In this research, the reaction of pyromellitic diimides bearing acidic functional group(s) in the side chain with aliphatic amines was found to produce organic salts with excellent photochromic properties in polar organic solvents like dimethylformamide. Upon exposure to sunlight, the solution color changed from pale-yellow to green, showing the characteristics of pyromellitic diimide radical-anion. The study of photo-coloration mechanism using experimental and theoretical methods revealed that the acid-base reaction of the mentioned pyromellitic diimides with an aliphatic amine causes unequal increases in the HOMO and LUMO energy levels. Larger enhancement in the HOMO compared to the LUMO energy level resulted in a decrease of HOMO-LUMO energy gap. Under this condition, leading to a facilitated intramolecular electron transfer, the light-induced single electron transfer from the side chain to diimide carbonyl group produced a green radical-anion. The prepared salts could be considered as intrinsic electrochromic materials besides possessing the photochromic behavior owing to their good electrical conductivity and electrochromic properties.
