Perylene Diimide-Based Hj- and hJ-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials

The exciton band dispersion in pi-stacks of conjugated organic chromophores is a critical factor in determining the photophysical response and transport properties. In such stacks, the exciton band width and, in particular, the curvature at the band center, is determined by an interference between short-range coupling due to wave function overlap and long-range Coulomb coupling arising from transition dipole-dipole interactions. The interference can be completely destructive, yielding a dispersionless flat band resulting in an unusual situation where the aggregate displays monomer-like properties, despite having closely spaced chromophores. Coupled chromophores such as these are called null aggregates and the perfect balance of interactions that leads to them are referred to as null points. Here, we study two perylene diimide (PDI) derivatives where positive long-range coupling induces H-aggregate behavior, whereas counteracting short-range coupling induces J-aggregate behavior. As such, both derivatives display so-called HJ-aggregate properties but are shown here to straddle a null point. In N-phenyl PDI pi-stacks, the stronger Coulomb coupling tilts the scales in favor of overall H-like behavior resulting in Hj-aggregates, characterized by a weak 0-0 vibronic photoluminescence (PL) peak, which increases with temperature. By contrast, in tetraphenyl PDI pi-stacks, the short-range coupling dominates, resulting in hJ-aggregates, as characterized by dominant 0-0 emission. Furthermore, in tetraphenyl PDI, the 0-0/0-1 PL ratio remains approximately twice the monomer value, independent of temperature, indicating strong Peierls-like dimerization. Identifying the null points in PDI derivatives provides reference geometries for band shape engineering through, for example, chemically induced or pressure induced changes in molecular packing.