[acs symposium series] inorganic chemistry: toward the 21st century volume 211 || solar electricity:...

1 Solar Electricity: Lessons Gained from Photosynthesis

J A M E S R. B O L T O N

The University of Western Ontario, Department of Chemistry, Photochemistry Unit, London, Ontario N6A 5B7 Canada

Nature has developed a mechanistically very complex yet conceptually very simple process for the conversion and storage of solar energy. In this lecture I shall first examine the reaction and mechanism of photosynthesis deriving insights into how nature has achieved this remarkable process. I shall then go on to describe various attempts to mimic the primary steps of photosynthesis. Finally, I shall speculate on how these insights into the mechanism of photosynthesis might be used to design a new type of solar cell for the conversion of light to electricity.

The Photosynthesis Reaction The reaction of photosynthesis is clearly the most important

chemical reaction since life could not exist for long without it. The overall reaction is

CO2(g) + H2O(l) light C6H12O6(s) + O2 (g)

where C6H12O6(s) is D-glucose, the major energy-storage product of photosynthesis from which all plant and animal biomass is derived. The thermodynamic parameters for the photosynthesis reaction at 298K (25°C) are ΔΗ=467 kJmol-1; ΔG=496 kJmol-1 and E°=1.24V (1).

It is helpful to think of the photosynthesis reaction as the sum of an oxidation half reaction and a reduction half reaction as shown in Figure 1. In fact, nature does separate these half reactions, in that the reduction of CO2 to carbohydrates occurs in the stroma of the chloroplast, the organelle in the leaf where the photosynthesis reaction occurs, - whereas, the light-driven oxidation half reaction takes place on the thylakoid membranes which make up the grana stacks within the chloroplast. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) carries the reducing power and most of the energy to the stroma to drive the fixation of CO2 with the help of some additional energy provided

0097-6156/83/0211 -0003$06.00/0 © 1983 American Chemical Society

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21 ST C E N T U R Y

C02(g) 8 hv

2NADPH+2H*

/ carbon \f 1 fixation Y V cycle Λ

+ 3 ATP

2NADP+

^ / l i g h t Y driven y 1 electron A

JV transportJ\

Λ stroma +3ADP +3Pi

thylakoid membrane

2H20(I)

0?(g)

l/6C6H| 206(s)+ H20(l)

Figure 1. The separation of the half reaction in the chloroplast of the photosyn-thetic plant cell. The dark reaction (left) and the light-driven reactions (right) are shown. Key: NADP\ oxidized form of nicotinamide adenine dinucleotide phosphate;

ATP, adenosine triphosphate; and Pif inorganic phosphate.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


1. B O L T O N Solar Electricity 5

by adenosine triphosphate (ATP) which i s also generated on the thylakoid membrane. [Readers interested i n the structure and mechanism of photosynthesis should refer to references (2-4)·] Although the mechanism of the carbon-fixation cycle i s very interesting biochemistry, i t i s the mechanism of the oxidation half reaction which w i l l concern us most, because i t i s here that the conversion of sunlight to chemical energy takes place. We s h a l l now explore some of the d e t a i l s of this remarkable process.

Mechanism of the Primary Photochemical Reaction of Photosynthesis

The primary photochemistry of photosynthesis takes place within very specialized reaction-center proteins situated i n the thylakoid membrane (see Figure 2) of the chloroplast (2) . Most (>99%) of the chlorophyll and other pigments act as an antenna system to gather l i g h t photons and channel them to one of the two reaction centers. In essence the antenna chlorophyll system acts as a photon concentrator, concentrating the photon flux by a factor of ^300 over what i t would be without an antenna system. In both Photosystems I and II the photochemically active component (P700 or P680) i s thought to be a chlorophyll a species. P700 i s l i k e l y a dimer of chlorophyll a. The primary photochemical step then involves the transfer of an electron from the donor (P700 or P680) to an acceptor species. In Photosystem II the acceptor Q1

i s thought to be a molecule of plastoquinone. In Photosystem I

the acceptors are not as well characterized; Ax may be a molecule of chlorophyll a i n a special environment and A 2 i s thought to be an iron-sulfur center.

Hence, i t appears that nature has chosen the fastest and perhaps simplest of a l l photochemical reactions, namely, photochemical electron transfer, to trap the energy of the elusive sunbeam. In effect, the reaction-center proteins of photosynthesis are solar c e l l s converting l i g h t to e l e c t r i c i t y which i s then used to drive the r e l a t i v e l y slow biochemical reactions which lead ultimately to D-glucose. These reaction-center solar c e l l s are very effective - the y i e l d for electron transfer i s almost unity, and the overall solar energy conversion to e l e c t r i c a l energy i s VL6% (5). This i s as good as or generally much better than most commercially available s i l i c o n solar c e l l s .

Now l e t us examine more closely the details of how the

0

0

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


6 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

NADPH

Figure 2 . Model of the thylakoid membrane showing the various components involved in electron transport from H20 to NADP\

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key: Z, the watersplitting enzyme which contains Μη; P680 and Qh the primary donor and acceptor species in the reaction-center protein of Photosystem II; Qi and Qt, probably plastoquinone molecules; PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside; Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I; P700 and At, the primary donor and acceptor species of the Photosystem I reaction-center protein; At, Fe-SA. and Fe-SB, membrane-bound secondary acceptors which are probably Fe-S centers; Fd, soluble ferredoxin Fe-S protein; and fp, is the flavoprotein that functions as the enzyme that

carries out the reduction of NADP+ to NADPH.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



electron i s transferred from one side of the thylakoid membrane to the other. Figure 3 i l l u s t r a t e s a view of our current knowledge of the composition and structure of the reaction-center protein from photosynthetic bacteria which performs a photochemical electron-transfer reaction analogous to the reactions i n the two photosystems of green-plant photosynthesis (2, 7_9 8). The bacterial reaction-center protein contains 4 bacteriochloro-ph y l l molecules, 2 bacteriopheophytins, one nonheme iron and a ubiquinone molecule. Two of the bacteriochlorophyll molecules form a special pair called P870 which acts as the photochemical electron donor. The other two bacteriochlorophylls absorb at 800 nm and may act as the l a s t l i n k to the antenna system. Within <6 ps following the absorption of a photon, an electron moves from the P870 to one of the bacteriopheophytin molecules. Subsequently within V300 ps the electron moves to the ubiquinone. Thus within V300 ps the energy conversion step i s over resulting i n a charge separation between P870 + and Q of 20-30 Â. If transfer to secondary acceptors does not occur, as i t does e f f i c i e n t l y i n vivo at physiological temperatures, then the electron returns to v i a a tunnelling mechanism with a h a l f - l i f e of ^20 ms at low temperatures (7, 8). A similar picture, although more complex and less well characterized, has been assembled for the reaction center of Photosystem I i n green plants (9).

Now for some speculation - i t appears that nature has achieved the very d i f f i c u l t task of assuring rapid and e f f i c i e n t forward electron transfer while at the same time preventing the spontaneous back reaction. In effect, the reaction-center protein i s an almost perfect photodiode. However, this f a c i l i t y has a price i n that ^0.8 eV of energy i s l o s t i n the primary electron transfer while an electron i s moved 20-30 Â away from the primary donor. This process i s carried out i n several steps rather than one step - this has two advantages - less energy has to be d i s sipated i n each step and i t i s possible to move the electron away a greater distance and thus minimize the chance of i t s returning. These seem to be key features i n the energy-conversion process.

Spectral and Thermodynamic Considerations

Photosynthesis uses sunlight as i t s energy source, a source with a broad spectral d i s t r i b u t i o n of photon energies. Thus any device which operates using a sensitizer with a fixed excitation energy w i l l l i k e l y be more e f f i c i e n t the lower the excitation energy, because more solar photons may then be u t i l i z e d . However, i t i s important to point out that, just as the Second Law of Thermodynamics says that i t i s impossible to convert heat into work with 100% e f f i c i e n c y , there i s a similar thermodynamic r e s t r i c t i o n on the conversion of l i g h t to work or any other form of energy other than heat (6, 10, 11). The plant degrades the energy of a l l absorbed l i g h t photons i n the photosynthetically active region (380-720 nm) to the energy of the lowest excited

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



Excitation

Figure 3. The bacterial reaction-center protein model from Rhodopseudomonas sphaeroides; the structure and positioning

of components are highly speculative.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



state of the chlorophyll molecules i n the reaction centers which l i e at an energy of M..8 eV (174 kJ mol" 1, 700 nm) above the ground state. It i s a common misconception that i n prin c i p l e a l l of this energy i s available to be converted into work or chemical energy. In fact, the thermodynamic analysis indicates that even i n a perfect converter at 700 nm only ^1.4 eV or *W8% of this energy i s Gibbs energy and available to do work.

Now l e t us see what effect these thermodynamic l i m i t s have on the mechanism of photosynthesis. The simplest system which nature could have devised would be one i n which a single photosystem absorbs l i g h t and carries out the necessary reactions of photosynthesis. At most only one electron can be driven through the c i r c u i t for every photon absorbed. The thermodynamic l i m i t allows only VL.4 eV from each photon at 700 nm to operate the overall photosynthesis reaction which requires a minimum of 1.24 eV per electron driven through the c i r c u i t . Thus the reaction i s allowed thermodynamically; however, to achieve this only MD.16 eV could be lost i n any nonidealities i n the process. The problem i s analagous to the engineer being required to design an almost f r i c t i o n l e s s heat engine. The system requires more f l e x i b i l i t y and thus we conclude that the photosynthesis reaction cannot be operated with one photosystem. In fact, we have already seen that the energy loss i n the primary electron transfer i s ̂ 0.8 eV leaving only M..0 eV to drive the reactions of photosynthesis. This i s why the plant had to develop the much more complex mechanism of two photosystems operating i n series (see Figure 2). Thus two photons are absorbed for every electron transferred from water to NADP+ providing ^2.0 eV which i s plenty to run the reaction.

Efficiency of Photosynthesis

The efficiency of photosynthesis depends on how the question i s asked. Worldwide ^3 χ 10 2 1 J of solar energy i s stored per year (12) and the annual solar input to the biosphere i s *\> 3 χ 10 2 4 J -thus the worldwide efficiency of photosynthesis i s ̂ 0.1%. This may sound rather low but then consider that 3 χ 10 2 1 J i s about 10 times the annual energy consumption by a l l peoples on the earth! Crops growing under ideal conditions have achieved as much as ^4% efficiency for short terms (13). However, i t i s possi b l e to analyze the maximum efficiency expected for a plant from a knowledge of the absorption spectrum of a leaf and the experimental quantum yields for O2 production as a function of wavelength. Such an analysis (12) indicates a maximum gross efficiency of ^9% which drops to %5% when photorespiration, a light-stimulated back reaction, i s accounted for. Hence, i t i s seen that a considerable loss i n efficiency occurs between the primary conversion to e l e c t r i c a l potential (VL6%) and the end products (V5%). Of course, the end product i s usually a very stable and storable f u e l .

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



Lessons Gained from Photosynthesis

Here I s h a l l summarize some of the salient points gained from our examination of the mechanism of photosynthesis.

1. Light i s concentrated by a factor of ^300 by use of an extensive antenna system of pigments. This minimizes the need to duplicate reaction centers.

2. The primary photochemical reaction i s an electron transfer reaction which occurs within a highly structured reaction-center protein which spans the thylakoid membrane.

3. Photochemical electron transfer occurs from the excited singlet state of the primary donor.

4. One or more intermediate electron acceptors mediate the electron transfer across the reaction center from the donor to the ultimate acceptor.

5. Relatively long wavelength absorbers are u t i l i z e d to maximize the capture of the solar spectrum.

6. Two photosystems are required to run the f u l l reaction of photosynthesis.

A r t i f i c a l Solar Energy Converters Designed to Mimic Photosynthesis

Much has been written about a r t i f i c a l solar-energy converters - the reader i s referred to references 10, 12, 14-17 for detailed treatments. Here I s h a l l deal exclusively with those a r t i f i c i a l systems designed to mimic various aspects of the photosynthesis reaction.

The primary donor i n Photosystem I P700 i s thought to be a special pair of chlorophyll a molecules. Katz and Hindman (18) have reviewed a number of systems designed to mimic the properti e s of P700 ranging from chlorophyll a. i n certain solvents under special conditions where dimers form spontaneously (19) to co-valently linked chlorophylls (20). Using these models i t has been possible to mimic many of the o p t i c a l , EPR and redox propert i e s of the i n vivo P700 entity.

Most of the interest i n mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer i n solution from chlorophyll a. to p-benzoquinone (21, 22) which has been shown to occur v i a the excited t r i p l e t state of chlorophyll a. However, these solution studies are not very good models of the i n vivo reaction center because the i n vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other i n the reaction-center protein.

The next l e v e l of sophistication involved studies of systems where separate donors and acceptors interact across an interface. Chlorophyll-quinone photochemical studies have been conducted using liposomes (23-25) and acetate films (26). Calvin and his coworkers (27) have conducted a variety of experiments

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



demonstrating that e f f i c i e n t photochemical electron transfer can under certain conditions be sensitized across a r t i f i c a l vesicles using ruthenium (II) t r i s ( b i p y r i d y l ) as a sensitizer (see also 28). In addition micelles (29-32) and monolayers (33-35) have been used to separate donors and acceptors.

The f i n a l l e v e l of sophistication has involved covalently link i n g a donor and an acceptor. I s h a l l review these systems extensively as this i s where we have concentrated our own work. Kong and Loach (36, 37) were the f i r s t to report the synthesis of such an intramolecular donor-acceptor entity - namely ( I ) ;

however, none of the photophysical or photochemical properties of (I) were reported at that time. Tabushi et a l (38) reported the synthesis of (II) and found that the fluorescence of (II) was strongly quenched as compared to tetraphenyl porphyrin and

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



speculated that the quenching might be due to intramolecular electron transfer. Mataga and his coworkers (39) have synthesized the series of compounds (III) and also observed strong

fluorescence quenching. In another publication (40) they observed o p t i c a l changes by picosecond laser flash spectroscopy which they interpreted as evidence for formation of a charge transfer state v i a intramolecular electron transfer from the porphyrin to the quinone. The lifetime of the charge transfer state was found to increase markedly as η increased from 2 to 6. Ganesh and Sanders (41) reported the synthesis of a quinone-capped metallo-porphyrin but did not report on any of i t s photophysical or photochemical properties. Netzel and his coworkers (42, 43) have studied (IV) using picosecond spectroscopy and on the basis of

(IV)

a complex kinetic model concluded that the charge transfer state of (IV) comes from the porphyrin t r i p l e t state rather than the singlet state. Harriman and Hosie (44) studied a series of porphyrins i n which a variety of electron donors and acceptors were attached at the four meso positions. They found strong f l u o r escence quenching for good electron donors (such as N,N-dimethyl

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



aniline) as well as good electron acceptors (such as p-benzoquin-one). F i n a l l y Matsuo et a l (45) reported the synthesis of ruthenium t r i s ( b i p y r i d y l ) covalently linked to viologen units. They found almost complete quenching of the emission from the ruthenium complex and i n addition the covalently linked compound considerably enhanced electron transfer to relay systems of aligned viologen units on micelles and polymers.

Although the fluorescence quenching and opt i c a l absorption changes found on excitation of many of the above intramolecular donor-acceptor molecules i s highly suggestive of intramolecular electron transfer, none of the above studies provided conclusive proof. We have been very interested i n the problem of the syntheses of porphyrin-quinone molecules. When Kong and Loach (36) published their synthesis of (I) i n 1978, we repeated their synthesis and i n 1980 we reported (46) a light-induced electron paramagnetic resonance (EPR) signal which was shown, v i a a variety of control experiments, to arise from an intramolecular electron transfer from the porphyrin end to the quinone end of (I) . Under certain conditions the electron transfer i s reversible. Kong and Loach (46) have observed similar EPR signals from i n t r a molecular transfer i n the Zn complex of I; however, under a l l conditions the electron transfer was i r r e v e r s i b l e . Loach et a l (47) have reported a variety of EPR and fluorescence data on (I) and i t s Zn complex confirming the intramolecular nature of the electron transfer and from the fluorescence data they conclude that the electron transfer most l i k e l y occurs out of the singlet state.

Our current work i s concentrated on the series of porphyrin-quinone molecules (V). These are very similar to I i n that amide linkages have replaced the ester linkages. We chose to study these molecules not only because they provided a def i n i t e change

(V)

0

H ( C H J - N 2 η H

0 V /

C-CH, 2

0

0

η = 2, 3, 4

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



i n the structure of the linkage but also because they are much less susceptible to decomposition by hydrolysis. The rate constant for forward electron transfer, as inferred from the fl u o r escence li f e t i m e s , i s about three times slower i n (V) as compared to (I) (48). We have also observed o p t i c a l changes using nanosecond optical flash photolysis and low temperature photolysis which show cle a r l y the development of a charge transfer state which p a r a l l e l s the EPR observations (48). F i n a l l y , we have observed s a t e l l i t e s i n the EPR spectrum of (V) with η = 2 which arise from spin exchange between the porphyrin cation r a d i c a l and the quinone anion r a d i c a l providing conclusive evidence for intramolecular electron transfer (49). Also the quantum e f f i c iency for electron transfer i s found to be VL-2% (49).

A Possible Solar C e l l

Much fundamental work yet remains i n the study of i n t r a molecular donor-acceptor molecules to find out what structural parameters of the donor, acceptor and p a r t i c u l a r l y the linkage enhance the effi c i e n c y of forward electron transfer while at the same time i n h i b i t i n g the rate of reverse electron transfer. Progress so far i s very promising.

Assuming that an e f f i c i e n t D-A type of molecule can be synthesized, i t should be possible to deposit these molecules as a monolayer onto a glass s l i d e coated with a metal such as aluminum or a wide bandgap semiconductor such as Sn02. With the acceptor end of the molecule near the conductor and with contact to the other side v i a an electrolyte solution i t should be possible to stimulate electron transfer from D to A and then into the conductor, through an external c i r c u i t and f i n a l l y back to D through the electrolyte. This would form the basis of a new type of solar c e l l i n which the layer of D-A molecules would perform the same function as the p-n junction i n a s i l i c o n solar c e l l (50). Only the future w i l l t e l l whether or not this concept w i l l be feasible but i f nature can do i t , why can't we?

Literature Cited

1. Bolton, J.R. "Photosynthesis 77: Proceedings of the Fourth International Congress on Photosynthesis" (Hall, D.O.; Coombs, J. and Goodwin, T.W., eds.); The Biochemical Society: London, 1978; pp 621-634.

2. Clayton, R.K. "Photosynthesis: Physical Mechanisms and Chemical Patterns"; Cambridge University Press: Cambridge, England, 1980.

3. Hall, D.L.; Rao, K.K. "Photosynthesis" Third Edition; Edward Arnold Ltd.: London, 1980.

4. Gregory, R.P.F. "Biochemistry of Photosynthesis" Second Edition; John Wiley and Sons Ltd.: New York, 1977.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


1. BOLTON Solar Electricity

5. The 16% efficiency figure is obtained as follows: for an air mass 1.5 solar spectra distribution ~36% of the incident solar energy can be converted to excited state energy at 700 nm (6). If an average absorption coefficient of 0.9 is assumed, then 32.2% of the incident irradiance will appear as energy of the excited states of the reaction-center chlorophylls. Out of the 1.8 eV excitation energy at 700 nm ~1.0 eV is generated as electrical energy in the electron transfer reaction. If we assume a fill factor of 0.9 then ~16% of the incident solar power will be available as electrical power following the primary electron-transfer step.

6. Bolton, J.R.; Haught, A.F.; Ross, R.T. "Photochemical Conversion and Storage of Solar Energy" (Connolly, J.S., ed.); Academic Press: New York, 1981; pp 297-399.

7. Bolton, J.R. "The Photosynthetic Bacteria" (Calyton, R.K. and Sistrom, W.R., eds.); Plenum Publishing Corp.: New York, 1978; pp 414-429.

8. Blankenship, R.E.; Parson, W.W. "Photosynthesis in Relation to Model Systems" (Barber, J., ed.); Elsevier/North Holland Biomedical Press: Amsterdam, 1979; pp. 71-114.

9. Bolton, J.R. "Primary Processes of Photosynthesis" (Barber, J., ed.); Elsevier/North Holland Biomedical Press: Amsterdam, 1977; pp 188-201.

10. Bolton, J.R. Science 1978, 202, 705-711. 11. Bolton, J.R.; Haught, A.F.; Ross, R.T. Interamerican Photo

chemical Society Newsletter 1981, 4, 26-30. 12. Bolton, J.R.; Hall, D.O. Annu. Rev. Energy 1979, 4, 353-401. 13. Hall, D.O. "Solar Power and Fuels" (Bolton, J.R., ed.);

Academic Press: New York, 1977; pp 27-52. 14. Bolton, J.R., ed. "Solar Power and Fuels"; Academic Press:

New York, 1977. 15. Archer, M.D., ed. Special Issue of J. Photochem. 1979, 10,

(1). 16. Hautala, R.R.; King, R.B.; Kutal, C., eds. "Solar Energy

Chemical Conversion and Storage"; Humana Press: Clifton, N.J., 1979.

17. Connolly, J.S. "Photochemical Conversion and Storage of Solar Energy"; Academic Press: New York, 1981.

18. Katz, J.J.; Hindman, J.C. "Photochemical Conversion and Storage of Solar Energy" (Connolly, J.S.,ed.); Academic Press: New York, 1981; pp 27-78.

19. Shipman, L.L.; Cotton, T.M.; Norris, J.R.; Katz, J.J. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 1791.

20. Boxer, S.G.; Closs, G.L. J. Am. Chem. Soc. 1976, 98, 5406. 21. Tollin, G. Bioenergetics 1974, 6, 69-87. 22. Tollin, G. J. Phys. Chem. 1976, 80, 2274-2277. 23. Hurley, J.K.; Castelli, F.; Tollin, G. Photochem. Photobiol.

1980, 32, 79-86. 24. Hurley, J.K.; Castelli, F.; Tollin, G. Photochem. Photobiol.

1981, 34, 623-631.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


16 INORGANIC CHEMISTRY: TOWARD THE 21 ST CENTURY

25. Kuriharg, K.; Sukigara, M.; Toyosshima, Y. Biochim. Biophys. Acta 1979, 547, 117-126.

26. Cheddar, G.; Castelli, F.; Tollin, G. Photochem. Photobiol. 1980, 32, 71-78.

27. Calvin, M. "Photochemical Conversion and Storage of Solar Energy" (Connolly, J.S., ed.); Academic Press: New York, 1981; pp 1-26.

28. Tunuli, M.S.; Fendler, J.H. J. Am. Chem. Soc. 1981, 103, 2507-2513.

29. Grätzel, M. "Photochemical Conversion and Storage of Solar Energy" (Connolly, J.S., ed.); Academic Press: New York, 1981; pp 131-160.

30. Thomas, J.K.; Piciulo, P. "Interfacial Photoprocesses"; ACS Symposium Series 1980, 184, 97-111.

31. Schmehl, R.H.; Whitesell, L.G.; Whitten, D.G. J. Am. Chem. Soc. 1981, 103, 3761-3764.

32. Infelta, P.P.; Grätzel, M.; Fendler, J.H. J. Am. Chem. Soc. 1980, 102, 1479-1483.

33. Whitten, D.G.; Mercer-Smith, J.Α., Schmehl, R.H.; Worsham, P.R. "Interfacial Photoprocesses"; ACS Symposium Series 1980, 184, 47-67.

34. Möbius, D. Accounts Chem. Res. 1981, 14, 63-68. 35. Kuhn, H. J. Photochem. 1979, 10, 111-132. 36. Kong, J.L.Y.; Loach, P.A. "Frontiers of Biological Energet

ics: From Electrons to Tissues" Vol. 1 (Dutton, P.L.; Leigh, J.S.; Scarpa, Η., eds.); Academic Press: New York, 1978; p 73.

37. Kong, J.L.Y.; Loach, P.A. J. Heterocyclic Chem. 1980, 17, 737-744.

38. Tabushi, I.; Koga, N.; Yanagita, M. Tetrahed. Lett. 1979, 257-260.

39. Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S.; Migita, M.; Okada, T.; Mataga, N. Tetrahed. Lett. 1981, 22, 2099-2102.

40. Migita, M.; Okada, T.; Mataga, N.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1981, 84, 263-266.

41. Ganesh, K.N.; Sanders, J.K.M. J. Chem. Soc. Chem. Commun. 1980, 1129-1131.

42. Netzel, T.L., Bergkamp, M.A., Chang, C-W.; Dalton, J. J. Photochem. 1981, 17, 451-460.

43. Bergkamp, M.A., Dalton, J.; Netzel, T.L. J. Am. Chem. Soc. 1982, 104, 253-259.

44. Harriman, Α.; Hosie, R.J. J. Photochem. 1981, 15, 163-167. 45. Matsuo, T.; Sakamoto, T.; Takuma, K.; Sakura, K.; Ohsako,

T. J. Phys. Chem. 1981, 85, 1277-1279. 46. Kong, J.L.Y.; Loach, P.A. "Photochemical Conversion and

Storage of Solar Energy" (Connolly, J.S., ed.); Academic Press: New York, 1981; p 350.

47. Loach, P.A.; Runquist, J.Α.; Kong, J.L.Y.; Dannhauser, T.J.; Spears, K.G. ACS Advances in Chemistry Series (in press).

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


1. B O L T O N Solar Electricity 1 7

48. Bolton, J.R.; Ho. T.-F.; McIntosh, A.R.; Siemiarczuk, Α.; Weedon, A.C.; Connolly, J.S. "Proceedings of the Solar World Forum Congress of the International Solar Energy Society" Brighton, England; Pergamon Press: New York, (in press).

49. Mcintosh, A.R. ; Ho, T.-F.; Weedon, A.C; Siemiarczuk, Α.; Stillman, M.J.; Bolton, J.R. (in preparation).

50. Bolton, J.R.; Ho, T.-F.; McIntosh, A.R., Canada, U.S. and Other Country Patents applied for.

RECEIVED August 3, 1982

Discussion A.W. Adamson, U n i v e r s i t y of Southern C a l i f o r n i a : You men

t i o n e d a thermodynamic l i m i t a t i o n to the e f f i c i e n c y of convers i o n of s o l a r energy t o u s e f u l work. T h i s i s an i n t e r e s t i n g p o i n t , and I would l i k e t o hear more about i t . I can see an e n t r o p i e e f f e c t i n terms of the f o l l o w i n g h y p o t h e t i c a l c e l l f o r the system A = A*, where A* i s the e x c i t e d s t a t e of s p e c i e s A:

P t / C ( s ) / s o l u t i o n of A w i t h e q u i l i b r i u m c o n c e n t r a t i o n of A*

s o l u t i o n of A w i t h n o n - e q u i l i brium c o n c e n t r a t i o n of A* produced by i r r a d i a t i o n

C(s)/M/Pt

Here, C(s) i s a s o l i d reduced form of A, and the double l i n e denotes a l i q u i d j u n c t i o n of n e g l i g i b l e p o t e n t i a l . A l s o , A* i s a t h e r m a l l y e q u i l i b r a t e d e x c i t e d ( o r t h e x i ) s t a t e and, as such, i s e s s e n t i a l l y a d i f f e r e n t chemical s p e c i e s from A. We suppose, t h e r e f o r e , t h a t i t i s p o s s i b l e t o f i n d an e l e c t r o d e M t h a t i s r e v e r s i b l e t o the r e d u c t i o n of A* t o C ( s ) , but co m p l e t e l y p o l a r i z e d w i t h r e s p e c t t o the r e d u c t i o n of A t o C ( s ) . O p e r a t i o n of t h i s c e l l under steady s t a t e c o n d i t i o n s s h o u l d then g i v e the d e s i r e d r e v e r s i b l e work a v a i l a b l e from the p h o t o p r o d u c t i o n of A*.

J.R. Bolton: Your model i s interesting, but I believe i t complicates the issue. The system A,A* i s a two-component system. In the dark μ^ = μ Α* = \ΐχ*° + kTlnX^S where the y*s are chemical potentials and X^$ i s the equilibrium mole fraction of A*. In the l i g h t X^* increases; hence, the chemical potential available to do work i s X

Δμ = kTln —

Δμ can never be as large as E g , the excitation energy.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



A.B.P. L e v e r , York U n i v e r s i t y : Most model p h o t o c a t a l y s t s undergo e l e c t r o n t r a n s f e r r e a c t i o n s v i a t h e i r s p i n t r i p l e t s t a t e s . You p o i n t out t h a t t h e r e are advantages t o u s i n g the s p i n s i n g l e t s t a t e f o r e l e c t r o n t r a n s f e r as e x e m p l i f i e d by c h l o r o p h y l l . What k i n d of s t r u c t u r a l or e l e c t r o n i c f e a t u r e s s h o u l d be b u i l t i n t o model p h o t o c a t a l y s t s t o favour use of t h e i r s p i n s i n g l e t s t a t e s f o r e l e c t r o n t r a n s f e r quenching?

J.R. Bolton: In solution most photochemical electron transfer reactions occur from the t r i p l e t state because i n the c o l l i sion complex there i s a spin i n h i b i t i o n for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs i n such systems but the back electron transfer i s too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as i n a c o l l i s i o n complex, yet i t i s s t i l l close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer i s inhibited by the distance and molecular structure between the two ends. Our future work w i l l focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode".

A . J . B a r d , U n i v e r s i t y of Texas: The mechanism you propose i m p l i e s t h a t t h e r e are s p i n s e l e c t i o n r u l e s o p e r a t i v e which a f f e c t the r e l a t i v e r a t e s of the e l e c t r o n t r a n s f e r r e a c t i o n s . I s t h e r e any evidence t h a t such s p i n s e l e c t i o n r u l e s are i m p o r t a n t i n these k i n d s of r e a c t i o n s , e s p e c i a l l y i n the presence of m e t a l l i c c e n t e r s ?

J.R. Bolton: We have not carried out any experiments as yet on metalloporphyrins linked to quinones. The spin selection rules should be operative i n the radical pair. The singlet state of the radical pair should be able to return to the ground state with no spin i n h i b i t i o n ; however, the t r i p l e t state of the radical pair can return to the ground state only v i a spin interconversion or v i a the t r i p l e t state of the porphyrin.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001



T.J. Kemp, U n i v e r s i t y of Warwick; N o t i n g the v e r y low quantum y i e l d f o r i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i n low temperatures d i s p l a y e d by your p o r p h y r i n - q u i n o n e model compound, would i t not be p o s s i b l e t o ' s h o c k - f r e e z e ' a s o l u t i o n undergoing i r r a d i a t i o n at a h i g h e r temperature (and g i v i n g a workable c o n c e n t r a t i o n of paramagnetic s p e c i e s ) i n o r d e r t o determine a low-temperature spectrum w i t h the p a r t i c u l a r aim of o b s e r v i n g a p o s s i b l e Am = 2 t r a n s i t i o n ?

J.R. Bolton: This i s a good idea - we w i l l t r y i t .

M.S. W r i g h t o n , M.I.T.; What i s the b a s i s f o r an e s t i m a t e of 5% e f f i c i e n c y f o r p h o t o s y n t h e s i s and the 187o f o r a comparison w i t h s o l i d s t a t e p h o t o v o l t a i c s f o r e l e c t r i c i t y g e n e r a t i o n ?

J.R. Bolton: The 5% figure refers to the net energy e f f i ciency in the production of d-glucose taking account of photorespiration. The 18% figure refers to the e f f i c i e n c y of conversion of sunlight to e l e c t r i c a l energy at the level of the reaction-center protein. The details of the calculation are in my paper. However, as you pointed out, I neglected a f i l l factor and thus a more r e a l i s t i c value would be ~16%, s t i l l an impressive number for the natural photovoltaic c e l l .

T.J. Meyer, U n i v e r s i t y of N o r t h C a r o l i n a : You r e p o r t t h a t the d a t a t h a t you observe suggests a l a c k of s o l v e n t dependence, and t h e r e f o r e t h a t e l e c t r o n t r a n s f e r occurs through the c h e m i c a l l i n k r a t h e r than through the s o l v e n t . However, i n e i t h e r case t h e r e i s a charge t r a n s f e r process through d i s t a n c e and t h e r e should be a s o l v e n t dependence a s s o c i a t e d w i t h the e l e c t r o n t r a n s f e r a c t . Do you have any comments on t h i s p o i n t ?

J.R. Bolton: If the electron reaches the quinone v i a the linkage, then the transfer must involve the molecular orb i t a l s of the linking structure and thus the solvent w i l l have only a secondary effect. If the electron transfer occurs through the solvent, then the solvent should have a f i r s t - o r d e r effect on the rate.

Dow

nloa

ded

by F

OR

DH

AM

UN

IV o

n A

pril

22, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: M

arch

3, 1

983

| doi

: 10.

1021

/bk-

1983

-021

1.ch

001


[acs symposium series] inorganic chemistry: toward the 21st century volume 211 || solar electricity:...

Documents