Tunable Optical Property between Pure Red Luminescence and Dual Emission Depended on the Length of Light-Harvesting Antennae in the Dyads Containing the Cardo Structure of BODIPY and Oligofluorene

The design and synthesis of light-harvesting antenna (LHA) and alternative properties between pure red luminescence and dual emission are demonstrated. The dyads composed of size-defined oligofluorenes including trimer (O3), pentamer (O5), and heptamer (O7) as a light absorber and red-emissive boron dipyrromethene (BODIPY) derivative linked via the cardo carbon were prepared. As we expected, amplified emission via the LHA process in the red region was obtained (φ > 0.78). By increasing the number of fluorene repeats from 3-mer to 7-mer, the light absorption ability increased, leading to large LHA efficiency. Especially, owing to the steric distribution of the conjugation components on the basis of the cardo carbon, electronic interaction was suppressed in the dyad. Then, intrinsic emission properties such as sharp and intense spectra can be preserved. Interestingly, it was observed that O7 showed dual emission from both oligofluorene and BODIPY units. From the mechanistic analyses, it was revealed that energy transfer was suppressed only in O7 (O3, O5: >0.95, O7: <0.70). It was implied that the effective exciton diffusion length on the oligofluorene might be from 5 to 7 fluorene units. In O7, the exciton could be localized in the part of the oligofluorene unit. Therefore, some percentage of the exciton could show emission in the oligofluorene unit before energy transfer.