The present volume, no 42 in the series “Photochemistry” of the Specialist Reports published by the Royal Society of Chemistry follows what is now become the usual format. A series of specialist reports over the photochemical papers published in the biennium 2012–2013 forms the first part. In even-numbered volumes of the series, it is the organic aspect that is considered, along with the computational work (inorganic, physicochemical aspects and solar photochemistry are presented in odd-numbered volumes).

The second part is formed by highlight on recent finding in various fields of photochemistry, which it is hoped will afford a flavour of advanced research that may be a pleasant reading for practitioners of photochemistry. This year, contributions in this section range from theoretical aspects (calculation of spectra) to applicative aspects such as polymer chemistry with visible light and singlet oxygen in biological media.

A book has been published on the hardly overestimated topic of intermediates in photochemistry and of photochemistry for the mild generation of intermediates.¹  Photochemistry indeed opens a new perspective, since it leads to the direct generation of intermediates under mild, cool conditions, lifting what is the main limitation of thermal chemistry. It is thus possible to concentrate the attention on the chemistry of intermediates, disregarding their generation, a 180° change with respect to the traditional approach of thermal chemistry, where most of the attention has to be devoted to the latter aspect.¹ 

There is much to do in this direction, both for exploiting new intermediates, as is the case for the recently rapidly developing aryl cation chemistry (by irradiation of aryl chlorides and related reagents)³  and by a more extensive exploitation of their properties. A collected book on Förster resonance energy transfer has been published, and refers to a very active topic.² 

Visible light photocatalysis for synthetic purpose is probably the most rapidly advancing single area in organic photochemistry. The rapid development appears to be due to the mixing of two fast advancing fields, that is knowledge on (thermal) organic catalysis (per se an area that is taking a bigger and bigger role in synthetic chemistry) and the use of inorganic complexes (where energies and redox potential are well known from research on inorganic chemistry and solar energy conversion). These opens a large of synthetic perspective, while profiting of the versatility introduced by inorganic sensitizers with their easily modulated properties (energy and redox properties of the excited states). The new perspective opened by the use of inorganic complexes is exemplified by the reaction between the enamine of propionaldehyde and dicyanobenzene, which are respectively oxidized and reduced by the appropriate excited state of the catalyst (see Scheme 1).⁴ 

Another interesting example is the synthesis of tetrahydrofurans from styrene and allyl alcohols under photocatalysis by acridinium salts, where electron transfer and hydrogen transfer both have a role. Thus, styrene is oxidized by the acridinium cation and add the alcohol. In this way, a radical cation is formed that is prearranged for a 5-endo cyclization to the desired tetrahydrofuran after hydrogen abstraction from phenylmalonitrile (the corresponding radical is reduced to the anion, a process that is coupled with the re-oxidation of the acridine radical, see Scheme 2).⁵ 

The peculiarity of photocatalyst (1) and related compounds have been previously evidenced and are based on the fact that both a chiral complexing moiety and a photocatalytically active moiety (xanthone) are incorporated in the same molecule. 4-(But-3-enyl)oxyquinolones (2) have been shown to undergo intramolecular [2+2] photocycloaddition in the presence of the chiral catalyst. The cyclobutanes were formed in high yields (83–95%). The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for quinolone (2) which, in contrast to simple alkoxyquinolones such as 3-butyloxyquinolone and 4-methoxyquinolone, decayed rapidly (τ≤1 ns) through cyclization to a triplet 1,4-diradical.

Reaction under these conditions led to the enantioselective 2+2 cycloaddition of some 2-quinolones, both 4-alkenyloxy (2) and (less satisfactory) 3-alkenyloxy (3), as illustrated in Scheme 3 by using either stereoisomer of the photocatalyst (see Scheme 3).⁶ 

A nice review on photoremovable protecting groups in chemistry and biology has been published, where attention is given to the mechanistic aspects and how these determine the efficacy of the systems.⁷ 

It seems to the present author that organic photochemistry is experiencing a renaissance, where at last the great potential of the knowledge accumulated in the meantime is developed in actual synthetic direction. However, most of photochemistry remains devoted to different applications and aspects. Time and spatial resolution are obviously a characteristic of light-induced reactions and the direct monitoring of processes occurring on nanoparticles or single molecules is finding more and more attention, as an example the photoinduced oxidation, tracking in real time optical and morphological changes.⁸ 

Many interesting applications involve the formation of smart materials, often based on the photoisomerization of azobenzene derivatives incorporated in liquid crystals.⁹ 

Among applications, many new sensors have been introduced. Notable are self-cleaning organic/inorganic photo-sensors that have recently been developed.¹⁰ 

Very active is the area of macromolecular chemistry, with applications that range from new photoinitiators to new polymerization mechanisms (RAFT, reversible addition-fragmentation), useful for obtaining advanced microparticle design.¹¹ 

Photocatalytic (in most cases by titania) water depollution has long been a most important applicative topic, indeed in the last decade this has been the most investigated topic in the whole field of photochemistry. The detailed mechanism of the reaction occurring are not necessarily known in every case, since different oxidizing species may be involved. It is thus welcome that at least in simple cases in depth mechanic issues begin to be confronted. As an example, previous irradiation of titania(110) leads to adsorbed oxygen atoms, but these are not involved in the formation of methyl formate during the oxidation of methanol, which rather involves a two steps mechanism, and has been proposed to involve deprotonation of a surface-bonded formaldehyde and reaction of the resulting radical with a likewise surface bonded methoxy group (see Scheme 4).¹² 

A novel system apt for the simultaneous transformation of inorganic and organic pollutants based on the reaction of sulfite has been reported. UV irradiation of iron(iii) species in the presence of sulfites results in the formation of SO₄^• and OH^• radicals, which results consistently better performing than other photo “Fenton” systems.¹³ 

Particular applications in many cases involve the photo-Fenton oxidation, e.g. that of glycolaldehyde in aerosol liquid water¹⁴  and the use of pH-insensitive bimetallic catalyst for the abatement of dye pollutants.¹⁵ 

The presence of chemical warfare agents remaining after either a domestic terrorist attack or a military conflict is a growing threat. The role of photochemistry for destroying toxic chemicals under safe conditions is matter of growing concern. As an example, nerve agents and organophosphate pesticides are potent acetylcholinesterase active agents because of their phosphorylating mode of action. Many of these compounds are persistent chemicals with low volatility, and thus lead to possible surface contact due to prolonged exposure. Photochemistry has an advantage with respect to traditional treatment of chemical warfare agents, which involve treatment with bleach or other strong oxidants, and thus mobilize large volumes of liquid chemicals, or even photocatalysis that require a catalyst.

An example of such a strategy is based on short wavelength irradiation, which produces directly oxidizing species such as OH radicals. Indeed, irradiation at 184 and 254 nm (the main emission of low pressure (germicidal) mercury arcs) is effective in destroying the chemical warfare agent profenofos. Formation of O atoms, ozone and OH radicals has been evidenced. An alternative OH^• generation process is ozone photolysis in the presence of water vapor. This process uses simple instrumentation with ambient gases (O₂ and H₂O), does not cause a significant increase in the treated surface temperature, and combines radicals, ozone, and shortwave UV light that may further contribute to the degradation.¹⁶ 

A problem that is becoming of increasing concern is the fact that degradation almost never results in complete mineralization, and is thus important to evaluate whether products of incomplete degradation are formed and are of significant toxicity, as it has been done in the cases of trenbolone acetate metabolites¹⁷  and of some fluoroquinolone antibiotics.¹⁸ 

Except for substances absorbing strongly in the near UV, the fate of contaminants in water under ‘natural’ conditions seem to be mainly due to excitation of dissolved organic matter (DOM). These absorb only the small fraction of sunlight in the deep UV. It has not been easy to determine how this path contributes to the degradation of organic contaminants in surface waters. A recent study has individuated various oxidant species, viz. ¹O₂, HO^•, H₂O₂, and DOM triplet states (³DOM*). It resulted that ¹O₂, ³DOM* and H₂O₂ are formed with a quantum yield of the same order and that H₂O₂ is reduced to HO^•, the quantum yield of which is one order of magnitude lower.¹⁹  Modelization of the process shows that the contribution of DOM sensitization to organic contaminant removal during UV treatment should be significant only at high UV fluency, characteristic of advanced oxidation processes. Of the reactive species studied, ³DOM* is predicted to have the greatest general influence on UV degradation of contaminants.

Another field that is growing into maturity is that of photoactivable drugs, in particular when activation is viable upon red light, which penetrates tissue with no damage. There are not many sensitizers, however, that are able to translate the photonic energy to the cleavage of a chemical bond. An advancement in this direction is based on the discovery that an aminoacrylate group could be cleaved to release parent drugs after oxidation by single oxygen (this has been tagged a “photo-unclick chemistry”).²⁰  In the example below, the enamide moiety is cleaved by singlet oxygen liberating the molecule C4, of structure very similar to that of the drug colchicin (see Scheme 5).

In a completely different approach, a molecule of the antitumor drug camptotechin is incorporated into a polyhydroxyacid through a photofragmentable nitrobenzyl moiety (see formula below). Further elaboration of the system allows the fabrication of shell cross-linked micelles from a diblock copolymer to which the prodrug is linked and thus to fine tuning the chemistry occurring.²¹  

As it is well known, calculations on the solar spectrum impinging on the Earth surface demonstrate that the ideal photosensitizer for a system that would absorb the maximum fraction of energy (somewhat above 1000 nm) would absorb near infrared photons, and of course, all of the photons above that limit. Thus, new sensitizers are continuously prepared with a maximum in the NIR for the use in solar cells. However, organic light absorbers have relatively narrow bandwidths separated by deep minima, which makes it difficult to obtain panchromatic absorption in a single organic semiconductor.

This limitation can be lifted, however, as shown in the case of a polymer (4). The singlet excited state of this material is photosensitized in the presence of a dye (5) that absorbs in the visible with great efficiency (ultrafast charge injection, lifetime of the latter compound, 10 fs to 10 ps when interfaced with ZnO as a prototypal electron-acceptor compound) (see Scheme 6).²²  This principle can be exploited and leads to a panchromatic photoresponse in prototype polymer/oxide bilayer photovoltaic diodes.

Of general interest may be advanced studies on the photostability of food and beverages in particular the degradation of anthocyanins from grape and purple sweet potato²³  and the alteration of enzymes after analysis.²⁴