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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

Ultrafast1Bu → 2Ag internal conversion.(a) Transient absorption timeslices from sub-30 fs TA on monomericAXT in DMSO, showing smooth transition from 1Bu (PIA below1.5 eV) to 2Ag (PIA above 1.8 eV). No other electronicstate can be identified. Arrows indicate direction of spectral change.The corresponding decay kinetics (symbols) in (b) DMSO and (c) acetonecan be well described with an exponential time constant (lines) of105 or 125 fs, respectively.
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fig6: Ultrafast1Bu → 2Ag internal conversion.(a) Transient absorption timeslices from sub-30 fs TA on monomericAXT in DMSO, showing smooth transition from 1Bu (PIA below1.5 eV) to 2Ag (PIA above 1.8 eV). No other electronicstate can be identified. Arrows indicate direction of spectral change.The corresponding decay kinetics (symbols) in (b) DMSO and (c) acetonecan be well described with an exponential time constant (lines) of105 or 125 fs, respectively.

Mentions: As noted above, the processof singlet fission identified in Figure 3 islargely complete within the instrument response of the sub-ps TA experiment.To fully resolve the triplet formation, we performed sub-30 fs TAmeasurements on the full set of aggregates as well as monomeric AXTin acetone and DMSO. In dilute solution (Figure 6) we find that the monomer reproduces the behavior observed previouslyusing sub-ps TA, namely direct conversion of 1Bu into ahot 2Ag state which rapidly cools. The higher time resolutionenables a clean determination of the 1Bu → 2Ag internal conversion time constant, which we find to varywith solvent from 105 fs in DMSO to 125 fs in acetone with no discernibleintermediate states. It is unclear what environment is most appropriatefor comparison to AXT molecules embedded in (or at the surface of)aggregates, but this measurement establishes an approximate 100 fstime scale with which singlet fission must compete to proceed efficiently.


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)

Ultrafast1Bu → 2Ag internal conversion.(a) Transient absorption timeslices from sub-30 fs TA on monomericAXT in DMSO, showing smooth transition from 1Bu (PIA below1.5 eV) to 2Ag (PIA above 1.8 eV). No other electronicstate can be identified. Arrows indicate direction of spectral change.The corresponding decay kinetics (symbols) in (b) DMSO and (c) acetonecan be well described with an exponential time constant (lines) of105 or 125 fs, respectively.
© Copyright Policy
Related In: Results  -  Collection

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

fig6: Ultrafast1Bu → 2Ag internal conversion.(a) Transient absorption timeslices from sub-30 fs TA on monomericAXT in DMSO, showing smooth transition from 1Bu (PIA below1.5 eV) to 2Ag (PIA above 1.8 eV). No other electronicstate can be identified. Arrows indicate direction of spectral change.The corresponding decay kinetics (symbols) in (b) DMSO and (c) acetonecan be well described with an exponential time constant (lines) of105 or 125 fs, respectively.
Mentions: As noted above, the processof singlet fission identified in Figure 3 islargely complete within the instrument response of the sub-ps TA experiment.To fully resolve the triplet formation, we performed sub-30 fs TAmeasurements on the full set of aggregates as well as monomeric AXTin acetone and DMSO. In dilute solution (Figure 6) we find that the monomer reproduces the behavior observed previouslyusing sub-ps TA, namely direct conversion of 1Bu into ahot 2Ag state which rapidly cools. The higher time resolutionenables a clean determination of the 1Bu → 2Ag internal conversion time constant, which we find to varywith solvent from 105 fs in DMSO to 125 fs in acetone with no discernibleintermediate states. It is unclear what environment is most appropriatefor comparison to AXT molecules embedded in (or at the surface of)aggregates, but this measurement establishes an approximate 100 fstime scale with which singlet fission must compete to proceed efficiently.

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