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The Synthesis and Photophysical Properties of Alkene- and Stilbene-Linked Fullerene-Porphyrin Dyads
May 16, 2000
Paul J. Bracher, Berthold Nuber, Ahsan U. Khan,Stephen R. Wilson and David I. Schuster
NYU Fullerene Grouphttp://mozart.chem.nyu.edu
Donor / Acceptor Systems
Donors� Metal complexes
(e.g. ferrocene)
� Pre-Aromatics (e.g. tetrathiafulvalene)
� PORPHYRINS
Acceptors� Quinones
� Imides
� FULLERENES
Guldi, D. J. Chem. Soc., Chem. Commum., 2000, 321-327.
Imahori, H.; Sakata, Y. Eur. J. Org. Chem., 1999, 2445-2457.
HOMO
LUMO
LUMO + 1
Fullerene-Porphyrin Dyads� Donor-Linker-Acceptor Systems
Goals:� Understanding Photophysical Properties as a function of
Molecular Topology.
� Long-lived Charged Transfer States. Electron transfer from the porphyrin to the fullerene mimics the process of photosynthesis in plants.
� Development of Agents for Use in Photodynamic Therapy (PDT). Light-absorbing species that photosensitize the formation of singlet molecular oxygen (1g) are being used against tumors in cancer treatment.
Past NYU Dyads
O
HO OH
HO OH
O
N
NH
NHN
OO
O
OO
O
N HN
NNH
O
N
CH3CH3
CH3 O
N N
NNO O
O O
O O
M
Schuster, et al. J. Am. Chem. Soc. 1999, 121,
11599
Fong, et al. Org. Lett.1999, 1, 729.
MacMahon. Unpublished results.
The New Alkene-Linked Dyads
N
N
N
NH
H
PhPh
Ph
2
N
N
N
N
PhPh
Ph
Cu
Ph
1
� Alkene linkage through a pyrrole ring.
� Stilbene linkage through one of the porphyrin phenyl groups
Synthesis of Dyad 1
N
N
N
N
H H
Ph
Ph
Ph
N
N
NN
PhPh
Ph
Cu
O O
O O
Ph3P+
Ph
1. CuAc2
2. POCl3/DMF
N
N
N
NPh
Ph
Ph
Ph
CHO
Cu
Ph BuLi, H+
Synthesis of Dyad 1, ctd.
1. BuLi, C60
N
N
NN
PhPh
Ph
Cu
Ph
O
NN
NN
PhPh
Ph
Cu
Ph
NNHTs
NH2NHTs
NN
NN
PhPh
Ph
Cu
Ph
1
Synthesis of Dyad 2
O
CN
2. DIBAL-H
O
SS1. C6H5CHO N
H N N
NN
HHPh
Ph
Ph
S
S1. , BF3
SiO2, H2O
N N
NN
HHPh
Ph
Ph
O
BF3OEt2
SO2Cl2
2. DDQ
3. 2.5 M HCl
SHSH
O O
Ph3P
1. Zn2+
2. BuLi, toluene
+
Synthesis of Dyad 2, ctd.
N
N
N
N
PhPh
Ph
Zn
O O
N
N
N
N
PhPh
Ph
N-NHTs
1. BuLi, C60
H+1. NH2NHTs
2. Zn2+
2. H+
Zn
N
N
N
NH
H
PhPh
Ph
O
N
N
N
NH
H
PhPh
Ph
2
Characterization
� NMR Spectroscopy
� All reactions involve the appearance or vanishing of easily recognizable functional groups, so NMR suffices despite the presence of the copper, which broadens the peaks.
� UV-vis Spectroscopy
� The linker and dyads show similar absorption spectra in the UV-vis region
Porphyrin Synthesis Hurdles
� Low Solubility
� Low Reaction Concentrations
� Messy Extractions
� Arduous Column Chromatography
� Poor Yields (10-25% total)
Planned Studies
Synthetic:� Synthetic generation of pure cis and trans linked forms of both
dyads.
Photophysical:
� UV-vis & Fluorescence spectroscopy
� Transient absorption study of the excited states
� Singlet oxygen photosensitization quantum yields
� Photoinduced cis-trans isomerization in the linker moieties
Photophysical Pathways
1D*
h
1O2*
+
3A*
–
1) Energy Transfer 2) ISC
Electron Transfer
Quenching
Recombination
3O2
Fullerene (e– acceptor)
Porphyrin (e– donor)
Energy Transfer Pathway
1O2*
3A* Quenching
3O2
Cell Death
� Triplet excited C60 photosensitizes the formation of singlet oxygen (1g) with =1
� Singlet oxygen decay (1g3-
g) at 1270 nm can be detected using ultra-sensitive Ge-diode near-IR spectroscopy.
(PDT)
Khan, et al.. Proc. Natl. Acad. Sci. USA, 1979, 6047-6049.
Photodynamic Therapy (PDT)
Procedure:
1) Chromophore is injected into patient
2) The tumor-diseased area is irradiated
3) Cell death results from the generation of singlet oxygen
� Recently, the FDA approved Photofrin®, a concoction of porphyrins, as a cancer-treatment agent for photodynamic tumor therapy (PDT).
� Studies in Japan concluded that a C60 derivative suppressed tumor growth better than Photofrin® in mice.
Tabata, et al.. Jpn. J. Cancer Res., 1997, 1108-1116.
Electron Transfer Pathway
+ –
Following Irradiation:
e- transfer1D* BET
� The pathway can be monitored by transient absorption spectroscopy with the 670 nm decay of the porphyrin radical cation and the 1060 nm decay of the C60 radical anion.
� The goal is to produce long-lived charge-transfer states to mimic the photosynthetic reaction centers which are responsible for harvesting solar energy in plants.
Stilbene Triad Pathways ??
1D*
electron transfer
+ –electron transfer
+ –
1D*
energy transfer
energy transfer 1L*
1A*
cis-trans Photoisomerization
N
N
N
NH
H
PhPh
Ph
2a 2b
NN
NN
H
H
PhPh
Ph
� Introduces a significant change in molecular topology
� Separable by HPLC
� Such isomerization holds the potential for practical application as a molecular switch
Acknowledgments
� Technical Assistance & Helpful Discussions� Shaun MacMahon
� Prof. Nicholas Geacintov (NYU), for providing access to his fluorimeter and the Ge-diode IR spectrometer
� Prof. Dirk Guldi (Notre Dame, Radiation Lab), for transient absorption studies and related discussions.
� Funding� National Science Foundation, Grant CHE-9712735� NYU College of Arts and Science, S.F.B. Morse Grant
Paramagnetic Energy Plot
S0
T1
S1
D0
Q1
Q2D1
D2
N N
NN
H
H
Ph
Ph
Ph
Ph
N N
NN
Ph
Ph
Ph
Ph Cu
NN
N N
Ph
Ph Ph
Ph
Cu
P-*C60
P - C60
E
?D3