Introduction and review of the year 2016

Elisa Fasani, Stefano Protti and Angelo Albini

PhotoGreen Lab, Department of Chemistry, University of Pavia, V. Le Taramelli 12 27100 Pavia, Italy

E-mail: angelo.albini@unipv.it

DOI: 10.1039/9781788010696-00003

Important advancement in photochemistry in the year 2016 are illustrated by presenting awards, including the Nobel Prize in Chemistry, some historical perspective and some representative examples of photochemical research published in 2016.

1 Introduction

The present volume, no. 45 in the series "Photochemistry" of the Specialist Periodical Reports published by the Royal Society of Chemistry follows the format of previous issues from Volume 39 onwards. Accordingly, the first section consists of a series of reviews on the advancements in inorganic, physical and solar photochemistry reported in the biennium 2014–2015. The second part of the issue consists of highlights on recent topics, with the aim to provide the reader with a flavor of advanced research that may be a pleasant reading for practitioners.

In the attempt to better serve our readers, the introduction chapter includes, along with reviews, thematic issues and papers published in 2016, as well as a section on awards and prizes assigned to researchers operating in the different sectors of photochemistry and a presentation of the quote of the series "one hundred years ago" printed on the back cover.

This year we are glad to be in the lucky position of congratulating with colleagues and friends working in one of the key topics of photochemistry, the design and the operation of 'machines' at the molecular scale. Indeed, they exploited the incommensurable advantage of light to be not only useful to synthesize molecules with mechanical properties but also to play the role of trigger by furnishing the energy required to make molecular machine able to perform a movement.

Thus, a specific energy quantum allows to all chemists that built a ship in a bottle, to follow an invitation to "take to the sea and travel!". Similarly, we aim to consider these yearly books as a boat that adjusts a bit the route every year for better serving the photochemical community.

Accordingly, this volume consists of two parts. The reviews section is devoted to the photochemical and the photophysical properties of transition metal complexes, the properties of organic dyes, the application of solar photochemistry and to spectroscopical studies. On the other hand, the highlights section includes reports on the application of photocatalytic reduction of CO in synthesis, the photoisomerization of Azobenzene derivatives and on photoredox catalysis by acridine and xanthene dyes.

2 The sentence of the year, 1917

By the outbreak of the World War some key outlines of photochemistry had been established. The scientists had defined the field of application of photochemistry, that is photochemistry refers to reactions caused by light or other electromagnetic radiations different from light, thus excluding e.g. electron emission to give ions.

The most important photochemical reactions in nature was photosynthesis. Photography was the only artificial application extensively developed, but farther there was possibly no chemical substance that was not decomposed by irradiation by the suited wavelength. In photolysis, just as in electrolysis, primary and secondary reactions (that led to the actually isolable products) were to be distinguished. There were two limitations in studying quantitatively a photochemical reaction. One of these was theoretical: only rays absorbed had a chemical effect, as first recognized by Grotthuss. The independence of photochemical reactions on temperature was difficult to accept, in particular due to the success kinetic studies were having during those decades, however. As an example, if a ? = 0.2 µ was considered, the medium translation energy of a molecule at 20 °C was only the 163rd part of the energy of a quantum of this wavelength, and it became equal to it only at 47 370 °C. The absorption of such a wavelength thus had a peculiar strength that made it able to cleave a chemical bond. Noteworthy, already in 1810, Gay-Lussac and Thenard had stated that 'in order to explain all of the chemical effects of light it is sufficient to accept the hypothesis by Count Rutford that light makes nothing more than strongly enhancing the temperature of the particles on which it acted, while it enhanced minimally the temperature of the whole mass'. Introducing the quantum hypothesis involved a complete change of approach, as E. Warburg suggested in 1917:

'According to the old approach, it could be assumed that all of the molecules encountered participated in the same way to the absorption, as it is the case, e.g. for the water particles that are all moved in the same way when hit by a wave. With the new approach, it is impossible to understand how it may happen that a very weak irradiation causes an effect that might be obtained otherwise only by using high temperatures. Things were different when the quantum hypothesis was introduced, according to which the effect was concentrated on a relatively small number of molecules, as only as many molecules participate into the absorption as are the quanta absorbed,' provided that, of course, the quantum hv was larger than the work required for causing the observed chemical change, 2c/?q. Einstein had considered that all of the absorbing molecules were cleaved and had formulated what he had called the equivalence law, where the primary cleavage actually had S = ?/2c. Such a case was well represented by the photolysis of HBr at 0.309 µ, which had a unitary quantum yield, provided that secondary processes were taken into account.

Other examples included the photolysis of ammonia to give the elements and the cleavage of ozone. On the other hand, the catalytic effect was strong e.g. with H2/Cl2 mixtures (Chlorknallgas), where most of the experiments had been done under conditions, where it was difficult to be sure that only a negligible amount of water vapor was present. These were difficulties of the experiments and had led to a number of conflicting reports (e.g. Dux and Bodenstein had washed their instruments with the reagent gas mixture for one month before beginning their experiments), and nevertheless, Bunsen and Roscoe...