Photochemical reaction dynamics of 2,2'-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Direct visualization of photo-induced disulfide through oxidative coupling of para-aminothiophenol

Chemical transformations under visible irradiation are interesting in green preparation. Herein, a photo-oxidative coupling reaction of para-aminothiophenol (p-ATP) dimerizing to 4-aminophenyl disulfide (APDS) rather than 4,4'-dimercaptoazobenzene (DMAB) was achieved in water by visible light irradiation, producing monodispersed organic nanoparticles in situ with strong light scattering visualized by the dark-field microscopy (DFM) imaging technique, owing to the formation of disulfide.

Direct observation of o-benzyne formation in photochemical hexadehydro-Diels-Alder (hν-HDDA) reactions

Reactive ortho-benzyne derivatives are believed to be the initial products of liquid-phase [4 + 2]-cycloadditions between a 1,3-diyne and an alkyne via what is known as a hexadehydro-Diels–Alder (HDDA) reaction. The UV/VIS spectroscopic observation of o-benzyne derivatives and their photochemical dynamics in solution, however, have not been reported previously. Herein, we report direct UV/VIS spectroscopic evidence for the existence of an o-benzyne in solution, and establish the dynamics of its formation in a photoinduced reaction. For this purpose, we investigated a bis-diyne compound using femtosecond transient absorption spectroscopy in the ultraviolet/visible region. In the first step, we observe excited-state isomerization on a sub-10 ps time scale. For identification of the o-benzyne species formed within 50–70 ps, and the corresponding photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions, we employed two intermolecular trapping strategies. In the first case, the o-benzyne was trapped by a second bis-diyne, i.e., self-trapping. The self-trapping products were then identified in the transient absorption experiments by comparing their spectral features to those of the isolated products. In the second case, we used perylene for trapping and reconstructed the spectrum of the trapping product by removing the contribution of irrelevant species from the experimentally observed spectra. Taken together, the UV/VIS spectroscopic data provide a consistent picture for o-benzyne derivatives in solution as the products of photo-initiated HDDA reactions, and we deduce the time scales for their formation.

We report the transient ultraviolet/visible absorption spectrum of an o-benzyne species in solution for the first time.

Taking the plunge: chemical reaction dynamics in liquids

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

Femtosecond to microsecond observation of the photochemical reaction of 1,2-di(quinolin-2-yl)disulfide with methyl methacrylate

Multiple radical reaction steps have been observed in a continuous sequence with sub-picosecond to microsecond transient absorption spectroscopy.

Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution

An adduct-mediated triplet energy transfer from a triplet thione to an alkene is observed with time-resolved vibrational absorption spectroscopy.