A copolymer of N-isopropyl acrylamide and acrylic acid was modified in a polymer analogous reaction by an acridizinium chromophore capable of reversible photochemical dimerization. The acridizinium chromophore allows a faster and more complete photocrosslinking and a better photoreversibility as compared to stilbene and stilbazolium chromophores. Due to its hydrophilic nature the chromophore only moderately affected cloud points T-c of the copolymer and lower critical solution temperatures (LCST) of photocrosslinked films of the copolymer, even at high chromophore content (3.5% w/w). Such films can be swollen in water to form hydrogels. LCST and T-c depend on the content and on the counterion of the chromophore, on the solvent from which films are prepared, and on the degree of crosslinking. The latter dependence allows the exploitation for isothermal switching of film properties by exposure to light.