The effect of solvent vapor annealing on the fluorescence properties and morphology of titanyl phthalocyanine/ perylene phenethylimide thin-film molecular semiconductor bilayers (TiOPc/PPEI) is investigated. A combination of atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) is used in conjunction with bulk absorption and fluorescence measurements to correlate the morphological and photophysical properties of these bilayer systems. AFM data show that treatment of the vacuum-deposited amorphous PPEI and TiOPc/PPEI films results in the crystalline transformation of these materials and severely alters the contact between the TiOPc and PPEI layers. AFM data show extended solvent vapor annealing produces void spaces in the TiOPc coverage on the order of several hundred nanometers. Steady-state fluorescence intensity and fluorescence lifetime measurements are used as a measure of charge-transfer quenching efficiencies. Very efficient charge-transfer quenching is observed when amorphous layers of TiOPc are deposited onto PPEI resulting from uniform contact between the layers. Extended annealing results in decreased charge-transfer quenching efficiencies as a result of widely dispersed, localized interfacial contact points.

We present results of cw, time-resolved, and spatially resolved spectroscopic studies of emission and absorption in a model conjugated polymer, poly( p-pyridyl vinylene͒ ͑PPyV͒. The redshifted film spectra suggest the formation of aggregated regions. The ϳ4ϫ reduction in emission efficiency in films vs solution is attributed to a longer radiative lifetime for aggregate excitons, as is evidenced by time-resolved fluorescence measurements. We present direct optical imaging of aggregates in a conjugated polymer via near-field scanning optical microscopy. The aggregate emission and absorption are found to be localized to partially aligned regions of the film ϳ200 nm in size. ͓S0163-1829͑96͒51530-4͔

The formation of flexible molecular fibers via the solution-phase self-assembly of the pseudo-isocyanine dye (PIC) 1,1′-diethyl-2,2′-cyanine, and poly(vinyl sulfate) (PVS) is reported. The physical and electronic properties of these fibers spin-coated into thin films on fused-quartz substrates are studied by fluorescence and topographic imaging with near-field scanning optical microscopy (NSOM) and also by atomic force microscopy (AFM). The scanned-probe images demonstrate that fibers with lengths in the hundred micrometer range, widths of hundreds of nanometers, and thicknesses of a few tens of nanometers are readily formed in aqueous mixtures of PVS and PIC. Unprecedented flexibility in these fibers is exemplified by the formation of numerous curved and looped structures in the spin-coated thin films. A sandwich-like composite structure of alternating anionic PVS and cationic PIC layers is proposed as a model for the assembly of the dye and polymer in these fibers. The alternating layers in this model are held tightly together via the cooperative "cross-linking" of the PVS and PIC layers by electrostatic dye/ polymer interactions, and by hydrophobic van der Waals interactions between the PIC molecules. The intermolecular interactions in the PIC layer result in the formation of a liquid-crystalline-like, well-ordered layer of the PIC which exhibits the spectral characteristics of J-aggregates. The proposed layered structure apparently possesses "reactive" surfaces which link individual fibers into a yarnlike assembly. This cross-linking effect is supported by the presence of continuous circular fibers and by the gel-forming ability of the solutions from which these fibers are grown.

Near-field scanning optical microscopy (NSOM) and scanning force microscopy have been employed to spatially resolve the complex morphologies, spectroscopy, and charge transfer induced fluorescence quenching efficiencies of a (perylene phenethylimide)/(titanyl phthalocyanine) bilayer (PPEI/TiOPc). The PPEI/TiOPc bilayer is a typical example of a n-like/p-like molecular semiconductor heterojunction, which is a common component in photocells, LEDs, and other devices. NSOM-polarized fluorescence and transmission datasand separate bulk X-ray diffraction and spectroscopic measurementsson PPEI/TiOPc bilayers and PPEI and TiOPc single layers has lead to a nanoscopic and mesoscopic picture of how vacuum deposition and subsequent solvent-vapor-annealing controls the local structure of these films. The layers and bilayers are highly organized, containing localized crystalline regions which are preferentially oriented relative to the substrate and PPEI/TiOPc interface. In highly annealed bilayers, only a small fraction of the area of the interface makes good contact between the bilayers, and the contact regions are less than 100 nm 2 in most cases. The consequences of the observed morphology on the charge separation efficiencies at the interface is examined. It is shown that exciton migration both perpendicular and parallel to the molecular interface are involved in the charge separation mechanism. Extended methylene chloride, solvent-vaporannealing of PPEI films produces long needle-like PPEI crystals with a range of sizes, as follows: width (50-200 nm), length (1000-2000 nm), and height (50-200 nm). Annealing of the TiOPc yields nanocrystallites that are preferentially oriented relative to the interface with a height in the range of 10-100 nm and widths in the range of <10 nm to 30 nm.