Porphyrins stacked dimer

FIGURE 7.26. (a) Molecular structure of compounds 42—45 (b) TEM images of a bundle of nanorods formed by compound 43 (Upper), and closer view of the bundle (Lower), (c) Pictorial representation of the nanorods formed by self-assembly of salt 43 in water, (d) Nanotubules formed by compound 45 observed at different scales. The measures of the two focused tubules are 35 x 530 and 50 x 470 nm. (e) Most stable dimeric structures of 45. Note that Upper is 6 kcal/mol per molecule more stable than Lower. (Upper) Interlayered structure (see text). (Lower) Porphyrin stacked structure. Notice that the porphyrin stacked structure is 2 kcal/mol per molecule more stable than the interlayered structure. 

A related imidazole-appended zinc(II) porphyrin linked via an ethynylphenyl unit to a magnesium(II) phthalocyanine has recently been reported [38], The imidazole group binds to the magnesium center to form a stacked dimer in noncoordinating solvents such as chloroform even at submicromolar concentration (<10-6M). Upon addition of dimethyl sulfoxide, the stacked dimer transforms to an extended dimer in which the imidazole group binds to the zinc center on the basis of the hard and soft acid and base principle. The extended dimer exhibits a much stronger fluorescence (by a factor of 28) compared with the stacked dimer, and this coordination-induced sliding motion has been found to be reversible. 

Tetracationic me o-tetramethylpyridinium porphyrin and the anionic lipid L—dimyristoylphosphatidic acid mixed in a ratio of 1 4 form a monolayer on glass. It shows the spectrum of porphyrin monomers, again lying flat on the surface, whereas on water surfaces they appear as stacked dimers. Only those molecules that had been directly attached to the anionic lipid layer on water were transferred to the glass plate (Fig. 6.8.1). The tendency of porphyrin monomers... 

When the donor group is attached at a /3-pyrrole position rather than the meso position, the properties of the assemblies change. Knapp (57) synthesized a series of /3-pyrrole 2-pyridyl substituted zinc porphyrins (49, Fig. 16) and showed that dimerization takes place provided the meso-substituents are not too bulky H NMR and VPO measurements showed that the porphyrin, which bears heptyl substituents, dimerizes in solution, whereas the phenyl analog does not. The complexation-induced changes in chemical shift suggest a stacked structure for the dimer. 

Fujita and coworkers have also reported the encapsulation of multimeric porphyrin assemblies in the box-shaped cavities of ternary Pd6 coordination cages. Two types of cofacial porphine dimers A and B could be stabilized (133). In the smaller [Pd6(L14)2(L15)6]12+ cage 29, whose diameter is 10.4 A (Fig. 21), two porphyrin molecules can be stackedo directly on top of each other with an interplane distance of 3.4 A. In the larger cage 30, an additional molecule of L14 is intercalated between the two porphyrin bases. All complexes were found to be water-soluble in contrast to other 7i-stacked porphyrin dimers. The encapsulation... 

The red component of the dimer Soret band is ascribed to a twisted conformer "A" and the blue component to a stacked conformer "B". Computer calculations can predict two conformations A and B with dihedral angles of the porphyrin moieties of 80 and 56 ,respectively. S2 emission was detected for 0,0 -C (TPPZn)2 and o,o -C (TPP-... 

Attempts to co-crystallize amphiphilic porphyrins with micellar fibres failed completely. The van der Waals interactions between porphyrin planes ( stacking ) are so strong that unsubstituted porphin and (3-octamethylporphyrin are insoluble in practically all solvents. Porphyrins with larger substituents such as ethyl or acetic acid side chains allow face-to-face dimerization only, an action which occurs in all solvents and also probably in micelles down to a concentra-... 

Figure 5.20 a) The porphyrin Tr-system produces a thickness o/ 0.35 nm of the porphyrin box , of which the length and width measures only twice as much. Electron-rich pyrrole rings tends to combine with the electron-poor porphyrin cavity, b) Porphyrin forms polymer stacks, whereas c) octaethylporphyrin only dimerizes. 

Tetra-substitution at the same meso positions of the porphyrin core by bulky ort/ru-terphenyl groups conferred to the macrocycle an overall macro-disk-like shape ((26) M = 2H, Cu). " In this case, both the free-base and the copper complex exhibited an enantiotropic Colt phase from room temperature up to 59 °C for the ligand and 57 °C for the copper complex. The comparison between these macrocycles with the related phthalocyanine derivatives confirmed the perturbing role of the side groups on the stability of the Colh phase, consequent to a disturbed molecular stacking. X-Ray diffraction suggested that the columns would be constituted by the stacking of dimer units, with a periodicity of about 9.5 A. 

---