Insights into the effects of substitution position on the photophysics of mono-o-carborane-substituted pyrenes

Abstract

Three closo-o-carborane-functionalised pyrene compounds (1CB, 2CB, and 4CB) were synthesised and fully characterised. The molecular structures of all compounds exhibited perpendicularity between the C–C bond of the o-carborane and the pyrene groups. The three compounds displayed major absorption bands assignable to π–π* transitions within the pyrene group, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carborane units and the pyrene moieties. While 1CB and 4CB displayed strong ICT-based emissions involving the o-carborane moiety (λ_em = 500–700 nm) in THF at 298 K, 2CB showed less intense LE-based emissions centred at λ_em = 407 nm. Although the PL spectra of all compounds demonstrated enhanced ICT-based emission via inhibition of C–C bond variance within the o-carborane in rigid states (THF at 77 K and films), the quantum efficiency of 2CB in films (Φ_em = 5%) did not significantly increase compared to that in THF at 298 K, while the values for 1CB and 4CB in films were dramatically enhanced to 75% and 62%, respectively. The radiative decay constants of each ICT-based emission showed that non-radiative decay processes were significantly larger for 2CB than in 1CB and 4CB. The relative energies of the various S₀ conformations as the dihedral angle between the o-carborane cage and pyrene unit was changed indicated that the o-carborane cages in 2CB could rotate more easily than those in 1CB and 4CB. Furthermore, the involvement of the o-carborane moiety in the LUMO level of 2CB was significantly affected by this dihedral angle. These results suggest that the free rotation of the o-carborane cage of 2CB interrupted its ICT transitions, with experimental and theoretical findings confirming that large structural variations around the o-carborane cage for 2CB also induced ICT-based non-radiative decay processes associated with the o-carborane, further blocking the ICT transition itself.