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Nearly complete quenching of porphyrin fluorescence (both in solution and in adsorbed state over SnO2 nanestructure) is observed in these dyads, indicating that the photocurrent generation mechanism probably involves the formation of photoinduced intramolecular charge separation state.
The fact that there is a close correspondence between the photocurrent action spectrum and the absorption spectrum of the electrodes, confirms that light absorption by the porphyrin-C60 dyad 9 and 10 is the initial step in the generation of photo-induced charge transfer mechanism, like in the natural photosynthetic apparatus .
Microwave Synthesis of Tetraphenylporphyrin
N2 - We prepared bifunctional MgII porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br-, Cl-, and I- counteranions, respectively, increased in the order 1i
We prepared bifunctional MgII porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br-, Cl-, and I- counteranions, respectively, increased in the order 1i II porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h-1). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with 18O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br- on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the MgII and Br- and a conformational change (induced-fit) of the quaternary ammonium cation.
A Pathway for Heme Biosynthesis: the Work of David …
This is particularly true if the monopyrrole precursor has fairly large 3- and 4-substituents because these enforce a helical geometry upon the elongating molecule. For this reason too, attempts to prepare the unsubstituted porphyrin 17 from pyrrole and formaldehyde usually gives a very low yield, probably due to a preference for infinite elongation due to the small 3- and 4-substituents in 8 () and due to solubility problems of the product 17. However, several groups have synthesized substituted porphyrins and can be desubstituted and this simplifies the problems associated with the low solubility in almost all solvents, of 17. An example is the synthesis of the soluble tetra-tert-butylporphyrin 18 from the Mannich pyrrole 19, followed by removal of the four tert-butyl groups using aqueous sulfuric acid to give 17 in 64% yield () .
In the synthesis of TPP, the interpyrrolic carbons are provided by the aldehyde group of the corresponding arylaldehyde. In the case of octaalkylporphyrins (such as 3), the future meso- (i.e. interpyrrolic) carbons can either be added separately by being not attached to the pyrrole (as for TPP) or can be already present in the form of a 2-substituent on the pyrrole. It is important to note that monopyrrole tetramerizations can only produce a structurally unique product if the 3- and 4-substituents in the monopyrrole are identical ().
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The Mechanism of Pyrrole Synthesis ..
The synthesis of a porphyrin (e.g.7) with a completely unsymmetrical array of substituents must progress through open-chain tetrapyrrolic intermediates. Though a number of open-chain tetrapyrroles have been investigated and utilized with success  over the years, the most used open-chain precursors of choice have been shown to be 1,19-dimethyl-a,c-biladiene salts such as 64.
Synthesis of Porphyrin-C 60 Dyads with ..
Dipyrromethanes can be synthesized using fairly straightforward methods. These important intermediates for porphyrin syntheses are categorized as either symmetrical or unsymmetrical about the interpyrrolic 5-carbon. Symmetrically substituted dipyrromethanes such as 26 can be prepared in one step by self-condensation of bromomethylpyrroles such as 27 in hot methanol , or by heating 2-acetoxymethylpyrroles such as 28 in methanol/hydrochloric acid . The pyrrole 28 is obtained from the corresponding methylpyrrole 29 by oxidation of the 2-methyl with lead tetra-acetate. Catalytic hydrogenation cleaves the terminal benzyl esters to afford the corresponding 1,9-dicarboxylic acid 30 which can be formylated using the Vilsmeier reagent (dimethylformamide with either phosphoryl chloride or benzoyl chloride) to give 31 ().
Synthesis And Characterization Of Porphyrins | …
The porphyrin-C60 dyad 9 was synthesized by a dicyclohexylcarbodiimide-mediated condensation of the C60-acid 8 and the amino-porphyrin 3 (Scheme 4).
Synthesis and characterization of porphyrin 2
A majority of naturally occurring porphyrins (i.e. those not derived by degradation of chlorophylls or bacteriochlorophylls) do not contain meso-substituents, and there are certainly none that possess four such substituents. This has led many to question the utility of TPP as a “natural” mimic. Thus, over the years, OEP has been favored as a model for natural porphyrins more than has TPP. Synthetic approaches to porphyrins such OEP are a more complex because of the increased difficulty of preparing the 3,4-disubstituted pyrrole starting materials; pyrrole 8 can, of course simply be purchased and only needs to be distilled.
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