HJ-Aggregate Behavior of Crystalline 7,8,15,16-Tetraazaterrylene: Introducing a New Design Paradigm for Organic Materials

Absorption and photoluminescence properties of terrylene derivative 7,8,15,16-tetraazaterrylene (TAT) in its 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 by either self-assembly in solution or physical vapor deposition, based on pi stacks aligned roughly along the crystallographic alpha axis. Using a Holstein-style Hamiltonian including both Frenkel and charge-transfer (CT) excitons, the crystal absorption and steady-state photoluminescence (PL) spectra/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, which are in excellent agreement with our experiments. The H/J competition also results in a sharp reduction in the exciton bandwidth and the appearance of an exciton band minima at k >> +/-pi/2, where k is the dimensionless wave vector along the stacking axis. The presence of a band minimum for nonzero values of k bestows hybrid HJ behavior in the PL spectrum. We present a new design paradigm for organic electronic materials on the basis of the constructive or destructive interference of short- and long-range coupling, postulating the existence of HH, JJ, JH, and HJ aggregates with unique transport and radiative properties.