Limits...
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

Monomer transient absorption. (a) Selected sub-psTA timeslicesfollowing excitation at 2.6 eV with 26 μJ/cm2. Athree-state model is sufficient to describe these data. The initial1Bu PIA can only be clearly observed within the pump pulseduration (black), and it decays to form the well-known 2Ag PIA in the visible region. The region below 1.1 eV is magnified10× to show the weak 2Ag → 1Bu PIA.(b) The corresponding decay kinetics (lines) show direct, instrument-limitedconversion from 1Bu to 2Ag and rapid decay tothe ground state. These processes are completely independent of pumpfluence over nearly 2 orders of magnitude (symbols, data normalizedwith respect to pump intensity) The data for 0.78 eV are magnifiedby a factor of 4 for clarity.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4440407&req=5

fig2: Monomer transient absorption. (a) Selected sub-psTA timeslicesfollowing excitation at 2.6 eV with 26 μJ/cm2. Athree-state model is sufficient to describe these data. The initial1Bu PIA can only be clearly observed within the pump pulseduration (black), and it decays to form the well-known 2Ag PIA in the visible region. The region below 1.1 eV is magnified10× to show the weak 2Ag → 1Bu PIA.(b) The corresponding decay kinetics (lines) show direct, instrument-limitedconversion from 1Bu to 2Ag and rapid decay tothe ground state. These processes are completely independent of pumpfluence over nearly 2 orders of magnitude (symbols, data normalizedwith respect to pump intensity) The data for 0.78 eV are magnifiedby a factor of 4 for clarity.

Mentions: To establish a baselinefor the photophysical behavior of AXT, we first examine the excitedstate processes of monomeric AXT in pure organic solution. Becausethe carotenoids exhibit pronounced solvatochromism (Suporting Information, Figure S3), monomer solutions wereprepared in both acetone and DMSO. The key results of TA measurementson AXT monomers excited at the absorption maximum are presented inFigure 2. Our TA results are presented throughoutin units of ΔT/T, in whichthe absorption of photogenerated states appears negative. Positivefeatures can reflect either increased transmission of the probe throughthe sample due to bleaching of the ground state or probe amplificationarising from stimulated emission. Using 120 fs excitation pulses,the results for the two solvents were almost identical save for aslight red-shift of all photoinduced absorption (PIA) features inDMSO (Figure S3), so only the acetone datawill be addressed here. AXT shows the characteristic polyene behaviorillustrated in Figure 1c and can be describedin terms of just two states.


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)

Monomer transient absorption. (a) Selected sub-psTA timeslicesfollowing excitation at 2.6 eV with 26 μJ/cm2. Athree-state model is sufficient to describe these data. The initial1Bu PIA can only be clearly observed within the pump pulseduration (black), and it decays to form the well-known 2Ag PIA in the visible region. The region below 1.1 eV is magnified10× to show the weak 2Ag → 1Bu PIA.(b) The corresponding decay kinetics (lines) show direct, instrument-limitedconversion from 1Bu to 2Ag and rapid decay tothe ground state. These processes are completely independent of pumpfluence over nearly 2 orders of magnitude (symbols, data normalizedwith respect to pump intensity) The data for 0.78 eV are magnifiedby a factor of 4 for clarity.
© Copyright Policy
Related In: Results  -  Collection

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

fig2: Monomer transient absorption. (a) Selected sub-psTA timeslicesfollowing excitation at 2.6 eV with 26 μJ/cm2. Athree-state model is sufficient to describe these data. The initial1Bu PIA can only be clearly observed within the pump pulseduration (black), and it decays to form the well-known 2Ag PIA in the visible region. The region below 1.1 eV is magnified10× to show the weak 2Ag → 1Bu PIA.(b) The corresponding decay kinetics (lines) show direct, instrument-limitedconversion from 1Bu to 2Ag and rapid decay tothe ground state. These processes are completely independent of pumpfluence over nearly 2 orders of magnitude (symbols, data normalizedwith respect to pump intensity) The data for 0.78 eV are magnifiedby a factor of 4 for clarity.
Mentions: To establish a baselinefor the photophysical behavior of AXT, we first examine the excitedstate processes of monomeric AXT in pure organic solution. Becausethe carotenoids exhibit pronounced solvatochromism (Suporting Information, Figure S3), monomer solutions wereprepared in both acetone and DMSO. The key results of TA measurementson AXT monomers excited at the absorption maximum are presented inFigure 2. Our TA results are presented throughoutin units of ΔT/T, in whichthe absorption of photogenerated states appears negative. Positivefeatures can reflect either increased transmission of the probe throughthe sample due to bleaching of the ground state or probe amplificationarising from stimulated emission. Using 120 fs excitation pulses,the results for the two solvents were almost identical save for aslight red-shift of all photoinduced absorption (PIA) features inDMSO (Figure S3), so only the acetone datawill be addressed here. AXT shows the characteristic polyene behaviorillustrated in Figure 1c and can be describedin terms of just two states.

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