A Reversible Proton Generator with On/Off Thermoswitch

Maskless, Reusable Visible-Light Direct-Write Stamp for Microscale Surface Patterning

We report a straightforward method for creating large-area, microscale resolution patterns of functional amines on self-assembled monolayers by the photoinduced local acidification of a flat elastomeric stamp enriched with photoacid. The limited diffusivity of the photoactivated merocyanine acid in poly(dimethylsiloxane) (PDMS) enabled to confine efficient deprotection of N-tert-butyloxycarbonyl amino group (N-Boc) to line widths below 10 μm. The experimental setup is very simple and is built around the conventional HD-DVD optical pickup. The method allows cost-efficient, maskless, large-area chemical patterning while avoiding potentially cytotoxic photochemical reaction products. The activation of the embedded photoacid occurs within the stamp upon illumination with the laser beam and the process is fully reversible. Preliminary positive results highlight the possibility of repeatable use of the same stamp for the creation of different patterns.

A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

Basic-to-acidic reversible pH switching with a merocyanine photoacid

The application of merocyanine photoacids has previously been largely limited to neutral and acidic pH values. Here we introduce a new merocyanine photoacid with superior pH switching qualities. By increasing the pK_a in the dark (pKdarka) and the solubility we increased the reversible visible light induced pH jump to 3.5 units. Moreover, it is the first demonstration of a merocyanine photoacid able to generate a significant pH drop from a basic (pH 8.3) to an acidic (pH 5.2) environment.

Reversible photo control of proton chemistry

Spatial, temporal, and remote control of proton chemistry can be achieved by using photoacids, which are molecules that transform from weak to strong acids under light. Most of proton chemistry is driven by a high concentration of protons ([H⁺]), which is difficult to obtain using excited-state photoacids. Metastable-stable state photoacids (mPAHs) can reversibly generate a high [H⁺] under visible light with a moderate intensity. It has been widely applied in different fields, e.g. fueling dissipative assemblies, driving molecular machines, controlling organic reactions, powering nanoreactors, curing diseases, manipulating DNA and proteins, developing smart materials, capturing carbon dioxide in air etc. This article compares mPAH with excited-state photoacid as well as common acids e.g. HCl to explain its advantages. Recent studies on the thermal dynamics, kinetics, and photoreaction of mPAHs are reported. The advantages and disadvantages of the three types of mPAHs, i.e. merocyanine, indazole, and TCF mPAHs, are compared with regard to photo-induced [H⁺], switching rate, and other properties. The mechanisms of controlling or driving functional systems, which involve acid-base reactions, acid catalyzed reactions, ionic bonding, coordination bonding, hydrogen bonding, ion exchange, cation-π interaction, solubility, swellability, permeability, and pH change in biosystems, are described. Applications of mPAHs in the chemical, material, energy, biotechnology and biomedical fields published in the past 5 years are reviewed. Prospects in the development and application of mPAHs are discussed.

Thermodynamics and kinetics of protonated merocyanine photoacids in water

Metastable-state photoacids (mPAHs) are chemical species whose photo-activated state is long-lived enough to allow for proton diffusion. Liao's photoacid (1) represents the archetype of mPAHs, and is being widely used on account of its unique capability to change the acidity of aqueous solutions reversibly. The behavior of 1 in water, however, still remains poorly understood. Herein, we provide in-depth insights on the thermodynamics and kinetics of 1 in water through a series of comparative 1H NMR and UV-Vis studies and relative modelling. Under dark conditions, we quantified a three-component equilibrium system where the dissociation (K a) of the open protonated form (MCH) is followed by isomerization (K c) of the open deprotonated form (MC) to the closed spiropyran form (SP) - i.e., in the absence of light, the ground state acidity can be expressed as K GS a = K a(1 + K c). On the other hand, under powerful and continuous light irradiation we were able to assess, for the first time experimentally, the dissociation constant (K MS a) of the protonated metastable state (cis-MCH). In addition, we found that thermal ring-opening of SP is always rate-determining regardless of pH, whereas hydrolysis is reminiscent of what is found for Schiff bases. The proposed methodology is general, and it was applied to two other compounds bearing a shorter (ethyl, 2) and a longer (butyl, 3) alkyl-1-sulfonate bridge. We found that the pK a remains constant, whereas both pK c and pK MS a linearly increase with the length of the alkyl bridge. Importantly, all results are consistent with a four-component model cycle, which describes perfectly the full dynamics of proton release/uptake of 1-3 in water. The superior hydrolytic stability and water solubility of compound 3, together with its relatively high pK GS a (low K c), allowed us to achieve fully reversible jumps of 2.5 pH units over 18 consecutive cycles (6 hours).