Light-Switchable Buffers

Abstract

A visible light-switchable buffer system based on a merocyanine photoacid is presented. Para-substitution of the indolium side with a methoxy group affords a compound suitable for making hydrolytically stable aqueous buffers whose pH can be tuned between 7 and 4 using 500 nm light.

Keywords: buffers; molecular switches; photoacidity; protonated merocyanines; water.

Figure 1

a) Four‐state model describing the operation of MCHs in water. Under dark conditions, dissociation (K _a) of MCH is followed by isomerization (K _c) of MC to the corresponding SP form—i.e., K _a ^GS= K _a(1+K _c). Under steady light irradiation, photoproduct cis‐MCH undergoes ring‐closing with concomitant release of a proton (K _a ^MS). Liao's photoacid and all compounds discussed here bear a propyl‐1‐sulfonate group (n=2). b) Effect of buffering on 500 nm light‐triggered pH jumps reported in this work.

Figure 2

a) Mechanism of hydrolysis of Liao's photoacid below pH 9 (see ref. [11]). b) Profiles of apparent first‐order rate constant of hydrolysis of Liao's photoacid (blue), compound 1 (grey), 2 (green), 3 (red), and 4 (black) as a function of the pH. Solid black lines represent the best fits to Equation (S1). c) Extrapolated kinetic parameters for nucleophilic addition of water (top) and decomposition of the tetrahedral intermediate (bottom). Experimental conditions: [1–4]=25±2 μM, [phosphate buffers]=20 mM, T=25 °C.

Figure 3

UV/Vis spectra of 1 in the dark (a) and under steady light irradiation (b) as a function of the pH. The obtained absorbance (A) profiles of MCH (437 nm), MC (534 nm), cis‐MCH (375 nm), and SP (225 nm) are reported on the right. Solid black lines represent the best fit to Boltzmann's sigmoidal equation. Experimental conditions: [1]=24±2 μM, [phosphate buffers]=20 mM, T=25 °C. In the case of (b), spectra were acquired using a 500 nm LED‐light source (100 mW); its emission spectrum is highlighted using the corresponding RGB color code.

Figure 4

a) Apparent solubility of compound 1 in the dark as a function of the equivalents of NaOH (α) added to the system. b) The pH of the resulting buffer solutions at equilibrium in the dark as a function of α; solid black lines represent the best fit to Equation (1) and (2), respectively. c) Reversible jumps of a solution of 1 at pH=pK _a ^GS (α=1). d) pH tuning by modulation of the light source power. Experimental conditions: T=25 °C, 500 nm LED‐light source: (c) 195 mW and (d) 40.0, 8.2 and 7.0 mW.