Light‐Modulated Cationic and Anionic Transport across Protein Biopolymers**.

Light is a convenient source of energy and the heart of light‐harvesting natural systems and devices. Here, we show light‐modulation of both the chemical nature and ionic charge carrier concentration within a protein‐based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer‐tethered photoacids and photobases to undergo excited‐state proton transfer and capture, respectively. Electrical measurements show that both the photoacid‐ and photobase‐functionalized biopolymers exhibit an impressive light‐modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid‐functionalized biopolymer, water‐derived anionic hydroxides are the suggested charge carriers for the photobase‐functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications. [ABSTRACT FROM AUTHOR]