Light Harvesting and Energy Transfer

Oleksandr Mikhnenko

June 15

2006

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

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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