Colorimetric fluorescence response to carbon dioxide using charge transfer dye and molecular rotor dye in smart solvent system

Combination of a charge transfer dye and a molecular rotor dye in a smart solvent system resulted in a remarkable colorimetric fluorescence (FL) response to carbon dioxide (CO₂). Nile Red and allyl-2-cyano-3-(1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)acrylate (ACHQA) were used as the charge transfer dye and molecular rotor dye, respectively, and an equivalent molar mixture of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and 1-propanol was chosen as the smart solvent system. Nile Red and ACHQA in the DBU/1-propanol solvent mixture emitted red light mainly due to Nile Red, whereas the FL color gradually turned to green with the increasing amount of CO₂ injected. The FL color reverted into the initial state by simultaneously bubbling N₂ gas and heating at 50 °C.