Absorption and emission in pinwheel aggregates of oligo-phenylene vinylenemolecules

The effects of exciton-vibrational coupling and point defects on the absorp tion and emission of distyrylbenzene nanoaggregates are treated theoretical ly. Two aggregate types based on a two-dimensional array of cyclic tetramer s (pinwheels) are considered: type A aggregates, composed of chiral pinwhee ls, and type B aggregates, composed of achiral pinwheels. The low-energy vi bronic features in the experimental excitation spectrum arise from vibratio nally dressed K = (0,0) excitons, while the more intense blue shifted H-ban d is due to nearly free K = (0,0) excitons. The K = (0,0) features are pola rized primarily along the herringbone plane normal. The lowest Davydov comp onent is polarized in the herringbone plane and is due to the lowest energy K = (pi,pi) exciton. This state is also responsible for the aggregate emis sion. The 0-upsilon peaks for v >0 are mainly due to indirect transitions t o the ground electronic state containing v phonons, with wave vector sum eq ual to (pi,pi). These peaks are largely independent of defect fraction and are polarized primarily along the herringbone plane normal. In stark contra st, the 0-0 emission critically depends on the concentration of point defec ts and is polarized entirely in the herringbone plane. This wavelength depe ndent emission polarization is in full agreement with experimental observat ions. Type A aggregates are weakly emissive, with the 0-0 emission peak van ishing 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 concentratio n. Sufficiently large type B aggregates are superradiant, and may be used t o enhance the quantum yield in optical devices such as light-emitting diode s. (C) 2001 American Institute of Physics.