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.
A high-performance holographic recording medium was developed based on a unique combination of photoinitiated thiol−ene click chemistry and functional, linear polymers used as binders. Allyl reactive sites were incorporated along the backbone of the linear polymer binder to enable facile film casting and to facilitate cross-linking by photopolymerization of the thiol−ene monomers that also serve as the writing monomers in this distinctive approach to holographic materials. The allyl content and the ratio of the linear polymer to the writing monomers were varied to maximize and control the refractive index contrast. A blade-coatingbased film preparation method was developed to form films from the mixture of linear polymer and the thiol−ene monomers. This approach results in a holographic material with a peak to mean index contrast (Δn) that reaches 0.04. The refractive index contrast was stable for at least two weeks. Haze in holograms with a high writing monomer loading was significantly reduced when a higher allyl content was incorporated into the binder, resulting in the lowest haze around 0.2%. Finally, the media exhibit high resolution as demonstrated by the ability to record reflection holograms with 140 nm pitch and diffraction efficiency in excess of 90%.
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