Light Harvesting and Energy Transfer

1
Light Harvesting and Energy Transfer

• Oleksandr Mikhnenko
• June 15
• 2006

2
Outline

• Introduction
• The phenomenon of Resonance Energy Transfer (RET)
• Light harvesting in nature
• Dendrimers in light harvesting applications
• RET in zeolite L channels, applications
• Conclusions

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Resonance Energy Transfer is fast
Models of RET
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Förster vs. Dexter
Förster
Dexter
Singlet-singlet and triplet-triplet transfer
Singlet-singlet transfer only
R 30-100 Å
R 6-20 Å
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Light harvesting is inspired by nature
Purple bacteria
LH Light Harvesting complex RC Reaction
Center Energy is absorbed mainly in LH2
1) Excitation energy gradient
2) Sufficient overlap of the emission and
absorption spectra of the pairs LH2-LH1, LH1-RC
RET efficiency gt 90
Pullerits T, Sundstrom V. 1996. Acc Chem Res.
29381389
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Dendrimers

• In LH applications almost all the energy is
  absorbed on the periphery followed by transfer to
  the core

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Energy transfer pathways in dendrimers
a) Direct RET all the elements are the same.
b) Successive RET Excitation energy gradient is
required. Very fast and efficient
Energy gradient can be realized by varying sizes
of the basic elements
Colors of the basic elements are used to
emphasize their excitation energies.
R. Kopelman, M. Shortreed et al. 1997. Phys. Rev.
Lett. 78(7)1239-1242
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Mechanism of RET in dendrimers

• All the chromophores are covalently bounded
  (Dexter)
• Usually conjugation is broken between elements
  (Forster)
• Dendrimer specific effects
• statistical distribution of interchromophore
  distances
• morphology effects
• temperature effects etc.
• Dendrimers with purely Dexter or Forster RET
  mechanism has been synthesized

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Applications signal amplification
Typical fluorescent map of a dendrimer
Signal cores fluorescence Dendrimer acts as
antenna Absorption spectrum broads emission
remains that of core. -gt Relative band narrowing
occurs

• Spontaneous emission of the peripheral groups
• emission of the core after the energy transfer
  from the periphery
• emission of the core upon the direct excitation.

Gilat S. L., Frechet J.M.J. 1999. Angew. Chem.
Int. Ed., 381422-1427
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Low concentration sensors

• A typical photochemical sensor based on energy
  transfer.

• (b) Dendrimer based sensors can detect low
  concentrations

Concentrations of sensors and target species are
about the same
Minimal concentration of fluorescent tags
Cant detect low conc.
Balzani V., et al. 2000. Chem. Commun., 853854
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Two photon absorption (2PA)

• Two Photon Laser Scanning Microscopy requires
  good 2PA chromophores
• Inorganic quantum dots can be toxic for live
  tissues
• Dendrimers have high 2PA cross-section and good
  for organisms

Mongin O., et al. Chem. Commun., 2006 915917
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Triplet oxygen detection

• 2PA Laser Scanning Microscopy allows getting 3D
  image of oxygen distribution
• Dexter energy transfer is on the last step
• Laser wavelength is weakly absorbed by the tissues

Raymond P. et al. 2005. J. Am. Chem. Soc.
12711851-11862
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Catalysis

• The main problem the mass transport from the
  focal point of the light harvesting system
• Can enforce reaction with small reagent that
  easily diffuse to the dendrimer core.
• Example reactions that require singlet oxygen
• (for chemists 4 2cycloaddition of the
  photoproduced singlet oxygen to dienes with
  subsequent reduction to the allylicdiol )

Stefan Hecht S. and Frechet J.M.J. 2001. J. Am.
Chem. Soc., 1236959-6960
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Dendrimers brief summary

• Elegant artificial realization of the concept
  of light harvesting
• Applications are conceptually different with
  conventional devices
• - Conventional devices usually can not be made of
  dendrimers (photovoltaic cell)

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Zeolite L

• Dye molecules do not aggregate with each other
• They are on distances sufficient for Forster RET
• Different dyes are used to guarantee directional
  energy transfer

Calzaferri G. et al. 2001. J. Mater. Chem.,
12113
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Photovoltaic cell

• Unidirectional RET
• Excitation transfer to the substrate (proven)
• Electron-hole pair separation (no data in
  literature)

Huber S., Calzaferri G., 2004. ChemPhysChem.,
5239
Calzaferri G. et al., 2006. C. R. Chimie.,
9214-225
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Conclusions

• Energy transfer is an essential process in light
  harvesting.
• Light harvesting in dendrimers allows
  conceptually new applications
• fluorescent signal amplifications
• detection of ultra low concentrations
• enhancement of two-photon absorption
• catalysis.
• Zeolite L crystals can be used as a backbone for
  directional energy transfer.
• Idea of photovoltaic cell was suggested.

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Morphology and temperature dependences
Morphology dependence
Substituents
Temperature dependence excitation stems to the
periphery
Entropy plays the key role. Threshold temperature
is
Here U is the energy loss during light
harvesting, Z is coordination number of the core.
Adronov A., Fréchet J.M.J. 2000. Chem. Commun.,
17011710