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

Abstract

Herein, we compared the optical properties of four compounds with an o-carborane cage linked to 1H-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.