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.