Stereodynamics of intramolecular triplet energy transfer in carotenoporphyrins

Gust, Devens; Moore, Thomas A.; Bensasson, René V.; Mathis, Paul; Land, Edward J.; Chachaty, C.; Moore, Ana L.; Liddell, Paul A.; Nemeth, Gregory A.

doi:10.1021/ja00298a038

Cited by 58 publications

(26 citation statements)

References 3 publications

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“…Over the last three decades, many carotenotetrapyrrole synthetic dyads have been designed and synthesized in which the absorption of a photon is followed by the formation of the tetrapyrrole triplet species by intersystem crossing; subsequent T-TET from the tetrapyrrole to the carotenoid produces the carotenoid triplet species (9)(10)(11)(12). In these dyads, depending on their precise chemical properties and on the way the tetrapyrrole and the carotenoid molecules are linked, the T-TET kinetics ranges from tens of microseconds to the subnanosecond range.…”

Section: Significancementioning

confidence: 99%

Triplet–triplet energy transfer in artificial and natural photosynthetic antennas

Kish

Méndez-Hernández

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.artificial photosynthesis | photoprotection | DFT calculations | triplet-triplet energy transfer | resonance Raman P hotoprotection is central to the maintenance of photosynthesis. Under certain circumstances, chlorophyll excited triplet states may form from intersystem crossing and from electron-hole recombination reactions in reaction centers. Chlorophyll triplet species can sensitize the formation of singlet oxygen, a deleterious reactive oxygen species (1). In photosynthetic complexes, this sensitization reaction is precluded by sufficiently rapid transfer of the triplet excited state from chlorophyll (Chl) or bacteriochlorophyll (BChl) to carotenoid molecules. The triplet states of carotenoids involved in photoprotection are well below the energy of singlet oxygen and therefore do not sensitize singlet oxygen. This quenching reaction reduces the lifetime of the Chl or BChl triplet state by many orders of magnitude (1) and renders it kinetically incompetent to sensitize singlet oxygen. Moreover, carotenoids can quench singlet oxygen directly in the event that it is inadvertently formed.In the light-harvesting (LH) proteins from most (anoxygenic) purple bacteria, triplet-triplet energy transfer (T-TET) from BChl to carotenoid molecules occurs on the nanosecond timescale (2, 3). By contrast, in light-harvesting complexes (LHCs) from...

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Section: Significancementioning

confidence: 99%

Triplet–triplet energy transfer in artificial and natural photosynthetic antennas

Kish

Méndez-Hernández

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…A portion (0.30 g, 0.654 mmol) of 7'-apo-7'-(4-carboxyphenyl)-bcarotene [39] was dissolved in 70 mL of toluene, and 0.65 mL (7.8 mmol) of pyridine was added. The mixture was stirred and 0.2 mL (3.27 mmol) of thionyl chloride was added dropwise.…”

Section: Tricarotenoporphyrinmentioning

confidence: 99%

Increasing the Yield of Photoinduced Charge Separation through Parallel Electron Transfer Pathways

Maniga

Sumida

Stone

et al. 1999

J. Porphyrins Phthalocyanines

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A strategy for increasing the yield of long-lived photoinduced charge separation in artificial photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has been investigated. Excitation of the porphyrin moiety of a carotenoid ( C )–porphyrin ( P )–naphthoquinone (Q) molecular triad leads to the formation of a charge-separated state C ·+– P – Q ·− with an overall quantum yield of 0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives C – P ·+– Q ·− with a quantum yield of ~1.0. However, electron transfer from the carotenoid to the porphyrin radical cation to form the final state does not compete well with charge recombination of C – P ·+– Q ·−, reducing the yield. The related pentad C 3– P – Q features carotenoid, porphyrin and quinone moieties closely related to those in the triad. Excitation of this molecule gives a C ·+– P ( C 2)– Q ·− state with a quantum yield of 0.073. The enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with charge recombination. The yield does not increase by the statistically predicted factor of three owing to small differences in thermodynamic driving force between the two compounds.

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“…As presented in Figure 18, a corrected fluorescknce excitation spectrum failed to show any evidence that light absorbed by the polyene moiety resulted in fluorescence from the porphyrin. In other words, even though the edge of the polyene nelectron system is separated by only -5A from the edge of the porphyrin, there is no singlet-singlet energy transfer [73]. As far as we could ascertain, the excited carotenoid relaxed by internal conversion back to its ground state.…”

Section: A Carotenoporphyrinsmentioning

confidence: 77%

“…Concomitant with the rise of the carotenoid triplet absorbance at 550 nm, the porphyrin triplet absorbance at 440nm decayed with a time constant of 2ps. There was no appreciable change in these parameters when 24 was dissolved in a rigid plastic matrix [73]. For 26 the triplet energy transfer was much faster.…”

Section: Triplet-triplet Energy Transfermentioning

confidence: 94%

“…As will be seen in the next section of this review and as has been alluded to earlier, neither the time average solution conformations from NMR studies nor the absorption spectra directly address the transient acquistion of folded or stacked structures which could provide electron exchange interaction. For singlet energy transfer within the necessary 10-12ps as limited by the carotenoid singlet lifetime such large scale motions are simply too slow to be effective [28,73]. In other words, only those chromophore pairs in which the necessary electronic coupling is a static property of the system will demonstrate efficient singlet energy transfer from carotenoid to porphyrin or chlorophyll.…”

Section: B Carotenopyropheophorbidesmentioning

confidence: 99%

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