Jiří Pfleger

Free charge carrier formation in polymers under illumination

In addition to many extrinsic processes, free charge carriers in polymers can be photogenerated intrinsically via an intermediate stage of bound ion-pairs formed in consequence of band-to-band transitions or exciton autoionization. The excitation can be realized within the framework of (i) a chemical bond in the polymer backbone or (ii) a side group skeleton. An electron—hole pair with a short separation distance created by light on the same polymer chain or on the same side group usually recombines geminately very fast and does not contribute to the photocurrent. An inter- or intrachain charge transfer or transfer of the electron from the main chain to a side group is necessary to form a more stable ion-pair. Dissociation of the ion-pairs by Brownian motion subjected to a combination of Coulomb and applied electrical field, into free charge carriers, can be described in terms of the Onsager theory of geminate recombination. The photogeneration yield can be improved by an electron acceptor doping or by an electron accepting groups attached to side chains.

Poly(p-iodophenylacetylene): synthesis, characterization, polymer stability and photoelectrical properties

Abstract New functional acetylene, 1-ethynyl-4-iodobenzene, ( p -iodophenylacetylene) was prepared, characterized (i.r., n.m.r., u.v. and mass spectra) and transformed into high-molecular-weight polymers. Various WOCl 4 -based and MoCl 5 -based catalysts were used in the polymerization, the former leading to the red soluble and the latter to dark red insoluble poly( p -iodophenylacetylene) (PIPA), respectively. Both types of PIPA are non-crystalline and they differ in the configurational structure which, however, could not be assigned with certainty. A too high molecular weight and/or cross-linking is suggested as a reason of insolubility of PIPA prepared on Mo-based catalysts. The soluble PIPA was found to degrade autoxidatively in tetrahydrofuran solution at room temperature obeying the kinetic laws of polymer random degradation. The determined value of the rate constant of degradation, 2.6 × 10 −6 min −1 , is slightly higher than that found for unsubstituted poly(phenylacetylene) (PPA), under the same conditions. PIPA was found to possess a higher photo-conductivity than PPA at low and moderate electric fields. The Onsager model offers an adequate explanation for the measured dependence of the photogeneration efficiency of the applied electric field assuming the Gaussian distribution of the radii of charge-transfer states.

Colloidal systems of silver nanoparticles and high-regioregular cationic polythiophene with ionic-liquid-like pendant groups: Optical properties and SERS.

We report tuning of structure dependent optical properties of colloidal systems of borate-stabilized silver nanoparticles (Ag NPs) and polythiophene-based cationic polyelectrolyte with ionic-liquid like side groups: poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide} (PMHT-Br) towards obtaining local electromagnetic field enhancement effects. Surface-enhanced Raman scattering (SERS) studies showed that the strong electromagnetic field enhancement is related to the formation of aggregates of Ag NPs achieved at the components ratio providing the charge balance between Ag NPs and cationic polythiophene, at which Ag NPs are nearly single-polymer-layer coated, their zeta potential is close to zero and they easily form aggregates in which the mean inter-particle distance enables the occurrence of desired plasmonic effects. Fluorescence quenching is efficient only in the systems with low concentrations of PMHT-Br, in which almost all polymer chains directly interact with the Ag NPs surface.

Polymerization of p-nitrophenylacetylene with metathesis catalysts. Photoelectrical properties of phenylacetylene/p-nitrophenylacetylene copolymer

Preparation of substituted acetylene polymers carrying nitro group is reported for the first time. Homopolymers of p-nitrophenylacetylene (NPA) and its copolymer with phenylacetylene (PA) have been prepared by polymerizations induced by WOCl 4 /3Me 4 Sn as a catalyst in benzene and benzene/dioxane as solvents. WOCl 4 alone does not polymerize, oligomerize and/or cyclotrimerize NPA. The two-component catalysts WOCl 4 /2Ph4Sn and MoOCl 4 /3Me 4 Sn produce low amounts of oligomers only. The homopolymer of NPA is insoluble, whereas the copolymer containing one NPA unit per 4.38 units of PA is soluble in aromatic solvents and THF. The IR and NMR spectra of the homopolymer and the IR spectrum of the copolymer are reported. The photoconductive properties of the copolymer are also reported and compared with those of poly(PA). Introduction of NO 2 groups onto polymer chains was found to increase the quantum efficiency of charge carrier photogeneration but to deteriorate the charge carrier transport properties of the polymer.

Poly(silylene)s: Effect of Polar Acceptor Side Groups on the Charge Carrier Photogeneration and Transport

Abstract The influence of polar electron-acceptor chromophores (both admixed and covalently bonded to the polymer backbone) on photoelectrical properties of poly[methyl(phenyl)silylene] was studied. Whereas the charge carrier mobility decreased for any concentration of the chromophore, the photogeneration efficiency increased for concentrations up to about 4 mole %. For higher concentrations, some decrease was observed. The decrease in mobility was ascribed to the electron-dipole interaction and to the broadening of the density-of-states distribution affecting the hopping transport.

Surface-enhanced resonance Raman spectroscopy of porphyrin and metalloporphyrin species in systems with Ag nanoparticles and their assemblies.

The advantages of systems with Ag nanoparticles and their assemblies for surface-enhanced resonance Raman scattering (SERRS) spectral investigation, detection and determination of porphyrin species are demonstrated. SERRS spectral detection limits of the testing porphyrin species (including porphyrin aggregates) in these systems are shown to be, on average, 10(2)-10(3) lower than detection limits by resonance Raman scattering (RRS). Systems with Ag nanoparticles modified by anionic organosulfur spacers enable us to obtain SERRS spectra of unperturbed cationic porphyrin species. In the case of thiopheneacetate-modified Ag particles prepared by laser ablation, no negative effect of the spacer on the spectral detection limit of the porphyrin was observed. Systems with isolated Ag nanoparticles allow for obtaining SERRS spectra of porphyrin species upon excitation into the Soret electronic absorption band which leads to at least a 10-fold decrease in the detection limit.

Characterization and surface-enhanced Raman spectral probing of silver hydrosols prepared by two-wavelength laser ablation and fragmentation

A four step Ag foil laser ablation-Ag nanoparticle fragmentation procedure in ultrapure water was carried out both under argon and in air. Pulses of a high power Nd/YAG laser were used for laser ablation (1064 nm) and for the three step Ag hydrosol treatment in the absence of Ag foil in the sequence 1064-532-1064 nm. Transmission electron microscopy (TEM) and surface plasmon (SP) extinction spectra provide evidence of Ag nanoparticle fragmentation in the second and third step of the procedure carried out under argon. While polydispersity of Ag hydrosol increases in the second step, both the polydispersity and the mean size of the nanoparticles are reduced in the third step. Qualitative and quantitative surface-enhanced Raman scattering (SERS)/surface-enhanced resonance Raman scattering (SERRS) spectral probing of systems with Ag hydrosols and the selected adsorbates at 514.5 nm excitation shows that Ag hydrosols obtained in the second step of the preparation procedure carried out in air are the most suitable substrates for SERS/SERRS experiments performed at this excitation wavelength.

Roles of octabutoxy substitution and J-aggregation in stabilization of the excited state in nickel phthalocyanine.

Nickel phthalocyanine (NiPc) complexes are known to show a rapid nonradiative deactivation of the photoexcited state through the internal conversion. This could be exploited in practical applications, such as photoprotection and photodynamic therapy. The butoxy substitution of NiPc plays an important role for drug delivery but also greatly influences its photophysics. We prepared novel peripherally substituted 2,3,9,10,16,17,23,24-octabutoxy nickel(II) phthalocyanine and characterized the deactivation pathway of its photoexcited state in solution by femtosecond transient absorption spectroscopy and quantum chemical calculations. We bring experimental evidence for the kinetic model, in which the photoexcitation evolves in two independent branches. In the first branch, assigned to the monomer, it undergoes ultrafast intersystem crossing to a triplet state, which subsequently decays to the ground state through a pathway involving lower-lying triplet states, with a ground-state recovery lifetime of 814 ps. It is about three-times longer than the lifetime published for unsubstituted NiPc. In the second branch, the photoexcitation decayed to a triplet state with an orders of magnitude longer lifetime, with the quantum yield of about 4%. This state showed spectral features of J-aggregates. These findings are important for the applications that rely on singlet oxygen formation or fast nonradiative deactivation of the excited state.

Time-Resolved Transient Optical Absorption Study of Bis(terpyridyl)oligothiophenes and Their Metallo-Supramolecular Polymers with Zn(II) Ion Couplers

α,ω-Bis(terpyridyl)oligothiophenes spontaneously assemble with Zn(II) ions giving conjugated constitutional dynamic polymers (dynamers) of the metallo-supramolecular class, which potentially might be utilized in optoelectronics. Their photophysical properties, which are of great importance in this field of application, are strongly influenced by the dynamic morphology. It was assessed in this study by using ultrafast pump-probe optical absorption spectroscopy. We identified and characterized relaxation processes running in photoexcited molecules of these oligomers and dynamers and show impacts of disturbed coplanarity of adjacent rings (twisting the thiophene-thiophene and thiophene-terpyridyl bonds by attached hexyl side groups) and Zn(II) ion couplers on these processes. Major effects are seen in the time constants of rotational relaxation, intersystem crossing, and de-excitation lifetimes. The photoexcited states formed on different repeating units within the same dynamer chain do not interact with each other even at very high excitation density. The method is presented that allows determining the equilibrium fraction of unbound oligothiophene species in a dynamer solution, from which otherwise hardly accessible values of the average degree of polymerization of constitutionally dynamic chains in solution can be estimated.

Morphology and Kinetics of Aggregation of Silver Nanoparticles Induced with Regioregular Cationic Polythiophene

The aggregation kinetics of negatively charged borate-stabilized silver nanoparticles (NPs) induced by the cationic regioregular polythiophene polyelectrolyte poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide} (PMHT-Br) and the morphology of formed aggregates have been investigated via ultraviolet-visible light (UV-vis) spectroscopy, transmission electron microscopy (TEM), zeta (ζ) potential measurements, dynamic light scattering (DLS), and time-resolved small-angle X-ray scattering (SAXS). Two or three populations of NPs are formed within milliseconds upon mixing the components, which differ in the mean size, extent of polymer coating, and time stability. These characteristics are primarily controlled by the PMHT-Br to Ag-NPs ratio. Population of single NPs of a mean size of ∼5 nm is present in every system and is mostly stable for a long time. At low ratios, the single NPs are most probably almost free of polymer chains and the second population includes slow, but in a limited extent, growing NPs in which single NPs might be interconnected by polymer chains. At the ratios corresponding to the charge balance in the system (ca. zero ζ-potential of NPs), the NPs aggregate, forming a second population that continuously grows in size, and finally undergo sedimentation. At the high ratios, three long-time stable populations of NPs are observed, having mean sizes of ca. 5, 13, and 35 nm; all NPs should be fully coated with PMHT-Br, giving them a positively charged stabilizing shell.