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MOLECULAR LEVEL ARTIFICIAL PHOTOSYNTHETIC MATERIALS Special volume edited by GERALD J. MEYER

BALTIMORE, MARYLAND DEPARTMENT OF CHEMISTRY, JOHNS HOPIUNS UNIVERSITY

PROGRESS IN INORGANIC CHEMISTRY Series edited by KENNETH D. KARLIN

BALTIMORE, MARYLAND DEPARTMENT OF CHEMISTRY, JOHNS HOPKINS UNIVERSITY

VOLUME 44

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS, INC. New York Chichester Brisbane Toronto Singapore Weinheim

MOLECULAR LEVEL ARTIFICIAL PHOTOSYNTHETIC MATERIALS

PROGRESS IN INORGANIC CHEMISTRY

VOLUME 44

Advisory Board

JACQUELINE K. BARTON CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

THEORDORE L. BROWN UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS

JAMES P. COLLMAN STANFORD UNIVERSITY, STANFORD, CALIFORNIA

F. ALBERT COTTON TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS

ALAN H. COWLEY UNIVERSITY OF TEXAS, AUSTIN, TEXAS

RICHARD H. HOLM HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS

EIICHI KIMURA HIROSHIMA UNIVERSITY, HIROSHIMA, JAPAN

NATHAN S . LEWIS CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

STEPHEN J. LIPPARD MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS

NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS

EXXON RESEARCH & ENGINEERING CO., ANNANDALE, NEW JERSEY

RUHR-UNIVERSITAT, BOCHUM, GERMANY

TOBIN J. MARKS

EDWARD I. STIEFEL

KARL WIEGHARDT

MOLECULAR LEVEL ARTIFICIAL PHOTOSYNTHETIC MATERIALS Special volume edited by GERALD J. MEYER

BALTIMORE, MARYLAND DEPARTMENT OF CHEMISTRY, JOHNS HOPIUNS UNIVERSITY

PROGRESS IN INORGANIC CHEMISTRY Series edited by KENNETH D. KARLIN

BALTIMORE, MARYLAND DEPARTMENT OF CHEMISTRY, JOHNS HOPKINS UNIVERSITY

VOLUME 44

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS, INC. New York Chichester Brisbane Toronto Singapore Weinheim

Cover illustration of “a molecular ferric wheel” was adapted from Taft, K. L. and Lippard, S. J . , J . Am. Chem. SOC., 1990, 112, 9629.

This text is printed on acid-free paper.

An Interscience@ Publication

Copyright 0 1997 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10 158-001 2.

Library of Congress Catalog Card Number 59-13035 ISBN 0-471-12535-0

1 0 9 8 7 6 5 4 3 2 1

For the first time in history, solar energy devices that operate on a molecular level have become economically competitive with traditional solid state photovoltaics. This represents an exciting time for inorganic photochemists. Re- search in this field has not only fundamental implications but also the real potential for practical applications. While a wide variety of strategies are being explored for creating artificial photosynthetic devices at the molecular level, arguably the most promising utilize solid state materials. This is the subject of this special volume of Progress in Znorganic Chemistry entitled ‘ ‘Molecular Level Artificial Photosynthetic Materials. ” The title was chosen to reflect both the molecular nature of the chemistry as well as the materials science aspects.

The goal in the design of artificial photosynthetic materials is to convert solar energy into high-energy chemical products or directly into electricity. To ac- complish this feat in a real inorganic assembly it is necessary to integrate a variety of features into the material. These features include light absorption, vectorial electron- and/or energy-transfer processes, and subsequent conversion to useful products. In many respects, solid state media provides an ideal host for integrating such features into a single artificial photosynthetic device. For example, materials science techniques allow one to arrange and spatially isolate the photo- and redox-active species such that macroscopic control of vectorial electron- or energy-transfer processes is possible. Furthermore, many materials are photoactive, conductive, and/or catalytic themselves and may, therefore, play a direct role in the light conversion process.

To combine all the above features into one device, the assembly must pos- sess considerable molecular complexity, much like natural photosynthesis. The underlying science is therefore very difficult and progress is often slow. Many recent developments are based on observations from studies previously per- formed in fluid solution. In fact, the translation of known inorganic photochemistry and photophysics to the growing area of molecular materials is an important theme throughout this volume. In this manner, well-developed excited-state chemistry serves as a foundation for the rapidly growing area of molecular photonic materials. These studies then probe the structure-property relations of the supramolecular nature of extended solid state materials.

The selection of topics in this volume allows a comprehensive account of the developments in key areas of molecular artificial photosynthetic materials. The first contribution contains recent advances in supramolecular design. The use of supramolecular compounds is inspired by natural photosynthesis and is based on the notion that energy conversion can take advantage of specific photo-

V

vi PREFACE

and redox-active components in a single molecular compound. Specific applications and the use of novel time-resolved vibrational spectroscopies as mechanistic probes of intercomponent energy- and electron-transfer processes are described.

The second contribution reviews transition metals incorporated within Lang- muir-Blodgett (LB) films. The ease with which a wide variety of monolayer and multilayer assemblies can be fabricated make this technique particularly attractive. A description of the LB technique and film characterization is given. The optical and redox properties of novel transition metal compounds in monolayer assemblies is exploited in optoelectronic applications. The third contribution describes the use of layered metal phosphonate solids as hosts for molecular photosynthetic systems. Some novel molecular architecture is described wherein viologen derivatives are covalently bound in a zirconium phosphate lattice. Excitation with ultraviolet light leads to photochemistry and viologen radicals that are stable indefinitely in air. The close-packed nature of the solid prevents oxygen diffusion and provides a striking example of how molecular level modification can be exploited to control electron-transfer processes in solid state materials.

Light-induced charge separation in monolithic gels prepared by sol-gel techniques are described in the fourth contribution. Gel materials are an interesting class of solids that contain both a liquid and solid phase. The “soft” semisolid nature of gel materials affords the opportunity to simultaneously incorporate mobile and immobile components in an artificial photosynthetic material. The strategy has led to some extremely long-lived charge separated pairs. Gel net- works provide a medium where the possibility of achieving macroscopic control of directed electron-transfer processes is very real. The fifth contribution describes molecular compounds assembled in zeolites. Charge-transfer chemistry and electrochemically driven charge propagation studies cited provide insights into molecular processes and microenvironments, Visible excitation of zeolitic assemblies produces charge-separated pairs that can in turn drive the splitting of water into hydrogen and oxygen. Detailed mechanistic water oxidation studies demonstrate that the zeolite matrix inhibits the degradation pathways commonly observed in fluid solution.

The optical and excited-state properties of nanometer-sized semiconductor clusters is also reviewed. This exciting and rapidly growing area of science has significant practical applications in several different fields including solar de- toxification of pollutants, photocatalysis, electrooptics, imaging science, and photovoltaics. Electron-hole dynamics in these materials can be controlled by surface modification. A novel class of materials is comprised of one material capped with a second semiconductor material. For example, SnO, colloids capped with TiO, facilitate charge separation and improve iodide photooxida- tion efficiencies. Multicomponent semiconductor structures may pave the way

PREFACE vii

toward new materials useful in solar energy conversion. The final contribution compliments the previous one and describes the development of porous nanocrystalline TiO, films for a variety of photonic applications. The ability to control molecular level electron-transfer processes at this technologically important interface is in itself noteworthy. Furthermore, nanocrystalline TiO, films modified with Ru(I1) polypyridyl sensitizers efficiently convert light into electricity in conventional regenerative solar cells. These studies represent the first time a device that operates on a molecular level yields solar conversion efficiencies that are comparable to solid state devices.

In summary, the fine group of diverse contributions to this volume reveal the phenomenal progress and state of the art in molecular level artificial photosynthetic materials. The studies described in this volume are generally in their infancy. The prospects for future exploration and applications of these tech- nologies are without bound.

GERALD J. MEYER

Baltimore, Maryland

Contents

A Supramolecular Approach to Light Harvesting and Sensitization of Wide-Bandgap Semiconductors: Antenna Effects and Charge Separation

C. A. BIGNOZZI

J . R. SCHOONOVER

F. SCANDOLA

Dipartimento di Chimica, Universita di Ferrara, Ferra Italy

Los Alamos National Laboratory, Los Alamos, New Mexico

Dipartimento di Chimica, Universita di Ferrara, Ferrara Italy

Langmuir-Blodgett Films of Transition Metal Complexes M. K. DEARMOND and G. A. FRIED

Chemistry and Biochemistry Department, New Mexico State University, Las Cruces, New Mexico

Layered Metal Phosphonates as Potential Materials for the Design and Construction of Molecular Photosynthetic Systems

L. A. VERMEULEN Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois

Light-Induced Processes in Molecular Gel Materials F. N. CASTELLANO and G . J . MEYER

Department of Chemistry, Johns Hopkins University, Baltimore, Maryland

Charge-Transfer Processes in Zeolites: Toward Better Artificial Photosynthetic Models

P. K. DUTTA and M. LEDNEY Department of Chemistry, The Ohio State University, Columbus. Ohio

Native and Surface Modified Semiconductor Nanoclusters P. V. KAMAT

Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana

1

97

143

167

209

273

ix

X CONTENTS

Molecular and Supramolecular Surface Modification of Nanocrystalline TiOz Films: Charge-Separating and Charge-Injecting Devices 345

T. GERFIN and M. GRATZEL Institut de Chimie Physique II , Ecole Polytechnique Fkde'rale de Lausanne, Laussane, Switzerland

L. WALDER Institut fur Chemie, Universitat Osnabriick, Osnabriick, Germany.

Subject Index 395

Cumulative Index, Volumes 1-44 409

A Supramolecular Approach to Light Harvesting and Sensitization of Wide-Bandgap Semiconductors : Antenna Effects and Charge Separation

C. A. BIGNOZZI

Dipartimento di Chimica Universitd di Ferrara Ferrara, Italy

J. R. SCHOONOVER

Los Alamos National Laboratoy Los Alamos, NM

F. SCANDOLA

Dipartimento di Chimica Universitd di Ferrara Ferrara, Italy

CONTENTS

I. INTRODUCTION

A. Scope and Limitations B. Sensitization of Wide-Bandgap Semiconductors C. Regenerative Photovoltaic Cells

1. Electron Injection Quantum Yield 2. Electron Collection Efficiency 3. Ligh-Harvesting Efficiency

D. Molecular Sensitizers

11. A SUPRAMOLECULAR APPROACH

A. Functions from Natural Photosynthesis 1 . Photoinduced Charge Separation 2. Antenna Effect

Progress in Inorganic Chemistry, Val. 44, Series edited by Kenneth D. Karlin. Molecular Level Art$cial Photosynthetic Materials, Special volume edited by Gerald J . Meyer. ISBN 0-471-12535-0 0 1997 John Wiley & Sons, Inc.

1

2 BIGNOZZI, SCHOONOVER, SCANDOLA

B. Supramolecular Systems as Photochemical Molecular Devices C. Kinetics of Intercomponent Transfer Processes

1. Electron Transfer 2. Energy Transfer

1. Photoinduced Charge Separation 2. Antenna Effect

D. Use of Supramolecular Systems in Sensitization of Wide-Bandgap Semiconductors

111. MONONUCLEAR METAL COMPLEXES

A. The MLCT Sensitizers B. Excited-State Properties

1. Localization 2 . Excited-State Decay 3. Excited-State Redox Potentials

C. Behavior on Semiconductors 1. Surface Interaction 2. Electron Injection Quantum Yield 3. Electron-Collection Efficiency 4. Light-Harvesting Efficiency

IV , POLYNUCLEAR METAL COMPLEXES AS LIGHT-HARVESTING ANTENNAS

A. General Design Principles B. Experimental Detection of Intercomponent Energy Transfer

1. Energy Transfer between Adjacent Units 2 , Long-Range Intermolecular Energy-Transfer Processes 3. Energy-Transfer Mechanism

C. Behavior on Semiconductors

V. POLYNUCLEAR COMPLEXES AS CHARGE-SEPARATING SENSITIZERS

A. General Design Principles B. Experimental Studies of Intercomponent Electron Transfer

1. The Ru(I1)-Rh(II1) Polypyridine Systems 2. The Ru(I1) Polypyridine-Phenothiazine System

C. Behavior on Semiconductors

VI. CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

I. INTRODUCTION

A. Scope and Limitations

The principles of dye sensitization of wide-bandgap semiconductors are well established. The application of this process to light energy conversion has been

SUPRAMOLECULAR APPROACH TO SENSITIZATION 3

proposed as an attractive research goal for several decades, although progress in the field has been initially slow. Recently, however, some spectacular advances in conversion efficiency were obtained by Gerfin et al. (this volume, Chapter 7) with regenerative photoelectrochemical cells based on sensitized ti- tanium dioxide (Ti02) nanocrystalline photoanodes. This practical success has caused a sharp acceleration in the field and active research is now being per- formed in many laboratories on several aspects of this complex problem. This research ranges from theory to fundamental photophysics, from semiconductor fabrication to optimization of other cell components, and from dye design and synthesis to cell technology.

In this chapter, a somewhat peculiar viewpoint of the semiconductor sensitization problem is presented. For many years our laboratory has been active in the field of supramolecular photochemistry, with particular interest in the design, synthesis, and characterization of inorganic photochemical molecular devices, that is, supramolecular systems based on inorganic building blocks with suitable built-in light-induced functions. This chapter tries to provide an answer to the following question: Can semiconductor sensitization benefit from the use of supramolecular species, in replacement of conventional, simple molecular sensitizers? The answer is based on the notion, inspired by natural photosynthesis, that energy conversion can take advantage of specific light-induced functions taking place at a supramolecular level. In particular, the “antenna effect” can be used to increase the overall cross section for light absorption and to widen the action spectrum of the sensitizer, while ‘‘photoinduced charge separation” within the sensitizer prior to charge injection can serve to suppress the detrimental effect of charge-recombination processes.

This chapter contains some background material on sensitization of wide- bandgap semiconductors and factors affecting conversion efficiencies in regenerative sensitized photoelectrochemical cells (Section I). Also, some essen- tial aspects of natural photosynthesis, and a few notions of electron and energy- transfer kinetics are briefly summarized as an introduction to the proposed supramolecular approach (Section 11). In subsequent sections, experimental results concerning mononuclear complexes as molecular sensitizers (Section III), polynuclear complexes as supramolecular light-harvesting antennas (Section IV), and polynuclear complexes as supramolecular charge-separating sensitizers (Section V) are discussed in some detail. Consistent with the aim of this chapter, no attempt was made for any exhaustive coverage of the literature on this subject. Rather, representative experimental results were used, which were taken mainly from our own work in this area, to exemplify the proposed approach.

B. Sensitization of Wide-Bandgap Semiconductors

Wide-bandgap, n-type, semiconductors such as Ti02, ZnO, and SrTi03, are stable, easily processed materials that do not undergo decomposition upon ir-


radiation. Sensitization of semiconductors implies an extension of their photo- sensitivity in a wavelength range beyond the bandgap energy. Sensitization has been known since 1873, when Vogel (1) discovered that organic dyes deposited on semiconducting halide grains increased the sensitivity of these particles on the low-energy side of the visible spectrum.

In the sensitization process, an electronically excited dye molecule, which is adsorbed on the surface of the semiconductor, injects an electron into the conduction band allowing photoelectrochemical and photocatalytic processes to occur. The possibility of solar energy conversion through this process encour- aged large research activities, starting at the end of the 1960s (2-41). These studies led to important insights into the fundamental processes of dye sensitization, but for about two decades the efficiency of molecular photovoltaic devices based on such an effect remained low. This low efficiency was largely due to the limited light absorption by the monolayer of dye adsorbed onto smooth electrodes or electrodes with small surface roughness.

In recent years, substantial advances have been made with the introduction of high surface area nanocrystalline TiO, electrodes. Transparent TiO, films can be produced by depositing nanometer size TiO, particles on a conducting glass support (34). This film is heated to form a porous, high surface area, TiO, membrane. A monolayer of dye sensitizer can then be attached to the TiOz membrane giving a photoanode sensitive to visible light, and representing the key component of the photocell. The development of these photoanoides and a thin-layer solar cell (34), combined with an appropriate design of the dye sensitizer (32, 33, 36, 40, 41) has allowed for conversion efficiencies of solar energy into electricity on the order of 7-10%. The parameters determining the performances of molecular photovoltaic devices based on polycrystalline TiO, will be briefly described in Section I. C.

C. Regenerative Photoelectrochemical Cells

The cell consists of a molecular sensitizer anchored to the semiconductor surface, a solution containing a relay electrolyte, and a metallic counterelectrode, Fig. 1.

The relevant processes that occur following irradiation of the photoanode are summarized in Fig. 2. Light excitation promotes the sensitizer to upper lying electronic excited states that convert very rapidly and efficiently to the lowest lying electronic excited state. The excited dye injects an electron into the semiconductor from a normal distribution of donor levels (7) at a rate kin,, and be- comes oxidized. The electron is swept to the semiconductor bulk by the surface electric field and flows through an external circuit to perform work. The oxidized dye is then reduced by an electron donor present as relay electrolyte, at a rate kD. Reduction of the oxidized donor occurs at the counterelectrode and the solar cell is therefore regenerative. Radiative decay, (k,) , nonradiative de-


Figure 1. Ti02 layer.

Schematic of the dye cell construction showing the illumination through the dye coated

cay, (knr), and recombination of the photoinjected electron with the oxidized dye sensitizer, (kb) , represent loss mechanisms. Other possible loss mechanisms such as recombination of the conduction band electrons with the oxidized electron donor, or chemical reactions taking place from the oxidized or excited dye sensitizer, are not shown in Fig. 2.

Amps

8- ~

D

Pt

D+

Figure 2. Schematic representation of the elementary steps involved in a regenerative photoelectrochemical cell for light conversion based on dye sensitization of semiconductors. See text for definitions of kinetic parameters.


The parameter that directly measures how efficiently incident photons are converted to electrons is the incident photon-current efficiency, (IPCE). The wavelength dependent IPCE term can be expressed as a product of the quantum yield for charge injection (a ) , the efficiency of collecting electrons in the external circuit (q), and the fraction of radiant power absorbed by the material or light-harvesting efficiency (LHE), Eq. 1.

IPCE = (a)(q)(LHE) (1)

While CP and q , can be rationalized on the basis of kinetic parameters, LHE depends on the active surface area of the semiconductor and the cross section for light absorption of the molecular sensitizer.

The overall efficiency of the photovoltaic cell (qCell) is given by Eq. 2

where Voc, the open circuit photovoltage, represents the difference between the Fermi level of the semiconductor under illumination and the redox potential of the electrolyte; iph is the integrated photocurrent density; ffis the cell fill factor; and I , is the intensity of the incident light. The integrated photocurrent density represents the overlap between the solar emission and the monochromatic current yield (42).

1. Electron Injection Quantum Yield

In the absence of photodecomposition reactions, the quantum yield for electron injection from the excited sensitizer to the semiconductor is given by Eqs. 3 and 4.

cp = kinj/(kinj + k, + kn,) (3)

cp = kinjT (4)

where k , is the rate constant for the charge injection process, and r represents the excited-state lifetime of the dye sensitizer anchored to the surface of the semiconductor.

Charge injection from the excited dye will be activated if the donor energy is positive with respect to the conduction band edge. Electron injection will be, on the contrary, activationless if the donor level has an energy equal to or more negative than the conduction band edge. In this case, there will be maximum overlap between the excited donor levels and the wide conduction band acceptor. The two different kinetic regimes can be determined with knowledge of the

SUPRAMOLECULAR APPROACH TO SENSITIZATION I

energy of the conduction band edge, EE (43), and the energy of the donor levels of the sensitizer, EZ'*.

Capacitance measurements have been used to determine the flat-band potential and hence the position of the conduction band edge at the semiconductor (43). Recently, optical absorption measurements of the conduction band electrons as a function of the applied bias have been proposed as a method of de- riving the flat-band potential of nanostructured TiO, films (44-46).

The evaluation of the energy of the donor levels requires the notion that an electronically excited sensitizer (S*) is a new chemical species with well-de- fined thermodynamic and spectroscopic properties (47, 48). Consequently, the reducing or oxidizing ability of an electronically excited molecule can be expressed independently by the electronic configuration of the excited state as a function of the ground-state redox potentials and the zero-zero spectroscopic energy of the excited state by Eqs. 5 and 6

E(S+/S*) = E(S+/S) - Em

E(*S /S- ) = E ( S / S - ) + Em ( 5 )

(6)

E(Sf/S) and E(SIS-) are the ground-state oxidation and reduction potentials, respectively. The zero-zero spectroscopic energy, (Em), (48) can be estimated from spectral fitting of the room temperature emission profile of the excited sensitizer (49, 50) or from the energy of the first vibronic component in the 77 K emission spectrum (51). With these approximations, EA'* (7) is given by Eq. 7,

where X represents the reorganization energy accompanying the charge injection process (52). The X term arises from structural differences between the excited, (S*), and oxidized, (Sf), sensitizer and differences between orientation and polarization of solvent molecules around S*/S',

For an interfacial electron-transfer process between a ground-state molecular species and a metal electrode, the total reorganization energy is about one-half the reorganization energy of the self-exchange reaction ( Xse) (53), Eq. 8

s + s+ --t s+ + s (8)

For example, for [ R u ( b p ~ ) ~ ] ~ + , where bpy = 2,2'bipyridine, the reorganization energy associated with electron injection to a metal electrode can be evaluated as being on the order of 3400 cm-' with A,, being about 6700 cm-' (54). Reorganization energy values of ;Ase have been commonly used for electron-


transfer processes involving an electronically excited species (7, 19, 33, 37). Note, however, that reorganization energies for electron-transfer processes involving excited states might be somewhat different from those involving ground- state molecular species due to a different electronic configuration.

For the charge injection process involving the excited dye molecule and the metal oxide semiconductor, surface vibrational modes could be coupled to the electron-transfer process contributing to the total rearrangement energy. Reso- nance Raman spectroscopy has recently been applied, by Hupp and co-workers (55) , to the study of interfacial charge-transfer processes from hexacyanide complexes of Fe(I1) or Os(I1) to colloidal TiO,. Adsorption of these species on TiO, gives rise to the appearance of a metal-metal (M" + Ti") charge transfer band in the visible region (29, 55). Excitation into this band was observed to lead to enhanced Raman scattering of three surface modes. Application of time- dependent scattering theory (56) to these spectra allowed for the calculation of the individual component of the total vibrational reorganization energy (Xi) and an estimation of a contribution of about 650 cm-' for the vibrational reorganization energy of TiO,.

There is a fundamental interest in developing a detailed understanding of the factors at the microscopic level that may affect the rates of the elementary steps depicted in Fig. 2. For an activationless charge injection process, it is generally accepted that electron transfer from the excited sensitizer to the conduction band is irreversible due to extremely fast thermalization processes to the conduction band edge, Time-resolved photoluminescence measurements for dye molecules adsorbed on Ti0, have shown that charge injection processes occur in the pi- cosecond time domain (57-59).

2. Electron-Collection Eflciency

The fraction of injected charges that percolate through the TiO, membrane and reach the back contact of the photoanode is represented by the factor 7. The magnitude of 17 should be limited, in principle, by the film resistance and recombination reactions of conduction band electrons with the oxidized dye and the oxidized relay electrolyte. Time-resolved laser photolysis measurements have shown that the injected electrons can percolate without significant loss through the network of interconnected particles of the film, and hence 7 - 1 (36).

3. Light-Harvesting Eflciency

The light-harvesting efficiency, or absorption factor, can be related to the dye molar extinction coefficient [ E ( A), L mol-' cm-I; X = wavelength] and to the surface coverage (r, mol cm-2) by Eq. 9.


An absorption factor of unity is ideal for a solar energy device as all the incident radiant power is collected. The development of nanocrystalline TiOz has produced an enhancement of LHE. In Section I.D, possible strategies to increase the cross section for light absorption by the molecular design of sensitizers are discussed.

D. Molecular Sensitizers

From the simple scheme shown in Fig. 2, it is evident that the dye sensitizer is involved in the absorption of radiant energy, in injection of electrons into the semiconductor, and in bimolecular electron transfer with the relay molecules. Consequently, to be useful in a photoregenerative device the molecular sensitizer must fulfill several important requirements, including: (1) the ability to absorb firmly on the semiconductor; (2) high stability in both the oxidized and excited state; (3) efficient light absorption in the visible region; (4) a long-lived electronic excited state; (5) an excited-state redox potential negative enough for electron injection into conduction band; (6) a ground-state redox potential as positive as possible and compatible with (3) and (5); and (7) low kinetic barriers (small reorganizational energy) for excited- and ground-state redox processes, so as to minimize free energy losses in primary and secondary electron-transfer steps.

In early experiments, solutions of dye molecules such as chlorophylls (4), xanthene dyes (19), cyanine dyes (19), and metal complexes [such as tris(bipyridine)ruthenium(II) (9) and hexacyanoferrate(I1) (29)] were placed in contact with the semiconductor surface in a three electrode configuration. In these experiments, the small photocurrents observed were limited by the diffusion of the molecular sensitizer to the surface of the semiconductor (7). An important advance was made by Fujiara et al. (60, 61), who reported the de- hydrative coupling of a carboxylic acid group of rhodamine B with surface hydroxyl groups to form an ester linkage. Photocurrents were two orders of magnitude larger than those obtained from amide-linked silane-treated SnO, surfaces. Shortly thereafter, Goodenough and co-workers (62-64) applied this interfacial chemistry for water oxidation studies. These studies demonstrated general routes for attachment of dye sensitizers containing carboxylic acid functions to TiOz electrodes. These functions provide a stable linkage with the semiconductor and promote a strong electronic coupling between ligand-localized antibonding orbitals and the conduction band (36, 41).

Photosensitization of TiOZ with transition metal complexes (36-4 1), chlorophyll derivatives, and related natural porphyrins (65) are of considerable current interest. Besides these studies, an interesting new approach, involving cou-


pling of two semiconductor particles with different energy levels, such as CdS-ZnO and CdSe-TiO,, has been proposed by Kamat and co-workers (66, 67)

For practical applications in solar energy devices based on Ti02 electrodes, mononuclear (36, 37, 41) as well as polynuclear (32-34, 39, 40) ruthenium polypyridine complexes have proven to be efficient sensitizers. This fact is thought to be the result of a combination of factors, among which, light-harvesting efficiency and stability of the molecular sensitizer in the excited and ground state play a very important role.

11. A SUPRAMOLECULAR APPROACH

A potentially valuable approach to the sensitization of wide-bandgap semiconductors can be attempted along the lines of supramolecular chemistry.

Supramolecular chemistry, the chemistry beyond the molecule (68-72), deals with chemical systems made up of a discrete number of assembled molecular subunits or components. The forces responsible for the spatial organization of the molecular components may vary from weak (intermolecular forces, electro- static, or hydrogen bonding) to strong (covalent bonding), provided that the degree of electronic coupling between the molecular components remains small with respect to relevant energy parameters of the components. At the roots of supramolecular chemistry is the concept, largely inspired from biology, that supramolecular systems have the potential to achieve much more elaborate tasks than their molecular components (or, in general, simple molecules): while molecular systems are involved in simple chemical acts, supramolecular systems can perform functions.

The basic idea is that, in a regenerative photoelectrochemical cell for energy conversion, supramolecular systems with suitable built-in functions can be prof- itably used to replace the usual molecular sensitizers on the semiconductor surface. As shown later, this strategy could bring about improvements in the efficiency of the photosensitization process, and, ultimately, in the performance of the cell. Inspiration concerning what are useful functions for a supramolecular sensitizer can be obtained from the study of natural light-energy converting (photosynthetic) systems. Although in nature light is converted into chemical fuels rather than into electricity, it is likely that some of the basic functions involved in natural photosynthesis can prove valuable in artificial devices based on sensitized semiconductors.

A. Functions from Natural Photosynthesis

Photosynthesis, the conversion of light energy into chemical energy is ac- complished in nature very efficiently by green plants and other organisms such


reaction center reaction center photosystem II electron photosystem I

proton channel

antenna antenna

Figure 3. Schematic representation of the natural photosynthetic process.

as bacteria and algae, (73). Although these are extremely complex systems, spectacular advances have been recently made in the understanding of their behavior.

The general structure (73) of the photosynthetic machine present in the thy- lakoid membrane of green plants is schematically shown in Fig. 3. The global function of this complex device is the oxidation of water to oxygen on the internal side and the production of nicotinamide adenine dinucleotide phos- phates reduced form (NADPH) and adenosine triphosphate (ATP) (which will be used later for the synthesis of carbohydrates from carbon dioxide) on the external side of the membrane. To reach this goal, the photosynthetic membrane contains several protein complexes (schematized by contour lines), each of which performs a specific function. Among these, the most important are the so-called reaction centers, usually designated as “photosystems I and 11,’ ’ which completely span the membrane. The function of each of these photosystems is to generate, following light absorption, a pair of charges of opposite sign on opposite sides of the photosynthetic membrane, a process briefly indicated as photoinduced charge separation. The positive hole generated in photosystem I1 is used, with the help of a manganese-containing enzyme, for the oxidation of water. The negative charge developed by photosystem I is used, with the inter- mediacy of suitable proteins, in the reduction of NADP+ to NADPH. The components lying between the two photosystems have the role of connecting “in series” photosystems I and 11, that is, to allow the electron-transfer processes by which the negative charge developed by photosystem I1 to neutralize the positive charge developed by photosystem I. The other important chemical as- pect of this system is the development of a proton concentration gradient across


the membrane, resulting from the generation of protons in water oxidation and consumption of protons in NADP’ reduction. Thus, the membrane contains channels for proton transport and special protein complexes that use the proton gradient for the synthesis of ATP.

Interestingly, in photosynthesis, excitation of the two photosystems does not occur by direct absorption, but rather by energy transfer from the so-called light-harvesting antenna systems. One such system is placed close to each of the reaction centers (Fig. 3). The antenna systems contain a large number (hundreds) of chromophores and thus have a much larger cross section for light absorption than the reaction centers. Indirect excitation of the reaction center through the antenna is the way in which nature has solved the problem of light- harvesting efficiency, with a relatively small number of reaction centers, and under low-intensity illumination conditions.

Even from a simplified block-type description such as that of Fig. 3, it is apparent that the photosynthetic machinery of green plants is very complex. This finding is not surprising, given the complex nature of the overall chemical process to be achieved (oxidation of water, reduction of NADP’, and synthesis of ATP). From the basic viewpoint of energy conversion, however, two fundamental functions can be extracted from the analysis of this complicated machine: (1) photoinduced charge separation and ( 2 ) antenna effect. A closer in- spection of the structure of reaction centers and antenna systems may be useful to highlight the relationship between function and supramolecular structure in natural photosynthesis.

1 . Photoinduced Charge Separation

Photoinduced charge separation can be considered as the most primitive stage in the conversion of light energy into chemical energy in photosynthesis. It occurs within structures known as reaction centers. Photosystems I and I1 are examples of these reaction centers. Photoinduced charge separation is common to most natural photosynthetic systems (green plants, algae, and bacteria), although the number and detailed structure of the reaction centers, as well as the subsequent chemical use of the primary charge-separation products, may be different between different photosynthetic organisms (73). A giant step in the understanding of photoinduced charge separation has been made with the de- termination of X-ray crystallography of the structure at the molecular level of some reaction centers of bacterial photosynthesis (74-77). A simplified view of the structure of the reaction center of Rhodopseudomonas viridis (74, 77, 78) is sketched in Fig. 4(a).

Detailed photophysical studies of this reaction center have led to a rather precise picture of the sequence of events participating in photoinduced charge separation. The key molecular components are the bacteriochlorophyll ‘‘special

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pair” (P), a bacteriochlorophyll monomer (BC), a bacteriopheophytin (BP), a quinone (Q), and a four-heme c-type cytochromes (Cy) (although a second set of BC, BP, and Q is present in a structurally equivalent branch, only the labeled groups in Fig. 4(a) are used in the photoinduced charge-separation process). These chomophores are held in a fixed geometry by surrounding proteins that span the photosynthetic membrane, so that the twofold axis of P is perpendic- ular to the membrane, the periplasmic face lies approximately between P and Cy, and the cytoplasmic face at the level of Q. In the reaction center, excitation of P is followed by very fast ( - 3 ps) electron transfer to the BP “primary” acceptor [whether the interposed BC plays the role of mediator in a superex- change mechanism (79) or directly intervenes as an intermediate electron acceptor (80) is still a subject of experimental debate (81-85)]. The next step is fast (-200 ps) electron transfer from BC to Q (86), followed by slower ( - 270 ns) reduction of the oxidized P by the nearest heme group of Cy (87). At that stage, transmembrane charge separation has been achieved with an efficiency approaching unity. The rate constants of the various electron-transfer steps involved in charge separation are summarized in the energy level diagram of Fig. 4(b), together with those of the inefficient BC- -+ P+ and Q - -+ Pf charge- recombination steps (as determined from experiments with modified reaction centers lacking the possibility of the competing forward processes) (88).

Figure 4 illustrates the importance of the supramolecular structure of the reaction center. The achievement of efficient photoinduced charge separation over a large distance is made possible by optimization of the organization of the molecular components in space, the thermodynamic driving force of the various electron-transfer steps, and the kinetic competition between forward (useful) over back (dissipative) electron-transfer processes. How this comes about can be reasonably understood in terms of standard electron-transfer theory (see Section II.C.l). In particular, (1) the forward steps have driving forces close to the reorganizational energy of the protein matrix, resulting in practi- cally activationless electron-transfer processes (i.e., with the maximum rate compatible with distance), ( 2 ) the back electron-transfer steps are slower than the forward ones as they are much more exergonic, lying in the “inverted” kinetic regime; (3) the back electron-transfer steps decrease sharply in rate as the charge-separation progresses because of the exponential dependence of rates on distance.

2. Antenna Effect

The antenna units in the photosynthetic membrane of green plants (Fig. 3) have the role of efficiently absorbing the incident light and transfering the excitation energy to the reaction centers of photosystems I and 11. To that purpose, the antenna units contain a large number ( - 360 for photosystem 11) of chromophoric groups (pigments) (73). Very efficient energy-transfer processes take


place between these pigments, so that the photon energy absorbed by any pig- ment molecule can migrate through the antenna until it reaches the special pair of the reaction center. The pigments of green plant antenna units belong to the carotenoid and chlorophyll families, and several chemically different pigments are present within each of these families. The presence of different pigments has a dual purpose: (a) to allow the antenna to absorb light into a relatively broad spectral range; (b) to provide, upon suitable organization in space, a stepwise energy gradient capable of channeling the excitation energy toward the reaction center. Not much is known about the spatial organization of the pigments in the antena units of green plant photosynthesis, as these intramembrane units cannot be isolated without losing their function. As inferred from the structure of the antenna units of other photosynthetic organisms, such as, for example, the blue-green algae (73), it is likely that the pigments with higher energy excited states are preferentially located in the outer parts of the antenna, which from an energy viewpoint performs as a relatively flat funnel converging on the reaction center (Fig. 5).

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Figure 5 . Schematic representation of an antenna unit of a natural photosynthetic system.


B. Supramolecular Systems as Photochemical Molecular Devices

It has been seen in the previous sections that light-induced functions such as photoinduced charge separation and the antenna effect are obtained in nature by suitable assemblies of molecular components. The lesson learned from nature is that the achievement of the function relies on a very specific organization of the molecular components in the dimension of space, of their excited states and redox potentials in the dimension of energy, and of their elementary acts in the dimension of time. While the complexity of the natural systems (which is largely related to their multifunction capability and, ultimately, to their living nature) is absolutely out of reach for the synthetic chemist, it is reasonable to believe that single, simple functions similar to those found in nature can be duplicated by artificial supramolecular systems made of a discrete number of suitably chosen and properly organized molecular components. Artificial supramolecular systems with built-in light-induced functions can be considered as photochemical molecular devices (89, 90).

Photoinduced charge separation requires as a minimum basis a two-component system (dyad), made of a light-absorbing chromophore (photosensitizer) and an electron acceptor. (Fig. 6 ) . While any type of chemical forces could, in principle, be used to link the two molecular components, covalent bonding via an appropriate bridging group seems to be the best choice for a highly stable dyad. In such a system, the efficiency of the charge-separation process depends on the competition between the deactivation of the excited state of the photosensitizer (not shown in Fig. 6) and the photoinduced electron-transfer step

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(process 1 in Fig. 6). The lifetime of the charge-separated state, on the other hand, depends on the rate of the thermal charge recombination step leading back to the ground state of the dyad (process 2 in Fig. 6).

As taught by nature, a way to increase the lifetime of charge separation is to go to more complex systems, where sequences of electron-transfer steps between nearest neighbors take place. The simplest of such systems is a three- component system (triad).

In one of the possible schemes [Fig. 7(a)], which mimicks the sequence of the natural reaction centers, a primary photoinduced electron-transfer step from the photosensitizer to a primary acceptor [process 1 in Fig. 7(a)] is followed by thermal electron transfer from the reduced primary acceptor to a secondary acceptor [process 2 in Fig. 7(a)]. In the alternative scheme [Fig. 7(b)], which was the first to be used in artificial charge-separating triads (91, 92) photoinduced electron transfer [process 1 in Fig. 7(b)] is followed by thermal electron transfer from an electron-donor component to the oxidized photosensitizer [process 2 in Fig. 7(b)]. In both cases, the efficiency of charge separation depends critically, besides on the competition between photoinduced electron transfer and the photosensitizer excited-state lifetime, on the competition between the secondary electron-transfer step (processes 2 in Fig. 7) and the primary charge-recombination step (proceses 2’ in Fig. 7). The lifetime of the charge-separated state depends on the (allegedly slow) rate of the final charge-recombination process involving remote molecular components (processes 3 in Fig. 7). Many artificial triads for photoinduced charge separation have been developed along these lines, based on both organic (91-97) and inorganic (98- 104) molecular components. More complex systems (tetrads, pentads, etc.) have also been constructed (105, 106) with the aim of increasing the efficiency and/or the lifetime of charge

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(a ) (b) Figure 7 . Two prototype molecular triads for photoinduced charge separation.


Figure 8. Molecular triads for photoinduced energy migration. (a) A branched system with parallel energy-transfer processes. (b) A one-dimensional system with energy-transfer steps in series.

separation. It is evident that the performance of a triad, as of any other artificial device for photoinduced charge separation, rests on control of electron-transfer kinetics by synthetic design. Some basic concepts about the factors that affect electron-transfer kinetics are recalled in Section 1I.C. 1.

The antenna effect is based on energy-transfer processes that convey the excitation energy from a number of chromophoric components to a common (final) acceptor, where this energy can be utilized. Two very simple schemes are shown in Fig. 8, where a branched system with parallel energy-transfer processes [Fig. 8(a)] and a one-dimensional system with energy-transfer steps in series [Fig. 8(b)] are illustrated. The performance of an artificial antenna system of this type depends on two factors: (1) for each excited chromophore, the competition between the energy-transfer processes and the deactivation processes that determine the lifetime; (2) for each pair of neighboring chromophores, the occurrence of energy transfer in the “right” direction rather than in the opposite one. For more extended systems, for example, a one-dimensional array with multiple components between the two terminal systems, the last condition can probably be relaxed without loss in overall directionality : Reversible energy hopping between the intermediate units can occur, inasmuch as efficient trapping occurs at one of the terminal components. Several highly branched antenna systems based on organic (107-1 10) and inorganic (1 11-121) molecular components have recently been developed (“arborols” with up to 22 molecular components are now available). Some one-dimensional inorganic antenna systems based on cyanide bridges have been designed, and will be discussed in detail in Section IV. Here again, it is evident that synthetic control on the kinetics of the intercomponent energy-transfer steps is crucial toward the achievement of the function. Some basic concepts about the factors that affect energy-tranfer kinetics are recalled in Section 1I.C.

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