Electronic coupling and conformational barrier crossing of 9,9′-bifluorenyl studied in a supersonic jet

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Abstract
Fluorescence excitation, dispersed fluorescence, and picosecond time‐resolved fluorescence spectroscopies have been used to study the dimeric molecule 9,9’‐bifluorenyl, isolated under supersonic jet conditions. The excitation spectrum reveals a splitting in several principal resonances of the S 0→S 1 excitation spectrum, which can be attributed to electronic coupling between the two fluorenyl moieties. The splitting, which for different vibronic transitions correlates with Franck–Condon factors, is consistent with an exciton model that includes higher electronic states. The splitting is reasonably well reproduced by a multipole interaction potential. Calculations have verified that the electron exchange coupling is small. Furthermore, the relative intensities of the resonances allow an estimate of the equilibrium geometry, suggesting that the molecular long axes have an angular displacement of 60°–70°, consistent with the geometry found by x‐ray diffraction. A most interesting feature of this species is that it is weakly fluorescent in fluid solution, which has been attributed to activated conformational barrier crossing of the excited molecule. In contrast, the fluorescencequantum efficiency of the isolated molecule can be close to unity, the lifetime ranging from 18 to 20 ns at vibrational energies <1400 cm−1. Above this region, the fluorescence decay time decreases steadily, to ≊2 ns by 2550 cm−1, indicating the onset of a nonradiative relaxation channel. Since the molecule was also seen to exhibit vibrational relaxation at low energies (i.e., ≤400 cm−1), the relaxation dynamics observed above 1400 cm−1 reflect the existence of a conformational potential energy barrier in the isolated molecule.