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

3. FörsterDexter weak coupling spectral overlap reduced back transfer due to difference between excitation energies point chromophores dipole-dipole Coulombic interactionwave function overlap exchange interaction 30-100 Å 6-20 Å Models of RET Resonance Energy Transfer is fast

4. F ö rster vs. Dexter Förster Dexter Singlet-singlet transfer only Singlet-singlet and triplet-triplet transfer R ~ 30-100 Å R ~ 6-20 Å

5. 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 > 90% Pullerits T, Sundstrom V. 1996. Acc Chem Res. 29:381–389

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

7. 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

8. 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

9. Applications: signal amplification Typical 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

10. 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

11. 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

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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.

18. Morphology and temperature dependences Substituents Morphology dependence 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., 1701–1710