acid (0.06 g), and anhydrous aluminum chloride (0.85 g, dissolved in 5 cm3 sulfolane) was introduced to 10 cm3 of sulpholane, slowly heated to 200 210 °C, and kept for 1 2 h. After cooling, the excess of sulpholane was removed, and the residue was dissolved in 2 100 cm3 of hot water, filtrated after the addition of activated carbon, and precipitated by the addition of 10 cm3 of 30% hydrochloric acid. The crude dye was dissolved in 25 cm3 of distilled water, neutralized by sodium carbonate to 24921

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The Journal of Physical Chemistry C

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Scheme 1. Synthesis Methods of AlPcS4 (Acid) and AlPcS4 (Salt): (I) Sulphonation of the Non-Sulphonated Phthalocyanine and (II) Condensation of Sulphophthalic Acid

pH 7.0, and precipitated by ethanol (1:1). Finally, 0.7 g of chromatographically pure dye was obtained. Briefly, Scheme 1 illustrates the methods used in this article for the synthesis of AlPcS4 (acid) and AlPcS4 (salt): (I) sulphonation of the nonsulphonated phthalocyanine and (II) condensation of sulphophthalic anhydride. Method I provides a mixture of large number of regioisomers, while the final product of method II represents a reduced number of regioisomers because the position of the sulphonyl group in the substrate of the reaction is fixed. 2.2. IR Measurements. One of the particular concerns in water experiments is the large absorption coefficient that can lead to a large excitation density.55,83 86 To avoid this effect, the homemade stainless cell was constructed to control H2O and D2O content of the hydrated film. The cell was connected to a reservoir where various chemicals were placed. P2O5, saturated solution of CH3COOK, and NaCl in H2O (or D2O) and pure water (or pure D2O) were used to maintain a humidity of 0, 23, 75, 100%. The 10 mM aqueous solution of AlPcS4 or AlPc was carefully spread with a syringe onto the surface of the Si3N4 window of 500 nm thickness and slowly dried in contact with the solvent vapor phase within an enclosed space around the

window. The solvent was allowed to evaporate before the measurements started. The pH of the aqueous solutions of the acid (I) and salt (II) AlPcS4 of 10 mM are 1.980 ( 0.002 and 7.570 ( 0.002, respectively. The steady state IR spectra have been measured with an IR spectrometer (Specord M 80). 2.3. Emission Measurements. Emission spectra were measured with Ramanor U1000 (Jobin Yvon) and a Spectra Physics 2017-04S argon ion laser operating at 514 nm at power 100 mW. The spectra were recorded at 293 K. The spectral slit width was 6 cm 1, which corresponds to the 500 μm mechanical slit of the spectrometer. A λ/4 wave plate was used to change the linear polarization into the circular one to avoid the different polarization sensitivity of the gratings. The interference filter has been used to purify the laser line by removing additional natural emission lines that interfere with the Raman lines, especially in the case of the solid samples. 2.4. Absorption Measurements. UV vis absorption electronic spectra were measured with Varian-Cary 5E and PerkinElmer Lambda 750 spectrophotometers in 2 mm and 10 mm quartz cells (Hellma). The spectra were recorded at 293 K for the aqueous solutions (c = 10 5, 10 4, and 10 3 M) and in the hydrated films with controlled humidity of the samples. 24922



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Figure 2. Electronic absorption and emission spectra of the hydrated films of AlPcS4: (a) AlPcS4 (acid(I)) in aqueous solution c = 10 5 M; (b) AlPcS4 (salt(II)) in aqueous solution c = 10 4 M; and (c) AlPcS4 (salt(II)) in the solid phase film at different humidities of water (H2O), 0, 23, 75, and 100%.

Figure 1. Electronic absorption and emission spectra of aqueous solutions of AlPcS 4 as a function of concentration: (a) Absorption of AlPcS 4 (acid (I)), (b) emission of AlPcS 4 (acid (I)), (c) absorption of AlPcS 4 (salt (II)), and (d) emission of AlPcS 4 (salt (II)).

3. RESULTS AND DISCUSSION Figure 1 presents the electronic absorption and emission spectra of aqueous solutions of AlPcS4 as a function of concentration. All the absorption spectra (Figures 1a,c) exhibit the characteristic bands at around 644 nm and 675 (677) nm due to the π π* electronic transitions and the band at around 610 nm. 24923


The Journal of Physical Chemistry C The structure of the absorption bands in most MPs may arise from the following factors: (a) the electronic transitions associated with the vibrational manifold of monomeric species (0 0, 0 1, and 0 2 vibrational transitions at 677 nm, 644 nm, and 610 nm, respectively);87 (b) exciton splitting into Q x and Q y components in the dimer structures (644 nm and 677 nm), and 0 1 vibrational transition of monomeric species at 611 nm;88 90 and (c) splitting due to axial ligation of the central metal atom by water that decreases molecular symmetry.4,91 Detailed inspection into Figure 1 shows that the absorption spectra of AlPcS4 (both acid (I) and salt (II)) are independent of concentration in a wide range up to 10 5 M. This finding suggests that AlPcS4 does not form aggregates in this concentration range. At higher concentrations the contribution from aggregation becomes clearly visible in the bands at 644 nm and at 610 nm. The fluorescence spectra are strongly dependent on AlPcS4 concentration (both acid (I) and salt (II)), and the fluorescence maximum is gradually red-shifted with increasing concentration. Figure 2a,b compares the absorption and the emission spectra of AlPcS4. One can see that both the emission and the absorption exhibit a sharply resolved vibronic structure and that the mirror image symmetry is largely conserved. It strongly suggests that the observed structure of the absorption bands in AlPcS4 originates from the electronic transitions associated with the vibrational manifold of monomeric species (0 0, 0 1, and 0 2 vibrational transitions at 677 nm, 644 nm, and 610 nm, respectively) rather than the species resulting from aggregation. There are contradicting reports on the aggregation of tetrasulphonated aluminum phthalocyanines.92 94 Some reports claim that that AlPcS4 does not form aggregates even at high concentrations of 10 4 M and is a very stable compound in aqueous solutions.94 Yoon et al.92 attributed a red-shift of absorption and emission spectra of phthalocyanines at high concentrations to the formation of dimers. Later, Dhami et al.93 has modified this conclusion, and the observed results have been explained by the reabsorption of fluorescence by ground state molecules of phthalocyanine. Dimerization can be prevented by axial ligands that behave as steric inhibitors. It has been suggested3 that the doublet band structure evident in the fluorescence emission and absorption spectra of many metallophthalocyanines in various solvents is attributed to the splitting of the Q-band into the x and y components. It has been suggested4 that the splitting of the Q-band in the aluminum tetrasulphonated phthalocyanine is caused by the ligation of water molecules to the aluminum atom which decreases the molecular symmetry. It is interesting to compare the electronic absorption spectra of AlPcS4 in aqueous solutions (Figure 2a) and in a solid film (Figure 2c). One can see that instead of a sharp, intense Q-band structure with a clearly resolved vibronic structure that is observed in aqueous solution (Figure 2a), there is a broad Q-band in the range of 600 800 nm with red-shifted components in the solid phase film (Figure 2c). The absorption profile strongly depends on humidity. The components Qx and Qy of the Q-band are significantly red-shifted from 677 and 644 nm in the aqueous solution to 821 and 753 nm in the film. The VB band maximum position does not depend on the content of water in the film and is observed at 610 nm like in the aqueous solution. Therefore, the electronic properties of solutions and pure liquids cannot be directly extrapolated to the solid phases.

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Figure 3. IR spectra of OH stretching vibrations of water confined in AlPcS4 (acid (I)) film for various contents of H2O: 100% H2O (a), 75% H2O (b), 23% H2O (c), and 0% H2O (d), and the Raman spectrum of the OH stretching vibrations of H2O (e).

To better understand the role of water on electronic transitions in solid films of phthalocyanines, we first need to settle the problem of interactions between the macrocycle and water monitored by the OH stretching vibrations. It is well known25,26,95,96 that OH stretch modes of water molecules are highly sensitive in frequency and intensity to the degree of hydrogen bonding between molecules. Therefore, we have carried out IR measurements on the films of AlPcS4 as a function of water content by controlling the humidity of the sample. Figure 3 presents the IR spectra of the hydrated films of AlPcS4 (acid (I)) in the different environments of humidities of H2O: 0, 23, 75, and 100%. The pronounced differences can be seen in the region of the O H stretching vibrational modes of water at around 3000 3700 cm 1. The results show that the humidity controls the number of water molecules in the AlPcS4/water films as the absorbance in the region of O H increases with increasing water humidity. The OH bands at various humidities represent a mixture of bulk and interfacial water. At 0% humidity, there is no bulk water, and the OH stretch bands represent the interfacial water attached to the metal phthalocyanine macrocycle. The Raman spectrum of the OH stretch band of bulk water in Figure 3 clearly shows two main features centered at 3197 and 3410 cm 1, which have been fitted with a pair of Gaussian subbands. We have assigned the double structure of the OH band with the peaks at 3197 cm 1 and 3465 cm 1 to the symmetric and asymmetric like stretching vibrations of water molecules involved in H-bond interactions (O H 3 3 3 O) with the oxygens of the sulphonyl substituents. At higher content of water in the system (higher than 0% humidity), H-bond interactions between water molecules give contribution to the OH stretch intensity. The band at 3087 cm 1 has been assigned to the C H stretching vibrations of the macrocycle.97,98 The further experimental evidence will be described below, but it is fair to say that until now it is inconclusive whether our assignment of the double structure to the symmetric- and asymmetric-like stretching vibrations of water are correct. Because of the complexity of the structure and dynamics of water, there is no generally accepted view on IR and Raman stretching vibrations. The situation becomes even more complicated for interfacial water in confined environments like those presented here in hydrated film of phthalocyanine. The broadband structure in the OH stretching region has encouraged researchers to fit water vibrational spectra into a 24924


The Journal of Physical Chemistry C certain number of overlapping Gaussian subbands. The IR and Raman stretch spectra of bulk water have been fitted into two Gaussian,53 three Gaussians54 four Gaussians,61,99 and five Gaussians.100 Laenen et al.54 have assigned the subband I peaked near 3340 cm 1 with fwhm 40 cm 1 and τor of 10 ps to tetrahedral ice-like water. The subband II peaked near 3400 cm 1 with fwhm 60 cm 1, and τor of 13 ps was attributed to water with bridged hydrogen bonds. The subband III peaked near 3440 cm 1 with fwhm 140 cm 1, and τor of 3 ps was attributed to water with bifurcated hydrogen bonds. Skinner et al.61 have also interpreted the four subbands on the basis of various structures involved in H-bonds. Region I has been assigned to the ice-like molecules with approximately four hydrogen bonds, and the other regions represent molecules with one or two broken bonds. Ratcliffe et al.101 and Walrafen102 assigned the band at 3230 3260 cm 1 to Fermi resonance between OH stretching and overtone of the bending mode, the band at 3450 cm 1 to a symmetric stretching, and the band at 3630 cm 1 to an asymmetric stretching of H2O. Later, Walrafen and Chu103 assigned the band at 3250 cm 1 to the in-phase OH stretching motions of a hydrogen bonded aggregate consisting of a central H2O molecule and its nearest neighbors in local tetrahedral arrangements. Furthermore, they suggested the band at 3400 cm 1 represents the OH stretching motions of those H2O molecules that are still associated with H-bonded water molecules that partially lost the phase relationship. Recently, Khoshtariya et al.104 106 deconvoluted the OD stretching overtone and OH stretching regions of the IR spectra into five subbands and proposed another assignment for those individual species. Sun100 has deconvoluted the Raman band of water into the components at 3014, 3226, 3432, 3572, and 3636 cm 1 and has assigned them to DDAA, DDA, DAA, and free OH symmetric stretching vibrations, respectively, where the following abbreviations have been used: DDAA (double donor double acceptor), DDA (double donor single acceptor), DAA (single donor double acceptor), and DA (single donor single acceptor), and free OH vibrations. Recent IR-ATR spectra107 of isolated water in acetone (CH3)2CdO 3 3 3 H2O) and acetonitrile (CH3CtN 3 3 3 H2O) show two well resolved bands assigned to ν3 and ν1, the asymmetric and symmetric stretch vibrations, respectively. This indicates that although all water molecules are surrounded by hydrophilic solvents and form H-bonds (OH 3 3 3 O or OH 3 3 3 N) the idea of asymmetric and symmetric normal modes still works, and it is useful in the description of vibrations in the condensed phases. Thus, we think that the fitting with two Gaussians does a proper job of fitting, although it was shown that a better fit is accomplished by using one additional minor subband at the red edge and one at the blue edge.99,108 The fitting with two Gaussians is consistent with the picture obtained from ultrafast nonlinear vibrational spectroscopy.109 Dlott et al. have shown that the OH-stretching band of water consists of two distinct subbands at 3235 cm 1 and at 3460 cm 1 that evidence clearly distinguishable dynamical behavior. A broader red-shifted band νR(OH) and a narrower blue-shifted band νB(OH) decay with different lifetimes along different decay pathways. A tentative structural interpretation is proposed that the νR(OH) subband results from strongly hydrogen-bonded ice-like water. Furthermore, the double structure is suggested by the temperature dependence of the Raman isotropic profile of water that

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Figure 4. IR spectra of OH stretching vibrations of water in AlPcS4 (salt (II)) film for various contents of H2O: 100% (a), 75%, (b), and 0% (c), and the Raman spectrum of the OH stretching vibration of H2O (d).

shows one isosbestic point at 3355 cm 1 with the red component decreasing with the blue component increasing as the temperature increases. These spectral changes have been rationalized by assuming the existence of a pseudochemical equilibrium between H-bonded and non-H-bonded ensembles. Because of the origin of the two main features of the Raman profiles discussed above, the observed variations can be largely attributed to the thermally induced disordering effect on ice-like structures, and the involved pseudochemical equilibrium can be assumed to refer to ordered (O) and disordered (D) water environments (in this respect, only water molecules participating in highly symmetric tetrahedral structures are expected to contribute to the O population).110 However, it has been shown recently111 that the isosbestic point does not necessarily imply a mixture of various structures of water. Some papers interpreted the temperature dependence with the isosbestic point simply as the decrease of intensity of the band that corresponds to the symmetric vibration and the increase of intensity of the asymmetric vibration.72,111 Figure 3 shows that the intensity of the asymmetric-like OH stretching vibration decreases to a larger extent than that of the symmetric mode with the decrease of humidity. Indeed, for 100% humidity the asymmetric OH stretch at 3465 cm 1 dominates the OH region, while at 0%, the intensity of this subband becomes smaller than that of the symmetric OH stretch at 3197 cm 1. These spectral changes can be rationalized by assuming the existence of interfacial water molecules H-bonded to the sulphonyl group of the phthalocyanine and H-bonded ensembles of bulk water. The observed variations with decreasing humidity can be largely attributed to the ordering effect of interfacial structures by water molecules participating in H-bond interactions with the phthalocyanine macrocyle. The subband at 3197 cm 1 can be assumed to refer to the symmetric-like OH stretch vibration of the more ordered interfacial water environments, in contrast to the 3465 cm 1 subband representing the asymmetric-like OH stretch of the more disordered water environments, where the tetrahedral arrangements in the H-bond network of water have been broken by the presence of the phthalocyanine macrocycle. To learn more about the interactions in the phthalocyanine film, we have compared the IR spectra of the hydrated AlPcS4 films with the Raman spectrum of bulk water in the pure state. The Raman water spectrum exhibits two maxima at 3258 cm 1 and 3410 cm 1. After the deconvolution into two Gaussians, 24925



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Scheme 2. Possible Sites of Interactions of Water with the Phthalocyanine Macrocyle

Figure 5. IR spectra of OH (A) and OD (B) stretching vibrations of water in AlPcS4 (acid (I)), 100% H2O (a), 23% D2O (b), 75% D2O (c), 100% D2O (d), bulk D2O (e), and bulk H2O (f).

the subbands that are shown in Figure 3 have the maxima and the band widths of 3197 (fwhm 186 cm 1), and 3410 (fwhm 263 cm 1), respectively. It indicates that the red-shifted subband at 3197 cm 1 has the same frequency as that in bulk water. On the contrary, the subband at 3465 cm 1 in phthalocyanine film is significantly blue-shifted with respect to that in bulk water. Some insight into the nature of the overlapping bands can be gained by comparing the results for the AlPcS4 (acid (I)) film and the AlPcS4 (salt (II)) film. Figure 4 shows the IR spectrum of the OH stretching vibration of water in AlPcS4 (salt (II)) films. Generally, the main vibrational features in the O H region of water are similar to those of the acid form presented in Figure 3. The main distinction between the results in Figures 3 and 4 is related to markedly weaker dependence on water humidity for the salt form. The blue-shift from 3410 cm 1 to 3465 cm 1 with respect to the bulk water in the pure state of the asymmetric-like band is also observed for AlPcS4 (salt (II)). The results presented in Figures 3 and 4 demonstrate that the OH band profile in the region of 3100 3600 cm 1 for water confined in restricted environments of the films is much more complex than that of the bulk water. The ambiguity in the assignment of most vibrational transitions in the OH water stretching region is due to the large number of H-bond interactions of the phthalocyanine macrocycle exhibited in a rather narrow frequency range, where they overlap and influence each other. Scheme 2 illustrates the possible interactions of water with the phthalocyanine macrocyle: interaction with the central metal atom (Al) and H-bond interactions (O H 3 3 3 N) between the water molecule and the bridging nitrogens, as well as H-bond interactions (O H 3 3 3 O) between the water molecule and the sulphonyl substituent. It has been suggested that phthalocyanine sulfonate groups undergo pH-independent hydrogen-bonding to three water molecules.112 Detailed inspection into Figures 3 and 4 shows evident blueshift of the maximum peak of the asymmetric vibration of water from 3410 cm 1 (in bulk water) to 3465 cm 1 in AlPcS4 (acid (I)) film. In contrast, the OH symmetric stretching vibration at

around 3197 cm 1 in the hydrated film corresponds quite well to that in bulk water. The blue-shifted frequency at 3465 cm 1 of the OH vibration of water in the AlPcS4 film H-bonded to oxygens of the sulphonyl groups suggests that the O H 3 3 3 O interactions are weaker than that in pure liquid. Another explanation is that the OH asymmetric stretch shifts to higher frequencies because the water oxygen atoms at the film interface do not receive H atoms from neighboring water molecules through H-bonding. On the other side, the position of the symmetric OH stretch subband at 3197 cm 1 is very similar to that in bulk water. This in turn is not compatible with the interpretation of the most red-shifted subband as the ice-like molecules with approximately four hydrogen bonds113 or inphase OH stretching motions of a hydrogen bonded aggregate consisting of a central H2O molecule and its nearest neighbors in local tetrahedral arrangements.103 It is worth noticing that although the local tetrahedral arrangement has evidently been broken in the hydrated films of AlPcS4, particularly at 0% humidities, where only interfacial water is present, the symmetric OH stretch subband at 3197 cm 1 is very similar to that in bulk water. From this, we can state that many of the interpretations which were proposed for liquids are not transferable to interfacial water in solid films. Making the connection to underlying structures is more difficult owing to the rather complicated relationship between the vibrational spectrum and local structure. Some insight into this problem can be gained in this work by a comparison of results from water H2O, D2O, and HOD. HDO is formed instantaneously by isotope exchange of D and H through proton hopping. HDO cannot be isolated chemically, but it can be easily detected by IR and Raman spectroscopy. In HOD, there is only one stretch localized on the OH moiety in contrast to water where there are two stretching vibrations delocalized over the entire molecule. Moreover, the HOD species may be viewed as simpler than water because there is little overlap between the OH stretch and the bend overtone. The bend overtone with the fundamental peaked at 1443 cm 1 lies primarily outside the OH band. We will demonstrate that deuteration and substituent effects in AlPcS4 (acid (I)) and AlPcS4 (salt (II)) help to identify the vibrational transitions in the complex H-bond structure of the phthalocyanine films. Figure 5 compares the spectra of the 24926



Figure 6. IR spectra of OH (A) and OD (B) stretching vibrations of water in AlPcS4 (salt (II)), 100% H2O (a), 0% H2O (b), 75% D2O (c), 100% D2O (d), bulk D2O (e), and bulk H2O (f).

hydrated films of AlPcS4 (acid (I)) in different environmental humidities of deuterated water D2O: 0, 23, 75, and 100%. The pronounced differences can be seen in the region of the O H and O D stretching vibrational modes of water at around 3000 3700 cm 1 and 2200 2700 cm 1, respectively. The results from Figures 5 and 6 show that the external humidity introduced by the saturated aqueous salts controls the number of water molecules in AlPcS4/water films as the absorbance in the region of O H decreases (and in the region of O D increases) with increasing deuterated water humidity. One can see from Figures 5 and 6 that the spectral region 2000 3600 cm 1 in the AlPcS4 film is represented by a few components: at 3087, 3197, 3378, and 3465 cm 1. The comparison with the Raman spectrum of bulk water with its two components at 3197 and 3410 cm 1 shows that the vibrational properties of water at the AlPcS4 interface differ significantly. It indicates that H-bonding of water at the metal phthalocyanine interfaces is substantially more complex than that in the pure state of the bulk water. The deuteration effects due to the isotope exchange in water help us to interpret the IR spectra in the studied region. First, the double structure of the OH stretching bands observed for AlPcS4 (acid (I)) at 100% of H2O humidity (Figure 5a) becomes much more complex on deuteration of water (Figure.5b, c,d), and a new peak at 3378 cm 1 emerges at the increased level of deuteration. Additionally, new OD stretching bands appear in the region of 2000 2800 cm 1. The vibrational band shape becomes much more complex than the double structure of the O D symmetric- and asymmetric-like vibrations at 2390 cm 1 and 2491 cm 1 observed for the pure state of the bulk deuterated water D2O. The effects of deuteration on the band shapes of the O H (and O D stretching) vibrations in hydrated AlPcS4 (acid (I)) films can be easily rationalized by the idea of decoupling of the vibrations of the O H (or O D) bonds in water. Upon deuteration of H2O, the new HDO and D2O species appear due to the isotope exchange. The HDO species have both bonds decoupled, which results in the appearance of the additional peaks corresponding to the decoupled vibrations at 3378 cm 1 and 2497 cm 1 for the O H stretching and the O D stretching of the HDO species, respectively. Thus, we have assigned the peaks at 3197 cm 1 (2348 cm 1) and 3465 cm 1 (2549 cm 1) to the symmetric and

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Figure 7. IR spectra of OH (A) and OD (B) stretching vibrations of water in AlPcS4 (acid (I)), 100% H2O (a), 100% D2O (c), and 100% H2O AlPc (b).

asymmetric O H (O D) coupled vibrations in H2O (D2O), respectively, and the peak at 3378 cm 1 (2497 cm 1) to the decoupled O H (O D) vibration of the HDO species. It is worth emphasizing that the blue shift from 2497 cm 1 in bulk D2O to 2549 cm 1 in phthalocyanine films is observed. Having reached this point, we still need to identify the O H stretching vibrations of water involved in the H-bond interactions with the central atom (aluminum) and/or with the pyrole nitrogens and the bridging nitrogens (O H 3 3 3 N). The simplest way to achieve this is to compare the IR spectra of the sulphonated phthalocyanine and nonsubstituted metal complex phthalocyanine. The results are presented in Figure 7. Figure 7 demonstrates that the IR spectrum of the nonsulphonated AlPc at 100% H2O (Figure 7b) differs markedly from that of the sulphonated derivatives of AlPcS4 (acid (I)) (Figure 7a) and AlPcS4 (acid (I)) at 100% D2O (Figure 7c). Detailed inspection into Figure 7 shows that the complex broadband structure observed in the sulphonated phthalocyanine disappears, and the only band which does not disappear is at 3344 cm 1. This effect must be evidently related to the lack of interactions between water and the sulphonyl substituent, and the only band which does not disappear emerges at 3344 cm 1. It must be related to the H-bond interactions between the water molecule and the pyrole nitrogens and the bridging nitrogens (O H 3 3 3 N) or to the interactions with the central aluminum atom H O 3 3 3 Al. To identify the origin of the interactions demonstrated by the vibrational transition at 3344 cm 1 in Figure 7, we have recorded the IR spectra as a function of water content in the nonsulphonated phthalocyanine films. Figure 8 presents the IR spectra of the nonsulphonated AlPc as a function of external humidity. The results demonstrate the lack of dependence on the water content in the film in contrast to the results presented so far in Figures 3 7 for the sulphonated AlPcS4. The lack of dependence on the humidity clearly suggests that water molecules confined in the AlPc films differ markedly from those in the sulphonated AlPcS4 films. Moreover, in contrast to the results presented in Figures 5 and 6 there is no evidence of isotope exchange between the OH and OD species. This finding suggests that water in AlPc films must be attributed to more restricted environments than to water molecules forming H-bond interactions with the pyrole nitrogens 24927



Figure 8. IR spectra of the nonsulphonated AlPc as a function of the external humidity: (a) 0% H2O, (b) 100% H2O, (c) ambient, and (d) 100% D2O.

Figure 9. Emission spectra of 10 mM AlPcS4 (acid (I)) and AlPcS4 (salt (II)).

and the bridging nitrogens of phthalocyanine macrocycle. The assumption can be further substantiated by the splitting of the Q-band in aluminum tetrasulphonated phthalocyanine presented in Figure 9. It has been shown3,114,115 that the splitting of the Q-band in aluminum tetrasulphonated phthalocyanine is caused by the ligation of water molecules to the aluminum atom which decreases the molecular symmetry. It has been proposed that oxygen of the ligating water molecules interacts with the aluminum atom, whereas the hydrogens interact with the pyrole nitrogens. This structure has slightly lower energy than the structure with the water hydrogens interacting with the bridging nitrogens. Such a structure is completely insensitive to the external humidity of the environment and deuteration effects, and rationalizes the results presented in Figure 8.

4. CONCLUSIONS This article has illustrated important aspects of the vibrational properties of water confined in a restricted environment of aluminum tetrasulphonated phthalocyanine films. The effects of controlled content of water as well as deuteration and substituent effects have been studied to identify interactions between water and the aluminum complex of phthalocyanine macrocycle in the solid phase. The results contribute to a deeper

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understanding of interactions at the interface between water and phthalocyanine films. The essential findings can be summarized as follows: (1) We have demonstrated the role played by the peripheral substituents and the central metal atom on the vibrational properties of hydrated aluminum phthalocyanine (AlPcS4 and AlPc) films. (2) The O H stretching modes of water have been used as a probe of H-bond interactions in AlPcS4 and AlPc films. (3) We have identified the O H stretching vibrations of water involved in the H-bond interactions with the sulphonyl groups (O H 3 3 3 O) and with the central aluminum atom (H O 3 3 3 Al). (4) The band at 3197 cm 1 in the AlPcS4 film has been attributed to the O H symmetric stretching mode of water molecules that are H-bonded to the hydrophilic sites (sulphonyl substituents) of the phthalocyanine macrocycle. The band is not shifted with respect to the symmetric O H band of the bulk water. (5) The band at 3465 cm 1 in the AlPcS4 film has been attributed to the asymmetric stretching mode of water molecules involved in the H-bond with the sulphonyl group of the phthalocyanine macrocycle. The band is significantly blue-shifted with respect to the asymmetric O H band of the bulk water (3410 cm 1). (6) The subband at 3197 cm 1 can be assumed to refer to the symmetric-like OH stretch vibration of the more ordered interfacial water environments, in contrast to the 3465 cm 1 subband representing the asymmetric-like OH stretch of more disordered water environments, where the tetrahedral arrangements in the H-bond network of water have been broken by the presence of the phthalocyanine macrocycle. (7) The band at 3378 cm 1 has been attributed to the O H vibration of the ligating water involved in interaction with the aluminum atom in the center of the metal complexes of AlPcS4 and AlPc. (8) The bands at 3378 cm 1 and 2491 cm 1 have been attributed to the O H (O D) decoupled vibrations of the HDO species in the restricted environments of the AlPcS4 films. To summarize, the blue shift of the OH stretching frequency from 3410 cm 1 in bulk water to 3465 cm 1 in hydrated phthalocyanine films suggests that H-bond interactions (OH 3 3 3 O) between water and the oxygens of the sulphonyl groups are weaker than that in pure water. Another explanation is that the OH asymmetric stretch shifts to higher frequencies because the water oxygen atoms do not receive H atoms from neighbor water molecules through H-bonding at the sulphonated phthalocyanine films interface. However, it is worth noticing that although the local tetrahedral arrangement has evidently been broken in the hydrated films of AlPcS4, particularly at 0% humidities, where only interfacial water is present, the symmetric OH stretch subband at 3197 cm 1 is very similar to that in bulk water. This in turn is not compatible with the interpretation of the most red-shifted subband as the ice-like molecules with approximately four hydrogen bonds or in-phase OH stretching motions of a hydrogen bonded aggregate consisting of a central H2O molecule and its nearest neighbors in local tetrahedral arrangements.102 From this, we can state that many of the interpretations which were proposed for liquid water are not transferable to interfacial water in solid films. 24928



’ AUTHOR INFORMATION Corresponding Author

*Tel: +48 42 6313175. Fax: +48 42 684-00-43. E-mail: abramczy@ mitr.p.lodz.pl.

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Water Confined in Films of Sulphonated Phthalocyanines - American

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