Benzobisoxazole Cruciforms: Heterocyclic Fluorophores with Spatially Separated Frontier Molecular Orbitals
作者:Jaebum Lim、Thomas A. Albright、Benjamin R. Martin、Ognjen Š. Miljanić
DOI:10.1021/jo202107w
日期:2011.12.16
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 shifts is followed by a hypsochromic one with continued acidification. X-ray diffraction studies of three selected cruciforms revealed the expected similar to 90 degrees angle between the cruciform's substituents, and crystal packing patterns dominated by [pi...pi] stacking and edge-to-face [C-H center dot center dot center dot pi] contacts.