Synthesis and Photophysical Properties of Colorful Salen-Type Schiff Bases

A series of colorful Salen-type Schiff bases derived from the different diamine bridges including 1,2-ethylenediamine (EU -Et3), 1,2-cydohexanediamine (Cy1-Cy3), 1,2-phenylenediamine (Ph1-Ph4), dicyano-1,2-ethenediamine (CN1-CN3), phenazine-2,3-diamine (Phen 1-Phen3), and naphthalene-2,3-diamine (Naph1 and Naph2) have been designed and prepared. The presence of electron-accepting substituents, electron-donating substituents, donor-acceptor (DA) systems, and/or pi-extended systems leads to not only full absorption and emission spectra (300-700 nm) in the visible region but also high fluorescence quantum yields up to 0.83 in solution. The experimental results and density functional theory (DFT) calculations have proved that the highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) levels, and consequently the energy gaps of these Salen ligands can be well tuned. The LUMO levels of these Salen ligands are mainly affected by the diamine bridges, whereas both the HOMO and LUMO levels are influenced by the phenol fragments. Adding Cu2+ and Co2+ to CN1 solution leads to a drastic color change from pink into brownish red and purple, respectively, which is useful not only for the ratiometric detection but also for rapid visual sensing even by the naked eye. Moreover, the properties of the cis-trans isomer of CN1 are examined. The Salen ligands have coordination chemistry similar to other well-known tetradentate porphyrin ligands as well as much easier preparation and rich photophysical properties.