Interplanar interactions in the excited triplet states of carbazole dimers by means of time-resolved EPR spectroscopy

The interplanar interactions for fully overlapped (FO) and partially overlapped ( PO) carbazole dimers were investigated using zero field splitting (ZFS) parameters D and E in the excited triplet (T-1) states. The observed D value decreases from the monomer to the dimers due to electron delocalization in the T-1 dimers. The highest electron delocalization was observed within the two planes of the FO dimer. The interplanar interactions were analysed in terms of exciton (EX) and charge resonance (CR) types interactions by comparing the D values of monomer and the dimers. It is found that about 90% of excitation is delocalized over the two carbazole units in the T-1 state of the FO dimer whereas for PO this contribution is 40%. In a model system, D-CR and E-CR were calculated using the two approximations, point charge and half point charge, for the evaluation of the ZFSs obtained.