Hybrid Benzimidazole–Imidazole Carbodicarbene Ligands Control Deep-Red Emission from Photostable Borenium Ions

Abstract Carbodicarbenes (CDCs) are exceptionally strong neutral ligands used in main-group chemistry. Their combined σ- and π-donor abilities stabilize highly electrophilic main-group element centers while allowing the frontier orbitals and photophysical properties of their complexes to be tuned. However, how the structure of CDCs controls the emission of the coordination complexes they support has yet to be established. Herein we report benzimidazole–imidazole hybrid CDC ligands L1–L3. The Rh(I) dicarbonyl complexes of L1 and L2 show lower average carbonyl stretching frequencies ((ν̅CO) = 2001 and 1998 cm–1 in solution) than any carbone-type ligand reported to date, highlighting their exceptional donor strength. Comparison of L1–L3 with a bis-benzimidazole benchmark CDC on the same rigid boraolympicenium framework gives four borenium ions 1–4 that differ only in the CDC ligand, allowing the features of the CDC that dictate the properties to be disentangled. Notably, borenium ions 1, 2, and 3 are deep-red-emissive in fluid solution, retain their absorption and emission profiles under aerobic irradiation, and sensitize singlet oxygen. In contrast, the red-emissive benchmark borenium ion 4 decomposes under the same conditions, underscoring the critical importance of ligand design. Across the four boraolympicenium ions, neither emission energy nor fluorescence efficiency varies monotonically with donor strength. Borenium ion 3 contains the most electron-rich and sterically crowded ligand, emits at higher energy than 1 and 2, and is not the most efficient emitter. X-ray crystallography, conformer searches, and molecular dynamics reveal that 1–4 have substantially different accessible conformations despite nearly identical buried volumes around boron. Excited-state calculations show that emission energy and bandwidth depend on both the conformations sampled and the sensitivity of emission energy to the dihedral angle between the N-heterocyclic carbene wings of the CDC. The accessible conformations also determine the response to solvent polarity and viscosity, and the balance between fluorescence and nonradiative decay in fluid solution, PMMA, molecular host matrices, and the crystalline state. Restricting the range of accessible conformations in the crystalline state restores strong deep-red fluorescence, with quantum yields approaching ΦF = 0.5. These results provide important design considerations for CDC-supported main-group materials and suggest new methods to enhance the photostability and efficiency of donor-stabilized narrow energy gap chromophores.