Theoretical study of heteroatom and substituent effects on excited-state intramolecular proton transfers and electronic properties of amino-type hydrogen bonding molecules

Abstract

Excited-state intramolecular proton transfer (ESIPT) reactions and electronic properties of the NH-type hydrogen bonding molecules (APBI, APBO, APBT, and APIP) and their derivatives have been theoretically investigated. For the different heteroatoms at the same position, the oxygen (APBO) and sulfur (APBT) bring weak intra-HB leading to only normal emission while the nitrogen (APBI) makes moderatly strong intra-HB in APBI giving dual emission. For the nitrogen heteroatoms at the different positions, the tertiary nitrogen in APIP greatly enhances intramolecular charge transfer (ICT) resulting in a tautomer emission maximum (driven by ESIPT) at much longer wavelength compared to that of APBI with secondary nitrogen. Moreover, the introduction of a tosyl (Ts) into one of amino protons of the NH-type molecules yielding Ts-substituted derivatives can ensure their ESIPT providing a single tautomer emission. Furthermore, the addition of a moderatly strong electron withdrawing group (cyano) in tosylaminophenyl causes a slight blue-shift of the tautomer emission maximum while the addition of a moderatly strong electron donating group (amino) makes a noticeable red-shift of the emission maximum compared to their parents. Among the designed derivatives, APIP-tosyl-NH2 photoexcited at around 360 nm exhibits the longest wavelength of tautomer emission in near infrared region caused by strong ICT leading to large Stokes shift that can avoid self-reabsorption which is an important character for fluorescent probes.