Electronic properties of star-shaped oligofluorenes containing an isotruxene core:: Interplay of para and ortho conjugation effects in phenylene-based π systems

The synthesis, photophysics, cyclic voltammetry, spectroelectrochemistry, and electrogenerated chemiluminescence (ECL) properties of a series of unsymmetrical star-shaped oligophenylenes IT1-IT4 are reported. The electronic couplings among the three oligofluorene arms in IT1-IT4 are strong due to the para-ortho branched isotruxene core. The ortho conjugation effect results in band splitting in the absorption spectra for both the neutral and the radical ionic form with a stronger effect for the latter. However, such an ortho conjugation effect becomes weaker as the oligofluorene arms are longer. The same fluorescence maxima displayed by IT2-IT4 suggest that the exciton coherence size (or the bound electron-hole pair) is no larger than 16 phenylene rings. The little chain-length dependence of the first reduction and oxidation potentials for IT1-IT4 suggests that the reversible electron-transfer processes of the neutral species are mainly associated with the isotruxene core. The ECL of IT1-IT4 is from the singlet excited state, but the spectra are red shifted with respect to the fluorescence spectra of dilute solutions due to the reabsorption effect. Our results also reveal that the meta conjugation interactions in the previously reported C-3-symmetrical truxene-oligofluorene analogs T1-T4 (Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.; Perepichka, I. F.; Bradley, D. D. C.; Koeberg, M. J. Am. Chem. Soc. 2004, 126, 13695-13702) are rather weak.