Synthesis, Layer Assembly, and Fluorescence Dynamics of Poly(Phenylenevinylene) Oligomer Phosphonates
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H.E. KATZ, S.F. SHANE, W.L. WILSON, M.L. SCHILLING, AND S.B. UNGASHE, AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974. ABSTRACT The title oligomers have been incorporated in Zr-based layers on quartz substrates. Absorbance and emission spectra and fluorescence decays have been measured on these films and compared with data from solutions, powders, and PPV. The samples may be divided into those with "liquid-like" behavior and "solid-like" behavior; the latter is characterized by blueshifted absorbance, red-shifted emission, and more complex decay dynamics than the former. By these criteria, the layers and PPV itself are decidedly "solid-like". INTRODUCTION Poly(phenylenevinylene)s, (PPVs) are among the most extensively studied organic electroluminescent materials.1 Processable PPV derivatives and PPV itself have been used to form the active layers in thin film light-emitting diodes with promising efficiencies. By tuning the morphologies, bandgaps, and charge transport properties of the polymers, device stabilities have been increased and multiple wavelength emitters have been fabricated. Modification of the optical and electrical properties 2have been noted in devices containing layers of different types of PPV in particular sequences. Despite the rapidity with which PPV has been developed for electroluminescence applications, many questions remain. In particular, it is not clear that the polymeric character of PPV is essential to its electroluminescence; extensive data have been reported on3 electroluminescent devices consisting of small molecules evaporated onto substrates. Oligomeric PPV fragments should be useful in addressing this issue. They can be synthesized and purified to homogeneity, and are thermally stable. 4 Three-phenyl ring oligomers (pdistyrylbenzenes) fluoresce with high quantum yields, 4 and have been employed as laser dyes6 and scintillators, 5 and as the active materials in vacuum-deposited electroluminescent films and electroluminescent copolymers. 7 Wavelengths of absorbance, emission, and photoconductivity maxima as a function of PPV oligomer length begin converging at the fourring (distyrylstilbene) level.8 Longer oligomers could therefore be assumed to mimic PPV itself, with any photophysical differences attributable to intermolecular association and morphology. Transition metal phosphonate self-assembly 9 provides a convenient way to study these oligomers in the solid state. This protocol involves the alternating chemisorption of transition metal ions and organophosphonates or phosphates on phosphorylated substrates, resulting in dense, transparent, solvolytically and thermally stable films of controlled thickness. We have employed Zr+4 as the metal ion and various rigid chromophores, including azo dyes, 10 361 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
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