New two-photon absorption organic chromophores containing imino and hydroxyl groups: synthesis, ESIPT and chemosensors

In this work, a variety of new conjugated chromophores containing imino and hydroxyl groups are presented. Excited state intramolecular proton transfer (ESIPT) of these chromophores under one- and two-photon irradiation was surveyed. One-photon absorption spectra show the presence of internal hydrogen bond in the organic dyes containing ortho-hydroxyl group, while the corresponding dyes carrying para-hydroxyl group do not exhibit intramolecular H-bonding effect. In the most of aprotic and protic solvents, the extended chromophores bearing ortho-hydroxyl group exhibit well-separated dual emission bands with large Stokes shift (ca. 160 nm), as contrast, the molecules containing para-hydroxyl group exhibit only single band with normal Stokes shift (ca. 50 nm). Two-photon absorption (TPA) induced competitive ESIPT emission can also be observed by near-infrared (near-IR) femtosecond laser to the molecules carrying ortho-hydroxyl group. The experiments show that the molecules carrying ortho-hydroxyl group are able to undergo ESIPT under one- and two-photon excitation, while the molecules containing para-hydroxyl groups do not exhibit such properties. 2-((3′,4′-Dimethoxyl-phenylethyleneyl-phenyl-4-ylimino)methyl)phenol (C1) and 2-((3′,4′,5′- trimethoxyl-phenylethyleneyl-phenyl-4-ylimino)methyl)phenol (C3) exhibit regular and selective response to Zn2+ in DMF. The molecular modeling was further performed to analyze ESIPT occurrence theoretically.