Rotational coherence spectroscopy of 9,9′-bifluorenyl: assignment of exciton components

Polarized time-resolved fluorescence measurements on the two 000 components of the exciton doublet of 9,9′-bifluorenyl yielded distinct sets of rotational coherence transients. These measurements allowed assignment of the transition moment vector for the lower-energy exciton component to ‘A’ symmetry within the point group C2. Hybrid transients observed for the other component confirmed ‘B’ symmetry. Also, we find the interannular twist angle between the two fluorene units to be 62±5°. Parallel behavior of the 1:1 complex of bifluorenyl with water suggests that the water molecule hydrogen-bonds symmetrically to both aromatic components.