Interplay between the keto defect and the interchain interaction on the green emission of fluorene-based polymer

To investigate the interplay between on-chain keto defect and interchain interaction and its consequence on the blue emission of polyfluorene (PF), first- to third-generation dendronized PFs as well as single-fluorenone-unit doped PF (PFN), synthesized by Suzuki polycondensation, were used as model compounds for steady-state and picosecond time-resolved photoluminescence (PL) spectroscopic studies. For PF film, the broad-band green emission did not appear, although severe interchain interaction was observed. For PFN film, the green emission that peaked at approximately 540 nm decayed in a multiphasic manner, suggesting significant heterogeneity in the excitation migration toward the keto center. To further examine the interplay effect, a series of novel dendronized-PF/PFN blend films in a molar ratio of 40:1 fluorene-to-fluorenone unit were studied. With reference to pure PFN film, those of the green emission of the blends showed strong dependence on the order of dendronization, that is, a higher generation resulted in a shorter-lived green emission. These observations are discussed in terms of interchain and/or intersegment interactions between the fluorene segments and the keto defect. It is concluded that the keto unit and the keto-centered, interchain aggregation lead to severe color degradation in a synergistic manner, and that dendronization can effectively minimize the undesirable green emission.