Walter A. Svec

Molecular-wire behaviour in p-phenylenevinylene oligomers

Electron transfer from electron-donor to electron-acceptor molecules via a molecular ‘bridge’ is a feature of many biological andchemical systems. The electronic structure of the bridge component in donor–bridge–acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer,. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor–acceptor distance — effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p -phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor–acceptor separations as large as 40 Å, with rate constants of the order of 1011 s−1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.

[42] Analytical procedures for the isolation, identification, estimation, and investigation of the chlorophylls

Publisher Summary This chapter discusses the analytical procedures for the isolation, identification, estimation, and investigation of the chlorophylls. Chlorophylls are the preponderant photosynthetic pigments of the verdant tissues of vascular plants, liverworts, and various algae. The chlorophyll content of plants varies with their age or stage of development, seasons, mineral nutrition, and the intensity and duration of the illumination. The usual or normal chlorophylls have been obtained in various, isotopically labeled modifications by supplying the growing plants with isotopic compounds of carbon and hydrogen. Many fatty or fatlike substances are extracted from plant material with the solvents required for the extraction of the fat-soluble chlorophylls. Therefore, these natural contaminants must be considered in the selection of methods for investigation of the green pigments. The most conspicuous contaminants are the yellow and orange carotenoid pigments. Chlorophylls have many striking physical properties in common. They are green to gray-green and purple pigments with pronounced absorption bands in the blue-green and red spectral regions. They are strongly fluorescent, the wavelength of the emitted light usually corresponding to that of the principal absorption band in the red spectral region.

Extraction, Separation, Estimation, and Isolation of the Chlorophylls

Publisher Summary This chapter discusses extraction, separation, estimation, and isolation of the chlorophylls. From the analytical standpoint, the principal photosynthetically functional green pigments extractable from autotrophic and photo heterotrophic organisms with organic solvents are regarded as chlorophylls. These pigments are products of the autotrophic and photo heterotrophic growth of various organisms. The various green and gray-brown products formed from the natural green pigments when cells are injured or killed, subjected to various reagents, or exposed to various unfavorable conditions are regarded as chlorophyll alteration products. Most analytical methods are not suited to the identification and estimation of chlorophylls within the chloroplasts. In many chromatographic separations, green zones of the common chlorophylls a and b serve as reference standards. Additional green zones may be misinterpreted as indicative of new chlorophylls, although in fact they may be because of chlorophyll alteration products or to anomalous zone formation. Chromatographic separations may be qualitative or quantitative, on an ultramicroscale or on a preparative scale. Qualitative methods reveal the number, the sequence, and the identity of the pigments. Quantitative methods provide individual pigments that may be estimated fluorimetrically, calorimetrically, or spectrophotometrically. Chromatographic methods may be employed in many different modifications, namely, columnar chromatography, one-way paper chromatography, two-way paper chromatography, radial paper chromatography, and also with acceleration by centrifugal force, and thin-layer chromatography.

Optical control of photogenerated ion pair lifetimes : An approach to a molecular switch

A prototype molecular switch is demonstrated that works on the principle that the local electric field produced by one photogenerated ion pair (D1+-A1−) can influence the rate constants for photoinduced electron transfer and recombination in a second donor-acceptor pair (A2-D2). Two ultrafast laser pulses were used to control the rate of a photoinduced electron transfer reaction within a molecule that consists of two covalently linked electron donor-acceptor pairs fixed in a linear structure, D1-A1-A2-D2. This type of molecular architecture may lead to the development of electronic devices that function on the molecular length scale.

Ultrafast photoinduced electron transfer in rigid donor-spacer-acceptor molecules: modification of spacer energetics as a probe for superexchange

Abstract Four fixed-distance porphyrin-quinone molecules, 1-syn, 1-anti, 2-syn, and 2-anti, were synthesized. These molecules possess a zinc 5-phenyl-10,15,20-tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1-syn: 8.2 x 109 s−1; 1-anti: 1.7 x 1010 s−1; 2-syn: 8.5 x 109 s−1; 2-anti: 1.9 x 1010 S−1. The corresponding rate constants for the porphyrin cation - naphthoquinone anion recombination reaction are 1-syn: 1.4 x 1010 s−1; 1-anti: 2.5 x 1010 s−1; 2-syn: 5.0 x 1010 s−1; 2-anti: 8.2 x 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3-4 x faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.

Loroxanthin, a unique xanthophyll from scenedesmus obliquus and chlorella vulgaris☆

Abstract A unique xanthophyll, which had been detected before in certain green algae, has now been isolated from Scenedesmus obliquus and Chlorella vulgaris . This pigment, here called loroxanthin, has also been isolated in its deuterated form from fully deuterated Chlorella . It forms a triacetate, and with methanol plus HCl, it yields monomethyl and dimethyl ethers. It can be oxidized to an aldehyde, loroxanthal. It has been characterized as a hydroxy lutein with the additional hydroxy group on a chain methyl group, probably that on C 9 . The previously reported absence of this pigment in C. pyrenoidosa has been confirmed. It was present in two marine Cladophora species and in Ulva rigida but absent in Spirogyra sp. and in two marine siphonalean green algae. It is probably not identical with certain similar pigments reported in other vegetable sources and variously described as trollein and trollein-like.

Chlorophylls c1 and c2

Abstract Chlorophyll c has been separated into its components by chromatography on polyethylene, and the structures of the components established by proton magnetic resonance. The less sorbed chlorophyll c 1 is magnesium tetradehydropheoporphyrin a 5 monomethyl ester, and the more sorbed chlorophyll c 2 is magnesium hexadehydropheoporphyrin a 5 monomethyl ester. The relative amounts of the two components in preparations of c from diatoms and from several species of brown algae have been determined by chromatography and from the nuclear magnetic resonance spectra. The ratio, c 1 / c 2 , is about 0·6 in the various chlorophyll c preparations.

Selectively metalated doubly cofacial porphyrin trimers. New models for the study of photoinduced intramolecular electron transfer.

Abstract The preparation of a doubly cofacial porphyrin trimer consisting of a Zn mesoporphyrin - Zn mesoporphyrin - mesoporhyrin stack is described. The optical absorption and fluorescence properties of the molecule as a function of solvent are also reported.

Molecular weights and empirical formulas of the xanthophylls of Vaucheria

Abstract The xanthophylls of Vaucheria were heteroxanthin, C40H56O4; a partial ester or esters of vaucheriaxanthin, C40H56O5; a dihydroxy carotenoid, C40H54O2; and diadinoxanthin or a similar isomer, C40H54O3.

Heteroxanthin, diatoxanthin and diadinoxanthin from tribonema aequale☆

Abstract The principal crystallizable xanthophylls of Tribonema aequale are heteroxanthin, diatoxanthin, and diadinoxanthin. The latter pigment from Tribonema is identical with the diadinoxanthin from Euglena and the diatom Nitzschia . Description and identification of the pigments have been based upon comparative i.r., NMR and mass spectra.