Triphenylamine-Ethynylanthracene Dyads: Photophysics and Potential Applications in Biological Systems and Bioimaging

Anthracene-based dyes are widely known to be impressive emitters, whereas triphenylamine-based dyes are notable for sensory applications. This work focuses on compiling these two building blocks by connecting them through a triple bond to ensure maximum planarity and high fluorescence quantum yield. Three anthracene-triphenylamine based dyes were synthesized, one of which is a basic 4-(anthracen-9-ylethynyl)-<i>N</i>,<i>N</i>-diphenylaniline [AnTPA]. The other two molecules are 4-(10-((4-(diphenylamino)phenyl)ethynyl)anthracen-9-yl)benzonitrile [sTPA] and 4-((10-((4-(diphenylamino)phenyl)ethynyl)anthracen-9-yl)ethynyl)benzonitrile [tTPA], where the electron-withdrawing benzonitrile group is added via one single bond and triple bond, respectively. The photophysical properties of these molecules are compared with the help of experimental and theoretical studies. These dyes exhibit intramolecular charge transfer (ICT) in their excited states resulting in high Stokes shifts, with their emission covering a broad spectrum ranging from blue to orange-red region. sTPA shows the highest ICT characteristics among these three molecules despite having an orthogonal benzonitrile unit. Emission of the synthesized molecules in polar media is sensitive to the fluctuations in solvent temperature, with tTPA being the most sensitive one. The ratiometric fluorescence increment displays a linear relationship with temperature. Fluorescence anisotropy values of these dyes are very sensitive to the medium viscosity and rotational constraint. Additionally, AnTPA was found to be a good candidate to study protein molecules, as evident by its good binding efficiency with human serum albumin. These fluorophores penetrate and stay within lipid bilayers and can be used for fluorescence imaging of giant unilamellar vesicles. They sense the phase transition temperature of liposomes. In mammalian cell-imaging experiments, AnTPA shows negligible cytotoxicity in HEK293T and HeLa cells and remains inside the cells for up to 36 h with a strong signal, highlighting its potential as a long-term cell-tracing dye.