Porphyrins stacking assemblies

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. 

Figure 6.3.1 Absorption spectra of the covalent porphyrin stacked and lateral pen-tamers shown in Scheme 6.3.3. Stacks show only short-wavelength shifts lateral assemblies often give, although not in the present case, both short- and long-wavelength Soret bands.

Recently, an example of an efficient BHJ SC was assembled of commercially available components by careful D-A interface optimization (Table 30) [108, 109]. Here, self-assembled porphyrins stacks were used as the charge mediators to assist effective exciton dissociation at the photocathode. This approach allowed con-stmcting BHJ SC with the PCE as high as 7.13 %. 

The stacked porphyrin arrays assembled on DNA templates are reminiscent of the columnar liquid crystalline (LC) phases produced by discotic mesogens, including several porphyrin derivatives [67]. Different discotic compounds with complementary interactions have been shown to form mixed LC phases [68], a concept that has been extended to the DNA-templated porphyrin stacks [69]. The two porphyrins Hj-P4... 

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... 

Tamaru S, Takeuchi M, Sano M, Shinkai S (2002) Sol-gel transcription of sugar-appended porphyrin assemblies into fibrous silica unimolecular stacks versus helical bundles as templates. Angew Chem Int Ed 41 853-856... 

Under certain conditions, rather well-defined ring and honeycomb patterns can be formed by solution casting of thin films. Recently Nolte described the formation of isolated rings with diameters of several micrometers formed by stacked porphyrin molecules [287], Phase segregation or eruption of gas bubbles was assumed to be responsible for the peculiar assembly of the porphyrin wheels . However, the regular shape of the solute molecules was also considered to be of crucial importance, Another example is the honeycomb pattern of star shaped poly(styrene)-block-poly(-p-phenylene) films described by Francois [288], In this case, the peculiar molecular shape was emphasized to be of crucial importance besides the proper choice of solvent and evaporation conditions [289],... 

Another question is which one, the bis coordination of O2 to two CoP centers or the electronic effects induced by the peripherally coordinated Ru(III)Ru(III) Ru(II), would be responsible for the cobalt porphyrin activation in the multielectron-transfer process. In fact, considering that the Co(4-TCPyP) films are formed by stacks of supermolecules, it is not possible a priori to rule out the first hypothesis. This problem was solved by repeating the experiments using the electrostatic assembled Co(4-TCPyP)/ZnTPPS films in which the bis coordination of dioxygen to two cobalt porphyrin centers is not possible (176). The catalytic behavior is reproduced in the ion pair film, confirming that the activation is mainly due to electronic effects induced by the peripheral ruthenium clusters, in synergistic association with their normal role as intramolecular multielectron source. 

Electrochemical data indicate that self-assembled monolayers of 5 and 6 catalyze the two-electron reduction of O2 to H2O2. The monolayer from 6 is a more effective electrocatalyst for the reduction of O2 than that from 5 [300]. The different reactivity results from different interfacial architecture this is confirmed by infrared, X-ray photoelectron, and visible spectroscopic measurements [300] which revealed coplanar, inclined t -7z stacking of the porphyrin ring in the monolayer of 5 and head-to-tail orientation of the porphyrin ring in the monolayer of 6. Treatment of the monolayer of 8 with Co(OAc)2 in methanol resulted in electrocatalytic activity in the reduction of O2 [300]. In contrast, a monolayer of 7 treated similarly failed to catalyze dioxygen reduction [300], although treatment of a mixed monolayer of 7 and CH3(CH2)3SH with Co(OAc)2 results in electrocatalytic activity similar to that of 6. 

In the case of ort/io-disubstituted aldehydes, one obtains extremely dense n stacks with Soret band absorptions at 370 nm, whereas meta-disubstituted analogs give a lateral packing characterized by 450-nm Soret bands (Fig. 6.3.1). Such covalent porphyrin oligomers are useful references in structure elucidation of corresponding supramolecular assemblies, where one cannot hope for a... 

The exciton effects observed in absorption spectra of porphyrin monolayers are often different from those in stacked or lateral fibrous assemblies. One usually finds exclusively long-wavelength shifts of the Soret bands or no effect at all. Porphyrins either lie parallel to each other on the subphase (no shift) or are oriented in a slipped stack-of-cards configuration tilted by 10-20° angles against the subphase surfaces, which produce file observed 20-100 nm red shifts (Fig. 6.8.2). 

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