A colorimetric proton receptor for low pH values based on a pyrene–spiropyran conjugate

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).

A novel pyrene-based dual multifunctional fluorescent probe for differential sensing of pH and HSO3− and their bioimaging in live cells

We designed and synthesized a new fluorescent probe for detecting pH and HSO₃⁻ using pyrene as an electron donor (D) and pyridine as an electron acceptor (A).

Off-on-off fluorescent chemosensor for pH measurement with a terbium(iii) complex based on a tripodal salicylic-acid derivative

New Tb(iii) and Eu(iii) complexes have been synthesized as potential pH probes using a tripodal substituted-salicylic ligand (H3). Among them, this carboxylate ligand is found to be a good sensitizer for Tb(iii) emission owing to the superior match of the triplet energy level of the ligand with the (5)D4 emitting level of Tb(iii). The resulting · complex exhibited high sensitivity in the physiological pH range with significant "off-on-off" fluorescence changes in aqueous solution. Furthermore, this unique rare-earth complex has been successfully used to monitor pH variations within HeLa cells and with filter papers.