Inclusion Complexes of a Metastable‐State Photoacid with High Acidity and Chemical Stability

Metastable-state photoacid MCH1 (see image) and cyclodextrins (CDs) can form inclusion complexes [MCH1 ⋅ (CD)₂] [CD=2-hydroxypropyl-β-CD (HP-β-CD), γ-CD,β-CD and HP-γ-CD] with stronger ground and metastable-state acidity and slower thermal relaxation (SP1→trans-MCH1) rates than free MCH1. The inclusion complexes except [MCH1 ⋅ (HP-γ-CD)₂] have better stability against hydrolysis than free MCH1. Appropriate host molecules can regulate the properties of metastable-state photoacids to meet various needs.

Abstract

Metastable-state merocyanine photoacids (MCHs) have been widely applied to various chemical, material and biomedical areas to drive or control chemical processes with light. In this work, stoichiometry and association constants have been determined for inclusion complexes of a photoacid MCH1 ((E)-3-(2-(2-hydroxystyryl)-3,3-dimethyl-3H-indol-1-ium-1-yl)propane-1-sulfonate) with β-cyclodextrin (β-CD), 2-hydroxypropyl-β-CD (HP-β-CD), γ-CD and HP-γ-CD by means of UV-Vis absorption spectroscopic titrations. The inclusion complexes were studied to enhance acidity and chemical stability. Kinetic study showed that CDs stabilized the acidic metastable state and slowed its thermal relaxation. The acidity of the ground and metastable state (pK _a ^GS and pK _a ^MS) increased upon addition of CDs. The pK _a ^MS of [MCH1 ⋅ (γ-CD)₂] is as low as 0.92 in comparison with 2.24 for MCH1, which is close to the lowest pK _a ^MS values (1.20 and 1.03) reported previously, in which case the MCH1 was structurally modified with alkylammonium side chains. Addition of CDs also significantly enhanced the chemical stability of MCH1 against hydrolysis, which is one of the major concerns for the application of MCHs. In particular, the addition of HP-β-CD increased the half-life of MCH1 in aqueous solution more than four-fold. Moreover, the quantum chemical calculations confirmed the stoichiometry and analyzed the binding sites and hydrogen bonds of the inclusion complexes.

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