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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

Triplet identification. (a) Integrated decaykinetics from allspecies, at the peak of PIA, following excitation at 2.3 eV. All aggregates(solid) show an enhancement of the final state lifetime of 5 ordersof magnitude, relative to the monomer (dashed). (b) Comparison ofthe TA signal at 100 ns for all five aggregates reveals the same PIAsignature in each, combined with a GSB related to the UV–visabsorption. The final state must be the same in all aggregates. (c)Comparison of the absorption edge (dashed) and long-lived TA signal(solid) of monomer AXT following triplet sensitization (green) andaggregate II. The similarity of shape confirms the assignment of thelong-lived aggregate state to triplets, produced via singlet fission.
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fig4: Triplet identification. (a) Integrated decaykinetics from allspecies, at the peak of PIA, following excitation at 2.3 eV. All aggregates(solid) show an enhancement of the final state lifetime of 5 ordersof magnitude, relative to the monomer (dashed). (b) Comparison ofthe TA signal at 100 ns for all five aggregates reveals the same PIAsignature in each, combined with a GSB related to the UV–visabsorption. The final state must be the same in all aggregates. (c)Comparison of the absorption edge (dashed) and long-lived TA signal(solid) of monomer AXT following triplet sensitization (green) andaggregate II. The similarity of shape confirms the assignment of thelong-lived aggregate state to triplets, produced via singlet fission.

Mentions: TA measurements wereperformed on all five aggregates at four or more pump photon energies.Due to the overall similarity of the aggregate TA data, only the representativeaggregates II (weak H-aggregate) and IV (strong J-aggregate) willbe discussed in detail here. The spectra of other aggregates and otherpump photon energies can be found in the SupportingInformation, Figures S5–S10. Comparison of the TA spectrafor aggregate II in Figure 3a,b with the monomerdata in Figure 2 reveals two immediate differences:the PIA bands are broader and much less pronounced in II, and thefinal decay is significantly slower. As will be shown below in Figure 4, the final state in this aggregate is the triplet.Its PIA is dominant well before 3 ps (see below), and the spectrumdecays without further evolution out to the μs time scale. Theonly spectral changes that can be clearly discerned occur on an ultrafastinstrument-limited time scale: much like in monomeric AXT the initialexcited state absorbs in the NIR, seen as a broad, weak PIA in theearliest TA spectrum. As shown in Figure 3c,the decay of the NIR band is matched by the decay of the slight ground-statebleach (GSB) observed around 2.4 eV. This kinetic reflects the sub-psformation of triplet PIA, which strongly overlaps with the ground-stateabsorption and results in a negative overall signal.


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)

Triplet identification. (a) Integrated decaykinetics from allspecies, at the peak of PIA, following excitation at 2.3 eV. All aggregates(solid) show an enhancement of the final state lifetime of 5 ordersof magnitude, relative to the monomer (dashed). (b) Comparison ofthe TA signal at 100 ns for all five aggregates reveals the same PIAsignature in each, combined with a GSB related to the UV–visabsorption. The final state must be the same in all aggregates. (c)Comparison of the absorption edge (dashed) and long-lived TA signal(solid) of monomer AXT following triplet sensitization (green) andaggregate II. The similarity of shape confirms the assignment of thelong-lived aggregate state to triplets, produced via singlet fission.
© Copyright Policy
Related In: Results  -  Collection

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

fig4: Triplet identification. (a) Integrated decaykinetics from allspecies, at the peak of PIA, following excitation at 2.3 eV. All aggregates(solid) show an enhancement of the final state lifetime of 5 ordersof magnitude, relative to the monomer (dashed). (b) Comparison ofthe TA signal at 100 ns for all five aggregates reveals the same PIAsignature in each, combined with a GSB related to the UV–visabsorption. The final state must be the same in all aggregates. (c)Comparison of the absorption edge (dashed) and long-lived TA signal(solid) of monomer AXT following triplet sensitization (green) andaggregate II. The similarity of shape confirms the assignment of thelong-lived aggregate state to triplets, produced via singlet fission.
Mentions: TA measurements wereperformed on all five aggregates at four or more pump photon energies.Due to the overall similarity of the aggregate TA data, only the representativeaggregates II (weak H-aggregate) and IV (strong J-aggregate) willbe discussed in detail here. The spectra of other aggregates and otherpump photon energies can be found in the SupportingInformation, Figures S5–S10. Comparison of the TA spectrafor aggregate II in Figure 3a,b with the monomerdata in Figure 2 reveals two immediate differences:the PIA bands are broader and much less pronounced in II, and thefinal decay is significantly slower. As will be shown below in Figure 4, the final state in this aggregate is the triplet.Its PIA is dominant well before 3 ps (see below), and the spectrumdecays without further evolution out to the μs time scale. Theonly spectral changes that can be clearly discerned occur on an ultrafastinstrument-limited time scale: much like in monomeric AXT the initialexcited state absorbs in the NIR, seen as a broad, weak PIA in theearliest TA spectrum. As shown in Figure 3c,the decay of the NIR band is matched by the decay of the slight ground-statebleach (GSB) observed around 2.4 eV. This kinetic reflects the sub-psformation of triplet PIA, which strongly overlaps with the ground-stateabsorption and results in a negative overall signal.

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