Photophysical Behavior of Self-Organizing Derivatives of 10- and 12-Vertex p-Carboranes, and their Bicyclo[2.2.2]octane and Benzene Analogues

Four series of isostructural derivatives of 3-ring liquid crystalline derivatives containing p-carboranes (12-vertex A, and 10-vertex B), bicyclo[2.2.2]octane (C), or benzene (D) as the variable structural element were investigated for their mesogenic behavior and electronic interactions. Comparative studies demonstrated that the effectiveness of elements A-D in stabilization of the mesophase typically increases in the order: B<A<C<D. Absorption and emission spectroscopy for the four series and reference compounds augmented with DFT computational results revealed that the degree of electronic interactions with the connected benzene π system increases in the order C<A<B<D. Emission spectra exhibit large Stokes' shifts for 10-vertex derivatives (B) in solutions and in the solid state. Spectroscopic characterization was supplemented with polarization electronic spectroscopy and solvatochromic studies of selected series. Overall, 12-vertex p-carborane A acts as an electron withdrawing auxochromic substituent with interactions similar to those of bicyclo[2.2.2]octane, although capable of accepting some electron density in the excited state. In contrast, 10-vertex p-carborane B is much more interactive with the π-aromatic electron manifold and exhibits greater ability to participate in photo-induced CT processes. Absorption and emission energies and quantum yields (1-51 %) for carborane derivatives, representing the D-A-D system, were compared to those of their isoelectronic zwitterionic analogues (the A-D-A system). The analysis is supplemented with four single-crystal XRD structures.