Electrochemistry and Electrogenerated Chemiluminescence of π‑Stacked Poly(fluorenemethylene) Oligomers. Multiple, Interacting Electron Transfers

The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a series of π-stacked poly­(fluorenemethylene) oligomers (F<i>n</i>, <i>n</i> = 1–6) were investigated. The pendant cofacially oriented fluorene moieties are essentially in contact with each other by Van der Waals interaction promoting electronic delocalization in these species. All six compounds give successive cyclic voltammetric one-electron (1e) oxidations in 1:1 acetonitrile/benzene (MeCN/Bz), and the multiple 1e transfer properties of all these compounds were confirmed by chronoamperometric experiments with an ultramicroelectrode and digital simulations. The potentials for oxidation of the successive 1e transfers can be explained in terms of electrostatic interactions among the fluorenes. The monomer (F1) shows one irreversible wave, while F2 shows two reversible 1e waves. F3 shows only two reversible 1e oxidation waves, which is consistent with the large energy to remove a third electron because of the greater electrostatic repulsion, so the third wave is shifted toward more positive potentials. Both F4 and F5 show three reversible 1e oxidation waves, while F6 shows four reversible 1e waves. The removal of the first electron from an oligomer becomes easier as <i>n</i> increases. The stability of the radical cations also increases with <i>n</i>. The removal of consecutive electrons from F<i>n</i> can be correlated with the distance between fluorene moieties. No reduction peaks were observed except for some broad ones at ∼−3.2 V vs SCE in THF, which is consitent with the wide highest occupied molecular orbital–lowest unoccupied molecular orbital gap in these compounds (absorbance at about 300 nm). No characteristic annihilation ECL signal was observed for these compounds in 1:1 MeCN/Bz mixed solvent. However, the ECL of F6 in the presence of the coreactant C₂O₄^2– showed a long-wavelength ECL emission that was proposed to be electrolyzed byproduct from the radical cation