Optical studies of Poly[p-(2,5-didodecylphenylene)ethynylene] in Thin Films
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Optical studies of Poly[p-(2,5-didodecylphenylene)ethynylene] in Thin Films Lynn Rozanski1, David A. Vanden Bout2, and Uwe H.F. Bunz3,4 1 Chemistry and Biochemistry Department, University of Texas at Austin, The University of Texas at Austin, CHEM & BIOCHEM DEPT, 1 University Station A5300, Austin, TX, 78712 2 Chemistry and Biochemistry Department, University of Texas at Austin, Center for Nano- and Molecular Science, Austin, TX, 78712 3 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332 4 Department of Chemistry and Biochemistry, Columbia, 29208
ABSTRACT Unlike many other conjugated polymers popular for use in LEDs, poly[p-(2,5didodecylphenylene)ethynylene] (DPPE), shows less interchain interaction between polymer chains after annealing to an ordered film. Evidence of this decreased interchain interaction can be seen in the emission spectra of pristine and annealed films. Thin films of pristine DPPE show a broad featureless green emission, characteristic of an excimer-like state, whereas annealed films blue-shift and become structured, resembling dilute solution emission. DPPE packs into ordered domains after annealing, shown by birefringence, polarization anisotropy NSOM, and xray crystallography data. The lack of chromophore interaction in the ordered domains can be explained through a brick-wall-type packing motif, where the polymer backbone is insulated by the side chains of offset neighboring polymers. Electroluminescence spectra of pristine and annealed LEDs are nearly identical, both having green emission, unlike their different photoluminescence spectra. This is evidence that light emission in LEDs comes only from excimer sites within the film, which would indicate a decreased performance for the annealed devices compared to pristine devices. Preliminary data of pristine and annealed LEDs supports this theory, with most pristine devices having a slightly higher electroluminescence intensity compared to annealed devices. INTRODUCTION Phenylene ethynylene polymers and copolymers have attracted interest for use in light emitting devices (LEDs) due to their high quantum yield and chemical stability.1 Dialkyl-PPEs such as poly[p-(2,5-didodecylphenylene)ethynylene] (DPPE, shown below in Figure 1) and related copolymers of PPEs exhibit structured blue emission in dilute solution, but this emission red-shifts to a broad featureless green emission in pristine films.2 Early on, there was much debate over whether this red-shifted emission was from aggregates or excimers of the polymer chains.3 Aggregates and excimers have some similar traits, including a broad featureless emission and longer lifetimes4, however, in the case of DPPE films, the absorption spectrum remains the same after annealing, indicating that the shifted emission and longer lifetime are caused by excimers rather than ground-state aggregates.2a Annealed DPPE films are highly birefringent, and in previous work we have shown fluorescence anisotropy NSOM data, indicating a high degree of order wi
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