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

초록

Threecloso-o-carborane-functionalised pyrene compounds (1CB,2CB, and4CB) were synthesised and fully characterised. The molecular structures of all compounds exhibited perpendicularity between the C-C bond of theo-carborane and the pyrene groups. The three compounds displayed major absorption bands assignable to pi-pi* transitions within the pyrene group, as well as weak intramolecular charge-transfer (ICT) transitions between theo-carborane units and the pyrene moieties. While1CBand4CBdisplayed strong ICT-based emissions involving theo-carborane moiety (lambda(em)= 500-700 nm) in THF at 298 K,2CBshowed less intense LE-based emissions centred at lambda(em)= 407 nm. Although the PL spectra of all compounds demonstrated enhanced ICT-based emissionviainhibition of C-C bond variance within theo-carborane in rigid states (THF at 77 K and films), the quantum efficiency of2CBin films (phi(em)= 5%) did not significantly increase compared to that in THF at 298 K, while the values for1CBand4CBin 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 for2CBthan in1CBand4CB. The relative energies of the various S(0)conformations as the dihedral angle between theo-carborane cage and pyrene unit was changed indicated that theo-carborane cages in2CBcould rotate more easily than those in1CBand4CB. Furthermore, the involvement of theo-carborane moiety in the LUMO level of2CBwas significantly affected by this dihedral angle. These results suggest that the free rotation of theo-carborane cage of2CBinterrupted its ICT transitions, with experimental and theoretical findings confirming that large structural variations around theo-carborane cage for2CBalso induced ICT-based non-radiative decay processes associated with theo-carborane, further blocking the ICT transition itself.

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