Effects of molecular geometry on the efficiency of intramolecular charge transfer-based luminescence in o -carboranyl-substituted 1 H -phenanthro[9,10- d ]imidazoles

Herein, we compared the optical properties of four compounds with an o -carborane cage linked to 1 H -phenanthro[9,10- d ]imidazole at the ortho - (oPC), meta - (mPC), or para -position (pPC) of the 2-phenyl ring or at the C2-position (PC) of the phenanthroimidazole moiety. In the solid state, pPC and PC exhibited intense intramolecular charge transfer (ICT)-based emission centred at ∼560 nm, while oPC and mPC exhibited dual emission in high-energy ( λ em ≈ 385 nm) and low-energy ( λ em ≈ 580 nm) regions due to a locally excited transition and ICT-based emission, respectively. Thus, the quantum efficiency and radiative decay constant of ICT-based emission in the film state were much higher for pPC and PC than for oPC and mPC. Analysis of solid-state molecular structure revealed a notable geometric difference between these two pairs, showing that the C–C bond axis of the o -carborane was orthogonal to the plane of the appended aromatic group for pPC and PC but not for oPC and mPC. Theoretical modelling of the low-energy transition in the first excited states of pPC and PC upon the rotation of the o -carborane cage verified that the above orthogonality plays an important role in the maximisation of ICT-based luminescence. Our findings suggest that the molecular geometry around the o -carborane cage is a major factor determining the efficiency of ICT-based radiative decay in o -carboranyl-substituted π-conjugated fluorophores.