The aggregated and crystalline phases of π-conjugated molecules and polymers continue to receive widespread attention as semiconducting materials for field effect transistors, light emitting diodes and solar cells. In such materials, the exciton band dispersion is a critical factor in determining the photophysical response and energy transport properties. In this talk the prospect of exciton band-shape engineering in organic materials is explored with applications made to p-stacks of perylene diimide chromophores. 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.[2, 3] The crystal and solution phases of two perylene diimide (PDI) derivatives, N-phenyl PDI and tetraphenyl PDI are analyzed in detail. In the crystal phases of both derivatives 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. In N-phenyl PDI π-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 π-stacks, the short-range coupling dominates, resulting in hJ-aggregates, as characterized by dominant 0−0 emission. The prospect of band-shape engineering is discussed, for example, in terms of chemically induced or pressure- induced changes in molecular packing.
 A. Oleson, T. Zhu, I.S. Dunn, D. Bialas, Y. Bai, W.Q. Zhang, M.J. Dai, D.R. Reichman, R. Tempelaar, L.B. Huang, F.C. Spano, Perylene Diimide-Based Hj- and hJ-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials, Journal of Physical Chemistry C 123 (2019) 20567-20578.
 N.J. Hestand, F.C. Spano, Expanded Theory of H- and J- Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer, Chem. Rev. 118 (2018)7069–7163.
 N.J. Hestand, F.C. Spano, Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials, Acc. Chem. Res. 50 (2017) 341-350.