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Oligofluorenes as Polymeric Model Compounds for Providing Insight into the Triplets of Ketone and Ketylimine Derivatives

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journal contribution
posted on 20.02.2016, 11:12 by Patricia Robert, Andréanne Bolduc, W. G. Skene
A series of oligofluorenes ranging between one and three repeating units were prepared as structurally well-defined representative models of polyfluorenes. The photophysics of the oligofluorene models were investigated both by laser flash photolysis and steady-state fluorescence. The effects of the ketone and ketylimine functional groups in the 9-position on the photophysical properties, notably the triplet quantum yield (ΦTT) by intersystem crossing and the absolute fluorescence quantum yields (Φfl), were investigated. The singlet depletion method was used to determine both the ΦTT and molar absorption coefficients of the observed triplets (εTT). Meanwhile, the absolute Φfl were determined using an integrating sphere. It was found that both the ketone and ketylimine substituents and the degree of oligomerization contributed to quenching the oligofluorene fluorescence. For example, the Φfl was quenched 5-fold with the ketylimine and ketone substituents for the bifluorenyl derivatives compared to their corresponding 9,9-dihexyl bifluorenyl counterparts. Meanwhile, the Φfl quenching increased 14 times with the trifluorenyl ketone and ketylimine derivatives. Measured ΦTT values ranged between 22 and 43% for the difluorenyl derivatives with εTT on the order of 13 000 cm–1 M–1. The ΦTT decreased to <10% concomitant with doubling of the εTT when the degree of oligomerization was increased to 3. A new fluorescence emission at 545 nm formed at low temperatures for the ketone and ketylimine oligofluorene derivatives. The emission intensity was dependent on the temperature, and it disappeared at room temperature.