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Light Harvesting and Energy Transfer


									Light Harvesting and Energy

    Oleksandr Mikhnenko
    June 15

   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          wave function overlap    exchange interaction
               30-100 Å                                            6-20 Å
Förster vs. Dexter
               Förster                                  Dexter

 D*        A             D    A*   3
                                       D*       1
                                                    A            1
                                                                     D   3

  Singlet-singlet transfer only              Singlet-singlet and
                                             triplet-triplet transfer

           R ~ 30-100 Å                                 R ~ 6-20 Å
           Forster     1                    k Dexter ~ exp[ 2 R / L]
       k             ~ 6
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

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

                            Colors of the basic    Energy gradient can be realized
                            elements are used      by varying sizes of the basic
                            to emphasize their     elements
                            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
   Usually conjugation is broken between elements
   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 amplification
                                 Typical fluorescent map of a
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     (b) Dendrimer based sensors can
    based on energy transfer.              detect low concentrations

        Concentrations of
        sensors and target
        species are about
            the same

         concentration of
         fluorescent tags

           Can’t detect
            low conc.

                Balzani V., et al. 2000. Chem. Commun., 853–854
Two photon absorption (2PA)
   Two Photon Laser
    Scanning Microscopy
    requires good 2PA
   Inorganic quantum dots
    can be toxic for live
   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

   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
    (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

   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
   Unidirectional RET
   Excitation transfer to the substrate
   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
   Energy transfer is an essential process in light
   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.
Morphology and temperature dependences
Morphology dependence

                                       RET Efficiency .
                                                                1   anti-2      syn-2       3   4

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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