Evidence for two-photon absorption-induced ESIPT of chromophores containing hydroxyl and imino groups

This paper presents experimental and theoretical investigations into excited-state intramolecular proton transfer (ESIPT) in new chromophores with hydroxyl and imino groups under one- and two-photon excitation. The results show that internal hydrogen bonding exhibits a remarkable influence on the maximum absorption wavelength of 2-[(4'-N,N-diethylaminodiphenylethylene-4-ylimino)methyl]phenol (C1) and 2-[(4'-methoxyl-diphenylethylene-4-ylimino)methyl]phenol (C3). Compounds C1 and C3 exhibit well-separated dual fluorescence emission bands under one- and two-photon excitation. The second fluorescence peaks of C1 and C3 are characterized by much larger Stokes shift than the first normal peaks (ca. 140 vs. 30 nm). 4-[(4'-N,N-Diethylaminodiphenylethylene-4-ylimino)methyl]phenol (C2) and 4-[(4'-methoxyldiphenylethylene-4-ylimino)methyl]phenol (C4) display single emission bands with small Stokes shifts (ca. 30 nm) in various solvents under one- and two-photon excitation. Furthermore, the first emission maxima of C1 and C3 are almost identical to the maximum fluorescence emission wavelengths of C2 and C4, respectively. These results show that C1 and C3 can undergo ESIPT via a reasonable six-membered ring, while there is no ESIPT in C2 and C4 under one- and two-photon excitation. Compounds C1 and C2 have larger two-photon absorption cross-sections under various near-infrared laser frequencies tuned from 700 to 880 nm. Molecular geometry optimization of the phototautomers (enol and keto) was performed to analyze the experimental results. The possibility of using these chromophores for metal ions as chemosensors of was thoroughly investigated. In DMF C3 exhibits excellent sensing responses to Zn(2+) and Fe(3+) ions through a greatly increased greatly and a largely reduced emission, respectively. In methanol disappearance of ESIPT emission with added Zn(2+) ions confirms its existence. The binding constants of C3 with Zn(2+) and Fe(3+) ions in DMF are also estimated.