J. Phys. Chem. 1986, 90,6231-6242 size, while the reaction of methanol on active sites is believed to proceed on the mechanism that is almost the same as before the modification. The enhancement of product shape selectivity therefore should be caused by the diffusion control of molecules within the pore and the subsequent surface reactions but not by the primary reaction about the methanol activation. The deposition of coke is not stimulated on the SiHZSM-5, however, because the activity decline was not fastened even in highly modified SiHZSM-5 (12.0 wt %). This may be supported by a former study20 claiming that the coke was deposited only on the external surface of ZSM-5. We could indicate some possible reactions responsible for the high shape selectivity on the SiHZSM-5. Isomerization into small-size molecules is one of the significant reactions, and it is exemplified by the increase of p-xylene and pethyltoluene formation. Because on the zeolite with a small pore opening only the para isomer could diffuse out from the ZSM-5 pore, deviation from the thermodynamic equilibrium in the pore enhanced isomerization from the ortho and meta isomers to the para isomer. A complementary increase of small olefins in place of large compounds is also indicative of the reaction within the pore. Although it seems that large compounds such as A9 and Alo decompose into small olefins as ethylene and propylene, the direct conversion is unfavorable to the thermodynamics. It has been r e p ~ r t e d ~that l - ~ such ~ hydrocarbons as propylene and butene are (20) Dejaifve,
P.; Auroux, A,; Gravelle, A. C.; Vedrine, J. C. J . Catal.
1981, 70, 123.
6237
converted readily into various hydrocarbons, of which product distribution is similar to that observed in the methanol conversion. Depending on the method of catalyst a ~ t i v a t i o nethylene ,~~ is also converted into various hydrocarbon products. In other words, olefins once formed in the zeolite pore are converted rapidly into products, including aromatic compounds. From that viewpoint, olefinic residues are regarded as adsorbed intermediates in this reaction.25 In orer to explain the higher reactivities of olefins and the resultant similar product distribution, therefore, it may be supposed that these hydrocarbon residues are converted to each other by gaining or losing skeletal carbons. In the small-size pore, smaller molecules should diffuse out easily and the exit of larger compounds may be suppressed. Under these conditions, the formation of small molecules should be relatively stimulated, since residues of larger compounds have to be converted into those for small molecules. One could therefore indicate the interconversion between hydrocarbon residues as a plausible idea in explaining the suppression of large aromatics and the complementary increase of small olefins. Registry No. Si(OCH&, 681-84-5; ammonia, 7664-41-7; water, 7732-18-5; nitrogen, 1727-37-9;o-xylene, 95-47-6. (21) Anderson, J. R.; Foger, K.; Mole, T.; Rajadhyaksha, R. A.; Sanders, J. V. J . Catal. 1979, 58, 114. (22) Dejaifve, P.; Vedrine, J. C.; Bolis, V.; Derouane, E. G. J. Catal. 1980, 63, 31. (23) Itoh, H.; Hattori, T.; Murakami, Y. Appl. Catal. 1982, 2, 19. (24) Rajadyaksha, R. A.; Anderson, J. R. J . Catal. 1980, 62, 510. (25) Dessau, R. M.; Lapierre, R. B. J . Catal. 1982, 78, 136.
Chemical Reactivity in,Monolayers: Study of an Amphlphiiic Tetrapyridinoporphyrazine in Langmuir-Biodgett Films Serge Palacin,* Annie Ruaudel-Teixier, and AndrQBarraud CEA- IRDI-DESICP-Dzpartement Physico- Chimie, Service Chimie MolZculaire, 91 191 Cif Sur Yvette Cedex. France (Received: July 8, 1986)
This paper reports on the synthesis, association in solution, monomolecular layer formation, and Langmuir-Blodgett (LB) homolayer and alternate layer properties of an amphiphilic copper tetrapyridinoporphyrazine. The high conjugation of the four pyridinium rings with the central porphyrazine core has a great influence on the chemical properties in solution and in a monolayer. The degree of association is dependent on the Lewis acidity of the solvent, the oligomeric forms-stable in pure chloroform solution--being broken by any donor-acceptor interaction between the porphyrazine ring and Lewis acids such as acetic acid, nitrogen dioxide, or iodine. The amphiphilic porphyrazine forms stable monomolecular layers to film pressure up to 35 "am-'. The aromatic ring reacts with water, giving rise to a virtually two-electron-reducedporphyrazine through a donor-acceptor complex. This complex can be transferred onto solid substrates and is indefinitely stable in LB films. It is characterized by infrared, electronic, and ESR spectroscopy. The chemical reduction in the monolayer state is inhibited by Lewis acids present either in the bath or in the monolayer. ESR studies on homolayers and alternate layers built from mixtures of the amphiphilic porphyrazine and w-tricosenoic acid show that the porphyrazine rings lie flat on the substrate, and provide an estimation of the plane-to-plane and in-plane coupling between the copper ions.
Numerous studies on physical and chemical properties of substituted phthalocyanines have recently appeared in the literature.'-I2 The increasing use of phthalocyanines in photovoltaic
cells,13 semiconductor^,^^^'^ organic or sensitive coating films18-20leads scientists to search how to modify the features of P.;Minor, P. C. Inorg. Chem. 1981, 20, 4015. (9) Grigoryan, L. S.; Simonyan, M. V.; Sharoyan, E. G. Phys. Status Solidi 1984, 84, 597. (10) Ter Haar, L.W.; Hatfield, W. E.; Tsutsui, M. Mol. Cryst. Liq. Cryst. 1984, 107, 181. (1 1) Mossoyan-Deneux, M.; Benlian, D.; Pierrot, M.; Fournel, A.; Sorbier, J. P. Inora. Chem. 1985, 24, 1878. (12) Iwatsu, F. J . Crysr. Growth 1985, 71, 629. (13) Martin, M.; Andrt, J. J.; Simon, J. Nouv. J . Chim. 1981, 5 , 485. (14) Wright, J. D.; Chadwick, A. V.; Meadows, B.; Miasik, J. J. Mol. Cryst. Liq. Cryst. 1983, 93, 315. (15) Giraudeau, A.; Fan, F.; Bard, A. J. J . Am. Chem. SOC.1980, 102, (8) Lever, A. B.
(1) Schumann, B.; Woehrle, D.; Jaeger, N. J.; J. Electrochem. Soc. 1985, 132, 2144. (2) Skorobogaty, A.; Lancashire, R.;Smith, T. D.; Pilbrow, J. R.;Sinclair, G. R. J . Chem. Soc., Faraday Tram. 2 1983, 79, 1123. (3) Smith, T. D.; Livorness, J.; Taylor, H.; Pilbrow, J. R.; Sinclair, G. R. J . Chem. SOC..Dalton Trans. 1983, 1391. (4) Maillard, P.; Gaspard, S.;Krausz, P.; Giannotti, C . J . Organomer. Chem. 1981, 212, 185. (5) Vinogradskii, A. G. Russ. J . Phys. Chem. 1983, 57, 1747. (6) Gruen, J. D.; Faulkner, L. R.J . Am. Chem. Soc. 1983, 105, 2950. (7).Louati, A.; El Meray, M.; Andrt, J. J.; Simon, J.; Kadish, K.; Gross, M.; Giraudeau, A. Inorg. Chem. 1985, 24, 1175.
0022-3654/86/2090-6237%01.50/0
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0 1986 American Chemical Society
6238 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
Palacin et al.
l
a
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Figure 2. UV-visible spectra of CuSls in pure chloroform (dotted line) and in a 80/20 acetic acid-chloroform mixture (concentration 2 X M).
donor-acceptor properties (Figure 1).
Figure 1. Copper tetrao“yltetrapyridino[3,4-b:3’,4’-g:3’’,4”I:3”’,4”’-q] porph yrazinium bromide (CuS I 8).
these materials. In this regard, the effect of electron-withdrawing or electron-donating substituents in the benzo rings is of major interest. Some recent papers concerning the properties of phthalocyanines and metallophthalocyanines in Langmuir-Blodgett (LB) films’&24 describe the effects of weak electron-donating substituents such as alkyl chains or ethers. These soluble phthalocyanines lack a well-defined amphiphilic character and their chemical properties in LB films are only slightly different from the parent phthalocyanines. So, it appeared attractive to actually prepare amphiphilic phthalocyanines, with strong electron-withdrawing groups conjugated to the macrocycle, in order to study their chemical properties in mono-and multilayers. Our experience of amphiphilic porphyrin^^^*^^ leads us to use the tetrapyridinoporphyrazine ring, the pyridine nuclei of which can easily be changed into alkylpyridinium rings. The resulting copper tetraoctadecyltetrapyridino[3,4-b:3’,4‘-g:3‘’,4”-1:3”‘,4’”-q]porphyrazinium bromide (cus18) gives good LB films and exhibits interesting (16) Petersen, J. L.; Schramm, C. S.; Stojakovic, D. R.; Hoffmann, B. M.; Marks, T. J. J. Am. Chem. SOC.1971, 99, 286. (17) Dirk, C. W.; Inabe, T.; Schoch, K. F.;Marks, T. J. J. Am. Chem. Soc. 105. --. - - ,1539. ---(18) Barger, W. R.; Wohltjen, H.; Snow, A. W. Transducors 85 IEEE
1983. -.
1985,410. (19) Baker, S.; Roberts, G. G.; Petty, M. C. Proc. IEEE 1983, 130, 260. (20) Baker, S.; Petty, M. C.; Robert, G.G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (21) Snow, A. W.; Jarvis, N. L. J. Am. Chem. SOC.1984, 106, 4706. (22) Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 132-134. (23) Kalina, D. W.; Crane, S . W. Thin Solid Films 1985, 132-134. (24) Hann, R. A.; Fryer, J. R.; Gupta, S . K.; Eyres, B. L. Thin Solid Films 1985, 132-1 34. (25) Ruaudel-Teixier, A.; Barraud, A.; Belbeoch, B.; Roulliay, M. Thin Solid Films 1983, 39, 33. (26) McArdle, C . B.; Ruaudel-Teixier, A. Thin Solid Films 1985,
132-134.
Results Study of the Properties in Solution. Numerous articles have been published concerning phthalocyanine aggregation in solut i ~ n . ~ ’ -As ~ ~expected, the properties of CuSls in solution are mastered by aggregation. We have studied CuSls solutions in pure chloroform, chloroform-ethanol, and chloroform-acetic acid mixtures, in order to determine the species involved in our spreading solutions. All spectroscopic measurements were carried out with freshly prepared solutions. Varying the concentration of CuSls in pure chloroform solutions gives no change in the absorption spectra, in the range 10-3-10-7 M, indicating that the species in solution is stable over this concentration range. Increasing the protic character of the solvent does not shift significantly the Soret band (330nm) whereas the Q-band splits into two peaks at 670 and 690 nm, with a large increase in intensity. The CuSls solution, blue in pure chloroform, turns greenish as proticity of the solvent is increased. Figure 2 shows two typical examples: pure CHC13, 670 nm (log e = 4.43)- 330 nm (4.73),and CHC13/CH3COOH = 690.670nm (4.80)330 nm (4.73). According to Gaspard‘s results,30the UV-visible spectra obtained in protic mixtures can be attributed to the monomeric copper tetrapyridinoporphyrazine nucleus in solution. On the contrary, dilution of the CuSla chloroform solution by ethanol gives rise to an almost flat, silent spectrum, presumably due to the very low solubility of cus18 in ethanol. This solvent appears to increase molecular aggregation of the macrocycles. The effects of oxidizing a n d reducing additives in chloroformic solutions of CuSls are the following: The action of gaseous nitrogen dioxide leads to a green solution (Soret band: 360 nm (log t = 4.7);Q band: 670-690 nm (4.9)) stable for months, even under vacuum. Iodine and bromine vapors lead to similar results. On the contrary, the purple chloroform solution (Soret band: 330 nm; very broad Q band: 570 nm) obtained by bubbling a reductive species such as gaseous hydrogen sulfide or hydrazine turns back to the initial blue solution in a few hours. Similar (27) Monahan, A. R.; Brado, J. A,; Deluca, A. F.J. Phys. Chem. 1972, 76, 446. (28) Gruen, L. C. Ausr. J . Chem. 1972, 25, 1661. (29) Zachariasse, K. A.; Whitten, D. G.Chem. Phys. L e r r . 1973, 22, 527. (30) Gaspard, S . Thesis, University Paris VII, 1978.
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The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6239
results are obtained by phase-transfer reduction with sodium dithionite in water (transferring agent, tetra-n-pentylammonium chloride). Surface Pressure us. Area Curves. LB Films. The CuS18 monolayer on pure water collapses at about 42 mN-m-'. It can sustain a surface pressure of 38 mN.m-' for a long time. At the pressure of 35 "em-', the molecular area is 135 A2, which suggests that molecules are arranged with their macrocycles not perfectly parallel to the water surface (the tetrapyridinoporphyrazine nucleus would have filled a square about 150 A2 in area). When the subphase is changed for diluted hydrogen peroxide or diluted hydrochloric acid (approximately 0.05 M in both cases), the collapse begins at 35 mN.m-' and the molecular area just before the collapse is 150 A2, which is the area expected for a macrocycle lying flat on the water surface. If the spreading solution is diluted by good amphiphilic molecules like w-tricosenoic acid (wC23) or docosanol, the film is greatly reinforced. For example, a 45-fold excess of the wC23 molecule allows us to transfer the resulting film onto solid substrates at a pressure of 45 "em-'. This mole ratio corresponds to a calculated area ratio of 5.5 on the bath. As will be seen later, ESR studies show that the macrocycles lie flat in this case, indicating an area of 150 A2 per CuS18molecule. From the area measured at 35 mN.m-' for a unit of 45 wC23 1 CuS18 (875 A2) it turns out that 5 wC23 molecules out of 45 climb and stay on the porphyrazine nucleus. This result is rather surprising, although a better interchain cohesion, and consequently a stabilization of the film, results from the better filling of the interchain holes above the macrocycles (a better chain packing increases van der Waals forces). As a consequence, the deposition ratio increases on downstrokes, compared to pure CuS18,and becomes close to unity. The CuS18-docosanol film seems quite different. Molecular areas show simple additivity. This is consistent with a two-phase system, without mixing of the two molecules in the monolayer. As ,will be seen later, this result is confirmed by ESR measurements on LB films, which indicate a noticeable coupling between neighboring copper ions. All these films can easily be transferred onto solid substrates, giving good Y-type deposition. Spectroscopic Properties of LB Films. For every example, the UV-visible absorption spectrum was recorded between 200 and 900 nm, and the infrared absorption spectrum between 3700 and 1000 cm-'. Figure 3a shows the UV-visible spectrum of a 180-layer sample of pure CuS18. Compared with the spectrum of the spreading solution (Figure 2), it exhibits a tremendous hypsochromic shift of the Q band, whereas no measurable change takes place in the Soret band; the Q band, now centered a t 575 nm, becomes very broad and its intensity is decreased. Upon dissolution of this sample back in chloroform, the spectrum of the original spreading solution is not restored, a t least at short term. The original spectrum is only restored after a week's exposure to room air, or one night's exposure under vacuum. The infrared spectrum of pure CuSls LB films (Figure 4) is identical with that of solid CuS18,excent the stretching vibration of the C=N+ bond which has shifted to 1620 cm-', indicating a small weakening of the bond strength. LB films of CuS18diluted by docosanol exhibit the same UVvisible and infrared sgectra as LB films of pure CuS18,whatever the molar ratio. This result confirms the two-phase system proposed for the corresponding monolayers. On the other hand, LB films of pure CuS18built on a hydrogen peroxide or hydrochloric acid solution exhibit spectral features close to those of the starting product: Soret band at 320 nm, broad Q band centered at 670 nm, C=N+ stretching vibration at 1640 cm-'. This result shows that the composition of the subphase can modify the nature of the macrocycle under study. The same holds for the composition of the monolayer itself in the case of mixtures: Figure 5 shows the UV-visible spectrum of a freshly prepared 500-layer sample of CuS18 diluted by a 45-fold excess of w-tricosenoic acid. This spectrum appears quite
+
WAVELENGHT
(nm)
I WAVELENGHT
0
(nm)
Figure 3. (a, top) UV-visible spectrum of a 180-layer sample of pure Cuslg. (b, bottom) UV-visible spectra of a 170-layer sample of pure cuslg dissolved in chloroform: just after dissolving (-), after one hour (- - -), after one night (-
a-).
The solution was kept under vacuum.
Ir, lnil I
1
v, WAVENUMBER
Bw
9
?, U
IU
I
(cm-1)
Figure 4. Infrared spectrum of a 180-layer sample of pure cuslg.
6240 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
I
I 200
400 WAVELENGHT
I
I
Palacin et al.
,
6 0 0 8 0 0 (nm)
Figure 5. UV-visible spectrum of a freshly prepared 500 layer sample of CuSI8diluted by a 45-fold excess of w-tricosenoic acid.
similar to the spectrum of monomeric tetrapyridinoporphyrazine in solution, the Q band being divided into two peaks at 670 and 690 nm and exhibiting a greater intensity than the Soret band at 365 nm. However, this spectrum undergoes a noteworthy evolution during the first hours following the deposition: the double Q band vanishes into a broad band centered a t 670 nm and the Soret band slowly shifts to 330 nm. This evolution can be attributed to a reversible reaction between CuS18and ~ C 2 3 . ESR Study of LB Films. The spatial ordering of the porphyrazine rings in LB films was studied via anisotropic electron spin resonance, by recording spectra for different values of the angle 8 between the dc magnetic field and the direction of the normal to the sample plane. Whenever the coupling between neighboring copper ions is strong, the spectrum exhibits a weak, wide and poorly anisotropic signal. Of course, this coupling is present in pure CuS18LB films, whatever the initial bath. It is also found in layers of pure CuS18 alternate with layers of pure W C ~This ~ . result confirms a small overlap of the porphyrazine nuclei in the monolayer built on pure water. A noticeable coupling is also present in CuS18LB films diluted by docosanol, even though some anisotropy could be detected. This last result confirms that CuS18environments are almost the same in pure and docosanol-diluted LB films. Figure 6 shows the ESR spectra of a 500-layer sample of CuSls diluted by a 45-fold excess of wC23, for 8 = Oo and 8 = 90°. For 8 = 90°, the signal is composed of a single line at g = 2.06 onto which is superimposed the classical superhyperfine structure, arising from the first four nitrogen atoms neighboring the copper ion in the porphyrazine ring. The presence of this superhyperfine structure is indicative of complete lack of copper-copper coupling.7*31For 8 = Oo, the signal is composed of four equidistant peaks, approximately equal in magnitude. The amplitude and the g location of these four peaks were studied as a function of 8. The experimental curves were fitted to Lesieur's model.32 The copper tetrapyridinoporphyrazinerings are found parallel to the substrate within f 5 O . The ESR spectra are definitely stable in time and do not appear to follow the evolution of the visible spectrum already mentioned. This study was repeated with various dilution ratios of ~ C 2 3 and showed that the acid molecule prevents plane-to-plane spin coupling and in-plane molecular aggregation. Comparative ESR (31) Rollmann, L. D.; Iwamoto, R. T. J . Am. Chem. Soc. 1968,90, 1455. (32) Lesieur, P. Thesis, University Paris VI, 1986.
Figure 6. ESR study of a 500-layer sample of CuS18diluted by a 45-fold
excess of w-tricosenoic acid. R
Figure 7. (a, top) ESR spectrum for 0 = Oo of homolayers of C U S , ~ diluted by a six-fold excess of w C ~(b, ~ .bottom) ESR spectrum for 0 = 0 ' of alternate layers made of the above mixture and pure wCZ3.
measurements were performed on diluted homolayers ( lCuSla + 6 ~ C 2 3Le., an area ratio of 1:0.75) and alternate layers made of the above mixture for one layer and pure ~ C 2 3for the other one. Figure 7a,b shows the corresponding respective spectra at 8 =. ' 0 In the case of alternate layers, the spectrum in Figure 7b exhibits the classical characters of noncoupled spins, as in Figure 6. In the case of homolayers, the spectrum is composed of two superimposed components: one is the classical four-peak spectrum of noncoupled spins, and the other one is the single wide line characteristic of pure CuS18homolayers (strong spin coupling). These two sorts of signals originate in the facing or nonfacing macrocycles in the double polar plane. This experiment theoretically gives access to the coupling constant, but no quantitative
Chemical Reactivity in Monolayers treatment of the data could be made since no quantitative model exists yet. It is noteworthy that, for low dilution ratios of wC23 (up to 1:6), the UV-visible spectrum of diluted LB films is similar to the spectra of pure c u s l 8 LB films. All the ESR measurements were calibrated by comparison with the spectrum of cupric sulfate pentahydrate. This calibration points out that the number of spins/number of porphyrazine molecules equals 100% f 30%. So the resonant species cannot be an impurity or a minority product.
Discussion As expected, substituting pyridinium rings for the benzo rings of the parent phthalocyanine induces significant changes in the donoracceptor properties of the macrocycle, and at the same time it slightly modifies the energy levels of the molecular orbitals.33 Unfortunately, aggregation, strengthened by the hydrophobic chains, greatly complicates the study of the properties in solution. The present work concentrates on studying the properties of an amphiphilic porphyrazine, in a monolayer on aqueous subphases and in LB films. c u s l 8 gives good films and can easily be transferred onto solid supports, building a well-organized assembly of porphyrazine nuclei. Analysis of the ESR data recorded from LB films of C&l8 diluted with wCZ3confirms our previous results on tetrapyridinoporphyrins:when strong hydrophilic groups, like pyridinium rings, are bonded to the macrocycles, these large molecules lie flat on the water surface and also on the substrates. In the case of pure LB films of either pyridinoporphyrins or porphyrazines, this gives rise to two assemblies of electron-rich aromatic cores facing each other in the polar planes of the LB films. These two amphiphilic macrocycles also exhibit similar physical properties: our first X-ray analysis indicates the same periodicities for LB films of amphiphilic porphyrazine and amphiphilic porphyrins (40 A). Also, as in the porphyrin the regular porphyrazine matrix becomes conductive (a N 10-I C2-I.cm-l) under iodine, bromine, or nitrogen dioxide pressure. Nevertheless, the tetrapyridinoporphyrazinering seems to have quite different chemical properties, due to the high conjugation of the four cationic nitrogens with the central aromatic core. Besides its donor properties toward electron-deficient species such as iodine or nitrogen dioxide, which were clearly Seen in chloroform solutions and in LB films, cus18 in the monolayer phase appears to act as a Lewis acid toward the water bath, giving rise to a virtually reduced porphyrazine ring by accepting an electron doublet from water. According to Iwatsu,lz such reactions may be responsible for the emergence of abnormal crystalline forms of metallophthalocyanines in the presence of alcohols. The new species, which contains extra electrons compared to the starting ring, exhibits particular physical properties, which are definitely different from those of the parent compound. The spectroscopic features of homolayers built on pure water are the same as those obtained by chemical reduction of a CuSl8 solution. They are also similar to those recently described for the first two electrochemically reduced forms of copper octacyano~hthalocyanine.~ The charge transfer from water to the free eg levels of the tetrapyridinoporphyrazine leads to the classical UV-visible spectrum of the reduced phthalocyanine. The increase in the electronic density of the tetracationic porphyrazine ring on water decreases the electrostatic repulsion arising from the positive charges and explains the emergence of partially aggregated forms in mono- and multilayers and the reduction in molecular area. This is confirmed by the action of Lewis acid additives such as wC23, hydrochloric acid, or hydrogen peroxide which restore the intrinsic molecular area ( 1 50 A2) by inhibiting the reactions with water. ESR measurements also confirm this trend to molecular interactions: plane-teplane and in-plane couplings are large enough to perturb the ESR spectra deeply. The ESR spectra of pure LB films provide a second very important information: since the ESR signal is still present and involves all the copper ions, the copper (33) Gal'pem, E. G.; Luk'yanets, E. A.; Gal'pem, M. G. Izu.Akad. Nauk.
SSSR.Ser. Khim. 1913, 9, 1976.
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6241 porphyrazine nucleus undergoes a two-electron-donating complexation. A one-electron process would have led to the disappearance of the ESR signal of the copper ions. Moreover, doubly reduced copper porphyrazine species are well-known to be much more stable than the singly reduced form, exhibiting almost the same ESR characteristics as the parent c o m p o ~ n d . ~ ?It~is~ * ~ ~ noteworthy that the "reduced" complex is definitely stable in LB films. The donating agent is not removed by long exposure to vacuum or moderate heating. Water can however be replaced in LB films by stronger Lewis bases such as hydrogen sulfide, or Lewis acids such as nitrogen dioxide, iodine, or bromine. On the contrary, the complex, like all reduced forms of phthalocyanine, is slowly broken in solution. In this case the charge-transfer bond is replaced by aggregative interactions of the macrocycles. In the presence of Lewis acids like w-tricosenoic acid, hydrogen peroxide, or hydrochloric acid in the bath or in the monolayer, the formation of the charge-transfer complex with water is inhibited and cusl8 appears in its tetracationic form: electrostatic repulsions in the monolayer prevent in-plane overlapping, leading to the normal molecular area and, in the case of nonsurfactant additives, to a weakened cohesion of the monolayer. UV-visible spectra of LB films built with oxidative additives in the subphase are very similar to those of the spreading solution, the Soret peak and the Q band undergoing only a small hypsochromic shift and some broadening due to the plane-to-plane interaction of the macrocycles. The stretching vibration of the C=N+ bond, which is expectedly weakened in the reduced form (1620 cm-') appears normally a t 1640 cm-' in the pure material. However, the slow evolution observed for LB films of cus18 diluted with a great excess of w-tricosenoic acid shows that the macrocycle undergoes an actual reaction with the fatty acid, probably involving water molecules. As has been showed by numerous a ~ t h o r s , ~the ,~~,~~ ESR signal of the copper ion is not significantly affected by this reaction, which involves the only aromatic core, through a modification of the electron distribution and consequently changes the UV-visible spectrum. A slow removal of water might be the reason of this evolution. Like any phthalocyanine, cusl8 is a Lewis base and can form charge-transfer complexes with electron-accepting species, as it was obviously showed in chloroform solution with acetic acid or with nitrogen dioxide. So, we can reasonably affirm that, in the presence of an oxidative species in the bath or in the monolayer, c u s l 8 reacts as an electron donor toward it. However, the spectral features of the resulting complex are almost similar to those of the spreading solution, indicating that the charge transfer is weak in these cases. It is noteworthy that direct molecular interactions between the macrocycles undergo only small effects (=10 nm) on absorption wavelengths. The close contact of porphyrazine nuclei always gives structureless broad bands, but the effect of aggregation is more marked on ESR spectra; spin coupling hides both anisotropy of the signal and the superhyperfine structure.
Conclusion All our results evidence the outstanding donor-acceptor properties of copper tetraoctadecyltetrapyridinoporphyrazinium bromide, through reactions in solution and in monolayers toward Lewis bases such as water or hydrogen sulfide, or Lewis acids such as nitrogen dioxide or w-tricosenoic acid. Three different forms of the copper porphyrazine are observed: i. The parent tetracationic species, which is seen in the crystalline state and in stable aggregates in pure chloroform solutions; ii. A virtually oxidized form, resulting from a weak charge transfer from the aromatic core to the accepting species, which is seen in protic solvents, in donor-acceptor complexes with nitrogen dioxide or iodine in chloroformic solutions, and in LB films built in the presence of an excess of Lewis acids in the bath or in the monolayer itself; (34) Woehrle, D.; Gitzel, J.; Okura, I.; Aono, S. J . Chem. SOC.,Perkin Trans. 2 1985, 1171. ( 3 5 ) Raynor, B.;Robson, M.; Torrens-Burton, A. S. M. J . Chem. SOC., Dalton Trans. 1911, 2360. (36) Roberts, E. M.; Koski, W. S. J . Am. Chem. SOC.1961, 83, 1865.
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J . Phys. Chem. 1986, 90, 6242-6247
iii. A virtually reduced form, resulting from a strong twoelectron charge transfer from water to the copper porphyrazine, which is seen in LB films built on pure water and in solutions containing other reducing agents. Each of these species exhibits its own characteristic spectral features. This explains the large difference between spectra of LB films made in various conditions. Molecular interactions modify only slightly these spectra by broadening and slightly shifting the absorption electronic bands. Because of its lesser net charge, the reduced species exhibits a higher tendency to interact with its neighbors. Clearly, the very easy donating complexation of this porphyrazine macrocycle arises from the substitution of the pyridinium rings for the benzo rings of the phthalocyanine. A similar effect was observed by Louati et ai. with cyano substituents.’ The accepting complexation with Lewis acids undergoes a weaker charge transfer, since the spectral features and the ESR data remain similar to those of the parent tetrapyridinoporphyrazine. This macrocycle can therefore be considered as an amphoteric Lewis species. Besides these chemical properties, ESR measurements show that LB films of this copper porphyrazine are well ordered, with the macrocycle lying flat on the substrate. The combination of these two properties is a particuiarly powerful tool to study electron transfer between a donor and an acceptor in a controlled geometry. Since amphiphilic phthalocyanines are (i) particularly easy to handle by the LB method and (ii) particularly versatile from their two degrees of freedom in their donor-acceptor properties (peripheral substitution and choice of central ion), they exhibit the right characteristics to be used as an adjustable material for basic studies on molecular electronics. Experimental Section Syntheses. Copper tetrap”idin0[3,4-b:3’4’-g:3’’,4”-1:3’~’,4~~’qlporphyrazine (CuTPyPz) was prepared as described by Iwashuma3’ and purified by repeated recrystallizations from (37) Iwashuma, S.;Sawada, T. Res. Bull. Meisei Uniu., Phys. Sei. Eng. 1983, 19, 43.
concentrated sulfuric acid. It was characterized by infrared and UV-visible spectroscopy. I R (KBr): 1608 cm-I; 15 10, 1405 cm-’(vc-c); 1108 cm-’@CH); 750 cm-’(yCH). UV (HSO,): 705 nm (log t = 4.9); 680 nm (4.78); 375 nm (4.36). CuTPyPz was quaternized by refluxing in dimethylformamide for 7 h in the presence of a 20-fold excess of octadecyl bromide. The crude copper tetraoctadecyltetrapyridino[3,4-b:3’,4‘-g:”,4”-f:3‘‘f,4“’qlporphyrazinium bromide (CuSls) [Figure 11 was purified by repeated Soxhlet extractions with diethyl ether, in order to remove the excess of the long-chain reagent, and a final recrystallization from a chloroform-diethyl ether solution. A gel filtration chromatography on Sephadex LH20 was performed, in order to separate CuSls from the less quaternized macrocycles, but this purification remained unsatisfactory. Anal. Calcd for ClooH160N12CuBr3(OH)(H20)(NH3): C, 63.7; H, 8.8; N, 9.6; Cu, 3.4; Br, 17.7. Found: C, 63.8; H, 8.6; N, 9.5; Cu, 3.5; Br, 12.7. IR (KBr) 2926-2854 cm-’(yCH2), 1640 cm-’(C=N+ ).38 Mono- and Multilayer Experiments. Homolayers were built on a trough patented by B a r r a ~ d .The ~ ~ trough used in the present work for alternate layers was recently described by Barraud et a1.40 The subphase was Millipore Q-grade water, unless notice, and all experiments were conducted at 20.22 OC in a nitrogen atmosphere. The solid substrates, which were cleaned by the usual rigorous procedure, were either quartz (for ESR measurements) or calcium fluoride. Transfer speed generally ranged between 0,5 c m - m i d (especially for the first layers) and 2 cmmin-I; the coated substrates were dried between each dip under an additional dry nitrogen jet. Spectroscopic Study. Infrared spectra were carried out on a Perkin Elmer 180 under nitrogen atmosphere. A Cary 2390 was used for UV-visible absorption spectroscopy. ESR measurements were performed on a ER 200 D Bruker apparatus, in the X band. (38) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman-Hall: London, 1975; Vol. I. (39) Barraud, A.;Leloup, J. French Patent No. 83 19770. (40)Barraud, A.;Leloup, J.; Gouzerh, A.;Palacin, S. Thin Solid Films 1985, 132-134.
Thermodynamics of the Unsymmetrical Mixed Electrolyte HCI-SrCi,. Pltzer’s Equations
Applications of
Rabindra N. Roy,* James J. Gibbons,+ Lakshmi N. Roy, and Michael A. Greene Department of Chemistry, Drury College, Springfield, Missouri 65802 (Received: February 18, 1986; In Final Form: July 1 1 , 1986)
Emf measurements were carried out on solutions at temperatures from 278.15 to 318.15 K, for ten values of the constant ionic strength between 0.1 and 5.0 mol kg-I, using a cell without liquid junction of the type Pt;H,(g, 1 atm)lHCl(ml), SrCl2(m,)1AgCl,Ag(A). The results are interpreted in terms of ionic interactions specific to the mixtures by use of Pitzer’s as well as linear formalism of mixed electrolyte solutions. The estimates of Pitzer mixing parameters S6H,Srand $H,Sr,CI, representations of temperature-invariant estimates of 6SOH,s,/6Tand 6$Hsr,cJ6T, have been made. A brief table of the activity coefficients is also given.
Introduction In earlier studies,’-” we have investigated ion-ion interactions in mixtures of hydrochloric acid with bivalent (and trivalent) chloride salts by emf measurements, which are particularly useful for examining the thermodynamic properties of mixed-electrolyte solutions, since they directly yield the activity coefficients of the solute in which the electrodes are reversible. These systems of the type HCI + MClz + H 2 0have been successfully treated by ‘Present address: Analytical Services, DAYCO Technical Center, P.O. Box 3258, Springfield, MO 65808.
0022-3654/86/2090-6242$01.50/0
the equations of PitzerS for cases in which ion association is negligible and the effects of higher-ordbr electrostatic terms6 on (1) Roy, R. N.;Gibbons, J. J.; Peiper, J. C.;Pitzer, K.S.J . Phys. Chem. 1983, 87, 2365.
(2) Roy, R. N.;Gibbons, J. J.; Ovens, L. K.;Bliss, G. A.;Hartley, J. J. Chem. sot.* Faraday Trans. 1405. (3) Roy, R. N.; Gibbons, J. J.; Trower, J. K.; Lee, G. A.J . Solution Chem. 1980, 9, 535, (4) Roy, R. N.;Gibbons, J. J.; Bliss, D. P.; Baker, B.; Casebolt, R. G. J . Solution Chem. 1980, 9, 12. ( 5 ) Pitzer, K . S.J . Phys. Chem. 1973, 77, 268. J.
*
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783
0 1986 American Chemical Society
