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 Unpublished results.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