In the present paper, a “light-up” chemsensor with a high specificity for carbon dioxide detection using a pyrimido[1,2-a]benzimidazole derivative (P1H) in liquid media has been developed. The results show that P1H reacts with carbon dioxide activated by a basic ion to form a carboxylic acid compound (P1-COOH). These results also provide a possible method for carboxylation reactions of P1H using carbon dioxide based on a vinyl carbanion. The complete reaction mechanism cycle was also described using DFT calculations.