Tuning the singlet-triplet energy splitting by fluorination at 3,6 positions of the 1,4-biscarbazoylbenzene

Abstract

While it has been known in the past decades that fluorination induces stability and electron transporting ability, less is known is the effect of fluorination on singlet-triplet energy splitting. A series of carbazole derivatives with fluorination at the 3 and/or 6 positions are synthesized using oxidative biaryl coupling method. It was found that fluorine at the 3 and/or 6 positions of 1,4-biscarbazoylbenzene (CCP) reduces the singlet energies and lowers the Lowest Unoccupied Molecular (LUMO) level compared to the non-substituted molecules The triplet energies are almost unaffected by the fluorination. This is in contrast with tert-butyl substituents where the triplet energies are reduced. Density functional theory is used to examine the frontier molecular orbitals. It was found that fluorination of carbazole at 3 and/or 6 positions increase the effective conjugation as a result of mesometric effect while tert-butyl substituents lower the bandgap through hyperconjugation. Fluorination of carbazole at 3 and 6 positions strongly depleted the connecting benzene of electrons at the LUMO levels. This gives rise to partial spatial separation of the highest occupied molecular orbital (HOMO) and the LUMO levels in fluorinated CCP which reduces the singlet-triplet energy splitting.