Shedding New Light on Squaraines: Utilizing Squaraine Dyes as Effective Tools in Organic Synthesis

First discovered in 1965 by Treibs and Jacob, fluorescent squaraine dyes are boldly colored compounds well known for their applications in physical and analytical chemistry. Structurally, squaraine dyes contain a central, electron deficient cyclobutenedione core flanked by two electron rich aromatic substituents in a 1,3-orientation, leading to donor- acceptor-donor resonance stabilization. These unique electronic properties enable squaraine dyes to intensely absorb and emit light in the near-infrared region. Classically, these compounds are utilized extensively in materials applications throughout physical and analytical chemistry, including biological imaging, photodynamic therapy, nonlinear optics, photovoltaics, and ion sensing.

In contrast, there are surprisingly few reported studies that explore the use of squaraine dyes as synthetic building blocks, despite possessing multiple sites of potential reactivity. This thesis focuses on the development of functionally rich squaraine scaffolds as readily available starting materials for the construction of more architecturally complex small molecules and the exploitation of their inherent reactivity to design chemically driven analytical protocols. To date, the applications of squaraine dyes have been successfully extended towards: (1) the development of squaraine dyes as thermo- and chemoreversible imaging agents, (2) accessing highly functionalized oxindoles and benzofuranones via a phosphine-mediated ring expansion of appropriately substituted dianiline squaraine dyes, (3) the utility of amino-acid bound 1,3-squaramides in Brønsted acid catalysis, and (4) the design of squaraine dyes as chiral transition metal ligands in enantioselective catalysis.