Frank Spano, Temple University
Title: The HJ-aggregate Behavior of Crystalline 7,8,15,16-tetraazaterrylene (TAT): Introducing A New Design Paradigm for Organic Materials
Abstract: Absorption and photoluminescence properties of the terylene derivative 7,8,15,16-tetraazaterrylene (TAT) in solution and crystal phases have revealed rather unusual spectral characteristics that defy classification in terms of simple H- or J-aggregate coupled systems. TAT readily forms crystalline aggregates either by self-assembly in solution or chemical vapor deposition based on -stacks directed roughly along the crystallographic a-axis. Using a theoretical model based on a Holstein-style Hamiltonian including both Frenkel and CT excitons the crystal absorption and steady-state photoluminescence (PL) line shapes are shown to be determined by a competition between long-range Coulombic coupling which induces H-aggregate behavior, and short-range charge-transfer mediated coupling which induces J-like behavior. Such “HJ” aggregates display J-aggregate signatures in the low-energy region of the absorption spectrum and H-aggregate signatures at higher energies, in excellent agreement with experiment. The H/J competition also results in a low-energy exciton band shape with minima at k = , where k is the (dimensionless) wave vector along the stacking axis. The presence of a band minimum for nonzero values of k bestows H-like PL behavior consistent with the attenuated 0-0 emission peak observed in the experimental spectrum. We present a new design paradigm for organic electronic materials based on the constructive or destructive interference of short- and long-range coupling, postulating the existence of HH, JJ, and JH aggregates with unique transport and radiative properties.