The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

Photopolymerization, or the use of light to trigger polymerization, is one of the most exciting technologies for advanced manufacturing of polymers. One of the key components in the photopolymerization processes is the photoactive compound that absorbs the light, generating the active species that promotes the polymerization and largely determines the final properties of the material. The field of photopolymerization has been dominated by photoradical generators to mediate radical reactions. In the last decade, to expand the number of polymers that can be prepared by photopolymerization, intensive research has been devoted to the synthesis and utilization of photoactive molecules that are able to generate a base or an acid upon irradiation. These organic compounds are known to promote not only the ring-opening polymerization of various heterocyclic monomers such as lactones, carbonates, or epoxides but also to trigger the step-growth synthesis of polyurethanes. This Minireview highlights the recent advances in the development of organic photobase and photoacid generators, with the aim of encouraging the wider application of these photoactive compounds in the photopolymerization area and to expand the use of these polymers in advanced manufacturing processes.

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Recent Advances and Challenges in the Design of Organic Photoacid and Photobase Generators for Polymerizations.

This review focuses upon major achievements and recent advancements in the development of photoactive molecules which, under controlled illumination, release acidic groups either in solution or at the solid state. Successful photoacid generators are described the design of which are based on ionic, benzyl ester, imino ester, spiropyran and terarylene frameworks and the mechanism of acid generation in each case is discussed in detail. The development of such systems towards applications in lithography, surface etching, polymer design and two photon absorption processes are presented in order to give a state of the art overview of the development of new photoacid generators in the last three decades.

Recent progress in development of photoacid generators

Acenes, heteroacenes, conjugated polycyclic hydrocarbons, and polycyclic aromatic hydrocarbons (collectively referred to in this review as conjugated polycyclic molecules, CPMs) have fascinated chemists since they were first isolated and synthesized in the mid 19th century. Most recently, these compounds have shown significant promise as the active components in organic devices (e.g., solar cells, thin film transistors, light emitting diodes, etc.), and, since 2001, there have been a plethora of publications detailing synthetic strategies to produce CPMs. In this review, we discuss reductive aromatization, reductive dearomatization, and elimination/extrusion reactions used to from CPMs. After a brief historical discussion on early methods to synthesize CPMs, we detail the use of various reagents commonly used for the reductive (anti)aromatization of precursors containing 1,4-diols/diethers, including SnCl2 and iodide (I-); mechanistic aspects are outlined when possible. Extension of these methods to carbomers and cumulenes is briefly discussed. We then examine the use of low valent metal species to reduce endoxides to CPMs, followed by discussion of the methods to directly reduce acenediones and acenones to the respective acene. In the final section, we describe methods used to affect aromatization to the desired CPM via extrusion of small, volatile molecules.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cplu.201700168

Reductive Aromatization/Dearomatization and Elimination Reactions to Access Conjugated Polycyclic Hydrocarbons, Heteroacenes, and Cumulenes.

This contribution reports on the synthesis and the photochemical behavior of two new sulfonium-based photoacid generators (PAGs). We demonstrate that a para-to-meta substitution of a methyl (p-cyanobenzyl) sulfonium group in a 4-alkoxystilbene core strongly influences the photodissociation efficiency of the PAGs and leads to an increase of the quantum yield for acid generation by a factor 2.4. This substantial effect, which was also corroborated by a reactivity enhancement in cationic photopolymerization, is assigned to the modulation of the electronic interaction between two low lying excited states whose energy gap is strongly influenced by this substitution effect. Moreover, it was found that the position of the sulfonium moiety hardly affects the two-photon absorption properties of these push–pull chromophores. By the two-photon fabrication of microstructures, we finally show the potential use of the meta derivative as cationic two-photon initiator.

Enhancement of Acid Photogeneration Through a Para-to-Meta Substitution Strategy in a Sulfonium-Based Alkoxystilbene Designed for Two-Photon Polymerization

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.

Shota Suzuki

Papers

Photochemistry of naphthalimide photoacid generators.