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Early studies: from organic light harvesting assemblies to light powered nanoreactors. Jean M.J. Fréchet Material Science Division, LBNL and Department of Chemistry, University of California Berkeley, CA 94720-160. Natural Light-Harvesting Complex. - PowerPoint PPT Presentation
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Early studies: from organic light harvesting assemblies to light powered
nanoreactors
Jean M.J. FréchetMaterial Science Division, LBNL
andDepartment of Chemistry, University of California
Berkeley, CA 94720-160
Natural Light-Harvesting Complex
• Rings of chlorophylls and carotenoids (antenna) surround reaction center (RC)
• Energy Transfer from the antenna to the RC is quantitative
• Energy received at the RC is utilized to enable a series of electron transfer steps resulting in the production of ATP
Karrasch, S., Bullough, P.A. & Ghosh, R. The EMBO Journal, 1995, 14, 631-638
A highly simplified dendritic mimic
?
• Highly branched, well-defined macromolecule.• Can be tailored for functionality by modifying end-groups and core.• Rings of branching units and end-groups surround a central core.• Near quantitative energy transfer from peripheral units to the core?• What chemistry, if any, could be done at the core?
Light harvesting dendrimer
Energy Transfer
D A
S1D
S0D
S1A
S0A
D A
S1D
S0D
S1A
S0A
1. Through-bond electron-exchange (Dexter) - <10 Å
Donor excitation energy transferred to the acceptor via one of two mechanisms:
Energy Transfer
D A
S1D
S0D
S1A
S0A
D A
S1D
S0D
S1A
S0A
1. Through-bond electron-exchange (Dexter) - <10 Å
2. Through-space dipole-dipole interaction (Förster) (over distances of 10-100 Å)
Donor excitation energy transferred to the acceptor via one of two mechanisms:
Dendrimers and Energy Transfer
Emission
Absorption
Emission
Absorption
a) T. Förster, Ann. Physik, 1948, 2, 55; b) T. Förster, Z. Naturforsch. 1949, 4A, 319
kJ
n N RETD
D
9000 10
128
2
5 4 6
(ln )
S1
S0
S1
S0
?R
R = interchromophoric distance. J = overlap integral between donor emission and acceptor absoprtion (energy match between each donor and acceptor transition). 2 = orientation factor. D = fluorescence quantum yield of the donor. = fluorescence lifetime of donor. n = solvent index of refraction.The transition dipole moments of the dyes are reflected in J and D
R is the interchromophoric distance, kET, the rate constant for energy transfer,
falls off as the sixth power of R.
Dendrimers and Energy Transfer
Emission
Absorption
Emission
Absorption
a) T. Förster, Ann. Physik, 1948, 2, 55; b) T. Förster, Z. Naturforsch. 1949, 4A, 319
kJ
n N RETD
D
9000 10
128
2
5 4 6
(ln )
S1
S0
S1
S0
?R
Effects of Increasing Generation
Note that although the number of donors doubles with generation,the donor-acceptor distance is also increasing.
12
34
5
Generation1
23
4
5
6TerminalGroups
12TerminalGroups
24TerminalGroups
48TerminalGroups 96
TerminalGroupsx 2
x 2
x 2
x 2
RDA
GenerationGeneration
Generation
Generation
RDA
ET = 1
1 + R0
6
Light harvesting and conversion
Energy Conversion -Photoinduced Electron Transfer:
Harvesting antenna -Energy Transfer Relay:
“Up-hill” Conversion -Two Photon Energy Transfer:
broad absorption
narrow emission
energy transfer followed by electron transfer
charge separation to generate electrochemical potential
absorption of manylow energy photons
emission ofhigh energy photons
Artificial photosynthesis – still a distant target!
Coumarin-Labeled Dendrimers
1000 2000 3000 4000 5000 6000 7000
Mass (m/z)
G-4
G-3
G-21245 (1246)
2569 (2565)
5533 (5535)MALDI-TOF
G-3G-2
G-1
G-4
O
O
N
N
NN
O
OO
O
OO
O
O
OO
O
O
N
O
O
O
O
O
O
O O
N
N
N
N
O
O O
O
O
O
O OOO
N
N
N
N
O
OO
O
O
O
OO
O
O
N
N
N N
O
O O
O
O O
O
O
O
N
O
OO
O
N
N
O
O
O
O
O
N
N
O
O
O
O
N N
O
OOO O
O
O
O
NO
N
O
OO
O O
O
O
O
O
N
N
N
N
N
N
N
N
O
O
O
O
O
O
O
O
O
O
O O
O O
O
O
Overall light output for G4 dendrimer
300320
340360
380400
420440
460480
500
360380
400 420 440 460 480 500 520
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
ExcitationWavelength
(nm)Emission Wavelength (nm)
EmissionIntensity
(a.u.)
AcceptorEmission
(direct excitation)
Acceptor Emission(energy transfer)
ResidualDonor Emission
Sylvain Gilat, Alex Adronov
-10 0 10 20 30 40 50 60 70 80
-0.7
0.0
0.7
1.4
G1/C343 G2/C343 G3/C343 G4/C343no
rm. i
nten
sity
time (ps)
O
O
N
N
NN
O
OO
O
OO
O
O
OO
O
O
N
O
O
O
O
O
O
O O
N
N
N
N
O
O O
O
O
O
O OOO
N
N
N
N
O
OO
O
O
O
OO
O
O
N
N
N N
O
O O
O
O O
O
O
Compound Theoretical kET Observed kET ET Efficiency (%) ( 10-10) ( 10-10) Time Resolved
G-1 66 58 99.9 G-2 28 26 99.8 G-3 20 16 99.7 G-4 5.3 6.1 99.1
Time-Resolved Studies
Fred V. R. NeuwahlAlex Adronov
Acceptor fluorescence rise times
fluorescence lifetimes of donor dyes: ca. 2 ns
Synthetic light harvesting systems
NB
N
FF
NB
N
FF
NB N
FF
NB N
FF
NBN
FF
NBN
FF
NB
N
FF
NB
N
FF
Zn
N
N
N
N
N
N N
N
N
N
N
N
Ru Ru
NN
N
N
N N
N N
Ru
N
N
NN
Os
N
N
NN
N
N
N
N
Ru
Ru
N
N N
N
N
N
N
N Ru
NN
N
N
NN
N
N
N N
N NRu
Ru
N
N
NN
N
N
N
N
Ru
N
N N
N
O
O
OO
OO
O
O
O O
OO
N N
Ru
O O
O
OO
O
O
O
O
O
O
O
N
N
OO
O
OO
O
O
O
O
O
O
O
N
N
O
O
O
OO
OO
O
O
O
OO
O
O
O
O
O
OO
O
O
O
O
O
O
O O
O
O
O
NN
O
O
O
OO
OO
O
O
O
OO
O
O
O
O
O
OO
O
O
O
O
O
O
O O
O
O
O
Moore
Lindsey
Aida
Balzani
Vögtle/Balzani
Jason Serin
Cascade energy transfer has also been achieved
Highly rugged, photostableantenna (K. Muellen et al.)
Exploring alternatives structures.
Dye-labeled linear polymer analog
Dendrimer:the better controlled system
Linear polymers are likely to be much easier to prepare than dendrimers but are also likely to be prone to
site-site interactions
Quantitative comparison of fluorescence quantum yields
Dendrimer F Polymer
F G-1 0.77 “G-1” 0.11 G-2 0.70 “G-2” 0.25 G-3 0.68 “G-3” 0.42 G-4 0.62 “G-4” 0.45
*
0
50
100
150
200
250
300
440 490 540 590 640
Wavelength (nm)
Emis
sion
Inte
nsity
(a.u
.)
Excimer formation maybe the cause of low F
values in “low generation” polymers
Polymer with 20% acceptors
Dilute Monomer
N
O
O
O
N
O
O
O
=
=
Alex Adronov
N
O
O
O
O O
N
O
onm
Energy transfer on surfaces: Self-Assembled Monolayers (SAMs)
AAA
DD D
D
DD D
D
A
DD D
D DD D
D
DD D
DDD D
DDD D
DD
D DD
A A
A A
hh’
•Self-assembly of individual donor dendrons and acceptor dyes simplifies the preparation of antennae and future devices.
•Energy transfer to a reaction center in monolayer configuration has not been explored
Lysander Chrisstoffels
0
0.2
0.4
0.6
0.8
1
350 400 450 500 550 600
ex= 350 nm
Wavelength (nm)
Inte
nsi
ty A
.U.
Observed emission from monolayer of A and D-G2 (1:1) after excitation of the donors at 350 nm and after excitation of the acceptors at 420 nm.
ex= 420 nm
Efficient light-harvesting and energy transfer is achieved by self-assembly of donors and acceptors as mixed SAMs. Acceptor emission is amplified.
L. Chrisstoffels
Mixed SAMs: amplification of emission
D-G2A
NH
Si(OCH2CH3)3
O
OO
N
OON
N
OO
OO
N
N
O
OO
O
OO
ONH
Si(OCH2CH3)3
Towards catalytic nanoreactors
O
O
O
HO O
HO
HO
OHO
OO
O
HO
O
HO
HO
O
HO
O
O
O
O
O
O
O
OHO
OH
OH
OOH
OO
O
OH
O
OH
OH
O
OH
O
O
O
O
OO
O
O
O
O
O
O
HO O
HO
HO
OHO
OO
O
HO
O
HO
HO
O
HO
O
O
O
O
O
O
O
OHO
OH
OH
OOH
OO
O
OH
O
OH
OH
O
OH
O
O
O
O
O
O
O O
hv
O O
OHHO(H2N)2CS
G-3
OO
O
O
O OH
OH
OH
OH
O
OO
O
O
OHOHHO
OH
OO
OO
OO
O
O
OHO
HO
HO
HO
O
OO
O
O
HOHO OH
HO
O O
OO
OO
O
O
O OH
OH
OH
OH
O
OO
O
O
OHOHHO
OH
OO
OO
OO
O
O
OHO
HO
HO
HO
O
OO
O
O
HOHO OH
HO
O O
OO
O O
O
O
O
Model Compound
O
OO
OOO
OO
OO
O
OO
O
OO
O
HO OH
Key issue: lack of long term photostability
More rugged systems can be designed but how can we design self-repair???
O
O O
HO
HO
O
O
O
OO
HOHO
OO
OO
OO
HOHO
HOHO
OO
OH
OH
O
O
O O
OHOH
OO
O O
OO
OHOH
OH OH
N
N
N
N
Pd
O
O O
HO
HO
O
O
O
OO
HOHO
OO
OO
OO
HOHO
HOHO
O
O
OH
OH
O
O
OO
O OH
OH
OO
O
O
O
O
OH
OH
OH
OH
OO
OH
OH
O
O
O O
OHOH
OO
O O
OO
OHOH
OH OH
O
O
OH
OH
O
O
OO
O OH
OH
OO
O
O
O
O
OH
OH
OH
OH
O
O
HO
HO
O
O
OO
OHO
HO
OO
O
O
O
O
HO
HO
HO
HO
O
O
HO
HO
O
O
OO
OHO
HO
OO
O
O
O
O
HO
HO
HO
HO
h
1O2
3O2HO OH
O O
O O
O
O
O
O
N
O
S
N
N
O
S
N
O
N
SN
N
O
SN
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
O
OO
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
OO
OO
O
O
OO
O
O
OO O
O
O
OO
O
OOO
OOO
O
OOO
OO
O
O O
O
O
O
OO
O
O
OO O
O
O
OO
O
OOO
OOO
O
OOO
OO
O
O O
O
O O
O
O
NH
HN
N
N
O O
O
O
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
O
OO
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
OO
OO
O
O
OO
O
O
OOO
O
O
OO
O
O OO
O OO
O
OO O
OO
O
OO
O
O
O
OO
O
O
OOO
O
O
OO
O
O OO
O OO
O
OO O
OO
O
OO
O
N
O
S
N
N
O
S
N
N
O
SN
N
O
SN
We can also design multiphoton harvesting photoreactors
N
SN
O
O
N
SN
O
ON S
N
OO
N S
N
OO
NS
N
O O
NS
N
O O
N
SN
O
NH
HN
N
N
O
N
SN
O
O
Thin-film Solution in water or film
325 350 375 400
Ab
sorb
ance
Wavelength (nm)
Photobleaching of Anthracene
Water
1O2*
NaO
ONa
O
O
NaOO
OO
ONa
O
Singlet oxygen production monitored by photobleaching of anthracenedipropionic acid
Can be done both by one and two photon processes
NB: target application is in therapy not fuel production!
Mimicry of photosystem II with synthetic manganese complexes.
2 H2O O2 + 4H+ + 4e-
In Photosystem II, light drives the splitting of water to molecular oxygen, protons and reductive equivalents. To the plant, O2 is a just a waste product while the protons and reductive equivalents are used in the generation of valuable carbohydrates.
The primary photosynthetic processes involves absorption of light by different antenna pigments with funneling of the excitation energy to the chlorophylls of the photosynthetic reaction center, which initiate a chain of electron transfer reactions between the reaction center cofactors. An energy-rich charge-separated state is generated across the membrane, which represents the initial product of the solar energy conversion.
Curr. Opin. Chem. Bio. 2003, 7, 666
Mimicry of photosystem II with synthetic manganese complexes.
Target: mimic the electron donor side reactions of PSII in synthetic complexes in which manganese is linked to a photosensitizer such as a Ru(bpy)3
2+ complexes rather than the more chlorophyll-like porphyrins. When the Ru(bpy)3
2+ moiety was oxidized from RuII to RuIII by a laser flash in the presence of an electron acceptor, the RuIII complex oxidized the attached MnII to MnIII by intramolecular electron transfer, with time constants of < 50 ns–10 ms, depending on the complex [Eur J Inorg Chem 2001, 1019]. Can this reaction done at the level of single-electron transfer be used in the design of more sophisticated complexes that incorporate more than one manganese ion?
Curr. Opin. Chem. Bio. 2003, 7, 666
Ru–Mn complexes that show intramolecular electron transfer from the MnII to the photo-oxidized RuIII with time constants from <50 ns to 10 ms. The quenching rate decreases exponentially with the metal–metal distance for most complexes (solid circles), and at short distance, the excited state was so short-lived that the ruthenium could hardly be photo-oxidized by the external acceptor methyl viologen. Modifying the ruthenium ligands can reduce the quenching rate by 3 orders of magnitude (open square). Note that the bridging ligand was the same and that the subsequent electron transfer from MnII to the photo-oxidized RuIII was not slowed down.
Curr. Opin. Chem. Bio. 2003, 7, 666
Summary: we have a long way to go!
Nature
O
O
N
N
NN
O
OO
O
OO
O
O
OO
O
O
N
O
O
O
O
O
O
O O
N
N
N
N
O
O O
O
O
O
O OOO
N
N
N
N
O
OO
O
O
O
OO
O
O
N
N
N N
O
O O
O
O O
O
O
Dendrimers
OO
N
Si SiSi
SiO
O
NH
SiOO O
Si
O
NN
OOO
O
O
NH
Si
O
O
NH
Si OOO
O
NN
O OOO
O
SiO
O
NH
SiOO
OSi
O
NN
O OO
O
O
NH
SiO
O
OO
N
O
NH
SiO
OO
Si
O
NN
O
OO
O
O
OSi SiSi
O
Si
NHO
OO
N
NHO
OO
N
Surface self-assemblyLayer by layer assembly
NN
OOH
OHO
O
O
N
O
N
OO
N
N
O
N
OO
N
O
O
O
O
N
O
N
OO
N
N
O
N
OO
N
O
O
Ru2+
anatase TiO2
e-
ITO
Photovoltaics
hν
photoreactor
Outlook
Today, the most promising applications of organics are in photovoltaics.For solar to fuel, look at organic-inorganic hybrid systems. The organic portion (ligands for Mn, Ru, porphyrin centers, connectors, etc..) may hold the key to optimal activity of the inorganic component involved in electron transfer.The catalytic center remains a black box with much development still required.Explore self-assembly and layer-by-layer assembly.
