Search

The conjugated polymer poly-phenylenevinylene (PPV) and its derivatives have been the subject of numerous nonlinear optical1 and light-emitting diode2 studies. These polymers can photobleach in the presence of oxygen and the photo-oxidation can modify their optical properties.3 For instance, it was recently shown that the nonexponential photoluminescence decay of this polymer is caused by exciton quenching at defects.4 One type of defect has been identified as carbonyl groups formed on the polymer chain as a result of photo-oxidation.3 The present work explores the effect of C60 doping on the photo-oxidative stability and the nonlinear optical (NLO) response of a derivative of PPV called poly-[2-methoxy,5-(2’-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). Photoinduced electron transfer from the excited state of MEH-PPV to C60 has been reported to occur on a subpicosecond time scale.3 By direct excitation into the tail of the π-π* transition of MEH-PPV, one may be able to access this charge transfer state, stabilize MEH-PPV against photo-oxidation, and enhance the NLO properties of the composite material.

The interest in the nonlinear optical (NLO) properties of the fullerenes lies in their unique structure and symmetry which are responsible for the three-dimensional delocalization of π-electrons, a property often associated with enhanced NLO properties in organic materials. At NRL, our studies have focussed on the NLO behavior of pristine films of the fullerenes in the near-infrared1,2 and visible3 spectral region. Recently, the group at Kentucky succeeded in polymerizing C60 by irradiation of oxygen-free, face-centered-cubic films with visible or ultraviolet light.4 It was found that oxygen plays an important role in the photochemistry of C60. We have recently reported on the photo-assisted diffusion of O2 into solid C60 as well as its photo-oxidation.5,6 The present paper addresses the effect of altering the electronic/chemical structure, achieved by O2-doping and photopolymerization, on the NLO properties of C60 and the dynamics of the NLO response.

Optical limiting of 532 nm, 6 nsec duration laser pulses due to thermal defocusing and aberration of intense incident light is reported using absorbing dyes to heat solvents with large thermal figures of merit.

Recent degenerate-four wave mixing (DFWM) studies, performed on C60 [1] and phthalocyanines (Pc) [2] at 1064 nm have shown that these materials possess large optical nonlinearities. The highly delocalized π-electron system within these materials is believed to contribute significantly to the observed nonlinear optical response. In addition, it has been shown in the case of the phthalocyanines that the introduction or modification of low lying electronic states by metal substitution can enhance the optical nonlinearity. There is also evidence that optical pumping to long-lived excited states [2] contributes significantly to the nonlinear response in some of the phthalocyanines. Two photon excitation of these electronic states may also occur. It is the purpose of this paper to investigate the contribution that optical pumping and nonlinear absorption make to the nonlinear response of the material.

Organic light emitting devices (OLEDs) have been fabricated on various flexible substrates using conducting polymers as anodes. In this paper we report on the performance of OLEDs using films of highly conductive Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) anodes without an indium tin oxide under-layer. The conducting polymer anodes are formed using a simple patterning method.

We report on optical limiting of Q-switched Nd:YAG radiation at 532 nm in an f/5 defocusing geometry using liquid solutions of C60 in 1-chloronaphthalene. The nonlinear optical-limiting mechanism is C60 excited-state absorption and enhanced thermal lensing in the solvent. A limiting threshold energy of 65 nJ, corresponding to an incident fluence of ∼0.3 J/cm2 at the focus in the solution, was observed with a device absorption of 25%.

The covalent linking of acceptor molecules to electron donating conjugated polymer is an approach for the development of new photoactive materials for the fabrication of organic photoelectric conversion devices. With this strategy we have designed a polyalkylthiophene copolymer series containing in the side chain anthraquinone molecules as electron acceptor. The peculiar features of the copolymers are the good processability and the ease in tailoring the content of acceptor moieties. Their potential use as photoactive materials is investigated in terms of the photoinduced charge transfer properties, studied by FTIR photoinduced absorption and Light Induced Electron Spin Resonance spectroscopies. The results indicate the photoinduced electron transfer from the polythiophene backbone to the anthraquinone substituents and its tunability by changing the content of acceptor molecules. The photovoltaic response of these polymers is also discussed.