Steric modulation of o-carborane enables tunable single and dual emission in multi-resonance TADF compounds

Abstract

Precise control over the emissive excited states of luminophores is crucial for advancing high-performance optoelectronic materials. Herein, we present a facile strategy for tuning the emissive excited states of o-carborane-functionalized B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) compounds through steric modulation of 2-R substituents. A series of 2-R-o-carboranyl MR-TADF compounds, denoted DtBuCzB-CBR, were synthesized and characterized with substituents of varying steric demand at the 2-position of the o-carborane cage (R = H (1), Me (2), ⁱBu (3), and SiMe₃ (TMS, 4)). Single-crystal X-ray analyses of compounds 2 and 4 revealed a nearly perpendicular orientation of the o-carboranyl C–C bond relative to the MR core plane, with compound 4 showing greater C–C bond elongation and structural distortion than 2. Notably, the compounds exhibit either single (1) or dual emission (2–4) in both solution and rigid states, depending on the steric bulk of the 2-R substituent. Theoretical calculations suggest that the single emission originates from a locally excited short-range charge transfer (SRCT) state confined within the MR core, whereas dual emission arises from both the SRCT state and a lower-energy hybridized local and charge transfer (HLCT) state, facilitated by electronic coupling between the MR core and the o-carborane unit. The emergence and persistence of the emissive HLCT state correlate with increased steric hindrance at the 2-position, which induces elongation of the o-carboranyl C–C bond and restricts cage rotation in the excited state. These findings provide a new design principle for o-carborane-based luminophores with tunable excited-state emission characteristics.