Session H22: Electrically and Optically Active Polymers

Periodic Polymers for Technology

\textit{Periodic} polymeric materials comprised of solid polymer and air have interesting interactions with electromagnetic and mechanical waves giving rise to complex dispersion relations including zero density of states (band gaps). The key concern for photonic materials is dielectric contrast, whereas for phononic materials, it is density contrast and relative speed of longitudinal and transverse waves in the two media that is important. The creation of a material with a dual band gap, that is a complete band gap for light and a complete band gap for sound would allow strong coupling in localized defect regions. Progress in this area requires the ability to design and model targeted geometries and excellent control of structure fabrication. Top-down, and bottom-up approaches, involving interference lithography and self assembly respectively are demonstrating good success in fabricating the requisite structures and creating desired properties for photonics and phononics.   

First-principles investigation of high energy density in PVDF copolymers

PVDF and its copolymers exhibit excellent electromechanical properties and in the case of PVDF-CTFE also a very high energy density [1]. We have investigated the phase diagram of these systems and can quantitatively explain the observed energy density of PVDF-CTFE as due to a para to ferroelectric phase transition in a disordered, multidomain structure [2]. Our results show that pure PVDF prefers the $\alpha$ phase at zero field. Electric field lowers the free energy of the $\beta$ phase, resulting in a structural phase transition at a sufficiently high field. Copolymer admixture lowers the critical field and eventually leads to an energetic preference for the $\beta$ phase even at zero field. For PVDF-CTFE with CTFE content below 17 \%, the $\alpha$ phase is still preferred and the field-induced phase transformation reversibly stores large amounts of energy. For PVDF-TeFE, the total energy difference between the two phases is much smaller, resulting in substantially smaller energy density. [1] B. Chu et al., Science 313, 334 (2006). [2] V. Ranjan et al., PRL 99, 047801 (2007).   

High and Stable Light Induced Birefringence from Spacer-Free Dye-Polyelectrolyte Liquid Crystal Complexes

Azo materials are promising in photonic applications due to the well-known photoisomerization of azo groups, which, for example, allows efficient inscription of gratings using light induced birefringence (LIB). The incorporation of liquid crystal (LC) character in these materials can be desirable to improve LIB properties, such as in side chain liquid crystal polymers (SCLCPs). However, SCLCPs are costly, and flexible alkyl spacers tend to diminish LIB properties. Here, we present LC azo materials obtained by simple ionic complexation procedures involving commercially available (or easily synthesized) dyes and oppositely charged polyelectrolytes; for example methyl orange (MO) and methylated poly(4-vinyl pyridine) (PVPMe). The latter complex, which possesses neither flexible spacer nor tail, has a single-layer smectic A-like structure until degradation and provides exceptionally high and stable LIB properties. These materials can be inscribed with surface relief gratings. Moreover, we have successfully obtained a photoresponsive electrospun mat from a solution of the MO/PVPMe complex mixed with poly(ethylene oxide).   

Reducing radiation-induced conductivity in polymeric dielectrics by small molecule electron traps

Polymeric dielectrics, when exposed to ionizing radiation, undergo the formation of electron-hole pairs and consequently exhibit radiation-induced conductivity (RIC), severely limiting the insulating capability of polymeric dielectrics used in ionizing environments. RIC can be significantly reduced by the incorporation of small-molecule traps: in the appropriate concentration range, small molecules consisting of aromatic rings and strongly electron-withdrawing groups can reduce RIC by more than 95 {\%} in poly(ethylene terephthalate) films. The dopant structure is critical: the addition of one nitro group, the strongest electron-withdrawing substituent, leads to $>$ 98 {\%} RIC reduction when placed on fluorenone, pyrene, acenaphthene, and anthracene cores. Other substituents, such as cyano or amino, improve RIC reduction over the isolated cores, but not as effectively as the nitro group. The electron-withdrawing capability of each substituent side group can be quantified using the Hammett parameter.   

Dynamics of acoustic phonons in exciton self-trapping in a quasi-one-dimensional system

The localization of electronic excitations via electron-lattice interactions is an important fundamental process in molecular-based electronic materials. In our previous work, we directly time-resolved the electronic and vibrational dynamics of the exciton self-trapping process in the quasi-one-dimensional mixed-valence metal-halide linear chain (MX) complexes [Pt(en)$_{2}$][Pt(en)$_{2}$X$_{2}$], (X = Cl, Br, I) using femtosecond coherent phonon techniques. In this work, we present transient absorption measurements on PtBr(en) at low temperature that reveal a large amplitude, strongly damped oscillatory component at a frequency of 11 cm$^{-1}$ that is consistent with the generation of a coherent acoustic wave associated with the formation of the localized lattice deformation that stabilizes the self-trapped state. Comparison with models for polaron formation provides an estimate of the spatial extent of the local deformation of $\sim $ 5 unit cells. This work is supported by the NSF under grant DMR-0305403.   

THz time domain spectroscopy of low-frequency vibrations in a quasi-one-dimensional system

The mixed-valence halide-bridged transition metal linear chain (MX) complexes are prototypical quasi-one-dimensional systems, with a charge density wave ground state and localized electronic excitations analogous to those of conjugated organic polymers. In this work, we present studies of the low-frequency infrared-active vibrational modes of the MX complex [Pt(en)$_{2}$][Pt(en)$_{2}$I$_{2}$](PF$_{6})_{4}$ (en = ethylenediamine, C$_{2}$H$_{8}$N$_{2})$ in the frequency range 0.3 - 3 THz using terahertz time-domain spectroscopic techniques. Distinct polarization-dependent complex refractive indices are observed in single-crystal samples of this highly anisotropic material. The measurements reveal a strong absorption at a frequency of 2.24 THz (75 cm$^{-1})$ polarized along the chain axis, which we assign to the infrared-active $\nu _{3}$ vibrational mode, involving relative motion of the mixed-valence ions in the charge density wave structure. This work is supported by NSF grant DMR-0706407.   

Optical studies of Pt-rich $\pi $--conjugated Polymers

We have used a variety of steady state and ultrafast spectroscopies for studying the photophysics of platinum-containing conjugated polymers, which have potential applications as the active layer of light-emitting diodes. The heavy metal Pt atom that is incorporated in the polymer chain dramatically increases the spin-orbit coupling, and this influences both the intersystem crossing time, T$_{ISC}$, and the phosphorescence emission intensity. The Pt-polymers were newly synthesized, where the intrachain Pt atom was incorporated into the polymer either in each (Pt-1) or in every three (Pt-3) monomer units. We will discuss an interesting effect for the photoexcited triplets, which dramatically influence the phosphorescence spectral shape vs. temperature. We also observed the existence of circular polarization memory of the phosphorescence emission in Pt-1 polymers, in which the platinum atoms are separated by only one phenyl ring; but not in Pt-3.   

Electronic Structure of Photo-degraded Polypropylene Ultrathin Films

The structural degradation induced by synchrotron radiation in polypropylene ultrathin films has been investigated by ultraviolet photoemission spectroscopy (UPS) and molecular orbital (MO) calculations. The UPS results of pristine and degraded films show very good agreement with the calculated density of states obtained from model MO calculations. The UPS results of the degraded films show a new peak appearance as the highest molecular state in the photoemission spectrum. Model MO calculations and UPS results correlate the new peak to the generation of double bond conjugation. This pi (double) bond generation has also been proved with the near edge X-ray absorption fine structure (NEXAFS) measurements. The pre-edge feature in the C-1S NEXAFS spectrum was augmented after the film exposed to white light emitted from synchrotron radiation.   

Molecular Dynamics Simulation of Highly Rigid Polymers in Dilute Solutions

The dynamics of highly rigid polymers control their degree of conjugation and hence their electro-optical characteristics. Molecular dynamics (MD) simulations have been used to study the conformation of a dilute solution of dinonyl \textit{para}-polyphenyleneethylene (PPE) in toluene, which is a good solvent for the backbone of the polymer. The goal of this study is to identify the factors that affect the conformation of a single chain. PPEs in solutions and at interfaces form a rich variety of optically active structures from micelles to gels, whereas the conformation of a single chain controls their optical response. Experimental studies have shown that the conformation of PPE has been affected by the nature of the solvent, the degree of polymerization as well as the nature of the side chains. The degree of cooperatively of the solvent with the polymer as well as the relative conformation of the aromatic groups with in the backbone, as obtained from MD studies will be discussed, together with effects of varying the molecular weight and the nature of the side chains.   

Ab-initio calculations of quasiparticle and excitonic properties of low band gap, polythiophene-based polymers

Electron donor polythiophene-derived polymers coupled with electron acceptor C60 compounds are the basis for the state-of-the-art organic photovoltaic (OPV) technology. However, with an incident photon to converted electron efficiency of only 5\%, OPV cells are not yet competitive with conventional inorganic semiconductor technology. One of the limitations is the relatively high energy gap of polythiophene which precludes the absorption of infrared photons. In this work, using the GW approximation and solving the Bethe-Salpeter equation, we compare results of the quasiparticle and excitonic properties of thiophene-, vinylene- and cyanovinylene-based copolymers with lower energy band gaps than polythiophene. The energy band alignments of the polymers and the C60 molecule are also discussed.