7,14-Diaryl-substituted zethrene diimides as stable far-red dyes with tunable photophysical properties

Synthesis and physical characterizations of a series of 7,14-diaryl-substituted zethrene diimides (ZDIs) bearing different substituents (alkyl chain, oligoethyleneglycol ether chain, and aryl group) at the imide sites as well as at the bay regions are described in this study. The synthesis takes advantage of Pd-catalyzed cyclodimerization reaction that allows construction of zethrene core and substitution at the bay region in one single step. The partially cyclized ZDI is also separated as a minor product. The carboxylic acid group is introduced to the bay region for the purpose of further bioconjugation. The photophysical properties, electrochemical properties, and photostability of these ZDI dyes are investigated with UV/vis spectroscopic measurements, cyclic voltammetry measurements, and photoirradiation tests. These dyes exhibit tunable photophysical properties in the far-red spectral region with moderate fluorescent quantum yields and good stability. The enhanced stability as compared to the parent zethrene and the 7,14-substituted zethrenes can be attributed to the electron-withdrawing effect of the imide groups and the kinetic blocking of the most reactive sites at the bay region.