DIVULGAÇÃO CHEMISTRY AND LIGHT - PART 2: LIGHT AND ENERGY* Franco Scandola, Carlo Alberto Bignozzi Dipartimento di Chimica - Università di Ferrara - Via L. Borsari, 46 - 44100 - Ferrara - Itália Vincenzo Balzani Dipartimento di Chimica “G. Ciamician” - Università di Bologna - Vila Selmi, 2 - 40126 - Bologna - Itália Recebido em 19/8/96 The conversion of solar energy into more useful forms of energy, such as chemical fuels or electricity, is one of the central problems facing modern science. Progress in photochemistry and chemical synthesis has led to a point where light energy conversion by means of artificial mo- lecular devices can be rationally attempted. In this article, a general approach towards this challenging goal is presented. Keywords: photochemistry; light; energy. Life is a water mill: the effect produced by the falling water continuous supply of two absolutely vital goods: atmospheric is achieved by the rays of the sun. Without the sun the wheel of oxygen and the organic material that supports the food chain. life cannot be kept going. But we have to investigate more closely On the other hand, photosynthesis is also very important for which circumstances and laws of nature bring about this re- human civilization, as it is responsible for the accumulation of markable transformation of the sunrays into food and warmth. huge amounts of fossil fuels. Reaction 1 is highly endoergonic W. Ostwald: Die Mühle des Lebens, Thomas, Leipzig (1911) (as much as 114 kcal/mol can be obtained from the opposite “combustion” reaction) and can only be obtained thanks to the 1. INTRODUCTION energy input of the incoming photons. Photosynthetic systems are present mainly in green plants, but also in other organisms Light is made of photons, and photons are at the same time such as algae and bacteria1. The general structure of the pho- energy quanta and information bits. Accordingly, most pro- tosynthetic machine present in the tilakoyd membrane of green cesses involving the interaction of light with matter (in a gen- plants is schematically shown in Fig.1. The global function of eral sense, “photochemical processes”) can be distinguished this complex device is the oxidation of water to oxygen on the according to the predominant energy-carrying or information- internal side and the reduction of NADP+ to NADPH (which carrying role of photons. Living examples of this double-faced nature of light are provided by the two most important photo- chemical processes taking place in the biological world, photo- synthesis and vision. As pointed out in a previous article of this series (La Chimica e l’Industria 1995, 77, 24)**, photochemistry is nowa- days a well-established and developed research field. The progress in photochemistry, together with that in chemical syn- thesis, has led to the point where one can conceive artificial photochemical molecular devices capable of using light as an energy supply (to sustain energy-expensive functions) or as an input signal (to be processed and/or stored). In this article, we will try to outline a rational approach to the design of artificial systems of the first kind, in particular for the conversion of light energy into chemical or electrical energy. 2. LEARNING FROM PHOTOSYNTHESIS Photosynthesis is the basic process by which, under the action of sunlight, living organisms convert abundant low-en- ergy substrates into valuable chemicals of high free energy content (biological fuels). In green plant photosynthesis, the reaction is the conversion of water and carbon dioxide into oxygen and carbohydrates (eq. 1). (1) This is certainly the most important process for life on our planet. From a purely biological viewpoint, it represents a * Publicado em La Chimica e l'Industria 1995, 77, 959 ** Quim. Nova 1996, 19, 542 QUÍMICA NOVA, 20(4) (1997) 423 Figure 1. Schematic representation of the photosynthetic process in green plants. will be used later for the synthesis of carbohydrates from car- Photosynthesis and the Nobel Prize in Chemistry bon dioxide, see the Inset) on the external side of the mem- brane. To reach this goal, the photosynthetic membrane con- The understanding of natural photosynthesis is one of tains several protein complexes (schematized by contour lines), the most prominent achievements of modern chemistry, as each of which performs a specific function. Among these, the witnessed by the award of several recent Nobel Prizes in most important are the so-called reaction centers, usually des- Chemistry within this field of research. ignated as “photosystems I and II”, which completely span the In 1961 Melvin Calvin received the Nobel Prize for his membrane. The function of each of these photosystems is to unravelling of the complex cycle (known as Calvin cycle) generate, following light absorption, a pair of charges of oppo- which, in green plants, uses the energy stored in the primary site sign on opposite sides of the photosynthetic membrane, a products of the photosynthetic process (NADPH, ATP) to process usually indicated as “photoinduced charge separation”. effect the reduction of carbon dioxide to carbohydrates. The positive hole generated in photosystem II is used with the In 1978 Peter Mitchell received the Nobel Prize for help of a manganese-containing enzyme, for the oxidation of the “chemiosmotic theory”, which accounts for the synthe- water. The negative charge developed by photosystem I is used, sis of ATP in terms of the proton gradient created across with the intermediacy of suitable proteins, in the reduction of the membrane by the photochemical water oxidation and NADP+. The components lying between the two photosystems NADP+ reduction. have the role to connect “in series” photosystems I and II, i.e., In 1988, Johan Deisenhofer, Robert Huber, and to allow the electron-transfer processes by which the negative Hartmut Michel received the Nobel Prize for the crystalli- charge developed by photosystem II neutralizes the positive zation and determination of the X-ray structure of the re- charge developed by photosystem I. As an additional useful action center of Rhodopseudomonas viridis. Among other function, the photosynthetic membrane synthetizes ATP (an- outstanding aspects of their discovery, the precise knowl- other high-energy chemical to be used in the synthesis of car- edge about the spatial arrangement of the active molecular bohydrates) taking advantage of the proton concentration gra- components of the reaction center represented a real break- dient created by the photochemical oxidation and reduction through in the field, opening the possibility of a detailed reactions (see the Inset). understanding of the charge separation mechanism in terms Interestingly, excitation does not involve direct light absorp- of the kinetics of the various electron-transfer steps. tion by the two photosystems, but energy transfer from the so- The theoretical basis for the understanding of elec- called light harvesting antenna systems. One of such systems tron-transfer processes had been laid down by Rudolph A. is placed close to each of reaction centers (Fig. 1). The an- Marcus in the late sixties. To reach conclusive experimen- tenna systems contain a large number (hundreds) of chro- tal confirmation and general acceptance, however, the mophores and have thus a much larger cross section for light Marcus theory had to wait recent years, with the advent of absorption than the reaction centers. Indirect excitation of the fast and ultrafast kinetic methods to study reactions taking reaction center through the antenna is the way chosen by Na- place in pico- (10-12) and femto- (10-15) seconds. In par- ture to solve the problem of light-harvesting efficiency with a ticular, it is now recognized that the main reason for the relatively small number of reaction centers in low-intensity il- extraordinary efficiency of the charge separation taking lumination conditions. place in natural photosynthesis in the fact that the various Even from a simplified block-type description such as that charge recombination steps belong to the so-called Marcus of Fig. 1, it is apparent that the photosynthetic machinery of “inverted” kinetic regime, where highly exergonic electron- green plants is a very complex one. This is not surprising, transfer reactions, contrary to common expectations, were given the complex nature of the overall chemical process to be predicted by Marcus to be intrinsically slow. The impor- achieved. From the simple viewpoint of energy conversion, tance of this theoretical model as a tool for prediction and however, two fundamental functions can be extracted from the rationalization of electron-transfer rates was acknowledged analysis of this complicated machine: (i) photoinduced charge with the award to its author of the 1992 Nobe Prize in separation, and (ii) antenna effect. A closer inspection at the Chemistry. structure of reaction centers and antenna systems may be 424 QUÍMICA NOVA, 20(4) (1997) useful to highlight the relationship between function and struc- various electron-transfer steps, (iii) the kinetic competition ture in natural photosynthesis. between forward (useful) over back (dissipative) electron- transfer processes. How this comes about can be reasonably 16-20 2.1. Charge separation in reactions centers understood in terms of modern electron-transfer theory (see the Inset). A giant step in the understanding of photoinduced charge separation has been made with the determination by X-ray 2.2. Energy migration in antena systems crystallography of the structure at the molecular level of some reaction centers2-6 (see the Inset). These structure determinations As far as the antenna units are concerned, detailed struc- refer to bacterial, rather than green plant, photosynthesis but tural information has become available only very recently, the basic features are likely very similar in all cases. A simpli- when high resolution structures of some light-harvesting com- fied view of the structure of the reaction center of Rhodopseu- plexes of green plant and bacterial photosynthesis have been 21-24 domonas viridis2, 5, 6 is sketched in Fig.