Facile Synthesis and Photoactivity of Merocyanine‐Photoacid Polymers

Abstract

In recent years, merocyanine photoacids have been utilized to control various chemical processes using visible light and have found applications in materials, energy, and biomedical areas. Molecular merocyanine photoacids are commonly used in the previous works. Covalently linking the photoacids to polymers improves their compatibility with different media, avoids leakage problems, and allows a localized proton concentration to be produced. However, the phenolic and indolinium moieties of the photoacids make them difficult to be polymerized with common methods. In this work, the monomer of a merocyanine photoacid is converted to a spiropyran in situ by adding trimethylamine to a dimethyl sulfoxide (DMSO) solution of the monomer. Free radical polymerization yields the polymers of the spiropyran, which is acidified to regenerate the photoacid. The photoacid polymers prepared show good solubility, photoacidity, and reversibility. Irradiating a thin film of a photoacid polymer doped with methyl orange through a mask copies the pattern of the mask to the film. The pattern can be erased by heating the film at 80 °C for 10 min, and a new pattern is created by irradiation through a different mask.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

“…87 The proposed intermediacy of the protonated cisoid form was supported by Shiozaki shortly thereafter, 88 who showed a better-defined response to protonation of spiropyran in ethanol with sulfuric acid than with the more commonly used trifluoroacetic 68,69,[89][90][91][92][93][94][95][96][97] and hydrochloric acids. 18,[98][99][100][101][102][103][104][105][106][107][108][109] These observations indicate that distinct protonated cisoid and transoid structures differ in pK a . Indeed, the dependence of the acidochromic response on the acid employed enables access to pH-gated photochromism in both indolinobenzospiropyran as well as 6-nitroindolinobenzospiropyran when the acids are stronger than the phenolates formed upon ring opening (Scheme 15 and Fig.…”

“…New levels of interconnected complexity can thus be achieved when the pK a of a complementary pH sensitive compound is matched appropriately with the spiropyran photoacid, 35,96,97,105,128 including but not limited to reversible polymerization, 106,107 permeability, 120,129-131 photogelation 121 and swelling, 118,126,[132][133][134][135] photocontrolled chemopropulsion, 115 photocurrent generation, 113,136 and reversible photoassembly of nanoparticles 125,137,138 (Scheme 19).…”

“…Switching functions have been shown to be restricted in thin films, monolayers and microstructures incorporating the spiropyran photochrome, and often an additional stimulus e.g., pH or temperature change, is needed. 31,105,160,161 When moving toward closely packed ensembles such as crystals and thick layers, however, the changes that need to occur collectively upon ring-opening are strongly hindered both spatially and ionically. By default, the solid state is not an ideal environment in which to undergo changes in their charge and spatial properties, let alone the drastic changes in shape (elongation and planarization) and charge (densely packed zwitterionic charges without a solvation sphere) the spiropyran-merocyanine system undergoes.…”

Effects of Substituents on Metastable‐State Photoacids: Design, Synthesis, and Evaluation of their Photochemical Properties

“…Later, excited‐state photoacids have been systematically studied since the 1970s, and they have been applied in studies of fast proton‐transfer processes owing to the short lifetime of their proton‐dissociation states . Last, but not least, Liao and co‐workers reported a metastable‐state photoacid that could reversibly undergo proton photodissociation accompanied with a nucleophilic reaction to form a cyclic spiropyran upon irradiation with a moderate intensity of visible light and thermal recovery in the dark. Given its high efficiency (a pH change of 2.2 units), long lifetime ( t 1/2 =76 s), and good reversibility, this metastable‐state photoacid has attracted considerable attention and has been utilized in various fields, such as the control of acid‐catalyzed reactions, supramolecular assemblies, microbial fuel cells, bacteria killing, waste management of reactions, cationic sensors, odorant release, and biological function .…”

A Reversible Proton Generator with On/Off Thermoswitch

“…Most of the works on photoacids presented so far have been based on dissolved photoacids small‐molecules in solution . However, the challenging immobilization of photoacid molecules into polymer supports, inorganic scaffolds, or biomolecules would open up novel possibilities . The successful immobilization of photoacid units in polymer films has shown the additional benefit of light‐mediated spatiotemporal control of the pH value since the immobilized photoacid did not diffuse as easily as small molecules would .…”