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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
Förster Dexterweak coupling
spectral overlap
reduced back transfer due to difference between excitation energies
point chromophores
dipole-dipole Coulombic interaction wave function overlap exchange interaction
30-100 Å 6-20 Å
Models of RET
Resonance Energy Transfer is fast
Förster vs. DexterFörster Dexter
D* A D A* 3D* 1A 1D 3A*
Singlet-singlet transfer only Singlet-singlet and triplet-triplet transfer
R ~ 30-100 Å R ~ 6-20 Å
6
1~R
k Forster ]/2exp[~ LRk Dexter
Light harvesting is inspired by naturePurple bacteria LH – Light Harvesting complex
RC – Reaction CenterEnergy is absorbed mainly in LH2
1) Excitation energy gradient:
RCLHLH EEE 12
2) Sufficient overlap of theemission 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
In LH applications almost all the energy is absorbed on the periphery followed by transfer to the core
Energy transfer pathways in dendrimersa
b
a) Direct RETall the elements arethe same.
b) Successive RETExcitation energygradient 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
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
Applications: signal amplificationTypical fluorescent map of a
dendrimer
(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.
Signal == core’s fluorescence
Dendrimer acts as antenna
Absorption spectrum broads; emission remains that of core. -> Relative band narrowing occurs
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.
Concentrations of sensors and target species are about
the same
Minimal concentration of fluorescent tags
Can’t detect low conc.
(b) Dendrimer based sensors can detect low concentrations
Balzani V., et al. 2000. Chem. Commun., 853–854
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: 915–917
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. 127:11851-11862
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 + 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
+ 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
Calzaferri G. et al. 2001. J. Mater. Chem., 12:1–13
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
Photovoltaic cell Unidirectional RET Excitation transfer to the substrate
(proven) Electron-hole pair separation (no
data in literature)
Calzaferri G. et al., 2006. C. R. Chimie., 9:214-225 Huber S., Calzaferri G., 2004. ChemPhysChem., 5:239
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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1 anti-2 syn-2 3 4
RE
T E
ffici
ency
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Morphology and temperature dependences
Substituents
Morphology dependence
Temperature dependence: excitation stems to the periphery
Entropy plays the key role. Threshold temperature is: )1ln( ZUTkBHere 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