Synthesis, Photophysical and Electronic Properties of a D‐π‐A Julolidine‐Like Pyrenyl‐o‐Carborane

Abstract

We synthesized 2‐(1‐1,2‐dicarbadodecaboranyl(12))‐6,6,12,12‐tetramethyl‐7,8,11,12‐tetrahydro‐6H,10H‐phenaleno[1,9‐fg]pyrido[3,2,1‐ij]quinoline (4) a julolidine‐like pyrenyl‐o‐carborane, with pyrene substituted at the 2,7‐positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR‐TDDFT calculations, we compare o‐carborane and B(mes)2 (mes = 2,4,6‐Me3C6H2) as acceptor groups. Whereas the π‐acceptor strength of B(mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π‐donor strength of the julolidine‐like moiety, however, which raises the pyrene HOMO‐1 above the HOMO. In contrast to the analogous pyrene‐2‐yl‐o‐carborane, 2‐(1‐1,2‐dicarbadodecaboranyl(12))‐pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally‐excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine‐like pyrene moiety is observed.