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.