PERSPECTIVE www.rsc.org/pps | Photochemical & Photobiological Sciences The sunny side of chemistry: green synthesis by solar light Stefano Protti and Maurizio Fagnoni* Received 7th May 2009, Accepted 22nd July 2009 First published as an Advance Article on the web 25th August 2009 DOI: 10.1039/b909128a The storage of solar energy is one of the main challenges in the near future. A rather unexploited way to fulfil this goal is the solar light induced formation of new chemical bonds, i.e. the synthesis of chemicals. Solar photons can be considered the ideal green reagents since they are costless and leave no residue in the reaction mixture. In many cases the solar radiation could be successfully used in place of toxic or expensive chemical reagents to overcome the activation energy in organic synthesis. In this perspective, the emerging trends on the use of solar light for green synthesis are summarized, highlighting the advantages of this photochemical method. 1. Introduction “Che l’energia solare, luce e calore, fosse necessaria alla vita, ` e cognizione che si disperde nella pi` u remota antichit` a; negli scrittori e nei poeti di ogni epoca, che ne trovano indicazioni sicure e molteplici. Scopo nostro non ` e quello di occuparci dell’azione della luce nell’evoluzione organica e nella vita, ma di esaminare soltanto l’azione della luce sui fenomeni che vengono attribuiti all’affinit` a chimica, combinazioni e decomposizioni.” 1 “It is well known from the most remote ancient times that life needs solar energy, light and heat; we can find obvious and numerous signs in the work of writers and poets in every epoch. However, our aim is not to be interested in the action of light in the organic evolution and in life, but we want to look into the action of light on the phenomena attributable to the chemical affinity, combinations and decompositions.” 1 Department of Organic Chemistry, The University of Pavia, V. Taramelli 10, 27100, Pavia, Italy. E-mail: fagnoni@unipv.it Stefano Protti Stefano Protti studied chem- istry at the University of Pavia (Italy), graduated in 2003, and completed his PhD in 2006 devel- oping photochemical arylation reactions via phenyl cations. He then moved to LASIR laboratory (Lille, France), where he studied the photoreactivity of flavonoids. Returning to Pavia, he focused his work on the optimization of photochemical syntheses from the eco-sustainable point of view. In 2007 he won the CINMPIS prize for the best PhD thesis in organic chemistry. Currently he is a post-doc researcher at the iBitTec-S laboratory (CEA Saclay, France), where he studies photocatalyzed oxidation reactions. Maurizio Fagnoni Maurizio Fagnoni graduated from the University of Pavia (Italy) with a thesis under the guidance of Prof. Angelo Albini on the use of photoinduced elec- tron transfer reactions in organic synthesis. He spent part of his PhD period at the University of M¨ unster (Germany) working in the group of Prof. J. Mattay. In 1996 he moved to the Isti- tuto Ronzoni (Milan) where he worked on the synthesis of pep- tidomimetics and functionalized chitosans. In 1998 he returned to the University of Pavia where he was appointed associate professor in 2008. His interests lie in the application of photochemistry to organic (green) synthesis encompassing photocatalysis and phenyl cation chemistry. In this statement by Patern` o, published in 1909, the author recognized the importance of solar radiation for human beings and at the same time defined the role of photochemistry in investigating all of the chemical phenomena that the matter underwent upon (solar) light absorption. Solar energy is the prerequisite for maintaining life on Earth, as recognized many thousands years ago when the sun had been an object of worship in many ancient civilizations. Sunlight-induced transformations on Earth are significantly older than life itself. It has been demonstrated that the sun produces light with a distribution similar to what would be expected from a 5525 K (5250 ◦ C) blackbody (Fig. 1). Roughly, we can say that half lies in the visible part of the electromagnetic spectrum and the other half in the near-infrared part. Recently, the total solar irradiance has been measured, confirming a solar constant value of 1366.1 W m -2 as the average intensity hitting the Earth’s atmosphere. 2 As a matter of fact, the energy from sunlight reaching our planet in 1 h (4.3 ¥ 10 20 J) is more than that consumed globally in 1 year (4.1 ¥ 10 20 J, 2001 data). 3 This journal is © The Royal Society of Chemistry and Owner Societies 2009 Photochem. Photobiol. Sci., 2009, 8, 1499–1516 | 1499