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Some features of the molecular assembly of copper porphyrazines
Materials Science and Engineering C 22 (2002) 167 – 170 www.elsevier.com/locate/msec
Some features of the molecular assembly of copper porphyrazines L. Valkova a,b,c,*, N. Borovkov b, V. Kopranenkov d, M. Pisani c,e, M. Bossi c,e, F. Rustichelli c,e a
Ivanovo State University, 39, Ermak, 153025 Ivanovo, Russia Institute of Solutions Chemistry, 1, Akademicheskaya, 153045 Ivanovo, Russia c Instituto Nazionale di Fisica della Materia, Unita di Ancona, Via Ranieri, 65-60131 Ancona, Italy d Institute of Organic Intermediates and Dyes, 1/4, B. Sadovaya, 103787 Moscow, Russia e Istituto di Scienze Fisiche, Universiteta di Ancona, Via Ranieri, 65-60131 Ancona, Italy b
Abstract Floating layers and Langmuir – Blodgett (LB) films of copper porphyrazine (CuPaz) and its tetra-tert-butyl-substituted homologue (CuPaz’) are studied. Contrary to phthalocyanines, the monolayer phase in the porphyrazine layers is metastable and transforms directly into the tetralayer one under moderate compression. In diffraction patterns and electronic spectra of the LB films, supramolecular peaks indicating collectivizing of the molecular electron density in direction perpendicular to the main axis of the macrocycle are found. The data obtained indicate the prismatic 3-D supermolecule to be the simplest structural unit of the porphyrazine assembly. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Porphyrazine; Langmuir – Blodgett films; Molecular assembly; X-ray diffraction; Supermolecules
1. Introduction Azaporphyrine films are of great interest as chemically sensitive materials [1], organic semiconducting materials [2] and materials for nonlinear optics [3]. Because azaporphyrines are supramolecular compounds, the film properties are controlled largely by the structure of the molecular assemblies of azaporphyrines. The fundamental structural unit of the azaporphyrine assemblies is thought to be a 1-D stack of tilted molecules. Tilt angle ranges from ca. 0j in the stack of the m-type to ca. 45j in the stack of the h-type [2]. Charge migration in the assemblies occurs allegedly along the axis of the stacks due to p– p orbital overlapping [3]. Because overlapping is the highest in the m-stacks, such molecular organization is considered as the most promising. Therefore, great attention is paid nowadays to phthalocyanines bearing long alkyl radicals in the nucleus. Really, the stacks of such phthalocyanines consist of thousands of molecules [4]. They exceed by far the stacks of unsubstituted phthalocyanines whose size may be estimated from the data on iodine sorption [5] as ca. 10. However, our analysis of the literature data indicates that materials on the
* Corresponding author. Ivanovo State University, 39, Ermak, 153025 Ivanovo, Russia. E-mail address: [email protected] (L. Valkova).
basis of alkyl-substituted phthalocyanines are poorly functional. In particular, conductivity of the LB films of lutetium phthalocyanine substituted with n-butoxy radicals is five orders of magnitude lower than one of the films of the unsubstituted analogue [6]. Conductivity of the phthalocyanine LB films is known to manifest itself starting from a number of monolayers necessary to be transferred onto a solid support [6,7]. The least number of monolayers equal to ca. 5 is observed for unsubstituted phthalocyanines. A noteworthy point is the film of manganese phthalocyanine [8] that exhibits high resistive effect towards trimethylamine in spite of no phthalocyanine ion radical forms generated. Another problem concerning alkyl-substituted phthalocyanines is connected with the fabrication of the LB films. A comparative study [9] of substituted and unsubstituted phthalocyanines shows spontaneous aggregation of the former on the water surface. Obviously, aggregation causes poor reproducibility and nonhomogeneity of the LB films of alkyl-substituted phthalocyanines [10]. The data cited above are hardly rationalized in the light of the concept of p – p orbital overlapping as the only factor controlling the structure and properties of the azaporphyrine assemblies. To our mind, they indicate that the azaporphyrine assemblies generally consist of 3-D units, i.e. supermolecules, rather than the molecular 1-D stacks. The
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supermolecules seem to form due to collectivizing of molecular p-electron density by electron hopping [11] rather than the van der Waals interactions [12]. The hopping may occur along both face-to-face and edge-to-edge directions of the azaporphyrine macrocycles. Because the macrocycle plays the major role in charge migration, we suppose that four peripheral benzo-groups in the phthalocyanine molecule may be a kind of hindrance to the edge-to-edge hopping. Therefore, in this work, we study porphyrazines, namely CuPaz being the basis of all azaporphyrines and its tetra-tert-butyl-substituted derivative CuPaz’. The work aims to register the 3-D supermolecules of the porphyrazines and to reveal specific features of the porphyrazine assembly with respect to the phthalocyanine one.
2. Experimental Porphyrazines CuPaz [13] and CuPaz’ [14] were synthesized from maleic nitriles. The highly symmetrical isomer of CuPaz’ was separated chromatographically [15]. The porphyrazine layers were prepared on an MDT Langmuir trough (Zelenograd, Russia). Solutions (6.0 10 4 M) of CuPaz in a mixture of o-xylene and 3,4-lutidine (1:1) and CuPaz’ in either benzene or methylene chloride were spread on tridistilled water. The solvents were allowed to evaporate over 30 min, and the layers were compressed at the rate of 19 cm2 min 1. The films were fabricated at surface pressures of 12 mNm 1 for CuPaz and 20 mNm 1 for CuPaz’. Small-angle X-ray diffraction from the films formed on silicon by 30-fold dipping was recorded on a Rigaku diffractometer (Japan) using Cu Ka radiation. UV – vis spectra of the films formed on quartz by ca. 100-fold dipping were recorded on a Specord M400 spectrophotometer (Germany). Dimensions of the porphyrazine molecules were calculated from data on the van der Waals atomic radii. Analysis of the surface pressure – molecular area (p –A) isotherms was performed by method described earlier [16].
3. Results and discussion The initial stage of development of the porphyrazine assembly was studied by means of analysis of the isotherms (Fig. 1) using the pA – p plots (Fig. 2). The CuPaz monolayer (Fig. 2, line a1b1) is stable up to p ca. 3.5 –4.0 mNm 1, stability being independent of initial surface concentration of the spread material. The limiting molecular area in the ultimately compressed layer (Amol) determined as a slope of the a1b1 line is equal to 0.40 nm2. Taking into account the area of the rectangle describing the side projection of the CuPaz molecule (Aedge = 0.38 nm2), the tilt angle relative to the water surface may be estimated as ca. 72j. It indicates that molecular arrangement in the CuPaz monolayers is analogous with the a-polymorph of phthalocyanines. Under compression, the monolayer transforms to the polylayer whose order is determined as four from the ratio of the Amol values found in the monolayer and in the c1 point. In the same manner, the order of the polylayer confined by the c1 and d1 points is determined as eight. The bilayer phase of CuPaz (not shown here) is metastable and may be obtained only at low initial surface concentrations. Thus, specific features of CuPaz as compared with phthalocyanines [16] are low stability of the monolayer phase, phenomenon of the direct monolayer – tetralayer transition and formation of the high-order polylayers under moderate compression. Contrary to CuPaz, behavior of CuPaz’ (mixture of geometric isomers) on the water surface is dependent on initial surface concentration and the solvent nature. The upper limit of the monolayer phase never exceeds 5 – 6 mNm 1 (Fig. 2, line a2b2). The Amol value in the monolayer cast from benzene solution is 0.82 nm2 that may indicate the ‘‘m-polymorph’’ of CuPaz’. The order of the polylayer phase resulting from the CuPaz’ monolayer varies depending on initial conditions. If CuPaz’ solutions in
Fig. 1. Surface pressure – molecular area isotherms of CuPaz (1) and CuPaz’ (2). Initial surface concentrations are equal to 15.2 and 10.1 10 7 mol m 2, respectively.
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Fig. 3. UV – vis absorption spectra of the LB films of CuPaz’ recorded in 30 min after formation. The films are formed using solutions in benzene (1) and methylene chloride (2). Fig. 2. pA – p plots for isotherms of CuPaz (1) and CuPaz’ (2): a1b1 and a2b2—monolayer, b2c2—bilayer, b1c1 and c2d2—tetralayer; c1d1—octalayer.
aromatic solvents are used, the monolayer transforms to the labile bilayer (line b2c2). The latter transforms into the tetralayer (line c2d2) of high stability and low compressibility. If solutions in nonaromatic solvents are used, the secondary phase of CuPaz’ may be either bi- or tetramolecular depending on initial surface concentration. However, in this case, both bi- and tetramolecular phases are of low stability and transform under moderate compression into the high-order polylayers. To monitor the advanced stages of the assembly development of the porphyrazines, the LB films formed from the tetralayers were studied. Initial diffraction pattern of the CuPaz film (to be discussed in detail elsewhere) shows two peaks corresponding to periodicities of 1.1 –1.3 and 2.1 nm. Because the side of the rectangle describing the main projection of the CuPaz molecule is equal to 1.12 nm, these periodicities should be considered as molecular and supramolecular (bimolecular packing). In the course of time, structural rearrangement takes place resulting in the structure characterized by periodicity of 4.2 nm. Thus, the supramolecular structure corresponding to the tetralayer packing arises. On the other hand, initial diffraction pattern of the film of CuPaz’ shows two molecular peaks corresponding to the a- and h-polymorphs of phthalocyanines. A scheme of the assembly development may be proposed as follows. Because of the edge-to-edge interactions, the porphyrazine molecules aggregate on the water surface yielding the 3-D supermolecules. The stable 3-D supermolecules are constructed as a short tilted prism with a square base of four molecules. Thus, the assembly in the LB films of sterically unhindered CuPaz develops three-dimensionally. On the other hand, the bulky tert-butyl radicals in the CuPaz’ molecule attenuate the edge-to-edge interactions, so two routs of the assembly development become possible. The first one is 3-D (‘‘porphyrazine’’ rout); the other one is quasi 1-D (‘‘phthalocyanine’’ rout). The latter may be
defined schematically as ordering of the simplest 1-D or 3-D supermolecules due to correlation of their long axes without significant collectivizing of the p-electron density along the edge-to-edge direction. Predominant realization of one or another rout is set by external factors affecting the structure of the low-dimensional supermolecules in the freefloating monolayers. Assumption that two routs of the porphyrazine assembly development exist is supported by electronic spectroscopy of the CuPaz’ films. The UV – vis spectrum of the molecular form of CuPaz’ consists of the Q (581 nm) and Soret (336 nm) bands [14]. The ratio between their extinction coefficients is ca. 2.1. The spectra of the CuPaz’ (highly symmetrical isomer) films recorded in 30 min after formation (Fig. 3) show the supramolecular band in the near IR region (840 nm) and the shoulder on the red side of the Q band. The spectra differ
Fig. 4. UV – vis absorption spectra of the heated LB films of CuPaz’ recorded in air (1, 2) and saturated vapors of n-hexylamine (3). The films are formed using solutions in benzene (1, 3) and methylene chloride (2).
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in resolution and relative intensity of the Soret band. When the films are heated at 100 jC, the spectra change differently. In the spectrum of the LB film formed using solution in benzene (Fig. 4, curve 1), the bands at 586 nm (the Q one) and 620 nm (the former shoulder) are well resolved and the Soret band is the most intense. On the other hand, in the spectrum of the LB film formed using solution in methylene chloride (Fig. 4, curve 2), the Q band vanishes, the Soret band is strongly reduced and the IR band is the most intense. Reverse spectral changes may be induced by means of vapors of amines. In particular, the spectrum of the films in saturated vapors of n-hexylamine (Fig. 4, curve 3) not only exhibits well-resolved p –p* bands, but the vibronic side band as well. The intensity ratio between the Q and IR bands is ca. 2.2. Obviously, the spectral changes above are hardly interpreted in terms of the concept of exciton coupling [17] as resulting from variation of the ‘‘assembling number of the 1D stacks’’ [18,19]. It seems also fruitless to seek the analogy between the CuPaz’ spectra and ones of the phthalocyanine polymorphs [20]: the molecular tilt angles in the LB films of CuPaz’ (highly symmetrical isomer) correspond to the aand h- polymorph, the band at 620 nm corresponds to the hpolymorph and the IR band corresponds to the m-polymorph. Taking into account the structural data on the floating layers and LB films of the porphyrazines, the spectra may be rationalized as follows. The spectrum of the film in the amine vapors (Fig. 4, curve 3) being the closest to one of the molecular form seems to be the spectrum of the simplest stable supramolecular form of CuPaz’. The spectra in Fig. 3 with their slightly reduced IR bands characterize the assembly in its initial nonstationary state where different supramolecular forms are present. The difference between the spectra 1 and 2 indicates that development of the assembly occurs more rapidly in the film formed using solution in methylene chloride. It seems reasonable to attribute the bands at 620 and 840 nm to the face-to-face and edge-to-edge charge transfer, respectively. The latter manifesting itself in reduction of the Soret band seems to entail collectivizing of the pelectrons of the second occupied a1u-orbital of the CuPaz’ molecule.
4. Conclusions A stable structural unit of the molecular assembly of porphyrazines is the 3-D supermolecule constructed by both
face-to-face and edge-to-edge intermolecular interactions. As a consequence, structural development of the assembly in the LB films may occur by two routs resulting in the films with different properties.
Acknowledgements The work is supported by the Education Ministry of the Russian Federation (Scientific Programme 015 of ‘‘Universities of Russia’’, Project UR.01.01.010, 2002) and by Istituto Nazionale di Fisica della Materia (Italy).
References [1] A.W. Snow, W.R. Barger, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines. Properties and Applications, vol. 1, VCH, New York, 1989, p. 18. [2] J. Simon, J.-J. Andre, Molecular Semiconductors, Springer-Verlag, Berlin, 1985. [3] G. de la Torre, M. Nicolau, T. Torres, in: H.S. Nalwa (Ed.), Supramolecular Photosensitive and Electroactive Materials, Academic Press, New York, 2001, p. 23. [4] P.G. Schouten, J.M. Warman, M.P. de Haas, C.F. van Nostrum, G.H. Gelink, R.J.M. Nolte, M.J. Copyn, J.W. Zwikker, M.K. Engel, M. Hanack, Y.H. Chang, W.T. Ford, J. Am. Chem. Soc. 116 (1994) 6880. [5] A.G. Vinogradskij, A.N. Sidorov, Khim. Fiz. 3 (1984) 380. [6] R. Jones, R.A. Hunter, K. Davidson, Thin Solid Films 298 (1997) 228. [7] H.-Y. Wang, J.B. Lando, Langmuir 10 (1994) 790. [8] S. Gupta, T.N. Misra, Sens. Actuators, B 41 (1997) 199. [9] R. Jones, R.A. Hunter, K. Davidson, Thin Solid Films 250 (1994) 249. [10] Y.-L. Lee, Y.-Ch. Chen, Ch.-H. Chang, Y.-M. Yang, J.-R. Maa, Thin Solid Films 370 (2000) 278. [11] M.E. Azim-Araghi, D. Campbell, A. Krier, R.A. Collins, Semicond. Sci. Technol. 11 (1996) 39. [12] C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525. [13] R.P. Linstead, M. Whalley, J. Chem. Soc. (1952) 4839. [14] V.N. Kopranenkov, L.S. Goncharova, E.A. Luk’yanets, Russ. J. Gen. Chem. 47 (1977) 2143. [15] V.N. Kopranenkov, D.B. Askerov, A.M. Shul’ga, E.A. Luk’yanets, Khim. Geterotsikl. Soedin. (1988) 1261. [16] L. Valkova, N. Borovkov, M. Pisani, F. Rustichelli, Langmuir 17 (2001) 3639. [17] E.S. Dodsworth, A.B.P. Lever, P. Seymour, C.C. Leznoff, J. Phys. Chem. 89 (1985) 5698. [18] M. Fujiki, H. Tabei, T. Kurihara, J. Phys. Chem. 92 (1988) 1281. [19] W.J. Schutte, M. Sluyters-Rehbach, J.H. Sluyters, J. Phys. Chem. 97 (1993) 6069. [20] J.H. Sharp, M. Lardon, J. Phys. Chem. 72 (1968) 3230.
Some features of the molecular assembly of copper porphyrazines L. Valkova a,b,c,*, N. Borovkov b, V. Kopranenkov d, M. Pisani c,e, M. Bossi c,e, F. Rustichelli c,e a
Ivanovo State University, 39, Ermak, 153025 Ivanovo, Russia Institute of Solutions Chemistry, 1, Akademicheskaya, 153045 Ivanovo, Russia c Instituto Nazionale di Fisica della Materia, Unita di Ancona, Via Ranieri, 65-60131 Ancona, Italy d Institute of Organic Intermediates and Dyes, 1/4, B. Sadovaya, 103787 Moscow, Russia e Istituto di Scienze Fisiche, Universiteta di Ancona, Via Ranieri, 65-60131 Ancona, Italy b
Abstract Floating layers and Langmuir – Blodgett (LB) films of copper porphyrazine (CuPaz) and its tetra-tert-butyl-substituted homologue (CuPaz’) are studied. Contrary to phthalocyanines, the monolayer phase in the porphyrazine layers is metastable and transforms directly into the tetralayer one under moderate compression. In diffraction patterns and electronic spectra of the LB films, supramolecular peaks indicating collectivizing of the molecular electron density in direction perpendicular to the main axis of the macrocycle are found. The data obtained indicate the prismatic 3-D supermolecule to be the simplest structural unit of the porphyrazine assembly. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Porphyrazine; Langmuir – Blodgett films; Molecular assembly; X-ray diffraction; Supermolecules
1. Introduction Azaporphyrine films are of great interest as chemically sensitive materials [1], organic semiconducting materials [2] and materials for nonlinear optics [3]. Because azaporphyrines are supramolecular compounds, the film properties are controlled largely by the structure of the molecular assemblies of azaporphyrines. The fundamental structural unit of the azaporphyrine assemblies is thought to be a 1-D stack of tilted molecules. Tilt angle ranges from ca. 0j in the stack of the m-type to ca. 45j in the stack of the h-type [2]. Charge migration in the assemblies occurs allegedly along the axis of the stacks due to p– p orbital overlapping [3]. Because overlapping is the highest in the m-stacks, such molecular organization is considered as the most promising. Therefore, great attention is paid nowadays to phthalocyanines bearing long alkyl radicals in the nucleus. Really, the stacks of such phthalocyanines consist of thousands of molecules [4]. They exceed by far the stacks of unsubstituted phthalocyanines whose size may be estimated from the data on iodine sorption [5] as ca. 10. However, our analysis of the literature data indicates that materials on the
* Corresponding author. Ivanovo State University, 39, Ermak, 153025 Ivanovo, Russia. E-mail address: [email protected] (L. Valkova).
basis of alkyl-substituted phthalocyanines are poorly functional. In particular, conductivity of the LB films of lutetium phthalocyanine substituted with n-butoxy radicals is five orders of magnitude lower than one of the films of the unsubstituted analogue [6]. Conductivity of the phthalocyanine LB films is known to manifest itself starting from a number of monolayers necessary to be transferred onto a solid support [6,7]. The least number of monolayers equal to ca. 5 is observed for unsubstituted phthalocyanines. A noteworthy point is the film of manganese phthalocyanine [8] that exhibits high resistive effect towards trimethylamine in spite of no phthalocyanine ion radical forms generated. Another problem concerning alkyl-substituted phthalocyanines is connected with the fabrication of the LB films. A comparative study [9] of substituted and unsubstituted phthalocyanines shows spontaneous aggregation of the former on the water surface. Obviously, aggregation causes poor reproducibility and nonhomogeneity of the LB films of alkyl-substituted phthalocyanines [10]. The data cited above are hardly rationalized in the light of the concept of p – p orbital overlapping as the only factor controlling the structure and properties of the azaporphyrine assemblies. To our mind, they indicate that the azaporphyrine assemblies generally consist of 3-D units, i.e. supermolecules, rather than the molecular 1-D stacks. The
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supermolecules seem to form due to collectivizing of molecular p-electron density by electron hopping [11] rather than the van der Waals interactions [12]. The hopping may occur along both face-to-face and edge-to-edge directions of the azaporphyrine macrocycles. Because the macrocycle plays the major role in charge migration, we suppose that four peripheral benzo-groups in the phthalocyanine molecule may be a kind of hindrance to the edge-to-edge hopping. Therefore, in this work, we study porphyrazines, namely CuPaz being the basis of all azaporphyrines and its tetra-tert-butyl-substituted derivative CuPaz’. The work aims to register the 3-D supermolecules of the porphyrazines and to reveal specific features of the porphyrazine assembly with respect to the phthalocyanine one.
2. Experimental Porphyrazines CuPaz [13] and CuPaz’ [14] were synthesized from maleic nitriles. The highly symmetrical isomer of CuPaz’ was separated chromatographically [15]. The porphyrazine layers were prepared on an MDT Langmuir trough (Zelenograd, Russia). Solutions (6.0 10 4 M) of CuPaz in a mixture of o-xylene and 3,4-lutidine (1:1) and CuPaz’ in either benzene or methylene chloride were spread on tridistilled water. The solvents were allowed to evaporate over 30 min, and the layers were compressed at the rate of 19 cm2 min 1. The films were fabricated at surface pressures of 12 mNm 1 for CuPaz and 20 mNm 1 for CuPaz’. Small-angle X-ray diffraction from the films formed on silicon by 30-fold dipping was recorded on a Rigaku diffractometer (Japan) using Cu Ka radiation. UV – vis spectra of the films formed on quartz by ca. 100-fold dipping were recorded on a Specord M400 spectrophotometer (Germany). Dimensions of the porphyrazine molecules were calculated from data on the van der Waals atomic radii. Analysis of the surface pressure – molecular area (p –A) isotherms was performed by method described earlier [16].
3. Results and discussion The initial stage of development of the porphyrazine assembly was studied by means of analysis of the isotherms (Fig. 1) using the pA – p plots (Fig. 2). The CuPaz monolayer (Fig. 2, line a1b1) is stable up to p ca. 3.5 –4.0 mNm 1, stability being independent of initial surface concentration of the spread material. The limiting molecular area in the ultimately compressed layer (Amol) determined as a slope of the a1b1 line is equal to 0.40 nm2. Taking into account the area of the rectangle describing the side projection of the CuPaz molecule (Aedge = 0.38 nm2), the tilt angle relative to the water surface may be estimated as ca. 72j. It indicates that molecular arrangement in the CuPaz monolayers is analogous with the a-polymorph of phthalocyanines. Under compression, the monolayer transforms to the polylayer whose order is determined as four from the ratio of the Amol values found in the monolayer and in the c1 point. In the same manner, the order of the polylayer confined by the c1 and d1 points is determined as eight. The bilayer phase of CuPaz (not shown here) is metastable and may be obtained only at low initial surface concentrations. Thus, specific features of CuPaz as compared with phthalocyanines [16] are low stability of the monolayer phase, phenomenon of the direct monolayer – tetralayer transition and formation of the high-order polylayers under moderate compression. Contrary to CuPaz, behavior of CuPaz’ (mixture of geometric isomers) on the water surface is dependent on initial surface concentration and the solvent nature. The upper limit of the monolayer phase never exceeds 5 – 6 mNm 1 (Fig. 2, line a2b2). The Amol value in the monolayer cast from benzene solution is 0.82 nm2 that may indicate the ‘‘m-polymorph’’ of CuPaz’. The order of the polylayer phase resulting from the CuPaz’ monolayer varies depending on initial conditions. If CuPaz’ solutions in
Fig. 1. Surface pressure – molecular area isotherms of CuPaz (1) and CuPaz’ (2). Initial surface concentrations are equal to 15.2 and 10.1 10 7 mol m 2, respectively.
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Fig. 3. UV – vis absorption spectra of the LB films of CuPaz’ recorded in 30 min after formation. The films are formed using solutions in benzene (1) and methylene chloride (2). Fig. 2. pA – p plots for isotherms of CuPaz (1) and CuPaz’ (2): a1b1 and a2b2—monolayer, b2c2—bilayer, b1c1 and c2d2—tetralayer; c1d1—octalayer.
aromatic solvents are used, the monolayer transforms to the labile bilayer (line b2c2). The latter transforms into the tetralayer (line c2d2) of high stability and low compressibility. If solutions in nonaromatic solvents are used, the secondary phase of CuPaz’ may be either bi- or tetramolecular depending on initial surface concentration. However, in this case, both bi- and tetramolecular phases are of low stability and transform under moderate compression into the high-order polylayers. To monitor the advanced stages of the assembly development of the porphyrazines, the LB films formed from the tetralayers were studied. Initial diffraction pattern of the CuPaz film (to be discussed in detail elsewhere) shows two peaks corresponding to periodicities of 1.1 –1.3 and 2.1 nm. Because the side of the rectangle describing the main projection of the CuPaz molecule is equal to 1.12 nm, these periodicities should be considered as molecular and supramolecular (bimolecular packing). In the course of time, structural rearrangement takes place resulting in the structure characterized by periodicity of 4.2 nm. Thus, the supramolecular structure corresponding to the tetralayer packing arises. On the other hand, initial diffraction pattern of the film of CuPaz’ shows two molecular peaks corresponding to the a- and h-polymorphs of phthalocyanines. A scheme of the assembly development may be proposed as follows. Because of the edge-to-edge interactions, the porphyrazine molecules aggregate on the water surface yielding the 3-D supermolecules. The stable 3-D supermolecules are constructed as a short tilted prism with a square base of four molecules. Thus, the assembly in the LB films of sterically unhindered CuPaz develops three-dimensionally. On the other hand, the bulky tert-butyl radicals in the CuPaz’ molecule attenuate the edge-to-edge interactions, so two routs of the assembly development become possible. The first one is 3-D (‘‘porphyrazine’’ rout); the other one is quasi 1-D (‘‘phthalocyanine’’ rout). The latter may be
defined schematically as ordering of the simplest 1-D or 3-D supermolecules due to correlation of their long axes without significant collectivizing of the p-electron density along the edge-to-edge direction. Predominant realization of one or another rout is set by external factors affecting the structure of the low-dimensional supermolecules in the freefloating monolayers. Assumption that two routs of the porphyrazine assembly development exist is supported by electronic spectroscopy of the CuPaz’ films. The UV – vis spectrum of the molecular form of CuPaz’ consists of the Q (581 nm) and Soret (336 nm) bands [14]. The ratio between their extinction coefficients is ca. 2.1. The spectra of the CuPaz’ (highly symmetrical isomer) films recorded in 30 min after formation (Fig. 3) show the supramolecular band in the near IR region (840 nm) and the shoulder on the red side of the Q band. The spectra differ
Fig. 4. UV – vis absorption spectra of the heated LB films of CuPaz’ recorded in air (1, 2) and saturated vapors of n-hexylamine (3). The films are formed using solutions in benzene (1, 3) and methylene chloride (2).
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in resolution and relative intensity of the Soret band. When the films are heated at 100 jC, the spectra change differently. In the spectrum of the LB film formed using solution in benzene (Fig. 4, curve 1), the bands at 586 nm (the Q one) and 620 nm (the former shoulder) are well resolved and the Soret band is the most intense. On the other hand, in the spectrum of the LB film formed using solution in methylene chloride (Fig. 4, curve 2), the Q band vanishes, the Soret band is strongly reduced and the IR band is the most intense. Reverse spectral changes may be induced by means of vapors of amines. In particular, the spectrum of the films in saturated vapors of n-hexylamine (Fig. 4, curve 3) not only exhibits well-resolved p –p* bands, but the vibronic side band as well. The intensity ratio between the Q and IR bands is ca. 2.2. Obviously, the spectral changes above are hardly interpreted in terms of the concept of exciton coupling [17] as resulting from variation of the ‘‘assembling number of the 1D stacks’’ [18,19]. It seems also fruitless to seek the analogy between the CuPaz’ spectra and ones of the phthalocyanine polymorphs [20]: the molecular tilt angles in the LB films of CuPaz’ (highly symmetrical isomer) correspond to the aand h- polymorph, the band at 620 nm corresponds to the hpolymorph and the IR band corresponds to the m-polymorph. Taking into account the structural data on the floating layers and LB films of the porphyrazines, the spectra may be rationalized as follows. The spectrum of the film in the amine vapors (Fig. 4, curve 3) being the closest to one of the molecular form seems to be the spectrum of the simplest stable supramolecular form of CuPaz’. The spectra in Fig. 3 with their slightly reduced IR bands characterize the assembly in its initial nonstationary state where different supramolecular forms are present. The difference between the spectra 1 and 2 indicates that development of the assembly occurs more rapidly in the film formed using solution in methylene chloride. It seems reasonable to attribute the bands at 620 and 840 nm to the face-to-face and edge-to-edge charge transfer, respectively. The latter manifesting itself in reduction of the Soret band seems to entail collectivizing of the pelectrons of the second occupied a1u-orbital of the CuPaz’ molecule.
4. Conclusions A stable structural unit of the molecular assembly of porphyrazines is the 3-D supermolecule constructed by both
face-to-face and edge-to-edge intermolecular interactions. As a consequence, structural development of the assembly in the LB films may occur by two routs resulting in the films with different properties.
Acknowledgements The work is supported by the Education Ministry of the Russian Federation (Scientific Programme 015 of ‘‘Universities of Russia’’, Project UR.01.01.010, 2002) and by Istituto Nazionale di Fisica della Materia (Italy).
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