Benzobisoxazole Cruciforms: Heterocyclic Fluorophores with Spatially Separated Frontier Molecular Orbitals

Abstract

We report the synthesis of nine conjugated cruciform-shaped molecules based on the central benzo­[1,2-<i>d</i>:4,5-<i>d</i>′]­bisoxazole nucleus, at which two conjugated currents intersect at a ∼90° angle. Cruciforms’ substituents were varied pairwise among the electron-neutral phenyl groups, electron-rich 4-(<i>N</i>,<i>N</i>-dimethylamino)­phenyl substituents, and electron-poor pyridines. Hybrid density functional theory calculations revealed that the highest occupied molecular orbitals (HOMOs) are localized (24–99%) in all cruciforms, in contrast to the lowest unoccupied molecular orbitals (LUMOs) which are strongly dependent on the substitution and less localized (6–64%). Localization of frontier molecular orbitals (FMOs) along different axes of these cruciforms makes them promising as sensing platforms, since analyte binding to the cruciform should mandate a change in the HOMO–LUMO gap and the resultant optical properties. This prediction was verified using UV/vis absorption and emission spectroscopy: cruciforms’ protonation results in hypsochromic and bathochromic shifts consistent with the preferential stabilization of HOMO and LUMO, respectively. In donor–acceptor-substituted systems, a two-step optical response to protonation was observed, wherein an initial bathochromic shift is followed by a hypsochromic one with continued acidification. X-ray diffraction studies of three selected cruciforms revealed the expected ∼90° angle between the cruciform’s substituents, and crystal packing patterns dominated by [π···π] stacking and edge-to-face [C–H···π] contacts