Phthalocyanine Dimer

The reason of why such reactions are discussed separately stems from the fact that metal-metal (both a direct or a bridge mediated) interaction causes certain specific properties of polynuclear complexes, and cannot be, therefore taken as a simple sum of their independent mononuclear constituents. A particular case is some phthalocyanine dimers in which the monomers are bound via a close k-tz system contact of the phthalocyanine ligands. Such dimers are included in this Section too in spite of the fact that there is neither a direct metal-metal bond nor a bridge connection. 

Similarly, the analog 54 also forms a self-assembled dimer [54]2 in non-polar solvents. In the presence of pyridine, the dimer dissociates into the monomeric species 54 Py [53,54], Further addition of AgPF6 leads to the formation of a novel Ag-linked phthalocyanine dimer [54]2Ag PF6 (Scheme 5) [55]. On the basis of its electronic absorption and magnetic circular dichroism properties, it has been suggested that this dimer adopts a planar and nms-con formation. Addition of NH4C1, which can remove the silver ions as AgCl, regenerates the monomeric 54 Py. 

The photophysical properties of the self-assembled dimer [54]2 have been studied in detail with various spectroscopic and computational methods [53,54], In the lowest excited singlet (Si) state, the excitation delocalizes over the two macrocycles, and the non-radiative decay is enhanced by dimerization. By contrast, the exciton interaction is very weak in the lowest excited triplet (Ti) state. In the excited-state absorption spectrum recorded in 1-decane, two sharp bands at 658 and 694 nm are observed, which can be attributed to the Q bands of the Tx self-assembled phthalocyanine dimer. 

The monophthalocyanines 106 and 107 show a weak aggregation tendency in chloroform. The latter has a self-dimerization constant of 1,175 M-1. By contrast, the donor-acceptor bis(phthalocyanine) 99 exhibits a much stronger aggregation tendency with a dimerization constant of 1.1 x 106 M-1 in chloroform. It is believed that in addition to the hydrophobic effect, the two phthalocyanine halves of compound 99 may be considered as donor and acceptor subunits that interact with each other. As revealed by electron microscopy, 99 forms one-dimensional nanoaggregates through intermolecular interactions between its complementary donor and acceptor phthalocyanine units as shown in Fig. 8. The dimerization constant of 99 is about one order of magnitude lower than that observed for the hetero-dimerization of 106 and 107, which may be due to the cyclophane step that hinders the formation of columnar aggregates of double phthalocyanine dimer. 

This self-assembly strategy has recently been extended to construct an interesting biomimetic model for the bacterial photosynthetic reaction center complex [97], In this system, a cofacial zinc(II) phthalocyanine dimer is formed via the interactions between K+ ions and the four 15-crown-5 units fused to the phthalocyanine ring. 

Fig. 11 Supramolecular structure of a cofacial phthalocyanine dimer - C60 conjugate (reproduced from [97] with permission from the American Chemical Society)...

Fig. 57 Proposed structure of the phthalocyanine dimer (65)2, reprinted from [155] with permission from Elsevier Science...

Measurements of the oxidation potentials yielded results which reflected the close proximity of the two phthalocyanine planes. The first two oxidation potentials (one-electron oxidations from each phthalocyanine ring) were 100 mV apart, suggesting the delocahzation of the cation radical over the two phthalocyanines jt-electronic framework. This behavior shows some similarity to that of the porphyrin dimers, and is expected to favor energy- and electron-transfer reactions. Another characteristic feature of this phthalocyanine dimer is that its fluorescence quantum yields are almost the same as those of the corresponding monomers (0.45, 0.26, and 0.76 for Zn(OBu), Zn(f-Bu), and Mg(f-Bu) phthalocyanines, respectively). Such a highly fluorescent phthalocyanine dimer has never been reported before. 

The structure of the phthalocyanine dimer was determined by theoretical calculations, utilizing Fraga s atomic pair potential function (R-i-4-6-12 type). A face-to-face, slipped structure is reported for the dimer. Structural and oxidation state effects on the association energy are discussed. A growth mechanism of phthalocyanine clusters and p-crystalline phthalocyanine is proposed. 

Stacking face-to-face, slipped (see Fig. 1) and face-to-face (see Fig. 2) configurations of the phthalocyanine dimer were revealed as the most important orientations in the crystal structures of P-phthalocyanine [11] and halogen-doped salts [12]. The association energies and distances of these minima are reported in Table 1. 

Two basic stacking structures are important for the description of the phthalocyanine dimer (see Table 1) face-to-face slipped (a-c minima. Fig. 1) and face-to-face (d-f minima. Fig. 2). The stabilities of the slipped a-c dimers are much greater than those of the face-to-face dimers. The horizontal slipping effects reduce the vertical distance between the moleculai planes by 0.2 A. Changes in the rotational angle yield a small effect on the association energies. 

Two clear advantages appear, when oxidation state effects are used for describing the phthalocyanine dimer (i) The association energy is increased, (ii) the interaction energy is less sensitive to the effect of tiie geometrical parameters (distance between the molecular planes and rotation angle). 

Table 1. Energies and geometrical parameters of the phthalocyanine dimer.

Fig. 1. Stacking face-to-face, slipped structures of the phthalocyanine dimer. A conical projection on the ZY plane is shown. The Z and Y axes denote the vertical and horizontal directions, respectively. The X axis lies in the viewing direction. Structures a (b, c) correspond to the a (b, c) minima in Table 1.

Table 2. Association energies (-E, kJ/mole) for various oxidation states of the phthalocyanine dimer.

Kobuke and colleagues described a complementary zinc porphyrin-magnesium phthalocyanine dimer that can be considered as a supramolecular switch (Figure 24). This switch is based on the variation of fluorescence induced by a change of ligands. More precisely, an imidazole-appended porphyrinatozinc-phthalocyaninatomagnesium complex was shown to spontaneously dimerize into a supramolecular short dimer (state I) in which the imidazole units are bound to the central magnesium atom of the Pcs. On addition of 2 equivalents (with respect to the dimer) of dimethyl sulfoxide (DMSO) or 1-methylimidazole to the dimer, a new extended dimer (state II) is formed in which... 

Ali H, Baillargeon P, van Lier JE (2008) Synthesis and properties of C-C conjugated phthalocyanine dimers. Tetrahedron Lett 49 7253-7255... 

Sumimoto M, Kawashima Y, Yokogawa D, Hori K, Fujimoto H (2012) Influences of dispersion and long-range corrections on molecular structures of three types of lithium phthalocyanine dimer. Int J Quantum Chem. doi 10.1002/qua.24072... 

Figure 8.6.1. Cofacial phthalocyanine dimer with a 4-atom bridge, adsorbed on an electrode, is a catalyst for electroreductions. The central metal atom is cobalt. (Reproduced by permission from C. C. Leznoff and the National Research Council of Canada).

---