Strategic molecular design of -carboranyl luminophores to manifest thermally activated delayed fluorescence.

In this paper, we propose a strategic molecular design of -carborane-based donor-acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9-fluorene-based compounds with - and -carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (-type: , , , and , and -type: - = [-form of ]·[NBu]) were prepared and characterized. The solid-state molecular structure of exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, exhibits a noticeable intramolecular charge transition (ICT)-based emission in all states (solution at 298 K and 77 K, and solid states); however, emissions by other -compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, - did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by -carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around -carborane cages change in -compounds, which indicates that the structural formation of is nearly fixed around its S-optimized structure. This differs from that for other -compounds, wherein the free rotation of their -carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other -compounds. Furthermore, displays short-lived (∼0.5 ns) and long-lived (∼30 ns) PL decay components in solution at 298 K and in the film state, respectively, which can be attributed to prompt fluorescence and TADF, respectively. The calculated energy difference (Δ ) between the first excited singlet and triplet states of the -compounds demonstrate that the TADF characteristic of originates from a significantly small Δ maintained by the rigid structural fixation around its S-optimized structure. Furthermore, the strategic molecular design of the -carborane-appended π-conjugated (D-A) system, which forms a rigid geometry due to severe intramolecular steric hindrance, can enhance the radiative efficiency for ICT-based emission and trigger the TADF nature.