Benzobisoxazole cruciforms: heterocyclic fluorophores with spatially separated frontier molecular orbitals

We report the synthesis of nine conjugated cruciform-shaped molecules based on the central benzo[1,2-d:4,5-d']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-(N,N-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.