Synthesis and optical properties of triphenylene-based dendritic donor perylene diimide acceptor systems

A donor-acceptor charge transfer system based on two discotic mesogens has been synthesized. The donor is either a triphenylene (POG0) or a triphenylene-based conjugated dendron (POG1), while the acceptor is a perylene diimide (PDI) core. The donors are covalently linked to the bay positions of the PDI core through an ether linkage. In chloroform, due to the short donor-acceptor distance and the matching frontier orbital levels, photoinduced charge transfer from either the donor excitation or the acceptor excitation are both thermodynamically and kinetically favored, resulting in efficient quenching of both donor and acceptor fluorescence. In a less polar solvent, hexane, while charge transfer is still the dominant mechanism for decay of the excited electronic state of POG1, photoinduced charge transfer is no longer energetically favorable for POG0 when the acceptor PDI core is excited, making the PDI core of POG0 weakly fluorescent in chloroform but strongly so in hexane. In solid film, POG0 is highly aggregated through both PDI-PDI and triphenylene-triphenylene homotopic stacking. POG1, on the other hand, aggregates through triphenylene dendrons with limited PDI-PDI core stacking, presumably due to the steric hindrance caused by bulky triphenylene moieties which block the access to the PDI core. The efficient photoinduced charge transfer, coupled with the homotopic stacking that forms separated electron-transporting PDI-stacked columns and hole transporting triphenylene-stacked columns, suggests that the reported donor-acceptor systems based on dual-discotic mesogens are potentially new efficient photovoltaic materials.