16 CHIMIA 2018, 72, No. 1/2 ChemiCal eduCation doi:10.2533/chimia.2018.16 Chimia 72 (2018) 16–22 © Swiss Chemical Society Teaching Fundamental Aspects of Natural and Artificial Photosynthesis in Higher Education Urs Leisingera, Urs Leuteneggera, and Roger Albertob* Abstract: Photosynthesis or the storage of solar energy in chemical bonds is a process which, in its essence, goes far beyond a conversion of CO and water into dioxygen and glucose or other organic products. Photosynthesis is 2 a process which comprises elementary features of most of the chemical reactions and some physical processes we are looking at in higher education; light absorption, proton transfer, redox reactions and making and break- ing of bonds amongst others. Metals and main group elements are involved and the entire process is embed- ded in a biological environment that involves proteins and membranes. In this article, we will focus on two key aspects of natural photosynthesis, namely the absorption of light (photons) and the separation of the excitons into electrons and holes based on P680 along with the electrochemical energetics. Further, we will present an artificial light driven catalytic process which mimics PSI for the reduction of water to H , the inorganic congener 2 of nicotinamide adenine dinucleotide phosphate (NADPH). This light-driven reductive process shall show the mechanistic complexity of the ‘easy’ reaction 2H+ + 2e– → H . 2 Keywords: Biomimetic · Electron transfer reactions · Excited states · Photosystems I and II · Reaction mecha- nisms · Water oxidation and reduction Urs Leisinger studied environmental Urs Leutenegger obtained his Diploma Roger Alberto studied chemistry sciences at the Swiss Federal Institute of in Natural Sciences at the ETH Zürich in at the ETH Zürich culminating in a Technology in Zurich (ETH), including 1985. He received his PhD in Organic PhD under the supervision of Prof. G. a practical semester at the Plant and Chemistry under the supervision of Prof. Anderegg on technetium fluorides in Pollution Research Laboratory in Tallinn, A. Pfaltz at the ETH Zürich in 1990. His 1988. After postdoctoral stays as an Estonia. He received his PhD for work on research work focused on enantioselective Alexander von Humboldt fellow with the response to oxidative stress in green catalysis with semicorrin-transition metal Prof. W. A. Herrmann at the TU Munich algae under the supervision of Rik Eggen, complexes. He joined the group of Prof. and Prof. A. Sattelberger at Los Alamos Eawag Dübendorf, in 2001. In 2004, he W. C. Still at Columbia till summer 1990 National Laboratory (LANL) 1989–91, obtained his Certificate in Education at beforeworkingforBaslerandHoffmann,an he became a group leader at Paul Scherrer the ETH and since 2001, he works as a engineering company in Zürich. In 1993 he Institute (PSI) from 1992-1999, associate chemistry teacher at the Kantonsschule obtained his Certificate in Education from Professor at the University of Zürich in Zug. the ETH. After teaching chemistry at the 1999 and finally full professor in 2005. Kantonsschule Rychenberg in Winterthur, He has undertaken guest professorships he moved to the Kantonsschule Zug, where in Sendai Japan, Paris and Singapore. he has been teaching chemistry since 2000. From 2012–2016 he was head of the Department of Chemistry. His research interests include technetium and rhenium, bioorganometallic chemistry, basic organometallic chemistry in water, field sensor probes for MR imaging. He is the chair of the University Research Priority *Correspondence: Prof. Dr. R. Albertob Program ‘Light to Chemical Energy E-mail: [email protected] Conversion’ (LightChEC) and was aKantonsschule Zug, Lüssiweg 24, CH-6302 Zug bDepartment of Chemistry, University of Zurich, awarded the Alexander von Humboldt Winterthurerstr. 190, CH-8057 Zurich research award in 2017. ChemiCal eduCation CHIMIA 2018, 72, No. 1/2 17 1. Introduction exergy of the photons, i.e. maximal energy as Fischer-Tropsch or Sabatier to liquid that can be used for chemical work accord- fuels are well established. Artificial water Natural photosynthesis in plants is a ing to the second law of thermodynamics, splitting is a field of intense research and process in which diluted solar light energy the recovery is even higher, reaching about a number of systems making this possible is converted into concentrated chemical 40%.[8] have been published, the artificial leaf be- energy. Ultimately, photocatalysis pro- However, this is theory and for a true ing probably the most prominent one.[14] vides all the energetic compounds that are efficiency the full incident solar spectrum Other assemblies which combine electro- needed to drive our society. It is a highly has to be considered. Light energy losses lytic water splitting with photovoltaics[15] complex and multi-step biochemical pro- in the plant start with light scattering in reach solar to dihydrogen&dioxygen effi- cess which can be generally described as the photosynthetic tissues and elsewhere ciencies up to 13%[16] but many of these the absorption of photons and the storage (approximately 30%), the radiation that systems are limited due to photodegrada- of their energy in chemical form. In the can be absorbed by chromophores is about tion of their components. There are various best-known oxygenic photosynthesis of 53% (see Fig. 2 below).[7] Moreover, losses possibilities to assemble the components cyanobacteria, algae and higher plants, the of light energy in the photosynthetic an- in water splitting, wired and non-wired, final products are essentially dioxygen O , tenna due to the funnelling of only about homo- or heterogeneous and advantages 2 NADPH and ATP as intermediates for the 1.8 eV per photon into photochemical re- and disadvantages are discussed with re- glucose formation and glucose C H O it- actions reduce efficiency further by 25%. spect to economy, ecology efficiency and 6 12 6 self.[1,2] The initial step of photosynthesis Including the subsequent chemical fixation other parameters.[17] Research is in most in the so-called reaction centres is photon- with an estimated efficiency of about 30%, fields still far from providing a long-term induced charge separation, generating re- accumulated losses total up to an approxi- stable (years), cheap and efficient water ducing equivalents in the form of electrons mate solar energy storage efficiency of ≈ splitting device. Still, from the available and simultaneously oxidizing equivalents 9%. Minimal respiratory losses of 30% systems, we can learn about fundamental in the form of holes.[3] The reducing equiv- add up, capping the efficiency of biologi- chemical and catalytic processes. alents are used to generate organic building cal photosynthesis at a maximum of 6%. In this article, selected basic reaction blocks for biomolecules from inorganic, This compares to the highest solar energy schemes found in natural photosynthesis stable compounds such as CO whereas conversion efficiency reported for crops of mainly in PSII will be discussed on a level 2 the latter holes are filled by electrons from about 3.7%.[9,10] This value includes the useful for higher education in gymnasia. water H O oxidation to form the ‘waste energy required to construct and maintain Reduction equivalents, conceptually gen- 2 product’ and strong oxidant O . The over- the photosynthetic apparatus, whereas the erated in a water oxidizing system such 2 all photosynthesis process can be summa- efficiencies quoted for technical devices as PSII, will be applied to artificial, PSI rized as: normally do not include the energetic mimicking systems to show how demand- maintenance costs.[2] ing the generation and the understanding 6 CO + 6 H O → C H O + 6 O According to these estimations, losses of a molecule-based architecture is. It is 2 2 6 12 6 2 ∆H° = 2800 kJ mol–1 in the photosystems and losses in the sub- not the intention of this essay to provide R ∆G° ≈ 2875 kJ/mol sequent photochemistry reduce the yields a comprehensive view about the state of R each by a factor of approximately 3. The research in natural photosynthesis or to Every hour about 4.2·1020 J energy remaining losses that reduce the yield by summarize the latest achievements in arti- reaches the earth surface, approximately a much higher factor, thus, are a conse- ficial photosynthesis. Rather, fundamental the energy that is turned over by humans quence of oversaturation of the photosyn- processes shall be discussed as relevant for in technical processes on our planet in one thetic apparatus with light at radiation in- photochemistry and catalysis. year (4.1·1020 J). However, only around tensities above about 10% of full sunlight, 0.1% of this influx is deposited in form incomplete ground cover and the fact that of biomass, approximately equal amounts biological photosynthetic systems are 2. Aspects of Natural on land ecosystems and in the oceans,[4] a situated in a complex biological context Photosynthesis disputed source of energy for satisfying and drive various biological processes.[11] our energy demand in the future.[5,6] If this Under non-ideal conditions, photosynthet- The initial steps of natural photosyn- amount of solar energy could be converted ic efficiency is generally 1%, more realisti- thesis essentially rely on two components, and stored to a sufficient extent with ef- cally about 0.1% as pointed out by Barber photosystem II (PSII) where water is oxi- ficient devices of some kind, energy prob- et al. in a very interesting study.[4] It is not dized and photosystem I (PSI) where the lems could be solved and the increase of an issue of this article to compare natural electrons from PSII are temporarily stored CO in the atmosphere be stopped or even photosynthetic efficiency with artificial in form of the reductant NADPH and then 2 reversed. Efficiency is a major issue. It can systems, but to learn from it. To improve used for the reduction of CO or as an elec- 2 be high as in wind turbines (Betz law, up photosynthetic efficiency was probably tron source in various anabolic reactions to 60% electric energy with respect to the not a demand in nature since the avail- (‘biological H ’).