A dual photobase system for directing the pathway of pH-sensitive chemical reactions with light

Light-gated chemical reactions allow spatial and temporal control of chemical processes. Here, we suggest a new system for controlling pH-sensitive processes with light using two photobases of Arrhenius and Brønsted types. Only after light excitation do Arrhenius photobases undergo hydroxide ion dissociation, while Brønsted photobases capture a proton. However, none can be used alone to reversibly control pH due to the limitations arising from excessively fast or overly slow photoreaction timescales. We show here that combining the two types of photobases allows light-triggered and reversible pH control. We show an application of this method in directing the pH-dependent reaction pathways of the organic dye Alizarin Red S simply by switching between different wavelengths of light, i.e., irradiating each photobase separately. The concept of a light-controlled system shown here of a sophisticated interplay between two photobases can be integrated into various smart functional and dynamic systems

Manipulating the electronic properties and structure of MoO3 nanosheets with light via an excited-state proton transfer mechanism

Light is an attractive source of energy to regulate stimuli-responsive chemical systems, enabling control over chemical and physical processes. Here, we use light as a gating source to control the redox state, the formation of a localized surface plasmonic resonance (LSPR), and the structure of molybdenum oxide (MoO3) nanosheets, which are important for a wide array of applications. However, the light excitation is not of the MoO3 nanosheets but rather of a pyranine (HPTS) photoacid, which in turn undergoes an excited state proton transfer (ESPT) process. We show that the ESPT process from HPTS to the nanosheets and the intercalation of protons within the MoO3 nanosheets triggers the reduction of the nanosheets and the formation of an LSPR peak, a process that is reversible, meaning that in the absence of light, the LSPR peak disappears and the nanosheets return to their oxidized form. We further show that this reversible process is accompanied by a change in the nanosheet size and morphology

Self-Propulsion of Droplets via Light-Stimuli Rapid Control of Their Surface Tension

Biology demonstrates many examples of response and adaptation to external stimuli, and here we focus on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. We use light as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultra-fast response, we use photoacids and photobases, capable of donating or capturing a proton, respectively, in their excited-state. We control the movement and directionality of the droplet’s self-propulsion by introducing the photo-acid/base either in bulk solution, on the surface of the droplet, or inside the droplet. We show that proton transfer between the photo-acid/base and the droplet results in a rapid change in the droplet’s surface tension, which induces the self?propulsion movement. The high versatility of our systems together with a record-breaking ultra?fast response to light makes them highly attractive for the design of various controlled cargo?carrier systems.<br /