Langmuir 2007, 23, 2555-2568
2555
“Two-Point” Assembling of Zn(II) and Co(II) Metalloporphyrins Derivatized with a Crown Ether Substituent in Langmuir and Langmuir-Blodgett Films Krzysztof Noworyta,*,†,‡ Renata Marczak,‡ Rafal Tylenda,‡ Janusz W. Sobczak,‡ Raghu Chitta,§ Wlodzimierz Kutner,‡,| and Francis D’Souza*,§ Department of Mineral, Analytical and Applied Chemistry, UniVersity of GeneVa, quai Ernest-Ansermet 30, CH-1211 GeneVa, Switzerland, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland, Department of Chemistry, Wichita State UniVersity, Wichita, Kansas 67260-0051, and Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszynski UniVersity in Warsaw, Dewajtis 5, 01-815 Warsaw, Poland ReceiVed September 13, 2006. In Final Form: NoVember 15, 2006 The effect of “two-point” interactions of Zn(II) and Co(II) metalloporphyrins, bearing 15-crown-5 ether peripheral substituents, on their assembling in Langmuir and Langmuir-Blodgett (LB) films was investigated. That is, simultaneously, the central metal ion of the porphyrin was axially ligated by a nitrogen-containing ligand in the emerged part of the Langmuir film on one hand, and a suitably selected cation pertaining in the subphase solution was supramolecularly complexed by the crown ether moiety in the submerged part of the film on the other. The compression and polarity properties of the Langmuir films of the derivatized free-base 5,10,15-triphenyl-20-(benzo15-crown-5)porphyrin, H2(TPMCP), and the corresponding cobalt(II) and zinc(II) metalloporphyrins, denoted as Co(TPMCP) and Zn(TPCMP), respectively, as well as inclusion complexes of the metalloporphyrins with selected cations were investigated. For the axial ligation of Zn(II) and Co(II), pyrazine (pyz) and 4,4′-bipyridnine (bpy) aromatic as well as piperazine (ppz) and 1,4-diazabicyclo[2.2.2]octane (DABCO) cyclic heteroaliphatic ligands were selected. The films were formed on the water subphase solution in the absence and presence of LiCl, NaCl, or NH4Cl. The Langmuir films were built of monolayer J-type aggregates of tilted porphyrin macrocycles. The porphyrins formed rather labile complexes with the cations in the subphase. Nevertheless, the XPS analysis revealed that these cations were LB transferred together with the porphyrins onto solid substrates. In the Co(TPMCP) Langmuir films formed on the water subphases, Co(II) was complexed by aromatic but not cyclic heteroaliphatic ligands, while, in these films formed on the NaCl subphase solutions, the metalloporphyrin was also complexed by DABCO. In Langmuir films spread on alkaline subphase solutions, both aromatic and heteroaliphatic ligands formed complexes with Co(TPMCP) of different stoichiometries. The X-ray reflectivity and GIXD measurements performed on selected LB films revealed some structure-building effects of the axial ligation.
1. Introduction The formation of supramolecular films with well-defined geometries and orientations is of considerable recent interest, as these films are essential for developing nanostructured molecular electronic and biomimetic devices.1,2 In this regard, one of the most convenient approaches is to tune the organization of molecular films formed on the surface of the desired substrate by self-assembly. Particularly, the combination of self-assembly with the Langmuir and the Langmuir-Blodgett (LB) techniques is very fruitful in the formation of well-organized molecular films at the air-water interfaces and in their transfer onto solid substrates of choice, respectively. Accordingly, several welldefined supramolecular two-dimensional systems have been prepared at the interfaces.1 Often, the well-defined architectures of these systems have been obtained through interactions between * To whom correspondence should be addressed.
[email protected];
[email protected]. † University of Geneva. ‡ Polish Academy of Sciences. § Wichita State University. | Cardinal Stefan Wyszynski University in Warsaw.
E-mail:
(1) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (2) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: New York, 2001. (b) Petty, M. C.; Bryce, M. R.; Bloor, D. Introduction to Molecular Electronics; Oxford University Press: New York, 1995. (c) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (d) Swift, J. A.; Pivovar, A. M.; Reynolds, A. M.; Ward, M. D. J. Am. Chem. Soc. 1998, 120, 4850.
water-insoluble film components, dispersed onto surfaces of aqueous subphase solutions, and various solutes present in the solutions. As the solutes, alkali, alkaline earth, and a few transition metals are commonly used to control the film architecture because of their high solubility and structural simplicity. If a well-defined surface structure is to be useful for the construction of molecular electronic devices, then the filmforming molecules should contain redox active sites, photoactive sites, or other active sites. Moreover, physicochemical transformations of these sites should be triggered upon the change of experimental or ambient conditions.1,2 Toward achieving this goal, porphyrins3 are particularly attractive candidates as constructing blocks of the supramolecular assemblies at the airwater interface because of their remarkably rich photochemical, electrochemical, and structure-building properties. These properties can be tailored by selecting proper central metal ions in the porphyrin macrocycles, axial ligands, and peripheral substituents. For assembling Langmuir films, these substituents should be capable of binding entities present in the subphase solution. Moreover, the axial ligation properties of the metalloporphyrins can be utilized for the self-organization of the molecules. In the present study, we have devised a novel “two-point” self-assembly approach to form well-defined Langmuir and LB films of metalloporphyrins (Scheme 1). In this approach, the (3) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2001; Vol. 1-10.
10.1021/la0626858 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007
2556 Langmuir, Vol. 23, No. 5, 2007 Scheme 1
central metal ion is axially ligated and the peripheral macrocyclic substituent forms an inclusion complex with cations present in the solution (Scheme 2). Accordingly, meso-tetraphenylporphyrin was derivatized with a 15-crown-5 ether substituent on one of its meso-phenyl rings. Zinc(II) and cobalt(II) were selected as the central metal ions of the porphyrin macrocycle, since the resulting metalloporphyrins are known to bind one4 and two axial ligands,5 respectively, to ultimately form penta- and hexacoordinated complexes.6 Besides, the crown ether moiety is likely to bind an alkali metal or an ammonium cation in the subphase solution.7 That way, it enhances the stability and structuring of Langmuir films of a hydrophobic entity, such as a crown ether appended fullerene.8 Owing to the presence of these two binding sites in the metalloporphyrin molecule, one may expect the formation of well-defined, self-assembled Langmuir films, as in the case of other heteroporphyrin systems.9 As demonstrated here, this is indeed the case for the crown ether appended metalloporphyrins in the presence of nitrogencontaining ligands in films and of Li+, Na+, or NH4+ in the subphase solutions. 2. Experimental Section 2.1. Chemicals. Reagent grade pyrazine (pyz), 4,4′-bipyridine (bpy), piperazine (ppz), 1,4-diazabicyclo[2.2.2]octane (DABCO), benzo-15-crown-5, pyrrole, and benzaldehyde were purchased from (4) (a) Bobrik, M. A.; Walker, F. A. Inorg. Chem. 1980, 19, 3383. (b) Walker, F. A.; Simonis, U.; Hong, Z.; Walker, J. M.; Ruscitti, T. M.; Kipp, C.; Amputch, M. A.; Castillo, B. V.; Cody, S. H.; Wilson, D. L.; Graul, R. E.; Yong, G. J.; Tobin, K.; West, J. T.; Barichievich, B. A. New J. Chem. 1992, 16, 609. (c) D’Souza, F.; Rath, N. P.; Deviprasad, G. R.; Zandler, M. E. Chem. Commun. 1999, 635. (5) (a) Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1150. (b) Lamar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1790. (c) Hambright, P. Chemistry of Water Soluble Porphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 3, pp 129-210. (6) Sanders, J. K. M.; Bampos, N.; Clyde-Watsow, Z.; Darling, S. L.; Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. Axial Coordination Chemistry of Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 3, pp 1-48. (7) (a) Matsumura, H.; Watanabe, T.; Furusawa, K.; Inokuma, S.; Kawamura, T. Bull. Chem. Soc. Jpn. 1987, 60, 2747. (b) Plehnert, R.; Schroter, J. A.; Tschierske, C. Langmuir 1998, 14, 5245. (8) (a) Wang, S.; Leblanc, R. M.; Arias, F.; Echegoyen, L. Thin Solid Films 1998, 327-329, 141. (b) Nierengarten, J.-F. New J. Chem. 2004, 28, 1177.
Noworyta et al. Aldrich and used without further purification. Analytical grade NaClO4 and NaCl were used as received from Fluka (Buchs, Switzerland). Analytical grade LiOH was used as received from Hopkins & Williams, Ltd. (Chadwell Heath, Great Britain). Analytical grade chloroform and methanol, as well as LiCl, NaCl, and NH4Cl, were supplied by POCH (Gliwice, Poland). The water used for the preparation of subphase solutions for Langmuir films was first distilled and then purified (resistivity exceeded 18.2 MΩ cm) with a Milli-Q filtering system of Millipore Corp. (Bedford, MA), which was equipped with one carbon and two ion-exchange cartridge stages. 2.2. Syntheses of 5,10,15-Triphenyl-20-(benzo-15-crown-5)porphyrin, H2(TPMCP), and Its Metal Derivatives, Co(TPMCP) and Zn(TPMCP). The free-base porphyrin derivative was prepared according to the published method.10 For that purpose, first, benzo15-crown-5 was converted into 4′-aldehyde.11 A mixture of pyrrole (50 mmol), benzaldehyde (35 mmol), and benzo-15-crown-5 aldehyde (15 mmol) in propionic acid (300 mL) was refluxed for 2 h. The desired product was separated from the other derivatives and then purified on a basic alumina LC column using a mixture of chloroform and methanol as the eluent. The Zn(II) and Co(II) derivatives were prepared and purified according to the typical procedure.12 2.2.1.5,10,15-Triphenyl-20-(benzo-15-crown-5)porphyrin[H2(TPMCP)]. 1H NMR in CDCl , δ ppm: 8.9-8.83 (m, 8H, β-pyrrole), 8.21 (m, 3 6H, ortho-phenyl), 7.77-7.67 (m, 11H, meta-, para-phenyl and a,b-phenyl protons of crown ether), 7.14 (d, 1H, c-phenyl proton of crown ether), 4.38-4.34 (m, 2H, 1′ ether protons), 4.26-4.23 (m, 2H, 1 ether protons), 4.07-4.03 (m, 2H, 2′ ether protons), 3.933.90 (m, 2H, 2 ether protons), 3.86 (s, 4H, 3,3′ ether protons), 3.82 (s, 4H, 4,4′ ether protons), -2.77 (s, br, 2H, imino). UV-vis (CHCl3) (λmax, nm): 420, 516.5, 551.5, 591.5, 650.5. 2.2.2. 5,10,15-Triphenyl-20-(benzo-15-crown-5)porphyrinatozinc [Zn(TPMCP)]. 1H NMR in CDCl3, δ ppm: 8.89 (m, 8H, β-pyrrole), 8.22-8.17 (m, 6H, ortho-phenyl), 7.74-7.69 (m, 9H, meta-, paraphenyl), 7.57 (d, 1H, a-phenyl proton of crown ether), 7.50 (s, 1H, b-phenyl proton of crown ether), 6.83 (d, 1H, c-phenyl proton of crown ether), 3.74 (s, br, 2H, 1′ ether protons), 3.52 (s, br, 2H, 1 ether protons), 3.02 (s, br, 4H, 3,3′ ether protons), 2.88 (s, br, 4H, 4,4′ ether protons), 2.75 (s, br, 2H, 2′ ether protons), 2.59 (s, br, 2H, 2 protons). UV-vis (CHCl3) (λmax, nm): 423, 549.5, 592. 2.2.3. 5,10,15-Triphenyl-20-(benzo-15-crown-5)porphyrinatocobalt [Co(TPMCP)]. 1H NMR in CDCl3, δ ppm: 15.99 (s, br, 8H, β-pyrrole), 13.37-12.54 (m, br, 6H, ortho-phenyl), 10.02-9.67 (m, 11H, meta-, para-phenyl and a,b-phenyl protons of crown ether), 9.35 (s, br, 1H, c-phenyl proton of crown ether), 5.66 (s, br, 2H, 1′ ether protons), 5.36 (s, br, 2H, 1 ether protons), 4.88 (s, br, 2H, 2′ ether protons), 4.52-4.39 (m, br, 4H, 3,3′ ether protons), 4.33 (s, br, 4H, 4,4′ ether protons), 4.21 (s, br, 2H, 2 protons). UV-vis (CHCl3) (λmax, nm): 412, 529. 2.3. Instrumentation and Procedures. The Langmuir films were prepared and examined using a 601 BAM trough (total area 490 cm2) from Nima Technology (Coventry, Great Britain). The surface pressure was measured with an accuracy of (0.1 mN m-1 using a PS4 sensor (Nima) of a Wilhelmi plate type. The surface potential was measured with a Kelvin probe type KP-2 sensor (Nima). The trough was also equipped with a D1L linear dipper for film transfer onto a solid substrate using the Langmuir-Blodgett (LB) technique. The surface of the subphase solution was cleaned by repeated compressing, by aspirating traces of surface active impurities, and then by expanding until changes of surface pressure of the blank subphase solution remained below 0.2 mN m-1 in the cleaning cycles (9) (a) Conoci, S.; Guldi, D. M.; Nardis, S.; Paolesse, R.; Kordatos, K.; Prato, M.; Ricciardi, G.; Vicente, M. G. H.; Zilbermann, I.; Valli, L. Chem.sEur. J. 2004, 10, 6523.. (b) Guldi, D. M.; Zilbermann, I.; Anderson, G. A.; Kordatos, K.; Prato, M.; Tafuro, R.; Valli, L. J. Mater. Chem. 2004, 14, 303. (10) Thanabal, V.; Krishnan, V. J. Am. Chem. Soc. 1982, 104, 3643. (11) Hyde, E. M.; Shaw, B. C.; Shepherd, I. J. Chem. Soc., Dalton Trans. 1978, 1696. (12) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: New York, 1977.
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
Langmuir, Vol. 23, No. 5, 2007 2557 Scheme 2
of compression and expansion. A sample solution of ∼1 mM (30100 µL) or ∼0.1 mM (300-1200 µL) porphyrin or metalloporphyrin in chloroform was evenly spread onto the subphase solution surface in a dropwise manner. The chloroform was allowed to evaporate for 15 min from the spread solution sample. Either water or LiCl, NaCl, or NH4Cl solutions of different concentration and pH values were used as the film subphase. The mixed metalloporphyrin-ligandchloroform solutions were prepared immediately prior to measurements by mixing, in appropriate mole ratios, samples of stock solutions of 0.1-1 mM metalloporphyrin in chloroform with a stock solution of ∼1 mM ligand in chloroform. That way, the total number of moles of the resulting solution samples was kept constant within the range 0.15-3 µmol but the mole fraction of metalloporphyrin differed for a given experimental series. Isotherms of surface pressure (π) and surface potential change (∆V) against area per molecule (A) were recorded simultaneously for the Langmuir films during compression at the speed of 25 cm2 min-1.13 Surface morphology changes of the Langmuir films during compression and expansion were imaged with Brewster angle microscopy (BAM) using a 20-µm resolution MiniBAM microscope from NFT-Nanofilm Technologie, GmbH (Goettingen, Germany). The Langmuir films were transferred onto different solid substrates, such as quartz slides, Au-coated glass slides, and highly oriented pyrolytic graphite (HOPG) plates, using the LB technique, which involved vertical stroking with the 5 mm min-1 immersion and emersion speed at constant surface pressure.
The UV-vis spectra of the LB films transferred onto quartz slides were recorded with a 0.1-nm resolution using a UV-2501PC or UV-3100 spectrophotometer from Shimadzu Corp. (Tokyo, Japan). The UV-2501PC spectrophotometer, equipped with a homemade polarizing accessory, was also used for linear dichroism measurements. The X-ray photoelectron spectroscopy (XPS) measurements were performed with an Escalab-210 spectrometer from VG Scientific (East Grinstead, U.K.) using Al KR X-ray radiation. Spectra were analyzed with the Avantage software from Thermo Electron (East Grinstead, Great Britain). The grazing incidence X-ray diffraction (GIXD) and X-ray reflection measurements were performed on a X04SA beamline at the Swiss Light Source Synchrotron facility (Villigen, Switzerland) using an in situ surface diffractometer equipped with a pixel detector. The X-ray beam of the 12.4 keV (λ ) 0.1 nm) energy was used for all experiments. The grazing angle of the incidence beam was 0.1 or 0.15°. All measurements were performed with the use of a Capton hood under helium atmosphere to minimize background scattering and possible sample decomposition caused by irradiation in the presence of oxygen. Samples were arranged vertically in all experiments. In-plane diffraction scans were accomplished with an X-ray beam parallel or perpendicular to the dipping direction used during the LB film transfer onto a glass or HOPG substrate, which allowed probing of the arrangement of the molecules in both directions. All experiments were performed at ambient temperature, (22 ( 1)°C.
3. Results and Discussion
Figure 1. Isotherms of (a) surface potential change and (b) surface pressure against area per molecule for Langmuir films containing (1 and 1′) 32 nmol of H2(TPMCP), (2 and 2′) 53 nmol of Co(TPMCP), as well as (3 and 3′) 48 nmol of Zn(TPMCP) formed on a water subphase. Compression speed was 50 cm2 min-1.
The compression and polarity properties of the Langmuir and LB films of H2(TPMCP), Co(TPMCP), and Zn(TPMCP) were examined in the absence and presence of selected aromatic or cyclic heteroaliphatic ligands containing nitrogen atoms (Scheme 1) in the films and Li+, Na+, or NH4+ in the subphase solutions. First, the cation effect was examined on the structures of the Langmuir and LB films in the absence of ligands. Then, the structuring effects of the ligand and, finally, both the cation and the ligand were investigated. 3.1. Langmuir and LB Films of Porphyrins in the Absence of Axially Coordinating Ligands. Initially, the π-A and ∆V-A isotherms were simultaneously recorded for the H2(TPMCP), Co(TPMCP), and Zn(TPMCP) films formed on the aqueous subphases (Figure 1). Then, the effect of complex formation between the crown ether moiety and the different cations in the submerged part of the film was studied. It appeared that the parameters of the π-A compression isotherms (Figure 1b), such as the area per molecule determined from the tangents of their highest slopes extrapolated to zero surface pressure (A0), the collapse pressure (πc), and the dynamic compressibility (κ ) -(1/A0)(dA/dπ)T, where T is the temperature in kelvin), were to some extent dependent both on the presence of a central metal ion in the porphyrin macrocycle and on its nature (Table 1). (13) (a) Noworyta, K.; Kutner, W.; Deviprasad, G. R.; D’Souza, F. Synth. Met. 2002, 130, 221. (b) Marczak, R.; Hoang, V. T.; Noworyta, K.; Zandler, M. E.; Kutner, W.; D’Souza, F. J. Mater. Chem. 2002, 12, 2123.
2558 Langmuir, Vol. 23, No. 5, 2007
Noworyta et al.
Table 1. Determined and Estimated Values of the Parameters of the π-A and ∆V-A Isotherms for the Langmuir Films of Porphyrins and Their Supramolecular Complexes with Cations isotherm parameters determined
estimated vertical orientation
porphyrin
A0,1; A0,2 (nm2)
(κ; κ2) × 10-2 (m mN-1)
πc (mN)
µ⊥ (D)
A0 (nm2)
Water Subphase 1.2 0.7 1.1 0.7 1.0 0.7
horizontal orientation
dC-C (nm)
µ⊥b (D)
A0 (nm2)
dC-C (nm)
µ⊥b (D)
0.4 0.4 0.4
3.6 3.5 3.5
2.4-2.7a 2.4-2.7a 2.4-2.7a
1.8 1.8 1.8
1.2 1.9 1.8
2.4-2.7a 2.4-2.7a
1.8 1.8
H2(TPMCP) Co(TPMCP) Zn(TPMCP)
1.1 0.9 1.3
1.2 2.3 1.5
Co(TPMCP) Zn(TPMCP)
1.1 1.3
1.2 2.5
Co(TPMCP)
1.3; 0.9
2.7; 1.1
29
0.1 M LiCl Subphase 0.8 0.7
0.4
2.4-2.7a
1.8
Co(TPMCP)
1.1; 0.9
2.4; 1.4
26
0.1 M NaCl Subphase 0.9 0.7
0.4
2.4-2.7a
1.8
2.2 2.1; 1.3 2.1; 1.5
0.1 M NH4Cl Subphase 21 1.7 0.7 28 1.5 0.7 20 1.2 0.7
0.4 0.4 0.4
2.4-2.7a 2.4-2.7a 2.4-2.7a
1.8 1.8 1.8
H2(TPMCP) Co(TPMCP) Zn(TPMCP)
1.1 1.1; 0.9 1.4; 1.3
26 10 19
0.01 M LiOH + 0.09 M LiCl Subphase 28 0.7 0.4 15 0.7 0.4
a Depending on the orientation of the porphyrin macrocycle and the 15-crown-5 ring in the film. b Based on semiempirical calculations at PM3 parametrization.
The determined κ values indicate the formation of the socalled “expanded liquid” films.14 For the water subphase, these values increase in the order H2(TPMCP) < Zn(TPMCP) < Co(TPMCP), indicating a decreasing order of the density of the molecule packing in these films. The area per molecule determined from the π-A isotherms for films floating on water subphases increases in the order Co(TPMCP) < H2(TPMCP) < Zn(TPMCP) (Table 1). The estimated A0 values are dependent upon the organization of the molecules in the films and range from 0.7 nm2 for the vertical orientation of the porphyrin macrocycle plane to 2.4-2.7 nm2 for its horizontal orientation with respect to that of the interface plane. The latter area value varies in the indicated range with the change in orientation of the crown ether moiety against the macrocycle plane. The collapse pressure for the H2(TPMCP) films is more than twice as high as that for the Co(TPMCP) films (Table 1). Moreover, the compressibility of the H2(TPMCP) films is significantly smaller than that of the Co(TPMCP) films. Apparently, films of the free-base porphyrin are more stable and rigid than those of the metalloporphyrins in the absence of an alkaline metal ion or a ligand. The surface potential change increases markedly in the ∆V-A isotherms (Figure 1a), nearly coinciding with the pressure buildup at the foot of each of the π-A isotherms (Figure 1a). The increase of ∆V was almost the same for all systems studied and was close to 400 mV. Moreover, the initial values of ∆V for each ∆V-A isotherm are close to zero, characteristic of the initial formation of either nonaggregated films or films of almost horizontally oriented porphyrin molecules. The values of the dipole moment component normal to the interface plane (µ⊥) determined from the ∆V-A isotherms using the Helmholtz equation, µ⊥ ) 0A∆V (0 is the electric permittivity of free space and is the electric permittivity of the film, assumed to be unity),15 are summarized in Table 1. Interestingly, the µ⊥ values are small and close to one (14) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold Publishing Co.: New York, 1954. (15) (a) Smith, T. J. Colloid Interface Sci. 1992, 23, 27. (b) Vogel, V.; Mo¨bius, D. J. Colloid Interface Sci. 1988, 126, 408.
another for all porphyrins despite remarkable differences in the area per molecule that they occupy. The values of µ⊥, calculated using semiempirical quantum chemistry calculations with PM3 parametrization (Table 1) for the vertical orientation of porphyrin macrocycles, are very similar for all porphyrins. On the other hand, the value calculated for the horizontally oriented macrocycle for H2(TPMCP) is smaller than those for the metalloporphyrins. For all porphyrins, the experimental µ⊥ values are lower than those calculated for the vertical orientation and closer to the values calculated for the horizontal orientation of the macrocycle. This result indicates that porphyrins floating on the water subphase are strongly tilted against the subphase plane. The tilt angles for all porphyrins seem to be relatively similar, as evidenced by similar experimental and calculated µ⊥ values for all porphyrins. Based on the determined parameters of the compression isotherms, one can postulate the most plausible orientation of the porphyrin molecules in the Langmuir films. That is, the determined A0 values for all three porphyrins studied are on one hand too small to account for the horizontal orientation of the macrocycle planes and on the other hand too high for vertical orientation. Most likely, the monolayer films are formed with the porphyrin macrocycle planes tilted against the interface plane, in accord with the inference that follows from the determined and calculated µ⊥ values. The effect of cation complexation on the properties of the Langmuir films was initially studied by using, first, the water subphase and, then, the 0.1 M NH4Cl subphase (Figure 2). Literature data on the 15-crown-5 complexation of various cations in water16 indicate that NH4+ cations should influence the Langmuir film properties more than the cations of alkali metals because of the higher stability of the (15-crown-5)-NH4+ complex in solution compared to the corresponding complexes of these cations. Comparison of the parameters determined from isotherms recorded for films floating on the water and 0.1 M NH4Cl subphases (Table 1) does not show, however, drastic changes in the organization of molecules in the films. That is, the area per (16) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. ReV. 1985, 85, 271.
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
Figure 2. Isotherms of surface pressure against area per molecule for Langmuir films containing (1) 50 nmol of H2(TPMCP), (2) 42 nmol of Co(TPMCP), and (3) 50 nmol of Zn(TPMCP) formed on the 0.1 M NH4Cl subphase. Compression speed was 25 cm2 min-1.
molecule for the H2(TPMCP) film on the subphase containing NH4+ remains the same as that on water while the collapse pressure is lower for the latter subphase, indicating the formation of a less stable film. Higher compressibility determined under the former than the latter solution conditions indicates that this film is also more of a liquid type on the NH4+-containing subphase than it is on water.
Langmuir, Vol. 23, No. 5, 2007 2559
Figure 3. Isotherms of surface pressure against area per molecule for Langmuir films containing (1) 54 nmol of Co(TPMCP) on water, (2) 42 nmol of Co(TPMCP) on 0.1 M LiCl, (3) 42 nmol of Co(TPMCP) on 0.1 M NaCl, and (4) 42 nmol of Co(TPMCP) on 0.1 M NH4Cl. Compression speed was 25 cm2 min-1.
The determined values of µ⊥ for the porphyrin films floating on the 0.1 M NH4Cl subphase are compiled in Table 1. Apparently, these values are higher than those for the films floating on water for all studied porphyrins.
order Li+ < Na+ < NH4+.16,17 Therefore, the presence of NH4+ in the subphase was expected to most strongly influence the isotherm shapes. Surprisingly, the parameters determined from the compression isotherms for films spread on different subphases appeared to be relatively close to one another (Table 1). For all studied subphases, two “liquid condensed” phases were formed. The area per molecule for the first phase was the highest for the Co(TPMCP) film floating on 0.1 M LiCl, while the values of A0 determined for films on 0.1 M NaCl and 0.1 M NH4Cl were almost identical. The area per molecule values determined for the second phase on all studied subphases were the same and were comparable to those observed for the Co(TPMCP) films floating on water, indicating similar organization of the molecules in the film. The Co(TPMCP) film spread on 0.1 M LiCl is more stable than the same film spread on either 0.1 M NaCl or 0.1 M NH4Cl, as evidenced by the higher collapse pressure for the former. Nevertheless, all films spread on the subphases containing cations are more stable than those spread on the water subphase. For the Co(TPMCP) films, the values of µ⊥ are the lowest in the case of 0.1 M LiCl and are higher for films on NaCl and NH4Cl, reaching 1.5 D for the latter. Generally, the effect of the structure of the double layer, developed at the air-subphase solution interface on the dipole moments of molecules in Langmuir films, is relatively small.18 Accordingly, at 0.1 M ionic strength, at an ∼1 nm2 area per molecule, and at a degree of association of the porphyrin-cation complex in the range 0.2 e R0 e 0.3 (in agreement with the calculations of ∆G values below) characteristic of the studied systems, the estimated value of potential at the outer Helmoltz plane with respect to the bulk solution is in the range 20 e φ2 e 50 mV. This small φ2 correction of the measured ∆V affects the determined µ⊥ values merely by ∼0.1 D. By using the shifts of the π-A isotherms resulting from complexation,19 one can determine the free energy of complex
To study the effect of the nature of the cation present in the subphase on the properties of the Langmuir films of the porphyrins in more detail, the Co(TPMCP) film was selected as a model system. Isotherms were recorded for the Co(TPMCP) films spread on water, 0.1 M LiCl, 0.1 M NaCl, and 0.1 M NH4Cl (Figure 3). The stability constants of the complexes of 15-crown-5 with the alkali metal or ammonium cations in water increase in the
(17) (a) Smetana, A. J.; Popow, A. I. J. Solution Chem. 1980, 9, 183. (b) Izatt, R. M.; Terry, R. E.; Haymore, B. L.; Hansen, L. D.; Dalley, N. K.; Avondet, A. G.; Christensen, J. J. J. Am. Chem. Soc. 1976, 98, 7620. (c) Cygan, A.; Biernat, J. F.; Chadzynski, H. Pol. J. Chem. 1979, 53, 929. (18) Cavalli, A.; Dynarowicz-Latka, P.; Oliveira, O. N.; Feitosa, E. Chem. Phys. Lett. 2001, 338, 88. (19) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press, Inc.: New York and London, 1961.
Interestingly, two phases are formed in the films of Co(TPMCP) and Zn(TPMCP) in the presence of NH4+ in the subphase. They manifest themselves by the change in the slope of the π-A isotherms. Depending on the nature of the porphyrin, the area per molecule is 0.1-0.2 nm2 higher for one phase than for the other. The resulting compressibility values indicate that both phases are of the “expanded liquid” type. However, the later formed phase is more rigid for both the Co(TPMCP) and Zn(TPMCP) films. The collapse pressure values for the porphyrin films floating on the subphase containing NH4Cl are higher than those on the water subphase and, in particular, those for the Co(TPMCP) film whose collapse pressure is almost three times higher. Clearly, the assembly of the porphyrin molecules in the films is different in the presence of NH4+ in the subphase than it is in its absence. The additional phase formed in the Co(TPMCP) and Zn(TPMCP) films is, presumably, composed of either slightly more tilted or less densely packed porphyrin molecules. The values of A0 determined for the second phase observed at higher pressure values for the Co(TPMCP) and Zn(TPMCP) films are the same as those for films floating on the water subphase, indicating similar structures of the films in both cases. On the other hand, the stability of the Co(TPMCP) and Zn(TPMCP) films is higher because the collapse pressure is higher.
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Table 2. Free Energy (∆G) and Stability Constant (K) Values of the Porphyrin-Ligand and Porphyrin-Cation Complexes Formed in the Langmuir Filmsb ∆G (kJ mol-1)
log K
log K a
Co(TPMCP); 0.1 M NH4Cl
-2.63
0.47
1.71
Co(TPMCP); 0.1 M NaCl
-2.25
0.40
0.70
Co(TPMCP); 0.1 M LiCl
-3.15
0.56
0.00
Co(TPMCP); 0.01 M LiOH + 0.09 M LiCl
-3.45
0.62
Co(TPMCP)-(pyz), XP ) 0.5; 0.01 M LiOH + 0.09 M LiCl
-2.06
0.40
Co(TPMCP)-(ppz), XP ) 0.6; 0.01 M LiOH + 0.09 M LiCl
-1.01
0.20
Co(TPMCP)-(DABCO), X ) 0.7; 0.01 M LiOH + 0.09 M LiCl
-1.48
0.26
Co(TPMCP)-(pyz), XP ) 0.7; 0.1 M NH4Cl
-1.14
0.20
Zn(TPMCP)-(pyz), XP ) 0.75; 0.1 M NH4Cl
-0.70
0.13
Zn(TPMCP)-(DABCO), XP ) 0.75; 0.1 M NH4Cl
-0.96
0.17
Zn(TPMCP)-(pyz), XP ) 0.75; 0.01 M LiOH + 0.09 M LiCl
-1.05
0.19
complex; subphase
a Literature data for the (15-crown-5)-cation complexes.16 b Determined for the mole fraction of the metalloporphyrin in the films (XP) corresponding to the maxima in the A0-XP plots.
formation (∆G) in Langmuir films:
∆G ) NA(
∫0πAc(π) dπ - XP∫0πAp(π) dπ)
(1)
where NA is Avogadro’s number, Ac(π) is the area per molecule dependence on surface pressure for the porphyrin film spread on the cation-containing subphase, AP(π) is the area per molecule dependence on surface pressure for the porphyrin film floating on the water subphase, and XP is the molar fraction of porphyrin (here, XP ) 1). The determined values of ∆G and the related stability constants (K) for the Co(TPMCP)-cation complexes formed in the Langmuir films are summarized in Table 2. The values of ∆G are relatively small for all studied systems, indicating the formation of relatively labile complexes. Comparison of the determined stability constants of the Co(TPMCP)-cation complexes in Langmuir films with those available in the literature for the (15-crown-5)-cation complexes formed in solution16 shows that the Co(TPMCP)-Li+ complex is more stable than expected, while the Co(TPMCP)-NH4+ complex is more labile. The stability of the Co(TPMCP)-Na+ complex in the Langmuir film is similar to that of the (15-crown-5)-Na+ complex in solution. Figure 4 shows the Brewster angle micrographs recorded at surface pressures of 0 and 10 mN m-1, and those close to the film collapse during compression of the Co(TPMCP) and Co(TPMCP)-(pyz) films spread on water and 0.1 M NH4Cl. The dark and bright regions in the micrographs of all films, imaged at π ) 0 mN m-1 (Figure 4a, 4a′, and 4a′′), indicate the two-dimensional gas and condensed phase domains, respectively, in coexistence. Interestingly, the domains of the condensed phase are more clearly seen for the Co(TPMCP) film floating on water
(Figure 4a) than for the Co(TPMCP) film floating on 0.1 M NH4Cl (Figure 4a′). Apparently, the molecules in the films on the water subphase are more aggregated than in those on the cation-containing subphase. The amount of the condensed phase is even smaller in the Co(TPMCP)-(pyz) film floating on 0.1 M NH4Cl (Figure 4a′′), thus indicating even more pronounced deaggregation of the film. As expected, the micrographs recorded at π ) 10 mN m-1 (Figure 4b, 4b′, and 4b′′) show brighter and darker regions characteristic of the formation of inhomogeneous condensed phases of different thicknesses. In the BAM micrograph of the Co(TPMCP)-(pyz) film floating on 0.1 M NH4Cl (Figure 4b′′), there are fibrous domains aligned in one direction. The micrographs recorded at π = πc exhibit very bright images (Figure 4c, 4c′, and 4c′′), corresponding to the formation of compact thick films. Unfortunately, the images are too bright to distinguish the features of the film morphology in more detail. The Langmuir films of all porphyrins, floating on water subphases, were transferred onto either quartz slides or Aucoated glass slides using the LB technique. An average transfer ratio was dependent on the porphyrin nature but did not exceed unity in each case. Typically, transfer ratios for upstroke transfers (emersions) were positive while those for downstroke transfers (immersions) were either very small or negative, indicating no transfer at all or even partial removal of the film from the substrate surface. In view of this behavior and taking into account the hydrophilicity of the quartz surface, one can assume the formation of LB films in which the hydrophilic crown ether moiety is adjacent to the quartz surface, which is hydrophilic, while the hydrophobic porphyrin moiety is stretched into air. Subsequent layers of LB films, formed predominantly during upstroke transfers, retain this orientation. That way, presumably, a noncentrosymmetric Z-type film is formed. The formation of the inclusion complexes between the 15crown-5 moiety and the cations in the submerged parts of the Langmuir films was confirmed by the results of the XPS measurements. In these measurements, the XPS spectrum, recorded in the range of the Na 1s electron, for the LB film of Zn(TPMCP) transferred from water was compared to that of the film transferred from the NaCl subphase onto a Au-coated glass slide (Figure 5). A background Na 1s signal was accounted for by recording the spectrum for a bare Au-coated glass slide. A peak at 1072.5 eV is present in this spectrum (curve 1 in Figure 5). Two peaks of binding energy corresponding to the Na 1s electron are present at 1072.3 and ∼1075.1 eV in the XPS spectrum of the Zn(TPMCP) LB film transferred from 0.1 M NaCl (curve 2 in Figure 5). In contrast, there is only one peak at 1071.8 eV in the spectrum of the Zn(TPMCP) film transferred from the water subphase (curve 3 in Figure 5). The binding energy of the latter peak is very similar to that recorded for the bare Au-coated glass slide. Therefore, we assign the peak at 1075.1 eV to Na+ complexed by the crown ether moiety of the porphyrin. The Na+-to-Zn(II) atom ratio of 0.65 was determined on the basis of calculated atomic concentrations corresponding to the Na+ and Zn2+ peaks in the XPS spectrum of the Zn(TPMCP) film LB transferred from 0.1 M NaCl. This lower than unity ratio is expected in view of the rather low stability of the Zn(TPMCP)-Na+ complex in the Langmuir film (Table 2). To maintain the electroneutrality of the LB film of the Zn(TPMCP)-Na+ complex, the counterion should also be transferred from the subphase solution onto the solid substrate. That was the case here, indeed, as manifested by the XPS spectrum (not shown), which revealed a much higher Cl atom content in the Zn(TPMCP) LB film transferred from 0.1 M NaCl than that
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
Langmuir, Vol. 23, No. 5, 2007 2561
Figure 4. Brewster angle micrographs of Langmuir films of (column a-c) Co(TPMCP) on water, (column a′-c′) Co(TPMCP) on 0.1 M NH4Cl, as well as (column a′′-c′′) Co(TPMCP)-(pyz) (XCo(TPMCP) ) 0.7) on 0.1 M NH4Cl at (row a-a′′) 0 mN m-1, (row b-b′′) 10 mN m-1, and (row c-c′′) collapse pressure.
Figure 5. XPS spectra of (1) the Au/glass slide support as well as the Au/glass slides with LB films of (2) Zn(TPMCP) recorded after 20 transfers from 0.1 M NaCl and (3) Zn(TPMCP) recorded after 20 transfers from water. The spectra in the binding energy range of the Na 1s electron are shown.
in the film transferred from water. The axial coordination of Clby Zn(II) might play a role in this transfer. The UV-vis spectra of all porphyrins in the LB films on quartz slides featured a red shift and a broadening of the Soret band with respect to those in the spectra of the porphyrins in chloroform solutions (Figure 6 and Table 3). This shift is characteristic of the formation of aggregates in which porphyrin macrocycles do not form perfectly parallel coaxial stacks but are significantly slipped with respect to one another.20-22 These (20) (a) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (b) Hunter, C. A.; Sanders, J. K. M.; Stone, A. J. Chem. Phys. 1989, 133, 395. (21) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371. (22) Adachi, M.; Yoneyama, M.; Nakamura, S. Langmuir 1992, 8, 2240.
aggregates, usually labeled as J-aggregates,32 have already been observed for numerous LB porphyrin films.22,23 The broadening of the Soret band was also frequently encountered before. It was attributed either to the presence of aggregates of different orientations in the LB films or to the simultaneous formation of different types of aggregates.24,25 Generally, this broadening is reported for LB films of porphyrins even without ligation. This effect is attributed to stronger porphyrin-porphyrin interactions in a solid, the formation of aggregates with a different number of porphyrins per aggregate, and the formation of slightly different excitonic states. In effect, a spectral shift resulting in band broadening is slightly different for each entity, as compared to that of the corresponding monomer. To discern the orientation of porphyrin macrocycles in the films, a series of UV-vis spectra in linearly polarized light was recorded for the LB films transferred onto quartz slides. That is, the film-coated slide was mounted in a dedicated holder of the polarizing accessory with the incident beam perpendicular to the film plane, and then the spectra were recorded for both p- and (23) (a) Yao, M.; Irie, T.; Ishida, N.; Inoue, H.; Yoshioka, N. Synth. Met. 2003, 137, 917. (b) Chou, H.; Chen, C.-T.; Stork, K. F.; Bohn, P. W.; Suslick, K. S. J. Phys. Chem. 1994, 98, 383. (24) Da Cruz, F.; Armand, F.; Albouy, P.-A.; Nierlich, M.; Ruaudel-Teixier, A. Langmuir 1999, 15, 3653. (25) Kano, K.; Takei, M.; Hashimoto, S. J. Phys. Chem. 1990, 94, 2181. (26) N’soukopoe´-Kossi, C. N.; Sielewiesiuk, J.; Leblanc, R. M.; Bone, R. A.; Landrum, J. T. Biochim. Biophys. Acta 1988, 940, 255. (27) (a) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862. (b) Donker, H.; Koehorst, R. B. M.; Schaafsma, T. J. J. Phys. Chem. B 2005, 109, 17031. (28) (a) Hu, X.; Schulten, K. Phys. Today 1997, 50, 28. (b) Gilman, P. B. In Photographic SensitiVity; Cox, R. J., Ed.; Academic Press: London, 1973. (c) Bohn, P. W. Annu. ReV. Phys. Chem. 1993, 44, 37. (29) CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: New York, 2003. (30) Musgrave, T. R.; Mattson, C. E. Inorg. Chem. 1968, 7, 1433. (31) Tabata, M.; Nishimoto, J. Equilibrium Data of Porphyrins and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Burlington, 1999; Vol. 9, pp 221-417. (32) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991, 95, 4074.
2562 Langmuir, Vol. 23, No. 5, 2007
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Table 3. Maximum (λmax) and Full Width at Half-Maximum (fwhm) Wavelengths of the Soret Bands for Porphyrins in Solution and in Langmuir-Blodgett Films Transferred onto Quartz Slides porphyrin in CHCl3 solution
Soret band features
in LB film on a quartz slide
subphase
Co(TPMCP) Co(TPMCP) Co(TPMCP)-(bpy), 2:1 Co(TPMCP)-(pyz), 1:1 Co(TPMCP) Co(TPMCP)-(pyz), 1:1 Co(TPMCP) Co(TPMCP)-(pyz), 3:2
water water water 0.01 M LiOH + 0.09 M LiCl 0.01 M LiOH + 0.09 M LiCl 0.1 M NH4Cl 0.1 M NH4Cl
Zn(TPMCP) Zn(TPMCP)-(DABCO), 3:1 Zn(TPMCP)
water 0.01 M LiOH + 0.09 M LiCl 0.1 M NH4Cl
H2(TPMCP)
water
Zn(TPMCP)
H2(TPMCP)
λmax (nm)
fwhm (nm)
430 442 443 442 440 439 442 441 422 441 439 438 420 436
16 32 28 32 25 27 21 21 10 30 20 20 12 44
molecules for a chosen electronic transition, β, can be then calculated using eq 3:
tan2 β )
2 - 2LDR
(3)
1 + cos2 R +1 LDR sin2 R
Equation 3 is obeyed if the transition dipole moments are evenly distributed in the cone of angle β around the normal to the film plane (i.e., LDR)0 ) 0). However, if the transition dipole moments are unevenly distributed, then corrected eq 4 should be used.
[ ( ) ( )
1 + cos2 R sin2 R 1 + cos2 R LDR + LDRLDR)0 - 1 (4) sin2 R
tan2 β ) 2LDR - 2/ LDR)0
Figure 6. UV-vis spectra of (a) (1) 2.6 µM H2(TPMCP) in CHCl3 and (1′) the LB film of H2(TPMCP) after 30 transfers, (b) (2) 6 µM Co(TPMCP) in a CHCl3-MeOH (1:1, v/v) mixed solvent solution and (2′) the LB film of Co(TPMCP) after 32 transfers, and (c) (3) 2.6 µM Zn(TPMCP) in a CHCl3-MeOH (1:1, v/v) mixed solvent solution and (3′) the LB film of Zn(TPMCP) after 30 transfers. The films were transferred onto quartz slides at a surface pressure of 10 mN m-1 and a speed of 5 mm min-1.
s-polarized light. Another pair of the p- and s-polarized light spectra was also recorded for the beam at an incidence angle, R ) 30°, against the normal to the film plane. Linear dichroism at a given incidence angle, LDR, can be calculated using eq 2:26
LDR )
Ap - As Ap + As
(2)
where Ap and As are the absorbances in p- and s-polarized light, respectively. The angle of the transition dipole moment of the
]
The UV-vis spectra recorded in polarized light of the Zn(TPMCP) film LB transferred from 0.1 M NH4Cl are presented in Figure 7 by way of example. This film was prepared by only one emersion. That way, the possible reorientation of the molecules in the film during the transfer of subsequent layers was avoided. There is almost no difference in the spectra recorded in p- and s-polarized light with the incident light perpendicular to the film plane (Figure 7a), indicating a random distribution of transition dipole moments in this film. However, if the light incidence angle is 30° (Figure 7b), the absorbance at the maximum of the Soret band in the spectrum recorded with p-polarized light is lower than that in the spectrum recorded with s-polarized light, indicating macrocycle tilting against the normal to the film plane. The tilt angle of the transition dipole moment for the Soret band of the Zn(TPMCP) film was calculated as β ) (62 ( 2)° using eq 3. Because the vector of the transition dipole moment for the Soret band is in the plane of the macrocycle,27 it was possible to postulate the orientation of the porphyrin molecules in the films. The calculated value of the tilt angle indicates that the Zn(TPMCP) macrocycle is markedly tilted and takes an almost flat orientation on the substrate surface. The tilt angles of the transition dipole moments for the Soret bands of other porphyrin films determined from the polarized light UV-vis spectra are presented in Table 4. Evidently, porphyrin molecules are strongly tilted in both the Co(TPMCP) and the Zn(TPMCP) LB films. The tilt angle is lower for films transferred from 0.1 M NH4Cl than that for films transferred from 0.1 M LiCl for both porphyrins. This angle difference may be due to the formation of a more stable porphyrin-NH4+ complex, as suggested by both the
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
Langmuir, Vol. 23, No. 5, 2007 2563
Figure 7. UV-vis spectra, in (1) p-polarized and (2) s-polarized light, of a LB film of Zn(TPMCP) at incidence angles of (a) 0° and (b) 30° with respect to the normal to the film plane. The films were transferred by a single stroke onto quartz slides at a surface pressure of 5 mN m-1 and a speed of 5 mm min-1 from the 0.1 M NH4Cl subphase (pH ) 5.2). Table 4. Tilt Angles (β) of the Porphyrin Macrocycles in the LB Films, Transferred by One Emersion at 5 mN m-1 onto Quartz Slides, Calculated on the Basis of the Linear Dichroism Measurements compound
subphase
β ( SD (deg)
Co(TPMCP) Co(TPMCP) Co(TPMCP) Co(TPMCP)-(pyz), X ) 0.5 Co(TPMCP)-(ppz), X ) 0.5 Co(TPMCP)-(DABCO), X ) 0.5 Zn(TPMCP) Zn(TPMCP)
0.1 M LiCl 0.01 M LiOH + 0.09 M LiCl 0.1 M NH4Cl 0.01 M LiOH + 0.09 M LiCl
69 ( 2 70 ( 2 60 ( 2 54 ( 2
0.01 M LiOH + 0.09 M LiCl
51 ( 2
0.01 M LiOH + 0.09 M LiCl
52 ( 2
0.1 M LiCl 0.1 M NH4Cl
68 ( 2 62 ( 2
presently derived compression isotherms (Table 1) and the literature data.16 The fact that the tilt angle for Langmuir films (estimated from the area per molecule values) is smaller than that for the LB films (determined from the linear dichroism measurements) may be due to some structural changes in the films during LB transfer. 3.2. Complexation of Metalloporphyrins by the Aromatic or Cyclic Heteroaliphatic Amino Ligands in the Langmuir and LB Films. Organized aggregate assemblies of various dyes, applied as functionalized materials or models of certain biological structures, have recently attracted a lot of attention.28 As a means of organizing molecular assemblies of porphyrins, 15-crown-5 appended metalloporphyrins were complexed in the present work with nitrogen-containing ligands in the emerged, i.e., air-
Figure 8. Dependence of the area per molecule at zero surface pressure on the mole fraction of Co(TPMCP) in mixed Co(TPMCP)ppz Langmuir films on the (a) water, (b) 0.1 M NaCl, and (c) 0.01 M LiOH + 0.09 M LiCl subphases. Dashed lines represent dependencies calculated according to the equation AX ) XCo(TPMCP)A0,Co(TPMCP), where A0,Co(TPMCP) is the area per molecule for the Co(TPMCP) film at zero surface pressure at XCo(TPMCP) ) 1.
stretching, part of the Langmuir films, in addition to cation complexation in the submerged part of the films, thus resulting in the “two-point” interactions in films. For complex formation, four different ligands, including two aromatic, namely, pyz and bpy, and two heteroaliphatic, namely, ppz and DABCO, were selected. Mixed chloroform solutions of metalloporphyrins and ligands were prepared immediately prior to the Langmuir film investigations. In these mixtures, the total number of moles was kept constant but the mole fraction of metalloporphyrin, XP, was varied. After allowing evaporation of the chloroform solvent from the metalloporphyrin-ligand solution spread onto the appropriate subphase, the π-A isotherms were recorded and the A0 values were determined. The A0-XP dependencies for the ligand-containing Co(TPMCP) mixed films, formed on the water, 0.1 M NaCl, and 0.01 M LiOH + 0.09 M LiCl subphases, are shown in Figures 8 and 9. All the ligands used, except bpy, are quite well soluble in water and, therefore, do not form Langmuir films by themselves. As tested herein, bpy does not form any Langmuir films either, despite its rather low water solubility at pH ) 7. In the absence of any structure-changing metalloporphyrin-ligand interactions, the area per molecule is expected to increase linearly with XP, i.e., AX ) XP A0,P, where AX and A0,P are the average area per molecule at surface pressure extrapolated to the zero value (i.e., the limiting area) for the mixed metalloporphyrin-ligand and pristine metalloporphyrin monolayer film, respectively. It appeared that only for the bpy or pyz aromatic ligand-containing Co(TPMCP) mixed films, formed
2564 Langmuir, Vol. 23, No. 5, 2007
Figure 9. Dependence of the area per molecule at zero surface pressure on the mole fraction of Co(TPMCP) in mixed Langmuir films on the 0.01 M LiOH + 0.09 M LiCl subphase containing (a) pyz, (b) bpy, (c) ppz, or (d) DABCO. Compression speed was 25 cm2 min-1. Dashed lines represent dependencies calculated according to the equation AX ) XCo(TPMCP)A0,Co(TPMCP), where A0,Co(TPMCP) is the area per molecule for the Co(TPMCP) film at zero surface pressure at XCo(TPMCP) ) 1.
on aqueous subphases, the A0,P-XP curves showed positive deviations from the predicted linear dependence with maxima at the XCo(TPMCP) values close to 0.50 and 0.65, respectively (not shown). However, it was not the case for the ppz (Figure 8a) and DABCO (not shown) cyclic heteroaliphatic ligands, for which A0 increased linearly with the Co(TPMCP) mole fraction, as predicted. Apparently, complexes of the 1:1 and 1:2 stoichiometries are formed in the presence of pyz and bpy, respectively, while no complexes at all are formed in the presence of ppz and DABCO. A ligated Zn(II) central metal ion of a porphyrin macrocycle is pentacoordinated, as opposed to a Co(II) ion, which is hexacoordinated in this complex environment.6 To explore the effect of axial ligation of the central metal ion on the Langmuir film properties, we performed a series of control experiments on the bpy-containing Zn(TPMCP) mixed films formed on water subphases. It appeared that no deviation from the linear dependence of A0 on XP was observed for these films (not shown), similar to those of the Co(TPMCP) films formed in the presence of the cyclic heteroaliphatic ligands (see above). If 0.1 M NaCl was used as the subphase, the shape of the A0,P-XP dependencies was substantially different. That is, they revealed higher than predicted A0 values for mixed Langmuir films containing aromatic ligands, although the maxima developed were located in the 0.7 e XCo(TPMCP) e 0.8 range. Interestingly, the A0,P-XP curve still did not reveal any deviation from linearity for the ppz-containing Co(TPMCP) mixed films (Figure 8b),
Noworyta et al.
whereas the same curve was deviated for the DABCO-containing Co(TPMCP) film in the entire XP range. Apparently, no complex formation in the ppz- or DABCOcontaining Co(TPMCP) films spread on water and in the ppzcontaining Co(TPMCP) film spread on 0.1 M NaCl may be related to competition of the porphyrin-ligand complex formation and the protonation of the nitrogen atoms of the ligands at the film-subphase interface. Obviously, protonated ligands cannot coordinate the metal centers. Indeed, the available literature data on dissociation constants of pyz, bpy, ppz, and DABCO seem to quite well confirm the observed behavior. In view of the acid dissociation constants of pyz (pKa ) 0.65)29 and bpy (pKa1 ) 3.03 and pKa2 ) 4.8),30 most of the ligand molecules are totally deprotonated in films on the CO2-saturated water (pH ≈ 5.5) subphase, leaving both nitrogen atoms available for the complexation of porphyrin metal centers. However, most of the ppz molecules (pKa1 ) 5.33 and pKa2 ) 9.73)29 are completely protonated and cannot ligate the porphyrin metal centers. Therefore, this behavior resembles that for the NaCl-containing subphase because the pH of this subphase is similar to that of pure water. The case of the DABCO molecules (pKa1 ) 3.0 and pKa2 ) 8.7)29 is intermediate because these molecules are only monoprotonated, i.e., only one nitrogen atom is protonated while the other is not, in the porphyrin films spread on water. This monoprotonation allows for the formation of separate porphyrinligand complexes but not of chains of complexes of ligandbridged porphyrins. To address the ligand protonation-deprotonation effect, the compression isotherms for the ligand-containing Co(TPMCP) films spread on alkaline subphases, i.e., those being 0.01 M in LiOH and 0.09 M in LiCl, were derived. The ionic strength of these subphases was kept at 0.1 M, being the same as that used previously, but their pH was higher than that used previously and was equal to 11.1. Therefore, deprotonation of the nitrogen atoms of all ligands was favored thus allowing for more efficient porphyrin-ligand complex formation. Indeed, the A0,P-XP curves for the pyz, bpy, ppz, and DABCO ligands present in the film at this subphase of high pH show positive deviation from linearity, indicating the incorporation of the ligand into the Langmuir film (Figure 9). Contrary to the behavior of films on the water and 0.1 M NaCl subphases, the A0,P-XP curves for all studied porphyrin-ligand films spread on alkaline subphases exhibit two regions of positive deviation from linearity. That is, one maximum is observed at around XP ) 0.30 for all ligands (Figure 9). However, the position of the second maximum is different for the different ligands and is located at around XP ) 0.55 for pyz and ppz (Figure 9a and 9c), and at around XP ) 0.70 for bpy and DABCO (Figure 9b and 9d). Most likely, complexes of two different stoichiometries are formed at the two mole fraction values. Interestingly, a Co(TPMCP)-ligand complex of the 1:2 metalloporphyrin-to-ligand ratio seems to be present in all films, while the formation of the 1:1 (pyz and ppz) and 2:1 complexes (bpy and DABCO) depends on the ligand nature. In an attempt to confirm the LB transfer of the ligand along with the metalloporphyrins from the ligand-metalloporphyrin mixed Langmuir films, XPS measurements were performed. Unfortunately, a close proximity of the peaks of the N 1s electrons of the nitrogen atoms of the metalloporphyrins and those of the ligands precluded univocal XPS data interpretation. For more insight into the influence of a strongly complexing cation on the properties of the Co(TPMCP)-ligand complexes in Langmuir films, a series of compression isotherms for the pyz-containing Co(TPMCP) films spread on 0.1 M NH4Cl was recorded. On the basis of these isotherms, a A0,P-XP curve was
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
constructed (not shown). This curve also exhibited two maxima, i.e., one at around XP ) 0.40 and the other at XP ) 0.70. The maximum at around XP ) 0.40 is also present in the A0,P-XP curves for the pyz-containing Co(TPMCP) films spread on the 0.01 M LiOH + 0.09 M LiCl subphase, while that at XP ) 0.70 is observed only for the 0.1 M NH4Cl subphase. This result shows that the nature of the cation present in the subphase influences the composition of the complexes in the film, allowing either the formation of one complex (in the case of the former subphase) or the formation of two complexes (in the case of the latter subphase). As control experiments, studies of the pyz- and DABCOcontaining Zn(TPMCP) Langmuir films spread on 0.01 M LiOH + 0.09 M LiCl (pH ) 11.1) and on 0.1 M NH4Cl (pH ) 5.2) were also performed (not shown). All these systems revealed relatively small positive deviations from linearity of the A0,P-XP curves with maxima that were difficult to distinguish. Apparently, a complex chain structure cannot be formed in the case of the pentacoordinated Zn(II) porphyrin. The ∆G values for the Co(TPMCP)-ligand complex formation in the Langmuir films were calculated using eq 1 with the appropriate porphyrin molar fraction, XP, and compiled in Table 2. For all studied systems, the free energy of complex formation is very low, indicating that these complexes are much less stable than those formed in solution.31 Notably, the ∆G values for the Zn(TPMCP)-ligand complexes are slightly lower than those for the corresponding Co(TPMCP)-ligand complexes. This result is in accord with the less clearly resolved deviation from linearity for the A0,P-XP curves for the Zn(TPMCP)-ligand complexes. However, understanding the reason for the positive deviation from linearity of the A0,P-XP dependence for the DABCOcontaining Co(TPMCP) films on 0.1 NaCl and not on water still bears further scrutiny. The ligand- and porphyrin-containing mixed Langmuir films, formed on the water subphase, were transferred onto the quartz slide substrates for UV-vis spectroscopic characterization. The resulting average transfer ratios were lower than unity and were dependent on the composition of the film. This transfer behavior was similar to that observed for pristine porphyrin films, indicating the formation of similar Z-type LB films. The UV-vis spectra recorded for the mixed Co(TPMCP) and bpy films as well as the Co(TPMCP) and pyz films are shown in Figure 10 by way of example. The spectral data for all studied complexes, present both in the chloroform solutions and in LB films, are summarized in Table 3. It is worth noting that the observed absorbance values at the Soret band maxima depend both on the molar fraction of porphyrin in the film and on the transfer coefficient of the film onto the quartz substrate. Those parameters are different for different films resulting in different absorbance values. The UV-vis spectra for all the metalloporphyrin-ligand mixed LB films show both a red shift and a broadening of the Soret band, as compared to that of the porphyrins in solution in the absence of ligand (Table 3). For films LB transferred from the aqueous subphase, this red shift is the most pronounced for the bpy-containing Co(TPMCP) film of the 1:1 stoichiometry. For other mixed LB films, there is a rather small red shift (∼1 nm) with respect to those for the LB films of Co(TPMCP) in the absence of ligand with no clear trend of changes. This behavior is typical for the formation of J-type aggregated films. However, the narrowing of the bands for the Co(TPMCP) and bpy mixed LB films, as compared to those for the pristine Co(TPMCP) LB film, may suggest some ordering incurred by the ligation of the metalloporphyrins.
Langmuir, Vol. 23, No. 5, 2007 2565
Figure 10. UV-vis spectra of mixed LB films of (a) Co(TPMCP) and bpy as well as (b) Co(TPMCP) and pyz. Co(TPMCP)-to-ligand mole ratios: (1 and 1′) pristine Co(TPMCP); (2 and 2′) 2:1; (3 and 3′) 1:1; (4 and 4′) 1:2. The spectra were recorded for LB films of 30 transfers from the water subphase onto quartz slides at a surface pressure of 10 mN m-1 and a transfer speed of 5 mm min-1.
Interestingly, the UV-vis spectra of the Co(TPMCP) and ligand mixed LB films, transferred from subphases containing cations, show a small blue shift of the Soret bands with respect to those of the pristine porphyrin films transferred under the same conditions. No narrowing of the Soret bands of the former films was observed under these conditions. Importantly, the blue shift of the Soret band for the ligand-containing porphyrin films transferred from 0.01 M LiOH + 0.09 M LiCl was more pronounced than that for the same films transferred from water. This behavior seems to indicate the stronger influence of cation complexation in the subphase than ligand complexation in the film on organizing porphyrin molecules at the air-solution interface. This inference is in accord with that following from the calculation of the ∆G values (Table 2), which appeared to be higher for the formation of the (15-crown-5 moiety)-cation complex than for the (metalloporphyrin moiety)-ligand complex. Moreover, the UV-vis spectra of the DABCO-containing Zn(TPMCP) film transferred from the 0.01 M LiOH + 0.09 M LiCl subphase exhibited both the red shift and the broadening of the Soret band, as compared to the solution spectra, thus also revealing the formation of J-type aggregates. The values of the tilt angle of the macrocycle plane, calculated on the basis of the polarized UV-vis spectra of the pyz-, ppz-, or DABCO-containing Co(TPMCP) films LB transferred from alkaline subphases (Table 4), are ∼20% lower than those determined for the Co(TPMCP) film transferred under the same conditions. Apparently, axial ligation incurs here the formation of layers of the porphyrin macrocycles oriented more vertically against the interface plane. However, the expected face-to-face ligand-bridged porphyrin stacks are not formed as there is no blue shift of the Soret band with respect to that of the monomer
2566 Langmuir, Vol. 23, No. 5, 2007
Figure 11. (a) X-ray reflectivity curve and (b) in-plane X-ray diffraction pattern of the Co(TPMCP)-(bpy) 2:1 (mole/mole) LB film after 28 transfers from the water subphase onto a glass slide at 10 mN m-1.
in solution, as expected for the face-to-face aggregates (so-called H-type aggregates).21,32 GIXD is a powerful technique allowing studies of structures of both mono- and multilayer films deposited on various solid substrates as well as at the air-liquid and liquid-liquid interfaces.33 Because of the very low intensity of the diffracted beam, however, very intense X-ray radiation, such as that produced by a synchrotron, must be used to obtain data of sufficiently high signal-to-noise ratio. As representative systems for the X-ray reflectance and GIXD studies, the Co(TPMCP), Co(TPMCP)-(bpy) 2:1 (mole:mole), and Co(TPMCP)-(DABCO) 1:1 (mole:mole) LB films, transferred from the water or 0.1 M NaCl subphase onto glass slides or HOPG plates, were selected. First, X-ray reflectivity curves were recorded and the film structure in the direction perpendicular to the film plane was ascertained. Then, the low-angle GIXD curves with the beam perpendicular or parallel to the dipping direction were recorded. The X-ray reflectivity curve (Figure 11a) for the Co(TPMCP)(bpy) 2:1 (mole:mole) film, LB transferred from the aqueous subphase, shows one Bragg peak of the scattering vector, Qz ) 4.3 nm-1. This peak corresponds to an interplanar distance of 1.46 nm, which agrees well with the distance between layers composed of ∼2-nm-long porphyrin molecules of tilted mac(33) (a) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (b) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (c) Jensen, T. R.; Balashev, K.; Bjornholm, T.; Kjaer, K. Biochimie 2001, 83, 399. (d) Weissbuch, I.; Lahav, M.; Leiserovitz, L. Cryst. Growth Des. 2003, 3, 125. (34) (a) Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 133, 197. (b) Hann, R. A.; Gupta, S. K.; Fryer, J. R.; Eyres, B. L. Thin Solid Films 1985, 134, 35. (c) Kovacs, G. J.; Vincett, P. S.; Sharp, J. H. Can. J. Phys. 1985, 63, 346. (d) Miller, A.; Knoll, W.; Mo¨hwald, H.; Ruaudel-Teixier, A. Thin Solid Films 1985, 133, 83. (e) Mo¨hwald, H.; Miller, A.; Stich, W.; Knoll, W.; Ruaudel-Teixier, A.; Lehmann, T.; Fuhrhop, J.-H. Thin Solid Films 1986, 141, 261. (f) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344.
Noworyta et al.
rocycle planes, as determined from a geometrical model for the distance between layers of vertically oriented porphyrin molecules. Interestingly, there are no Kiessig fringes on the reflectivity curves pointing to the formation of a relatively rough film of inhomogeneous thickness. Similar results were obtained for all other studied films (Table 5). However, the out-of-plane interplanar distance for the DABCO-containing Co(TPMCP) LB film is slightly larger than that for the bpy-containing Co(TPMCP) film. That means that the porphyrin macrocycles are more vertically oriented in the latter film. In the GIXD curve (Figure 11b) for the bpy-containing Co(TPMCP) film, there are peaks at Q ) 4.2, 8.3, 11.1, and 12.2 nm-1 corresponding to the 1.50, 0.76, 0.57, and 0.52 nm interplanar distances between the porphyrin macrocycles in the film plane. The values of the parameters determined from the GIXD curves recorded with the X-ray beam perpendicular (Qx) and parallel (Qy) to the dipping direction are the same, indicating the formation of randomly oriented domains in the film. The interplanar distances determined from the reflectivity curves and the GIXD curves recorded for other films (Table 5) indicate that ligation of the porphyrin metal center leads to the formation of a more structured film as compared to that of pristine Co(TPMCP). For the latter film, there is only one peak at 19.9 nm-1. It corresponds to the 0.32-nm interplanar distance, which is in accord with the distance between the planes of porphyrin stacks.24 However, this result may imply one-dimensional, randomly oriented porphyrin stacking. On the other hand, two-dimensional domains are formed in the bpy- and DABCO-containing Co(TPMCP) LB films. In these domains, the distance between porphyrin molecules in the neighboring stacks is around 1.50 nm. This distance is slightly smaller than the width of the Co(TPMCP) molecule, estimated as 1.7 nm. However, it agrees well with the distance expected for parallel stacks of slightly rotated porphyrin macrocycles (so-called “slipped-deck-of-cards” structure) observed in many LB films of different porphyrins34 as well as in the porphyrin crystal structures.35 The 0.76-nm distance determined for the bpy-containing Co(TPMCP) film could correspond to the intramolecular distance between ligated porphyrin molecules in the stacks. Because of the small number of reflexes recorded, it is difficult to determine the exact parameters of the porphyrin crystal lattice. However, by arbitrarily indexing the 4.2 and 8.3 nm-1 reflexes as (10) and (01), respectively, one can calculate sets of theoretical reflexes for different elementary cells. The best agreement between experimental and calculated reflexes was found for an oblique cell in the film plane with the following lattice constants: a ) 1.52 nm, b ) 0.77 nm, and ∠ ) 80° (Scheme 3). A similar elementary cell was obtained earlier for the porphyrin domains in the mixed porphyrin-cadmium arachidate LB films.36 In the elementary cell proposed for the Co(TPMCP)-(byp) LB films, the neighboring porphyrin columns are slightly displaced along the direction parallel to the long axis of the column. The distances determined for the DABCO-containing Co(TPMCP) LB film are very similar to those for the Co(TPMCP)-(byp) film. The estimated lattice parameters for the former film are very similar to those for the latter (a ) 1.54 nm, b ) 0.76 nm, and ∠ ) 82°). Taking into account that the bpy molecule is approximately twice as long as the DABCO molecule, one might conclude that the porphyrin does not form ligand(35) (a) Fleischer, E. B.; Miller, C. K.; Webb, L. E. J. Am. Chem. Soc. 1964, 86, 2342. (b) Hamor, M. J.; Hamor, T. A.; Hoard, J. L. J. Am. Chem. Soc. 1964, 86, 1938. (c) Silvers, S. J.; Tulinsky, A. J. Am. Chem. Soc. 1967, 89, 3331. (36) (a) Peng, J. B.; Foran, G. J.; Barnes, G. T.; Crossley, M. J.; Gentle, I. R. Langmuir 2000, 16, 607. (b) Peng, J. B.; Barnes, G. T.; Gentle, I. R.; Foran, G. J.; Le, T. H.; Crossley, M. J. Langmuir 2001, 17, 1936.
Self-Assembly of Zn(II) and Co(II) Metalloporphyrins
Langmuir, Vol. 23, No. 5, 2007 2567
Table 5. Scattering Vector (Q) and Corresponding Distance (d) Values Determined from the X-ray Reflectivity and Grazing In-Plane X-ray Diffraction (GIXD) Measurements for Langmuir-Blodgett Films of the Co(TPMCP) and Co(TPMCP)-Ligand Complexes Transferred from the Water or 0.1 M NaCl Subphase sample; subphase
substrate
Qz (nm-1)
dz (nm-1)
Qxy (nm-1)
dxy (nm-1)
Qy (nm-1)
dy (nm-1)
Co(TPMCP)-(DABCO), 1:1 20 transfers from water
HOPG
4.2
1.50
4.1 8.3 11.0 (b)a 18.7
1.53 0.76 0.57 0.34
4.2 8.3 11.0 (b)a
1.50 0.76 0.57
Co(TPMCP)-(DABCO), 1:1 20 transfers from 0.1 M NaCl
HOPG
4.2
1.50
4.1 8.3 10.0 12.1 13.1
1.53 0.76 0.63 0.52 0.48
4.1 8.3 9.8 11.1 12.3
1.53 0.76 0.64 0.57 0.51
Co(TPMCP) 48 transfers from water
glass
4.3
1.46
NA
NA
19.9
0.32
Co(TPMCP)-(bpy), 2:1 28 transfers from water
glass
4.3
1.46
4.2 8.3 11.1 (b)a 12.2 (b)a
1.50 0.76 0.57 0.52
4.2 8.3 11.1 (b)a 12.7 (b)a
1.50 0.76 0.57 0.49
a
(b), broad peak.
Scheme 3
bridged aggregate stacking, as this would result in a smaller porphyrin-porphyrin distance in the DABCO-containing Co(TPMCP) film. The in-plane structure of the DABCO-containing Co(TPMCP) LB film, transferred from the 0.1 M NaCl subphase, is slightly different than that transferred from the aqueous subphase (a ) 1.63 nm, b ) 0.81 nm, and ∠ ) 70°). An increase of the elementary cell area is in agreement with the observed increase of the A0 value for the Co(TPMCP)-(DABCO) Langmuir films on the 0.1 M NaCl subphase as compared to that for the same film spread on the water subphase. Generally, from the X-ray reflectivity and GIXD measurements it follows that, indeed, ligation of the porphyrin metal center exerts some ordering effect but this ordering does not necessarily lead to the formation of ligand-bridged porphyrin stacks.
Closer inspection of the A0,P-XP plots for the pyz- or bpycontaining Co(TPMCP) mixed Langmuir films formed on aqueous subphases as well as those of the ligand-containing Co(TPMCP) films formed on 0.01 M LiOH + 0.09 M LiCl and the pyz-, bpy-, or DABCO-containing Co(TPMCP) mixed films formed on 0.1 M NaCl reveals that ligands present at a certain composition range incur remarkable structural changes in the films, manifested by higher than expected A0 values. However, the UV-vis spectral data do not reveal any dramatic changes of molecular orientation in the LB films. In particular, there is no indication of the formation of face-to-face, i.e., H-type, aggregates to a significant extent. The formation of these aggregates should result in a blue shift of the Soret band with respect to the position of this band for the corresponding monomer in solution or the formation of the second blue-shifted band20,21 that, clearly, is not the case here. The behavior of the studied films can be explained assuming that the formation of the complex between the metalloporphyrins and pyz or bpy does not lead to the linking of two neighboring metalloporphyrin molecules in the films. Higher A0 values for films containing these ligands, as compared to the films in which these ligands are absent, can be, therefore, interpreted in terms of either a change in the tilt angle of the macrocycle planes with respect to the interface planes in monolayer films or, in the case of multilayer films, the partial destruction of these films and the formation of mixed mono- and multilayer films. Additionally, the tilt angle with respect to the normal to the film plane, determined from linear dichroism, is ∼50° for all studied films. This angle value, calculated from the out-of-plane interplanar distance in the bpy-containing Co(TPMCP) multilayer film and determined from the X-ray reflectivity curve, is similar and equal to 46°. As this tilting prevents the formation of ligandbridged porphyrin chains, it is, therefore, clear that ligation of the porphyrin metal center leads to the formation of porphyrinligand complexes and, in consequence, more ordered films. Although cyclic heteroaliphatic ligands, due to their “chair” conformation and suitably oriented bond angles of nitrogen atoms, could connect neighboring porphyrins and assemble them in one-dimensional stacks, this structuring does not seem to be the case here.
4. Conclusions The free-base, Co(II), and Zn(II) tetraphenylporphyrins appended with the 15-crown-5 moiety form monolayer Langmuir
2568 Langmuir, Vol. 23, No. 5, 2007
films in which porphyrin macrocycles are tilted with respect to the interface plane or multilayer Langmuir films in which J-type aggregated porphyrin molecules are horizontally arranged. The Li+, Na+, and NH4+ cations are supramolecularly complexed by the 15-crown-5 moiety in the subphase-submerged part of the film thus affecting the structural properties of the Langmuir films. The resulting complexes can be readily LB transferred onto solid substrates. Co(TPMCP) forms complexes with the pyz and bpy aromatic ligands in the emerged parts of the Langmuir films on the water subphases. Moreover, the Co(TPMCP)-(DABCO) complex is formed in the Langmuir film on the 0.1 M NaCl subphase. All studied ligands form complexes with Co(TPMCP) in films on the alkaline 0.01 M LiOH + 0.09 M LiCl (pH ) 11.1) subphase. This complex formation only slightly influences the structure of the metalloporphyrin aggregates in the Langmuir films. Predominantly, J-type aggregates of tilted porphyrin macrocycles are formed in all metalloporphyrin-ligand-cation mixed LB films. The Co(TPMCP) molecules form onedimensional stacks in the LB films with the intermolecular
Noworyta et al.
distance of ∼0.32 nm, while Co(TPMCP)-ligand complexes form columnar stacks with the porphyrin molecules separated by 0.76 nm in the column. The columns are separated by 1.5 nm and are slightly displaced with respect to each other along the long axis of the column. Acknowledgment. We thank Dr. B. Patterson, Dr. P. Willmott, Dr. O. Bunk, and Mr. R. Herger of the X04SA beamline, Swiss Light Source Synchrotron facility, for experimental assistance and valuable conversations. W.K. is grateful to the Ministry of Education and Science of Poland for financial support through Project No. N204 046 31/1214. K.N., W.K., and F.D. thank the Swiss Light Source Synchrotron facility in Villigen, Switzerland, for support through Research Grant No. 20040271. We also thank the National Science Foundation (Grant 0453464 to F.D.) and the donors of the Petroleum Research Fund, administered by the American Chemical Society. LA0626858
