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Molecular Reorganization in Langmuir-Blodgett Films of Mesogenic Zn-Porphyrin Octaesters Paolo Facci*,† and Marco P. Fontana Department of Physics and INFM, University of Parma, Parco Area delle Scienze 7/A, I-43100 Parma, Italy
Enrico Dalcanale, Mirco Costa, and Tania Sacchelli Department of Organic and Industrial Chemistry, University of Parma, Parco Area delle Scienze 17/A, I-43100 Parma, Italy Received February 25, 2000. In Final Form: May 16, 2000 The role of chain length in affecting the molecular organization in thin films of mesogenic Zn-porphyrin octaesters ZnCnOAP has been investigated by means of Langmuir isotherms and area-per-molecule relaxation measurements at the air-water interface, as well as by unpolarized and polarized optical absorption spectroscopy and polarized ATR-FTIR spectroscopy after transfer onto the solid substrate. The results clearly identify three different classes of behavior depending on the chain length. Intermediate chain length molecules (ZnCnOAP, where n ) 6-12) feature clear coexistence regions in the isotherms as well as marked molecular relaxation due to self-organization effects in the coexistence region. Such behavior is absent in the short-chain derivative ZnC3OAP and strongly reduced in the longest side chain derivative ZnC14OAP, reflecting the tendency of intermediate chain length porphyrins to form columnar mesophases in the bulk. These data are confirmed by the time evolution of the optical absorption spectra for monoand multilayers transferred at surface pressures corresponding to different zones of the compression isotherm. The role of the chain length is also evident in the optical absorption spectra of transferred layers as a modulation of the blue shift of the Soret band connected with the formation of molecular stacks. Molecular anisotropy has been investigated by polarized optical (Soret band) and infrared (C-H stretching bands) spectroscopy, finding tilting angles of 40-45° for the macrocycles in transferred layers and evidence of quantum confinement effects.
Introduction Control over structural organization at the molecular level is an essential requirement for assembling new materials for molecular electronics applications.1 From this point of view the self-organizing properties of liquid crystals constitute an interesting feature, which can be exploited to produce highly ordered, anisotropic Langmuir-Blodgett (LB) films.2 Among the several mesophase organizations possible, columnar structures composed of flat conjugated molecules are well suited for conductive3 and photoconductive4 applications, as well as for highdensity data storage.5 All of them rely on extensive π-π overlapping among the stacked cores surrounded by an insulating mantle of hydrocarbon chains, which favors anisotropic conduction characteristics.6 † Present address: Dipartimento di Scienze Ambientali, Universita` della Tuscia, Viterbo, Italy.
(1) Alivisatos, P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E. Adv. Mater. 1998, 10, 1297. (2) (a) Laschewscky, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1574. (b) Mindyuk, O. Y.; Heiney, P. A. Adv. Mater. 1999, 11, 341. (3) (a) Boden, N.; Borner, R. C.; Bushby, R. J.; Clements, J. J. Am. Chem. Soc. 1994, 116, 10807. (b) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (4) (a) Schouten, P. G.; Warman, J. M.; de Haas, M. P.; Fox, M. A.; Pan, H.-L. Nature 1991, 353, 736. (b) Liu, C.-Y.; Pan, H.-L.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (5) Liu, C.-Y.; Pan, H.-L.; Fox, M. A.; Bard, A. J. Chem. Mater. 1997, 9, 1422. (6) Van Nostrum, C. F.; Nolte, R. J. M. Chem. Commun. 1996, 2385.
Porphyrins form an important class of molecules which have been widely investigated from many points of view: important systems such as heme- and chlorophyll-based proteins have porphyrin-based active sites;7 their catalytic properties have attracted interest as biomimetic oxidation catalysts;8 some meso and β-substituted porphyrins form liquid crystalline phases;9 LB films of porphyrin derivatives have been extensively studied, as prototypes of deposition of flat, disklike molecules.10 An interesting family of liquid crystalline porphyrins is that of porphyrin octaesters11 H2CnOAP (octa alkyl porphyrinooctaacetates) and their metal complexes MCnOAP (Figure 1), many of which form rectangular columnar mesophases Dr. The complexation of different divalent metal ions allows the strength of the π-π interactions and therefore the mesogenic properties to be tuned.11b In previous preliminary studies12 we have demonstrated for the specific member of this porphyrin family H2C8OAP (7) The Porphyrins; Dophin, D., Ed.; Academic Press: New York, 1978. (8) Metalloporphyrins in Catalytic Oxidations; Sheldon, R. A., Ed.; Marcel Dekker: New York, 1994. (9) Dalcanale, E. In Comprehensive Supramolecular Chemistry, Vol. 10; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Reinhoudt, D. N., Eds.; Pergamon: Oxford, 1996; Chapter 20. (10) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Petty, M. C. Langmuir-Blodgett Films. An Introduction; Cambridge University Press: Cambridge, 1996. (11) (a) Gregg, B. A.; Fox, M. A.; Bard, A. J. Chem. Commun. 1987, 1134. (b) Paganuzzi, V.; Guatteri, P.; Riccardi, P.; Sacchelli, T.; Barbera`, J.; Costa, M.; Dalcanale, E. Eur. J. Org. Chem. 1999, 1527, 7.
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Figure 2. Langmuir isotherms of ZnCnOAP for n ) 3, 6, 8, 10, 11, 12, and 14. The data are each shifted 5 mN m-1 vertically for clarity. Figure 1. Structure of the porphyrin octaesters studied in this work.
the existence of a well-defined coexistence region in the isotherm, implying a spontaneous first-order transition between face-on and edge-on molecular orientations in the monolayer. Such behavior could be related to the fact that in the bulk H2C8OAP forms a well-defined columnar mesophase (104 < Dr < 160 °C).11b The tendency of the molecules to self-organize in columnar stacks could be relevant to their peculiar phase transition behavior. Ordered metalloporphyrins may yield highly anisotropic (e.g., one-dimensional) conduction, which makes them potentially useful for molecular electronics applications. From this point of view it is important to investigate the structure and dynamics of porphyrin monolayers at the air-water interface; the characteristics of such monolayers should be strongly dependent on the same interactions, which lead to liquid crystallinity of the bulk material. In particular the interplay between electrostatic π-π interactions and entropic steric effects connected to the alkyl chain length may be expected to influence domain morphology and molecular orientation at the air-water interface, and also the molecular ordering in the deposited LB films. As part of our program aimed at the fabrication of anisotropic conductive molecular films, in this paper we address the issue of the role of the electronic and steric interactions in determining the structure and phase transitions in monolayers of porphyrin octaesters in which the relative weight of such interactions is varied by varying the molecular composition. In this paper we consider a whole family of porphyrin-Zn complexes ZnCnOAP (Figure 1), in which the length of the alkyl chain is varied to yield a corresponding variation in molecular mesogenicity. We found a strong correlation between the length of the alkyl chain and the main features of the Langmuir compression isotherms. Furthermore, we were able to follow in detail molecular reorganization in the deposited layers via polarized optical absorption spectroscopy. Experimental Section Film Deposition. ZnCnOAP (n ) 3, 6, 8, 10, 11, 12, and 14 carbon atoms) have been synthesized according to the procedure described in ref 11b. Typical working solutions were prepared by dissolving the crystallized porphyrins in chloroform at a concentration of 1 mg/mL. Other chlorinated and aromatic solvents have shown similar behavior both in dissolving and in spreading the molecules onto the water surface. Langmuir films were prepared in a commercial trough filled with ultrapure water (12) (a) Sartori, E.; Fontana, M. P.; Costa, M.; Dalcanale, E.; Paganuzzi, V. Thin Solid Films 1996, 284-285, 204. (b) Sartori, E.; Fontana, M. P.; Dalcanale, E.; Costa, M. Mol. Cryst. Liq. Cryst. 1996, 290, 31.
(Milli-Q grade, 18.2 MΩ cm resistivity). After spreading of typically 30-50 µL of porphyrin solution and waiting 10 min for solvent evaporation, the monolayer was compressed at a rate of 48 cm2 min-1. No detectable dependence of the shape of the isotherm was observed waiting at a surface pressure around 0 mN m-1 for a longer time. Π/A isotherms were recorded for the whole set of available chain lengths. Area-per-molecule relaxation measurements were performed at different surface pressures, to test the film behavior in different phases. Mono- and multilayers have been transferred onto solid hydrophilic substrates (glass and quartz slides for optical spectroscopy, ZnSe crystal for FTIRATR polarized spectra) according to the Langmuir-Blodgett method at different surface pressures (35, 8, and 3 mN m-1) using a dipping speed of 0.1 mm/s. Film transfer took place only during the upstroke of the dipper, with transfer ratios of 1.1 in the case of a surface pressure of 35 mN m-1 and of 0.95 at 3 mN m-1, but of course with much higher values when the layer was transferred in the transition region (8 mN m-1) where the film was not in an equilibrium phase; the described behavior was followed up to at least 10-monolayer transfer. No film detachment was observed during the downstroke. Substrates have been cleaned by being soaked in 30% H2O2-70% H2SO4 for a couple of hours before use. Optical Characterization. Optical absorption spectra were acquired in the wavelength range 300-650 nm. Polarized spectra were measured by inserting a Glan-Thompson linear polarizer mounted on a goniometer (accuracy 1°) in the beam pathway, placing the sample at 45° to the light propagation vector and acquiring spectra in vertical (V) and horizontal (H) polarization. From these two measurements the in-plane and out-of-plane absorptions were then determined. FTIR-ATR polarized spectra were acquired in horizontal ATR (attenuated total reflection) configuration with a linear polarized placed in the radiation pathway, using a 45° ZnSe crystal coated with the porphyrin layer. Spectra were acquired in V and H polarization from which the linear-infrared dichroism could be determined.
Results and Discussion To investigate the role of chain length in ZnCnOAP as to the behavior of these molecules once organized in dense monomolecular films, Π/A isotherms have been measured. Figure 2 reports the isotherms for n ) 3, 6, 8, 10, 11, 12, and 14. Several features result from these measurements. First of all, all the curves but n ) 3 and, partially, n ) 14 display an isotherm characterized by an extended coexistence zone of different molecular orientations relative to the liquid surface. This confirms our previous studies12 wherein for the first time we observed a spontaneous phase transition in the monolayer between side-on and edge-on orientations. This phase behavior disappears or tends to vanish, in the case of n ) 3 and n ) 14, respectively, for which the mesogenic properties are either absent or strongly reduced. The measured isotherms moreover can be grouped into three different subsets according to the value of the areaper-molecule displayed at high pressures (>25 mN m-1)
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where all molecules are arranged edge-on. While shorter chain length derivatives (n ) 3 and 6) show a limiting area-per-molecule value of 56.4 ( 0.5 Å2, n ) 8 results in an increased value (63.3 ( 0.5 Å2) which further increases to 78.3 ( 0.5 Å2 for n ) 10, 11, and 12 and increases even more (90.1 ( 0.5 Å2) in the case of n ) 14. This trend is indeed consistent with an entropic role (increase in conformational disorder as the chain length increases) of the eight alkyl chains: the chain length influences the extent of close packing which is reachable in the dense monolayer. Finally, we note the peculiar behavior of the isotherms with regard to the onset of the surface pressure as a function of the chain length. In fact, the area-per-molecule at which a significant surface pressure onset takes place does not scale simply with the chain length. This fact points out once more the role of the alkyl chain in determining the molecular arrangement at the air-water interface not only at the steric level, but also as an entropy contribution. There seems to exist a balance between the interaction of the hydrophilic macrocycle with the water surface and the hydrophobic alkyl chain contribution. The isotherms reveal that the chains do not lie in an extended configuration irrespectively of their length; rather there exists a chain limiting length (n ) 11) for which the areaper-molecule corresponding to the onset of the surface pressure is maximum (330 Å2). Above and below this chain length, the corresponding area-per-molecule is lower. This fact implies that the conformation of the molecules at the air-water interface cannot be described with a simple starlike model; rather, as a function of n it can be even more complicated. To try to shed light on the molecular mechanisms affecting the different behavior of the isotherms in the various regions of the compression isotherms, we have performed area-per-molecule relaxation measurements at different surface pressure values. In our previous work on H2C8OAP we have reported strong hysteresis effects in the monolayer, which implies that care must be taken to take into account the nonequilibrium behavior of this system, as also evidenced by small maxima present in some isotherms (Figure 2).2b We concentrated our attention on those cases which display a marked coexistence region and chose the case of ZnC10OAP as paradigmatic. Figure 3 reports the relaxation kinetics measured at 3, 8, and 35 mN m-1 surface pressure. While the trend at 3 mN m-1 (a) shows a fairly constant behavior with a fractional increase of the area-per-molecule which can be tentatively ascribed to the stabilization of the interaction of the macrocycle with the liquid surface (side-on arrangement), possibly also connected with the relaxation of the alkyl chains relative to the water surface, the behavior at 8 mN m-1 (b) is completely different and shows a very strong relaxation which brings the area-per-molecule at a value comparable with that of the closely packed film. This fact can be explained considering that some other interactions besides the compression enter the game, namely, the tendency of the molecules to form tilted columnar phases where π-π interactions are maximized. It is worth noting that the kind of functional dependence of the relaxation curve itself is quite unusual for ssaysa “passive” relaxation mechanism, characterized by a multiexponential decay (for instance, such as that shown at 35 mN m-1, Figure 3c, where molecules are already packed in stacks, as will be evident also from the spectroscopic data). In fact it features a marked variation in the slope of the curves, giving rise to a sigmoidal-like trend which indicates
Facci et al.
Figure 3. Area-per-molecule relaxation kinetics measured at 3 (a), 8 (b), and 35 (c) mN m-1 surface pressure for ZnC10OAP.
Figure 4. Comparison between absorption spectra of a monolayer of ZnC3OAP and ZnC10OAP deposited at 35 mN m-1.
the activation of some mechanism which accelerates the relaxation phenomenon. This mechanism once more can be individuated in the balance and interplay between electrostatic and steric interactions, which also gives liquid-crystalline properties to the porphyrins in the range 6 e n e 12. This kind of trend has been found only in the coexistence region of 6 e n e 12 porphyrins, while for higher surface pressures or for n ) 3 and 14 a normal multiexponential relaxation has been measured. Validation of the inferences resulting from the analysis of Π/A isotherms and from area-per-molecule relaxation measurements can be obtained in the case of films transferred onto solid substrates by means of optical absorption spectroscopy. In Figure 4 are reported the absorption spectra of a monolayer of ZnC3OAP and ZnC10OAP deposited at 35 mN m-1 for comparison. Both spectra display the typical
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Figure 6. FTIR-ATR linear dichroism (LD) of a multilayer of ZnC10OAP transferred at 35 mN m-1.
Figure 5. Absorption spectra for one monolayer of ZnC10OAP transferred at 3 and 35 mN m-1 (a) and at 8 mN m-1 (b), showing the temporal evolution of the Soret band.
features of the metalloporphyrins i.e., the two Q-bands and the Soret band. In both of them the Soret band is clearly blue shifted in comparison to the solution counterpart, where it is typically located at 417 nm. This wellknown solid-state phenomenon together with a marked broadening of the Soret band arises because of the overlap of the π orbitals when molecules stack together. These data therefore corroborate the idea coming from the areaper-molecule measurements and confirmed by previous AFM (atomic force microscopy) data12b on ZnC8OAP that at higher surface pressures molecules are stacked in columns lying on the film plane in edge-on arrangement. It is interesting to note, however, that the extent of the shift in the Soret band is different in ZnC3OAP (32 nm) and ZnC10OAP (24 nm). This effect can once more be accounted for by the entropic role of the alkyl chains. In fact, shorter chains allow a tighter stacking of the macrocycles, a consequent larger overlap between the π orbitals, and a resultant larger shift in the Soret band.13 In the case of mesogenic porphyrins, such as ZnC10OAP, we have also transferred monolayers in correspondence with different zones of the compression isotherm. Figure 5a reports the absorption spectra for one monolayer transferred at 3 and 35 mN/m. A marked difference between the two spectra is evident. While the higher pressure one shows the features already noted in Figure 4, the 3 mN m-1 sample does not show any shift in the Soret band with respect to the typical value for isolated molecules. This fact implies that no interactions between π orbitals take place as confirmed also by the suppression of the lower energy Q-band.14 This situation therefore is indicative of a film in which molecules lie flat on the substrate in the way inferred already by the data at the air-water interface. This result is also important since it implies the possibility of choosing the molecular orientation of the porphyrins by tuning the phase from which one transfers Langmuir-Blodgett films. (13) (a) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S., Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (b) Kroon, J.; Sudholter, E.; Schenning, A.; Nolte, R. J. M. Langmuir 1995, 11, 214. (14) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1989, 93, 4227.
The situation which is relative to the samples transferred in the coexistence region is indeed very interesting. In this case both the shifted and nonshifted bands are present in the spectra, indicating the presence of molecules characterized by two different states of aggregation (Figure 5b); it is very interesting to note that this spectrum evolves in time, tending to a situation in which the population of the stacked molecules increases, decreasing the other family. The presence of an isosbestic point at 410 nm is very meaningful in this sense, confirming that only two kinds of different molecular environments are available for the molecules. This self-organizing behavior is consistent with earlier evidence on free-base porphyrins which display a marked self-organization trend in a time scale of several days as shown by AFM studies.12b The molecular orientation in Langmuir-Blodgett films has also been investigated by means of polarized spectroscopies able to provide information on the molecular order parameter. We have performed both FTIR-ATR linear dichroism (LD) on a multilayer of ZnC10OAP and polarized absorption spectroscopy in the UV-vis. Figure 6 shows the FTIR-ATR LD; although a clear signal is visible in the C-H stretching region, indicating a preferential orientation of the alkyl chains, no quantitative conclusions can be drawn from these data due to the particular disposition of the chains all around the molecule. The data just confirm that the chains are preferentially oriented in the vertical direction. Figure 7 reports the polarized transmission data for ZnC3OAP (a) and ZnC10OAP (b) Langmuir-Blodgett multilayers in the optical range. It is evident that a dichroic signal is present in both spectra. Analyzing the data around the maximum of the Soret band, it is possible to give an estimate of the tilting angle of the transition moment connected with this band since it lies in the macrocycle plane. Calculations of the tilting angle yield a value of 45° for ZnC3OAP and 41° for ZnC10OAP along the film plane. It is worth noting at this point that the shape of the curve around the Soret band shows a difference in the two polarizations which cannot be explained in terms of simple dichroic behavior. Rather, it seems that the spectra arising from the in-plane polarization show a much broader onset of the band toward the lower energies. This effect can be interpreted by considering the peculiar supramolecular organization that is exhibited by these molecules. At 35 mN m-1 in fact, they show a columnar organization with the columnar structures lying on the substrate surface. In these structures a marked molecular packing has been demonstrated as well as a consequent strong overlap of the π orbitals. In these structures electrons are delocalized along the major axis of the column and are confined in the transverse plane. The difference in the polarized spectra
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spectra on structures bearing a different orientation of the columnar structures and, possibly, by ab initio theoretical simulations.
Figure 7. Polarized transmittance data for ZnC3OAP (a) and ZnC10OAP (b) Langmuir-Blodgett multilayers in the optical range, transferred at 35 mN m-1.
could reflect the different extent of electron delocalization in the columnar supramolecular structures, resembling much the behavior of quantum-confined anisotropic inorganic semiconductors.15 Of course, at this stage, this explanation is only tentative and should be supported by other observations, for instance by similar polarized
Conclusions In this paper we have investigated the self-organization of mesogenic Zn-porphyrin octaesters at the air-water interface. Depending on the surface pressure applied, either side-on or edge-on orientation of the molecules in the deposited monolayers has been obtained. Moreover, in the coexistence region the porphyrins underwent a spontaneous first-order phase transition from side-on to edge-on orientation, where the molecules stack in columns in the same tilted geometry as in the bulk mesophase. This effect can be used to deposit Langmuir-Blodgett structures of mesogenic porphyrin derivatives which are highly anisotropic and with a molecular orientation which can be tuned by selecting the appropriate deposition pressure. The preliminary finding of quantum localization effects on the optical behavior indicates the possibility of influencing the electronic distribution by tuning the highly anisotropic molecular architecture of porphyrin octaesters. These results can have an impact on materials science since flat polycyclic molecules and their supramolecular aggregates are among the most promising candidates for molecular electronics applications, behaving like organic semiconductors. In this sense it is possible to envisage the realization of monodimensional organic semiconductors oriented in space according to the needs of the application. Acknowledgment. This work was supported by MURST through the projects “Molecular materials for NLO” and “Structure and dynamics of monolayers of biological interest”.
(15) Facci, P.; and Fontana, M. P. Solid State Commun. 1998, 108, 5.
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