Cross-Conjugated Cruciform Fluorophores

In optoelectronic devices, chromophores can be designed at the molecular level to create materials with properties desired for advanced applications. Organic fluorophores in particular can be constructed with macroscopic properties that arise from two distinct contributions: (i) the collective impact of the molecular backbone and substituents and (ii) the connectivity within the molecule (that is, the spatial molecular architecture). Accordingly, the exploration of novel conjugated architectures is a productive area of current research. Different two-dimensional, “X-shaped” conjugated materials have been synthesized for a variety of applications. They include spiro compounds, paracyclophanes, swivel-type dimers, bisoxazole-derived cruciforms, tetraethynylethenes, and tetrasubstituted tolanes. A subset of these compounds are constructed from two “perpendicular” π-conjugated linear arms connected through a central aromatic core; examples of these include tetrakis(arylethynyl)benzenes, tetrakis(styryl)benzenes, and tetrasubstituted thiophenes. In this Account, we evaluate 1,4-distyryl-2,5-bis(arylethynyl)benzenes or cruciforms (XFs). Electronic substitution of this “X-shaped” cross-conjugated scaffold tunes both the energy levels of the frontier molecular orbitals (FMOs) and their spatial distribution in XFs. The resulting fluorophores exhibit FMO separation, imbuing XFs with unusual yet desirable properties for sensory applications. Using model analytes, we examine how the underlying FMO arrangement and the nature of analyte interaction elicit observable responses. These studies provide a foundation for accessing functional responsive ratiometric cores, demonstrating the importance and unique potential of FMO-separated fluorophores. We also highlight the essential contribution of serendipity in materials development. Moving beyond one-dimensional molecular wire-type fluorophores to two-dimensional “X-shaped” materials provides access to materials with unexpected and exciting properties. XFs represent such novel conjugated architectures, and their successful development has frequently has hinged on inspiration from structural components and principles developed in diverse research areas.