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The nature of singlet exciton fission in carotenoid aggregates.

Musser AJ, Maiuri M, Brida D, Cerullo G, Friend RH, Clark J - J. Am. Chem. Soc. (2015)

Bottom Line: The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers.In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales.This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

View Article: PubMed Central - PubMed

Affiliation: †Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom.

ABSTRACT
Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

No MeSH data available.


Related in: MedlinePlus

Regimes of triplet–tripletannihilation in aggregate II.(a) Integrated decay kinetics of II following sub-ps excitation at2.6 eV. The same multiexponential function fits the decay at all fluences,requiring the recombination process to be entirely geminate. Lifetimes(0.9, 6, and 49 ps) are not assigned to individual processes but ratherreflect the wide distribution of triplet pair decay rates. (b) Integrateddecay kinetics of II on longer time scales, following ns excitationat 2.3 eV, show strong fluence dependence indicative of non-geminateTTA.
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fig5: Regimes of triplet–tripletannihilation in aggregate II.(a) Integrated decay kinetics of II following sub-ps excitation at2.6 eV. The same multiexponential function fits the decay at all fluences,requiring the recombination process to be entirely geminate. Lifetimes(0.9, 6, and 49 ps) are not assigned to individual processes but ratherreflect the wide distribution of triplet pair decay rates. (b) Integrateddecay kinetics of II on longer time scales, following ns excitationat 2.3 eV, show strong fluence dependence indicative of non-geminateTTA.

Mentions: We first consider the ps-ns regime (Figure 5a), during which the triplet PIA spectrum decays uniformly with atmost a slight narrowing (Figure 3a). The multiexponentialdecay we observe on these time scales is far too short for the intrinsictriplet lifetime. Instead, in agreement with studies of zeaxanthinaggregates26,39 we attribute this behavior toannihilation processes, specifically triplet–triplet annihilation(TTA). This is a highly efficient process—though some fractionof the triplets survives out to μs time scales, at least 90%of the excited population annihilates between 1 ps and 1 ns. At thesame time, this initial TTA regime is completely independent of pumpfluence and must thus correspond to the annihilation of geminate tripletpairs. These facts enable an important insight into the energeticsof singlet fission in carotenoids.


The nature of singlet exciton fission in carotenoid aggregates.

Musser AJ, Maiuri M, Brida D, Cerullo G, Friend RH, Clark J - J. Am. Chem. Soc. (2015)

Regimes of triplet–tripletannihilation in aggregate II.(a) Integrated decay kinetics of II following sub-ps excitation at2.6 eV. The same multiexponential function fits the decay at all fluences,requiring the recombination process to be entirely geminate. Lifetimes(0.9, 6, and 49 ps) are not assigned to individual processes but ratherreflect the wide distribution of triplet pair decay rates. (b) Integrateddecay kinetics of II on longer time scales, following ns excitationat 2.3 eV, show strong fluence dependence indicative of non-geminateTTA.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4440407&req=5

fig5: Regimes of triplet–tripletannihilation in aggregate II.(a) Integrated decay kinetics of II following sub-ps excitation at2.6 eV. The same multiexponential function fits the decay at all fluences,requiring the recombination process to be entirely geminate. Lifetimes(0.9, 6, and 49 ps) are not assigned to individual processes but ratherreflect the wide distribution of triplet pair decay rates. (b) Integrateddecay kinetics of II on longer time scales, following ns excitationat 2.3 eV, show strong fluence dependence indicative of non-geminateTTA.
Mentions: We first consider the ps-ns regime (Figure 5a), during which the triplet PIA spectrum decays uniformly with atmost a slight narrowing (Figure 3a). The multiexponentialdecay we observe on these time scales is far too short for the intrinsictriplet lifetime. Instead, in agreement with studies of zeaxanthinaggregates26,39 we attribute this behavior toannihilation processes, specifically triplet–triplet annihilation(TTA). This is a highly efficient process—though some fractionof the triplets survives out to μs time scales, at least 90%of the excited population annihilates between 1 ps and 1 ns. At thesame time, this initial TTA regime is completely independent of pumpfluence and must thus correspond to the annihilation of geminate tripletpairs. These facts enable an important insight into the energeticsof singlet fission in carotenoids.

Bottom Line: The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers.In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales.This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

View Article: PubMed Central - PubMed

Affiliation: †Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom.

ABSTRACT
Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

No MeSH data available.


Related in: MedlinePlus