Contents:

Introduction.

Natural Photosynthesis.

Antenna Effect.

Chromophores used in Artificial photosynthetic arrays:

Porphyrins

Dendrimers

Fullerenes

Rylenes

Conclusion.

Introduction

165,000 TWof sunlighthit the earthevery day

165,000 TW of sunlightstrike the earth every day.

blue green orange red

Electromagnetic spectrum

NATURAL PHOTOSYNTHESIS

Schematic representation of bacterial light harvesting complexes.

N.W.Isaacs et al, Nature, 1995, 374, 517

Antennae

Light Excitation transfer

Pigment molecules

Electron transfer

e - acceptor

e- donor

Antenna Effect

Requisites for light harvesting:

Absorption of visible light.

Stability to photolysis.

Efficient energy transfer process.

High energy content.

Long lived excited state.

Appropriate kinetic factors.

Pen

RC

Pen

PenPen

C C

C C

h

h h

hh’

E E

EE

Pen : Energy transfer photosensitiser.

RC : Reaction Centre

C : Connectors

Intramolecular Light Harvesting systems : Testing the efficiency of energy transport

e¯

e¯h h h h h h h

ReI

PeI Pen Pen PenPen Pen Pen

R’eI

Fujihira et al, Thin Solid Films, 1989, 180, 43

Pen

Rel Pel R’el

Light harvesting on monomolecular layer assembly

Pel : Electron transfer photosensitiser

Pen: Energy transfer photosensitiser

Rel: Electron relay

Meyer.T.J.et al ,J. Am. Chem. Soc. 1989, I l l , 9101

Remote Photosensitisation

Synthetic multiporphyrin arrays have been frequently prepared to shed light on light harvesting processes.

Porphyrins are more stable and accessible synthetically than chlorophylls.

Absorption properties: strongly in the blue and weakly or moderately in the green regions.

Porphyrins:

Dyad

Q

P

Yanagita.M., et al, Tetrahedron Lett, 1979, 20, 257

1

2

3

C-P-Q Triad

Moore.T.A., et al., Nature, 1984, 307, 630

C

C-P-Q

C-1P-Q

C-P+-Q-

C+-P-Q-

0.0

1.0

2.0

Ene

rgy

(eV

)

1

2

3

5

4

PQ

= 2µs (in butyronitrile)= 300ns (in CH2Cl2)

Gust, D., et al, J. Am. Chem. Soc. 1997, 119, 1400.

C P C60

Triad

C-P-C60

C-1P-C60

C-P-1C60

C-P+-C60–

1[C+-P-C60– ] 3[C+-P-C60

–]

3[C-P-C60]

1

2

3

4

5 6 7

8

109

11

E (eV)

= 4.9µs (in MTHF)

N

N

N

Ru N

N

N

N

NH N

HN N

N N

NAu

Sauvage.J.P., et al, Chem. Soc.Rev., 2004, 33, 147

Triad

= 75 ns (in CH3CN)

+–

+–

Tetrad

Gust.D.,Moore.T.A., et al, Science, 1989, 244, 35

= 460ns

Fukuzumi.S, Guldi,D.M et al, J.Am.Chem.Soc. 2001, 123,

6617

Tetrad

+ –

= 0.38s

Pentad

Gust.D,Moore.T.A et al, Science, 1990, 248, 199

–+

C PZn PH QN QB

= 55 µs

Hexad

Gust. D., Lindsey.J, et al, J.Am.Chem.Soc. 1999, 121, 8604

energy transfer 50 ps

energy transfer 244 ps

charge separation 3 pscharge recombination 1330 ps

Lindsey, J., et al, J. Org. Chem. 1999, 64, 9101.

Increasing the spectral coverage

abs em

Lindsey, J. et al Org. Lett., 2000, 2, 17

Self Assembly of Light Harvester / Reactive Chromophore Assemblies

They are nanoscale macromolecules which are highly branched polymers with precisely controlled architectures.

Dendrimers:

Balzani V, Lehn J-M, et al, Angewandte Chemie, 1987, 26, 1266

Moore et al, Angewandte Chemie, 1993, 32, 246

Armaroli N. et al, Chem.Eur.J, 2003, 9, 37

Potential uses and positioning of metals within dendritic architectures

Newkome.G.R., et al,Chemical Reviews, 1999, 99, 1689

Aida.T. et al, Chem.Eur.J. 2002, 8, 2667

Zn-P

H2-P

h’

h

Balzani.V., et al, Coord Chemistry Reviews, 2001, 219–221, 545

Dendrimers with metal complexes in each branching centre.

Light harvesting and antenna effect

Balzani.V., et al, Coord.Chem.Reviews, 2001, 219–221, 545.

- Os2+

- Ru2+

Balzani.V., et.al, New. J. Chem., 1999, 23, 63

Dendrimer having a metal complex as the core

RYLENES

Wasielewski.M.,et al, J.Am.Chem.Soc, 2004, 126, 12268

Conclusion:Artificial photosynthetic arrays by theincorporation of various chromophores have been looked into.

These arrays mimic certain aspects of natural photosynthetic systems.

According to Giacomo Ciamician:“The photochemical processes, that hitherto have been the guarded secret of the plants, will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry but mankind is.”

Acknowledgements

Dr. Russell H. Schmehl

Dr. Mark J. Fink Dr. James P. Donahue

Group Members: Dr. Kumaresan Duraisamy Heidi Hester Srivathsa Vaidya Rupesh Narayana Prabhu David Karam Chemistry Department, Tulane University