Porphyrins, Stacked

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... 

Figure 23 Schematic representation of the intermolecular tunneling interaction between a pair p to a pair qin LB films containing loosely stacked porphyrins (ellipses).

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],... 

As a leading example for short-spaced dyads, a n-n stacked porphyrin-fullerene dyad (ZnP-Ceo) 21 should be mentioned, which was probed in light of their electron transfer and back electron transfer dynamics [361, 362], The close van der Waals contact ( 3.0A) is responsible for pronounced electronic interactions in the ground state between the two 7t-chromophores. For example, the ZnP Soret-and Q-bands in the n-n stacked dyad 21 show a bathochromic shift and lower extinction coefficients compared to free ZnP [361], In the n-n stacked dyad 21 the linkage of the two bridging units occurs in the trans-2 position at the fullerene. A charge-separated radical pair evolves from a rapid intramolecular electron transfer k 35 ps) between the photoexcited metalloporphyrin and the fullerene core in a variety of solvents (i.e., ranging from toluene to benzonitrile). Remarkably, the lifetimes in tetrahydrofuran (t = 385 ps) and DCM (t = 122 ps) are markedly increased relative to the more polar solvents dichloromethane (r = 61 ps) and benzonitrile (t = 38 ps) [362]. This dependency prompts to an important conclusion ... 

The rates of the back electron transfer processes, from the ZnP +-C6o state to the ground state, are clearly in the Marcus-inverted region. This is one of the important examples that provide experimental support to electron transfer kinetics clearly occurring in the inverted region. Using the same synthetic methodology, a similar family of n-n stacked porphyrin fullerene dyads (i.e., ZnP-Ceo and H2P-C60), in which the porphyrin chromophore is attached by a malonate bridge to the trans- position of the fullerene sphere, has been reported [363]. 

Photoinduced electron transfer across planar bilayer membranes doped with discrete cofacially stacked porphyrin trimers and tetramers has been demonstrated... 

Long-range Vhh hh and Jhh couplings have been extensively used by Therien and co-workers in the structure determination of the unusually rigid 7i-stacked porphyrin-bridge-quinone system. 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

The modified electrode with the attached CoTPP can reduce CO2 to CO whereas the electrode modified with adsorbed CoTPP cannot. The cyclic voltam-mogram of the modified porphyrin-amine-GC electrode in pH 6.8 phosphate buffer shows the appearance of the anodic hump after the electrode is polarized to potentials where the evolution of hydrogen takes place under N2 atmosphere see Figure 5.12. The ligand bonded to the glassy carbon does not show this hump, and then the authors associate it with the cobalt center. The hump would correspond to the oxidation of the formed hydride. Over the first-bonded porphyrin there are more than 100 layers of stacked porphyrins. Stacked Co(II)TPP could accept a hydrogen atom from CoTPP bonded GC to form HCo(II)TAPP. ... 

However, when the stacked electrode is cycled under CO2 the hump disappears indicating that here is not a formation of hydride in the stacked porphyrins. 

The HCo(ll)TPP bonded on GC reacts with CO2 giving CO. On the other hand if the electrode is heavily rinsed, the packed structure disappears. In this case, the electrolysis measurements show higher current eflhciencies for the non-stacked structure. In fact, according these results the stacked porphyrins promote the evolution of hydrogen instead the reduction of C02. ... 

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... 

---