Molecular design of amino-type hydrogen-bonding molecules for excited-state intramolecular proton transfer (ESIPT)-based fluorescent probe using the TD-DFT approach

Abstract

A molecular screening has been carried out for fluorescent probes harnessing excited-stated intramolecular proton transfer (ESIPT) of NH-type molecules having aminophenyl or tosylaminophenyl as a proton donor and benzimidazole, benzoxazole, benzothiazole, or imidazo[1,2-a]pyridine as a proton acceptor with different substituents using time-dependent density functional theory. Among the designed ESIPT molecules, 2-(2′-tosylaminophenyl)benzimidazole, 2-(2′-tosylaminophenyl)benzothiazole, and 2-(2′-tosylaminophenyl)imidazo[1,2-a]pyridine with dimethylamino in the tosylaminophenyl and/or cyano in the benzimidazole, benzothiazole, and imidazo[1,2-a]pyridine, respectively, were revealed to be the best five candidates because they passed the screening requirements, including photophysical, kinetics, and thermodynamic parameters. Here, these five candidates required less photo-absorption around 380 nm and emitted the tautomer peaks in the near infrared (NIR) region, leading to large Stokes shifts (∼200 nm) with no self-reabsorption, which are important characteristics for fluorescent probes. The NIR emission is caused by the intramolecular charge-transfer character of the strong electron-donating dimethylamino in the tosylaminophenyl moiety and heteroatoms in the benzimidazole/benzothiazole/imidazo[1,2-a]pyridine moiety as evidenced by the electron-density differences and frontier molecular orbitals. In addition, they exhibit high photo-acidity and photo-basicity (low PT barrier with highly exothermic) to warrantee the ESIPT. Therefore, the obtained screening information in this work could be beneficial for designing new ESIPT fluorescent-based probes.