Dendrimers and light: towards electroactive and photoactive dendrimers

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Dendrimers and lighttowards electroactive and
photoactive dendrimers
Jean M.J. Fréchet, Dept. of Chemistry, UC
Berkeley AFOSR MURI Program (Dr. C.
Lee) Graduate Students Alex Adronov, Adam
Freeman, Stefan Hecht, Patrick
Malenfant. Postdoctoral Fellow Lysander
Chrisstoffels. Collaborators Prof. M. Thompson,
USC Prof. P. Prasad, SUNY Buffalo Dr. S.
Gilat, Lucent Bell Labs Prof. G. Fleming, UC
Berkeley \ Dr. D. Robello, Kodak
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Natural photosynthetic processes
1. Light Harvesting
Chlorophyll Bound to Protein
2. Energy Transfer Relay
Reaction Center
Lipid Bilayer
3. Charge Separation
Frechet_at_cchem.berkeley.edu
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The dendritic antenna

• Design synthesis of a dendritic light
  harvesting antenna
• Demonstration of efficient through-space energy
  transfer
• Study of the effect of increasing dendrimer
  generation on ...the energy transfer efficiency

Frechet_at_cchem.berkeley.edu
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Antennas for Light Harvesting and Energy Transfer

• Efficient light harvesting and energy transfer
  (gt90) in dendritic systems have been
  demonstrated.
• Synthesized molecules have been fully
  characterized and energy transfer was studied by
  both steady-state and time-resolved techniques.
• The synthetic approach has a modular design that
  provides versatility in the choice of core
  acceptor and surface donor dyes (coumarins,
  oligothiophenes, two-photon chromophores).

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Summary of initial energy transfer results
Donor Dye
Acceptor Dye
G-4 Dendrimer
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Synthetic strategy
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Peripheral laser-dye donor functionalization
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Core acceptor functionalization
Identical reactions can be carried out to link
the penta- thiophene cores to the donor dendrons
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Thiophene-core fully dye-labeled dendrons
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MALDI-TOF of T7-labeled dendrons
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Spectral properties of the models
Emission Intensity (a.u.)
Extinction coefficient e (x 10-4, M-1 cm-1)
Wavelength (nm)
Large overlap between donor emission and
acceptor absorption enables efficient energy
transfer.
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Light harvesting G-1 to G-3
G-3
G-2
Extinction coefficient e (M-1 cm-1)
G-1
T7
Wavelength (nm)
Light Harvesting capacity doubles with
generation.
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Energy transfer G-1 to G-3
conc. 5.06 x 10-6 M
lexc 343 nm
G-3
Emission intensity (a.u.)
G-2
G-1
Direct Core Emission lmax 425 nm
T7
Wavelength (nm)
Beyond G-1, sensitized fluorescence becomes much
more intense than fluorescence from direct
excitation of the core.
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T5-core dendrons as antennas
conc. 3.93 x 10-6 M
lexc 343 nm
G-3
G-3
Emission intensity (a. u.)
G-2
Extinction coefficient (M-1cm-1)
G-2
G-1
Direct Core Emission
G-1
lexc 425 nm
T5
T5
Wavelength (nm)
Wavelength (nm)
The observed absorption and fluorescence emission
spectra of the G-1 to G-3 pentathiophene core
dendrons were very similar to those for the
series of heptathiophene core dendrons.
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Energy transfer efficiency
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Tunable emission
Emission intensity (a. u.)
Emission intensity (a. u.)
Wavelength (nm)
Wavelength (nm)
It is possible to tune the dendrimer emission
wavelength by changing the core functionality.
Also, by mixing the different types of dendrimers
(no dye at core, coumarin 343 at core, and
oligothiophene at core), it is possible to obtain
broadband emission by exciting at a single
wavelength (lmax of donor dye - 343 nm).
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Incorporation of two-photon dyes
Investigation of cooperativity effects of
two-photon chromophores when arranged in a
branched structure
EDC/DMAP

G-1
A collaboration with Prof. Paras Prasad.
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Two-photon dendrimers
K2CO3

18-Crown-6
G-2
Collaboration with Prof. Paras Prasad
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Future directionsdendritic energy transfer relay
380 nm
320 nm
470 nm
390 nm
480 nm 510 nm
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Surface-confined energy transfer

• Self-assembly of individual
• donor dendrons and acceptor dyes simplifies the
  preparation of antennas.
• Energy transfer on surfaces opens avenues for
  the fabrication of novel photonic devices.
• Variation of photoactive donors and acceptors
  allows for numerous applications, ranging from
  sensors to solar cells and new devices.

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Energy transfer on self-assembled surfaces
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N
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Frechet_at_cchem.berkeley.edu

• Self-assembled monolayers of chromophores with
  different aspect ratios were prepared on silicon
  wafers by using siloxane chemistry.
• Complete quenching of the donor emission as well
  as efficient energy transfer from the assembled
  coumarin-2 (donor) dyes to the coumarin-343
  (acceptor) dyes was observed.
• The photophysical properties are tuned by
  varying the molar ratio of assembled donor and
  acceptor chromophores on the surface.

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Synthesis of adsorbates
Donor chromophore
Acceptor chromophore
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Physisorption of coumarin-343 onto
amino-terminated SAMs
40000
420 nm
l
ex
II
30000
20000
10000
III
I
0
400
500
600

lem
(nm)
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Photophysical properties of coumarin-derivatized
SAMs
Excitation spectra and Emission spectra of SAMs
of 1 or 2
1
A.U.
0.75
2
1
2
0.5
0.25
1
0
320
370
420
470
520
570
l

(nm)
em
1
2
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Energy transfer within SAMs of mixed monolayers
of coumarin chromophores
Emission spectra from mixed monolayer of 1 and 2
(12 ratio)
80000
lex 370 nm
60000
40000
lex 420 nm
20000
0
350
400
450
500
550
600
1
2
(nm)
l
em
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Energy transfer on surfaces The next step

• Light harvesting event is followed by electron
  transfer.
• The excited state of secondary donor (De)
  transfers an electron instead of emitting light.
• Electron acceptor can inject an e- into a
  semi-conducting substrate.

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Light harvesting and electron transferthe
concept
3. Charge Transport
4. Subsequent Reaction
1. Light Harvesting Energy Transfer
2. Charge Separation
Electroactive core is capable of donating an
electron to attached acceptor this effects
charge separation that may be followed by charge
transport and subsequent reaction
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Light harvesting and electron transferthe
molecules
1. Light Harvesting
2. Energy Transfer
3. Electron Transfer
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Why use dendrimers in OLEDs?
Preorganization of active components and building
blocks
A collaboration with Prof. M. Thompson, USC
Site-isolated light emitting chromophore
Hole or electron transporting moiety
Insulating building block
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Dendrimers in single layer OLEDs
Metal cathode
ITO anode
Glass substrate
A collaboration with Prof. M. Thompson, USC
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Synthesis of Dye-Labeled Cores
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Synthesis of HT-Labeled Monodendrons
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Synthesis of Reactive Monodendrons
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Synthesis of Fully-Labeled Dendrimers
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Single-Layer OLEDs
A collaboration with Prof. M. Thompson, USC
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Device Fabrication
A collaboration with Prof. M. Thompson, USC
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Photo- and electroluminescenceof C343 labeled
dendrimers
TAA TAA2-G-1-OH
PBD
A collaboration with Prof. M. Thompson, USC
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Photo- and Electroluminescenceof T5 Labeled
Dendrimers
PBD
A collaboration with Prof. M. Thompson, USC
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Color Tunable OLEDs by Mixing Dendrimers
The small dendrimer affords partial site-isolation
T5
C343
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Towards enhanced properties
Surface (HT) chromophores
Central lumophore
Interior insulating monomer layers
Increasing the size of the dendrimer will
increase site isolation of the central lumophore
this should lead to enhanced color tunability of
devices containing mixtures of dendrimers.
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Increasing dendrimer size for enhanced
site-isolation
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Current status of project

• A new family of fully-labeled dendrimers has been
  successfully prepared via a modular approach.
• Photoluminescence studies indicate that efficient
  Forster energy transfer between peripheral and
  core chromophores occurs within these dendrimers.
• Analogous energy conveyance processes occur in
  single-layer OLEDs containing these dendrimers
  and exclusive emission from the core chromophores
  is observed.
• Site isolation of chromophoric dyes within the
  dendrimer affords some degree of color tunability.

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Porphyrin-core stars as photo- and electroactive
polymers
peripheral functionalities (chromophores or
solubilizing groups)
polymer backbone (UV-transparent and redox-stable)
porphyrin-core unit (energy sink and catalytic
site)
advantages ease and flexibility of
preparation/modification
efficient shielding of the core (site isolation)
solvent-induced change of
shape and size potential photoresponsive
devices and sensors, catalysts
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Toward Light-driven Supermolecular Catalysis
substrate
product
water-soluble micellar structure Þ
solvophobically-driven catalysis
Solar Energy Concentration and Conversion
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Encapsulated porphyrins - a retrospective
Myoglobin Mimics - Oxygen Binding in Artificial
Enzymes
Þ dioxygen binding affinity 1500 times
higher than hemoglobin (T-state)
Collman, Fu, Zingg, and Diederich Chem. Commun.
1997, 193
see also Jiang and Aida Chem. Commun. 1996, 1523
(polyether dendrimers)
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Encapsulated porphyrins - a retrospective
Hemeprotein Mimics - Tuning of Redox properties
by Isolation of the Core
Pollak, Leon, Fréchet, Maskus, Abruña Chem.
Mater. 1998, 10, 30
Jin, Aida, Inoue Chem. Commun. 1993, 1260 Aida
and coworkers Macromolecules 1996, 29, 5236
see also Diederich and coworkers Angew. Chem.
Int. Ed. Engl. 1994, 33, 1739 Angew. Chem. Int.
Ed. Engl. 1995, 34, 2725 Helv. Chim. Acta 1997,
80, 1773
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Synthesis of branched porphyrin-core star polymers
branched initiators
star polymers
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Characterization of porphyrin-core star polymers
1H NMR Analysis
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Modification of porphyrin-core star polymers
core modification (metalation)
end-group modification (esterification)
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Accessibility of the zinc porphyrin core
fluorescence quenching experiments employing
methyl viologen in acetonitrile
Stern-Volmer Analysis
Þ
Þ enhanced shielding of the core moiety by the
polymer backbone Þ degree of site isolation is a
function of the chain length
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Energy transfer in coumarin-terminatedporphyrin-c
ore star polymers
spectral overlap
distance
Förster, Fluoreszenz Organischer Verbindungen,
Vandenhoech and Ruprech Göttingen, 1951
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Influence of the chain length on the energy
transfer
quenched donor emission in chloroform
Þ correlation between energy transfer efficiency
and average donor-acceptor distance
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Influence of the solvent on the energy transfer
solvation-induced change of average
donor-acceptor distance
Û
bad solvent (MeCN) collapsed conformation
good solvent (CHCl3) extended conformation
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The role of concentration - trivial vs.
non-trivial energy transfer
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Photo and electroactive dendrimers some
conclusions

• Dendrimers are ideally suited for the design of
  nanometer scale antennas.
• Through space energy transfer is very efficient.
• Site isolation of chromophores at the core of
  dendrimer provides for unique behaviors not
  achievable with ordinary chromophores or
  polymers.
• Energy transfer on surfaces opens avenues for
  the fabrication of novel photonic devices.
• Applications include photonic devices, solar
  cells, OLEDs, sensors, light-powered reactors,
  etc.