Introduction. Electroluminescence (EL) properties of conjugated polymers have received widespread interest since 1990.1 A vast amount of effort was made on modification of chemical structure for purposes of color tuning or improvement of device performance.2 More recently, morphological effects in the light-emitting behavior of a representative conjugated polymer, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), have attracted some attention. This polymer has been considered as amorphous, with a glass transition (Tg) in the vicinity of 65 °C.3,4 A growing number of evidences have generally suggested the presence of “aggregate emission” (at wavelength λem ) ca. 640 nm) in solution-cast MEH-PPV films, which competes with single-chromophore exciton emission (λem ) ca. 580 nm) and low-efficiency excimer emissions (λem ) ca. 700 nm or higher).4-11 The term “excimer” refers to a neutral excited state shared between multichain segments in the absence of ground-state electron orbital overlap whereas “aggregates” have both the ground state and the excited state delocalized between multichain segments. These “aggregates”, already in existence in solutions above a moderate concentration threshold (on the order of 1% or less), may survive the film forming process (typically spin-coating) and affect significantly the light emission4,7,8 by the generation of interchain species that compete with the single-chain exciton.5-9 The “aggregation” in solution has previously been rationalized in terms of solvency power,7 although its exact physical identity is still a subject of controversy.11 Nevertheless, as emission from interchain species may be strongly enhanced after long-term annealing at an elevated temperature (i.e., several hours at 215 °C) after film formation,6 there arises a distinct possibility for the existence of a thermodynamically favored way of molecular packing in the bulk state. This would also be consistent with recent observations of increased optical heterogeneity in the submicron scale and decreased hole mobility upon heat treatment.10 Reported in this communication are our experimental findings that MEH-PPV is in fact mesomorphic in nature, with the tendency to form supramolecular assemblies with primitively layered structure (i.e., of smectic order) and yet maintaining its nematic-like optical texture upon heat treatment in the temperature range of 200-260 °C. Similarities in ultraviolet-visible light absorption and photoexcited emission behavior of heat-treated MEH-PPV as compared to reported aggregation effects in solution processed films appear to imply that the aggregates formed in solutions and the thermally induced supramolecular assemblies in the bulk state are structurally similar in terms of spatial arrangement of chain segments. Experimental Section. Poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, chemical structure given as inset in Figure 4a) was synthesized via the Gilch method following in general the procedure reported by Wudl et al.12 Polarized light microscopic (PLM) observations were made by use of a Nikon Optiphot-Pol microscope equipped with a temperaturecontrolled stage. A Siemens D5000 diffractometer equipped with a copper target (KR line, with wavelength λ ) 0.154 nm), a graphite collimator, and a standard sample holder was used to obtain 1-D (“powder” ) X-ray diffraction (XRD) profiles at room temperature under a step-scan rate of 0.05° per 2 s in the scattering angle range of 2θ ) 1°-41°. Transmission electron microscopic (TEM) studies were performed using a JEOL 3010 instrument under an acceleration voltage of 150 kV. Optical absorption (UV-vis) and photoluminescence (PL) spectra of the film specimens were obtained by use of a LabGuide USB2000 instrument. Film specimens were drop-cast from toluene solutions on a quartz or glass substrate. Routine drying (ca. 4 h at 80 °C under vacuum) and heat-treatment (5 min at an elevated temperature under a protective stream of nitrogen, followed by fast cooling to room temperature) procedures were typically adopted to follow the thermally induced structural change or its effects on absorption/emission behavior. Oriented films were obtained via manual shearing at ca. 230 °C, followed by fast cooling to room temperature. For TEM studies, specimens were detached from the substrate using aqueous HF solutions and shadowed with platinum. Results and Discussion. Given in Figure 1 is a series of optical micrographs taken during heating and subsequent cooling of MEH-PPV. The optical texture is clearly nematic-like, with reversible thermochromic changes (from orange-red to orange-yellow) upon heating toward or cooling from its isotropization temperature (Ti) around 290 °C. For poly(9,9-dihexyl2,7fluorene) (PdHF) with comparable level of side-chain Figure 1. Polarized light micrographs of solution-cast MEHPPV film at a fixed heating/cooling rate of 20 °C/min in the temperature range of ambient to 300 °C: (a) as-cast film at room temperature; (b) heated to 100 °C, (c) 180 °C, and (d) 260 °C before passing the isotropization temperature of 290 °C. During cooling from 300 °C, the optical texture and the thermochromism were reversibly observed. 4229 Macromolecules 2002, 35, 4229-4232