Light Harvesting and Energy
Transfer
Oleksandr Mikhnenko
June 15
2006
Outline
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Introduction
The phenomenon of Resonance Energy
Transfer (RET)
Light harvesting in nature
Dendrimers in light harvesting applications
RET in zeolite L channels, applications
Conclusions
Resonance Energy Transfer is fast
Models of RET
Förster
Dexter
weak coupling
spectral overlap
reduced back transfer due to difference between excitation energies
point chromophores
dipole-dipole Coulombic interaction
30-100 Å
wave function overlap
exchange interaction
6-20 Å
Förster vs. Dexter
Dexter
Förster
A
D*
D
A*
Singlet-singlet transfer only
R ~ 30-100 Å
k
Forster
1
~ 6
R
3
D*
1
A
1
D
Singlet-singlet and
triplet-triplet transfer
R ~ 6-20 Å
k Dexter ~ exp[2 R / L]
3
A*
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:
ELH2  ELH 1  ERC
2) Sufficient overlap of the
emission and absorption spectra of
the pairs LH2-LH1, LH1-RC
RET efficiency > 90%
Pullerits T, Sundstrom V. 1996. Acc Chem Res. 29:381–389
Dendrimers
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In LH applications
almost all the energy
is absorbed on the
periphery followed by
transfer to the core
Energy transfer pathways in dendrimers
a
a) Direct RET
all the elements are
the same.
b
b) Successive RET
Excitation energy
gradient is required.
Very fast and
efficient
Colors of the basic
elements are used
to emphasize their
excitation energies.
Energy gradient can be realized
by varying sizes of the basic
elements
R. Kopelman, M. Shortreed et al. 1997. Phys. Rev. Lett. 78(7):1239-1242
Mechanism of RET in dendrimers
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All the chromophores are covalently bounded
(Dexter)
Usually conjugation is broken between elements
(Forster)
Dendrimer specific effects
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statistical distribution of interchromophore distances
morphology effects
temperature effects etc.
Dendrimers with purely Dexter or Forster RET
mechanism has been synthesized
Applications: signal amplification
Typical fluorescent map of a
dendrimer
Signal == core’s fluorescence
Dendrimer acts as antenna
Absorption spectrum broads;
emission remains that of
core. -> Relative band
narrowing occurs
(a) Spontaneous emission of
the peripheral groups;
(b) emission of the core after
the energy transfer from
the periphery;
(c) emission of the core upon
the direct excitation.
Gilat S. L., Frechet J.M.J. 1999. Angew. Chem. Int. Ed., 38:1422-1427
Low concentration sensors
(a) 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
Can’t detect
low conc.
Balzani V., et al. 2000. Chem. Commun., 853–854
Two photon absorption (2PA)
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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: 915–917
Triplet oxygen detection
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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. 127:11851-11862
Catalysis
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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 + 2]cycloaddition 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., 123:6959-6960
Dendrimers: brief summary
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+ 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)
Zeolite L
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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., 12:1–13
Photovoltaic cell
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Unidirectional RET
Excitation transfer to the substrate
(proven)
Electron-hole pair separation (no
data in literature)
Huber S., Calzaferri G., 2004. ChemPhysChem., 5:239
Calzaferri G. et al., 2006. C. R. Chimie., 9:214-225
Conclusions
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Energy transfer is an essential process in light
harvesting.
Light harvesting in dendrimers allows conceptually
new applications:
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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.
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Idea of photovoltaic cell was suggested.
Morphology and temperature
Morphology dependence
dependences
90%
RET Efficiency .
80%
70%
60%
50%
40%
30%
20%
10%
0%
1
anti-2
syn-2
3
Substituents
Temperature dependence: excitation stems to the periphery
Entropy plays the key role. Threshold temperature is:
k BT  U ln( Z  1)
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., 1701–1710
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