Unique Photophysical Properties of 1,8-Naphthalimide Derivatives: Generation of Semi-stable Radical Anion Species by Photo-Induced Electron Transfer from a Carboxy Group

The development of anion sensors for selective detection of a specific anion is a crucial research topic. We previously reported a selective photo-induced colorimetric reaction of 1-methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium (MNEI) having a cationic receptor in the presence of molecules having multiple carboxy groups, such as succinate, citrate, and polyacrylate. However, the mechanism underlying this reaction was not clarified. Here, we investigate the photo-induced colorimetric reaction of N-[2-(trimethylammonium)ethyl]-1,8-naphthalimide (TENI), which has a different cationic receptor from MNEI and undergoes the photo-induced colorimetric reaction, and its analogues to clarify the reaction mechanism. The TENI analogues having substituents on the naphthalene ring provide important evidence, suggesting that the colorimetric chemical species were radical anions generated via photo-induced electron transfer from carboxylate to the naphthalimide derivative. The generation of the naphthalimide-based radical anion is verified by 1H NMR and cyclic voltammetry analyses, and photo-reduction of methylene blue is mediated by TENI. In addition, the role of the cationic receptor for the photo-induced colorimetric reaction is investigated with TENI analogues having different hydrophilic groups instead of the trimethylammonium group. Interestingly, the photo-induced colorimetric reaction is observed in a nonionic analogue having a polyethylene glycol group, indicating that the colorimetric reaction does not require a cationic receptor. On the other hand, we reveal that the trimethylammonium group stabilizes the radical anion species. These generation and stabilization phenomena of naphthalimide-based radical anion species will contribute to the development of sophisticated detection systems specific for carboxylate.