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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

Aggregation of astaxanthin. (a) AXT chemical structure.(b) NormalizedUV–vis absorption of five stable aggregates of AXT in 9:1 water:DMSO(I and II) or 4:1 water:acetone (III–V) as well as monomericAXT in acetone (dashed). Vertical lines indicate pump photon energiesused for TA measurements. (c) Model of exciton decay in monomericAXT. Ultrafast internal conversion from 1Bu to 2Ag is followed by fast nonradiative decay to the ground state. (d)Model of singlet fission and triplet annihilation in carotenoid aggregates,as measured here. Upon aggregation, the 1Bu state no longercouples to 2Ag. Instead, it converts directly into tripletpairs via singlet fission. The 2Ag state is lower in energythan the triplet pair and enables efficient recombination back tothe ground state.
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fig1: Aggregation of astaxanthin. (a) AXT chemical structure.(b) NormalizedUV–vis absorption of five stable aggregates of AXT in 9:1 water:DMSO(I and II) or 4:1 water:acetone (III–V) as well as monomericAXT in acetone (dashed). Vertical lines indicate pump photon energiesused for TA measurements. (c) Model of exciton decay in monomericAXT. Ultrafast internal conversion from 1Bu to 2Ag is followed by fast nonradiative decay to the ground state. (d)Model of singlet fission and triplet annihilation in carotenoid aggregates,as measured here. Upon aggregation, the 1Bu state no longercouples to 2Ag. Instead, it converts directly into tripletpairs via singlet fission. The 2Ag state is lower in energythan the triplet pair and enables efficient recombination back tothe ground state.

Mentions: Here we use broadband transientabsorption spectroscopy from thetens of femtoseconds to microseconds time scales to investigate thesinglet fission process in aggregates of the carotenoid astaxanthin(Figure 1a). We study a series of five distinctaggregates to determine both the role of intermolecular structureand the interplay between triplet formation and internal conversionto the 2Ag state. We show that intermolecular singlet fissionin polyenes follows the same mechanism—direct formation from1Bu (Figure 1d)—observedin the intramolecular polymer system reported previously.21 The initial rate of triplet formation is seento only weakly correlate with the strength of intermolecular couplingwith a fastest time constant of only 65 fs, among the fastest of anyreported system. These results raise important questions about theelectronic structure and intermolecular interactions of the polyenesand also point the way toward a universal mechanism for ultrafastsinglet fission.


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)

Aggregation of astaxanthin. (a) AXT chemical structure.(b) NormalizedUV–vis absorption of five stable aggregates of AXT in 9:1 water:DMSO(I and II) or 4:1 water:acetone (III–V) as well as monomericAXT in acetone (dashed). Vertical lines indicate pump photon energiesused for TA measurements. (c) Model of exciton decay in monomericAXT. Ultrafast internal conversion from 1Bu to 2Ag is followed by fast nonradiative decay to the ground state. (d)Model of singlet fission and triplet annihilation in carotenoid aggregates,as measured here. Upon aggregation, the 1Bu state no longercouples to 2Ag. Instead, it converts directly into tripletpairs via singlet fission. The 2Ag state is lower in energythan the triplet pair and enables efficient recombination back tothe ground state.
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Aggregation of astaxanthin. (a) AXT chemical structure.(b) NormalizedUV–vis absorption of five stable aggregates of AXT in 9:1 water:DMSO(I and II) or 4:1 water:acetone (III–V) as well as monomericAXT in acetone (dashed). Vertical lines indicate pump photon energiesused for TA measurements. (c) Model of exciton decay in monomericAXT. Ultrafast internal conversion from 1Bu to 2Ag is followed by fast nonradiative decay to the ground state. (d)Model of singlet fission and triplet annihilation in carotenoid aggregates,as measured here. Upon aggregation, the 1Bu state no longercouples to 2Ag. Instead, it converts directly into tripletpairs via singlet fission. The 2Ag state is lower in energythan the triplet pair and enables efficient recombination back tothe ground state.
Mentions: Here we use broadband transientabsorption spectroscopy from thetens of femtoseconds to microseconds time scales to investigate thesinglet fission process in aggregates of the carotenoid astaxanthin(Figure 1a). We study a series of five distinctaggregates to determine both the role of intermolecular structureand the interplay between triplet formation and internal conversionto the 2Ag state. We show that intermolecular singlet fissionin polyenes follows the same mechanism—direct formation from1Bu (Figure 1d)—observedin the intramolecular polymer system reported previously.21 The initial rate of triplet formation is seento only weakly correlate with the strength of intermolecular couplingwith a fastest time constant of only 65 fs, among the fastest of anyreported system. These results raise important questions about theelectronic structure and intermolecular interactions of the polyenesand also point the way toward a universal mechanism for ultrafastsinglet fission.

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