Molecular Aggregation and Luminescence Properties of Bulk Poly(2,5-di-n-octyloxy-1,4-phenylenevinylene)

Structural development in cast films of poly(2,5-di-n-octyloxy-1,4-phenylenevinylene) (DO-PPV) upon heat treatment was studied by means of polarized light microscopy, X-ray diffraction, ultraviolet−visible spectroscopy, and photoluminescence spectroscopy as well as scanning and transmission electron microscopy. Results indicated the dominance of a lamellar phase of biaxially refringent nematic-like texture upon short-term heat treatments at elevated temperatures (T_a) between 160 and 240 °C. A further increase in T_a results in dissipation of the mesomorphic order near the isotropization temperature T_i = ca. 300 °C. This lamellar phase, 1.8 nm in layer spacing at room temperature, typically consists of beads or grains approximately 10−20 nm in diameter that agglomerate into wormlike features in the absence of external stress. Mechanical shear right after high-temperature annealing results in disintegration of worms formed in the tranquil state; the constituent beads nevertheless reagglomerate into new wormlike entities whose long axes lie perpendicular to the shear direction, with the DO-PPV backbones and the lamellae lying parallel to the shear direction. The optical absorption edge at 590 nm and the photoexcited emissions at 600 and 630 nm remained independent of structural development at low to intermediate T_a's; however, structural dissipation near T_i results in a clear shift of absorption edge to 570 nm whereas the emission maximum blue-shifted to 585 nm. In combination of transient absorption and emission spectra during cooling of a dilute DO-PPV/p-xylene solution as well as published results of time-resolved emission studies, we conclude that the emission at 585 nm corresponds to signle chromophores in disordered DO-PPV chains, whereas the emissions at 600 and 630 nm correspond respectively to single chromophores of extended conjugation and excimer-like interchain species within the ordered lamellar domains.