Hydroxycruciforms: amine-responsive fluorophores

The synthesis of three hydroxy-substituted cruciforms (XF, 1,4-bis(4'-hydroxystyryl)-2,5-bis(4'-methoxyphenylethynyl)benzene, 1,4-bis(4'-methoxystyryl)-2,5-bis(4'-hydroxyphenylethynyl)benzene, and 1,4-bis(4'-hydroxystyryl)-2,5-bis(4'-hydroxyphenylethynyl)benzene) starts with a Horner reaction followed by a Sonogashira coupling and subsequent deprotection. The three herein described XFs contain either two or four free phenolic hydroxyl groups. All three XFs were subjected to photometric UV/Vis titrations in a methanol/water mixture. The respective pK(a) values were obtained by data deconvolution. As the three XFs display a significant change in emission color upon photoinduced deprotonation, the XFs were taken up in different solvents and exposed to twelve amines. The amine-dependent change in emissivity of the tetrahydroxy XF is sufficiently distinct in the eight solvents that all of the inspected amines are discerned by a linear discriminant analysis. The tetrahydroxy XF in different solvents forms a sensor array, the response of which is based on the excited-state proton transfer (ESPT) to amines and mediated by the choice of the battery of solvents that are utilized.