Ulf Stalmach

Towards highly luminescent phenylene vinylene films

Abstract Fluorescence and electronic absorption spectra, fluorescence decay curves and fluorescence quantum yields of a series of oligo (p-phenylene vinylenes) are investigated in solution, nanoaggregates and vapour-deposited or cast ultrathin films. The film constituting molecules are varied in chain length and modified by electron donating and withdrawing substituents and bulky alkyl spacers. PPP-MO calculations serve to rationalize the resulting spectral changes. In dilute solutions, fluorescence yields of the short oligomers with alkyl or oxyalkyl substituents approach the region of unity. The yields decrease with chain length, reaching a long-chain limit of ΦF=0.4–0.7. Introduction of electron withdrawing -CN or -SO2CF3 groups can reduce the yields to almost zero, due to facilitated excited-state torsions around the vinylene double bonds. In films, the situation changes drastically. Fluorescence yields of the parent compounds become very low because of molecular excition coupling which reduces the radiative rates and increases the nonradiative rates of charge separation or internal conversion. Introduction of bulky or polar substituents reduces excitonic coupling, but keeps the molecular environment rigid enough to suppress nonradiative torsional deactivation, so that finally the substituted oligophenylene vinylenes produce highly luminescent films with a present maximum of ΦF≈0.6 for 1,4-bis(α-cyanostyryl)-2,5-di-n-hexylbenzene (HTCo).

Preparation and Nonlinear Optics of Monodisperse Oligo(1,4-phenyleneethynylene)s

Oligo(1,4-phenyleneethynylene)s 1a−e, with solubilizing propoxy side chains, were prepared by use of Hagihara−Sonogashira coupling reactions. The synthetic strategy was based on a building block system and on the use of trimethylsilyl and triisopropylsilyl protecting groups that could be cleaved selectively. The extension of the conjugation with an increasing number of repeat units provokes a bathochromic shift of the long wavelength absorption and a superlinear increase of the second hyperpolarizability |γ|. The corresponding third harmonic generation (THG) measurements were performed using polystyrene matrices and variable laser wavelengths. We conclude that the conjugation length is much larger than 5 repeat units.

Electroluminescent Behavior of a Homologous Series of Phenylenevinylene Oligomers

Finally, the film emission spectra of both the DEHF/PER and the DEHF/CNSt copolymers are unaffected bythermal annealing at 200˚C for 48 h, suggesting that noemissive excimer is formed in the annealed copolymers.The dramatic effect of annealing on the DEHF homopoly-mer is shown in Figure 3b. Although excimer formation isless pervasive in the branched DEHF derivative relative tothe di-n-hexylfluorene (DHF) homopolymer, a broad andunstructured longer-wavelength excimer emission (500–550 nm) that increases with annealing time and tempera-ture is observed.In summary, the synthesis of thermally stable, easily pro-cessable random copolymers of 9,9-di-2¢-ethylhexylfluor-ene and either perylene or a-cyanostilbene can be accom-plished by nickel-mediated coupling reaction. Here, thebranched DEHF units provide adequate solubility whenunsubstituted perylene and a-cyanostilbene units are incor-porated into the conjugated main chain. We have demon-strated that formation of excimers in thin films of poly-(fluorene) derivatives upon thermal annealing can be sup-pressed by incorporation of lower-bandgap chromophoricsegments, which serve as efficient excitonic energy trapsupon excitation of the poly(fluorene) chromophoric seg-ments. This study provides indirect evidence of the possiblerole of the anthracene subunits in DHF/ANT copolymersin suppressing excimer formation.

Relationship between structure and electroluminescence of oligo(p-phenylenevinylene)s

The preparation of LEDs with poly(p-phenylenevinylene) (PPV) as emitting material is well established, However, due to the presence of a distribution of conjugated chain lengths in the polymer, systematic investigations of the electroluminescence with polymeric materials are difficult, as far as the optical emission is concerned. We are studying the relationship between structural variation of substituted oligo(p-phenylenevinylene)s and their electroluminescent behaviour using a series of distyrylbenzenes with a variety of substituents in order to investigate their influence on the electroluminescence (EL). Furthermore, we synthesized a homologous series of monodisperse oligo(2,5-dipropoxy-1,4-phenylenevinylene)s with up to 11 repeating units. This series covers the spectrum from monomer to polymer. The influence on the EL can be investigated by preparing single layer LEDs using vapor deposition or spincoating of the oligomers in a polystyrene (PS) matrix. The comparison of photoluminescence (PL)- and EL-spectra shows that the photophysical properties of the oligomers are strongly altered by aggregation phenomena

Synthesis and Electronic Spectra of Substitutedp-Distyrylbenzenes for the Use in Light-Emitting Diodes

The influence of substitution on the absorption and Luminescence spectra of oligo(phenylenevinylene)s has been studied using distyrylbenzene (DSB) as a model compound. The degree, character, and pattern of substitution was varied systematically, altering the electronic properties of the DSB, the wavelength of the emitted light could be tuned over a range of 100 nm. The syntheses of 6b—h were performed by twofold Wittig Horner-olefinations of bisphoshonates 1a, b with substituted benzaldehydes 2a—i, 6ivia Heck-reaction of the dibromosulfonylbenzene 3, 6k by Siegrist-reaction of 4 with N-phenylbenzaldimine and the Knoevenagel-reaction of benzyl cyanide with 5 led to 6l.

Light-emitting diodes based on phenylenevinylene oligomers with defined chain lengths

Abstract We present characteristic electroluminescence data on single-layer devices based on three different oligo phenylenevinylene compounds with defined chain lengths. Light-emitting diodes (LEDs) were prepared by subsequent vacuum evaporation of the oligomer film and the A1 top electrode onto an indium—tin oxide (ITO) substrate. The LEDs show light emission in the spectral range of yellow to orange depending on the conjugation length of the compound used. We report photoluminescence and optical absorption data of the thin sublimed films, together with the wavelength dependence of the electroluminescence and with the current—voltage characteristics of the LED devices. The redox behavior and, thus, the pertinent electronic energy levels of the phenylenevinylene oligomers were characterized using cyclic voltammetry. The experimental data can be interpreted within a consistent model description.