Porphyrin chromophores

Chlorophyll a (L.P. Vernon, 1966) contains an unsymmetrical porphyrin chromophore with two special features the double bond between C-17 and C-18 is hydrogenated and carhon atoms 13 and 15 hear a carboxylated, isocyclic cyclopentanone ting E. 

In numerous synthetic studies4 1 b chlorins have been obtained from completely unsaturated porphyrins by reactions that occur on the periphery of the porphyrin chromophore. The main problem in these methods is the lack of regioselecttvity because each of the four pyrrole subunits... 

Shimomura, O. (1984). Porphyrin chromophore in Luminodesmus photoprotein. Comp. Biochem. Physiol. 79B 565-567. 

Porphyrine chromophore units have also been introduced to the PPV backbone but the PLQY of such materials decreased rapidly with increasing ratio of the porphyrine units and no EL devices have been reported [177,178]. 

Oxidized porphyrin chromophores are also chemically and biologically important. Oxo-phlorins (7) possess a carbonyl group at one of the methine positions nominally, they are... 

Corrole contains an 1 S-7t-electron system and shows aromaticity similar to the porphyrin chromophore, with strong ring current effects on the peripheral and inner-proton NMR, relatively stable parent mass ion and intermediary C—C bond length between single and double bonds. 

Taking advantage of the fact that the triplet state of metalloporphyrins with tetrabenzopor-phyrin (H2TBP or H2I, fig. 10) lies around 12 500 cm-1, a team from Minsk succeeded in sensitizing Ybm luminescence in solutions of [Yb(TBP)L], where L = Cl or acac (acetyl-acetonate), in several solvents like benzene, dimethylformamide (dmf), pyridine, quinoline or a mixture of octane and benzene (Kachura et al., 1974). Energy transfer from the porphyrin chromophore was ascertained by the excitation spectrum of the Ybm luminescence being identical to the absorption spectrum of the complex. In benzene, the quantum yield of... 

The first biomimetic triad of the D-D-A type, C-P-Q triad 4, was reported in 1983 [42 -44, 46]. It consists of a porphyrin chromophore covalently linked to both a benzoquinone-type electron acceptor and a carotenoid polyene. The carotenoid serves as the secondary donor in the system. It also performs two auxiliary functions. It acts as an antenna by absorbing light in spectral regions where the porphyrin does not absorb strongly and transferring singlet excitation... 

Thus, it has been shown that the porphyrin chromophores act as an antenna system for transmitting its excited energy to the noncovalently associated fullerene moieties. Furthermore, the examination of excited-state characteristics revealed significant energy transfer properties of these complexes upon photoexcitation. 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

Novel triads that contained tetrathiafulvalenes as electron donors and porphyrin chromophores (refer to previous section for porphyrin-fullerene dyads) as donor units have been recently reported [246] (Scheme 12). Improved analogs of the latter triads were soon considered by connecting the tetrathiafulvalene unit to a 7r-extended conjugated network [247]. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

A zinc(n) meso-meso linked porphyrin oligomer 57 exists in a nonhelical conformation in solution, but may adopt a dynamic helical conformation upon complexation with an achiral urea 58 through complementary hydrogen bonding interactions [115]. In the presence of the chiral diamine (S)-59, the 57-58 complex forms a predominantly one-handed helical conformation, thus showing a characteristic ICD in the absorption region of the porphyrin chromophore. This system may be used to sense the chirality of chiral diamines. 

In 2002, a triad molecule including dithienylethene (DTE), porphyrin (P), and fullerene (DTE-P-C60) was first designed and synthesized as a photoinduced electron transfer (PET) switch by D. Gust et al. [33], as shown in Scheme 5. This molecule was synthesized by modified procedures reported literature Porphyrin chromophore was covalently linked to substituted dithienylethene by Sonogashira coupling reaction. Fullerene was introduced by the cycloaddition reaction of dyad aldehyde (dithienylethene-porphyrin aldehyde) with the fullerene and the N-methyl-glycine. 

It is of interest to note that the relative intensities of the MOR peaks corresponding to the Soret and a—/ bands (30) are close to that expected from Simpson s theory of the electronic structure of the porphyrin chromophore (29). 

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]. 

Porphyrin chromophores have received much attention, particularly as photoelectric devices and molecular wires. Efficient re-electronic communication between porphyrin macrocycles is pivotal in various complex functions. K.M. Smith et al. showed that neighboring acetylenic units on porphyrins provide a means for the efficient construction of aromatic superstructures triggered by the Bergman cyclization reaction conditions and give rise to novel [n]phenacenoporphyrins, which belong to a new class of highly re-extended porphyrins." ... 

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