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

Abstract

In this paper, we propose a strategic molecular design of closo-o-carborane-based donor–acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9H-fluorene-based compounds with closo- and nido-o-carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (closo-type: 2FC, 3FC, 4FC, and 4FCH, and nido-type: nido-4FC = [nido-form of 4FC]·[NBu₄]) were prepared and characterized. The solid-state molecular structure of 4FC exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, 4FC 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 closo-compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, nido-4FC did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by closo-o-carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around o-carborane cages change in closo-compounds, which indicates that the structural formation of 4FC is nearly fixed around its S₀-optimized structure. This differs from that for other closo-compounds, wherein the free rotation of their o-carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of 4FC results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other closo-compounds. Furthermore, 4FC 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 (ΔE_ST) between the first excited singlet and triplet states of the closo-compounds demonstrate that the TADF characteristic of 4FC originates from a significantly small ΔE_ST maintained by the rigid structural fixation around its S₀-optimized structure. Furthermore, the strategic molecular design of the o-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.