Absorption and emission in pinwheel aggregates of oligo-phenylene vinylene molecules

The effects of exciton-vibrational coupling and point defects on the absorption and emission of distyrylbenzene nanoaggregates are treated theoretically. Two aggregate types based on a two-dimensional array of cyclic tetramers (pinwheels) are considered: type A aggregates, composed of chiral pinwheels, and type B aggregates, composed of achiral pinwheels. The low-energy vibronic features in the experimental excitation spectrum arise from vibrationally dressed K=(0,0) excitons, while the more intense blue shifted H-band is due to nearly free K=(0,0) excitons. The K=(0,0) features are polarized primarily along the herringbone plane normal. The lowest Davydov component is polarized in the herringbone plane and is due to the lowest energy K=(π,π) exciton. This state is also responsible for the aggregate emission. The 0-v peaks for v>0 are mainly due to indirect transitions to the ground electronic state containing v phonons, with wave vector sum equal to (π,π). These peaks are largely independent of defect fraction and are polarized primarily along the herringbone plane normal. In stark contrast, the 0–0 emission critically depends on the concentration of point defects and is polarized entirely in the herringbone plane. This wavelength dependent emission polarization is in full agreement with experimental observations. Type A aggregates are weakly emissive, with the 0–0 emission peak vanishing in defect-free aggregates and increasing with defect concentration. The reverse holds for type B aggregates: the 0–0 intensity scales with the number of molecules in the aggregate and decreases with defect concentration. Sufficiently large type B aggregates are superradiant, and may be used to enhance the quantum yield in optical devices such as light-emitting diodes.