Water-soluble cruciforms: Response to protons and selected metal ions

The syntheses of three water-soluble cruciform fluorophores (XF) carrying aniline-N,N-bisacetic acid, 2-hydroxyaniline-N,N,O-trisacetic acid, and 1,2-phenylenediamine-N,N',N',N'-tetrakisacetic acid are reported. The XF skeleton was synthesized by a Horner reaction to assemble the distyrylbenzene unit followed by a Sonogashira coupling to attach the phenyleneethynylene modules. The photophysics of both the sodium salts and the ethyl esters of the three carboxylated 1,4-bis(aminostyryl)-2,5-bis(phenylethynyl)benzenes were investigated in chloroform and in aqueous buffered solution at a pH of 7.0 and compared to that of 1, 4-bis(dibutylaminostyryl)-2,5-bis(phenylethynyl) benzene (BDB). The attachment of the carboxylate units to the aniline nitrogens influenced the photophysics and the sensory responses of the XFs, as the combined effect of steric bulk and charge repulsion led to a blue-shifted absorption when compared to that of BDB. While the fluorescence of the water-soluble XFs is sensitive toward metal cations, the mode of sensing action is different from that of BDB, where direct complexation to the aniline nitrogen lowers the energy of the HOMO (but not of the LUMO), leading to a blue-shifted emission. In the case of the 2-hydroxyaniline-N,N,O-trisacetic acid and 1,2-phenylenediamine-N,N,N',N'-tetrakisacetic acid-functionalized XFs, interaction with metal cations in aqueous buffered solution is guided by a breakup of excimers that form in water at XF concentrations as low as 50 mu mol.L-1.