Electroabsorption spectroscopy of luminescent and nonluminescent ∏-conjugated polymers

Journal ArticleWe have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent p-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenylene ethylene), and polythiophene; the nonluminescent polymers include poly(diethynyl silane) and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton (1Bu) and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states (Ag), the strongest of which (mAg) is located at an energy about 1.3 times that of the 1Bu exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent Ag state (kAg) at an energy of about 1.6 times that of the 1Bu. We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im[x3(ω;ω,0,0)], using a summation over states model dominated by the ground state, the 1Bu exciton, two strongly coupled Ag states (mAg and kAg), and their most strongly coupled vibrations, using Frank-Condon overlap integrals

Electromodulated photoinduced absorption; a new spectroscopy in ∏-conjugated polymer/C60-blends

Journal ArticleWe have applied the electromodulated photoinduced absorption (EPA) technique to a variety of C 6 0 -doped and pristine luminescent conducting polymer films. EPA measures the electric field-related change of the absorption in the sample as a result of above-gap illumination. An electric field leads to two major effects. One is a possible Stark shift of photoexcitation (polarons, polaron pairs, or C 6 0 ') energy levels, and the other is a change in the photoexcitation decay kinetics, which leads to reduction or enhancement of PA features. This can be due to an electric field enhanced dissociation of the 1Bu exciton or due to a lifetime reduction of polaron pairs

Optical probes of pristine and C60-doped silicon-bridged PT polymer; a novel highly luminescent, low bandgap polymer

Journal ArticleSilicon bridged PT polymer (PTSi) is a new soluble polymer with a strong luminescence in the infrared band (0-0 transition at 1.6 eV). We have applied various spectroscopies such as absorption, electroabsorption (EA), photo induced absorption (PA), electromodulated PA (EPA), photoluminescence (PL) and photo induced absorption detected magnetic resonance (PADMR) to pristine and C60- doped PTSi thin films in order to eluvidate the photoexcitation and electronic states this material.. The absorption spectrum consists of three bands, one in the visible with a peak at 2 eV and the others in the UV at 4.5 eV and 6.2 eV respectively. The PL band at 1.6 eV contains also three pronounced phonon replica, each 150 meV apart. An electric field reduces the luminescence due to dissociation of excitons into polarons. The PA spectrum is dominated by a broad PA band peaked at 1.1 eV which is due to triplet-triplet transitions, with a shoulder at 1.7 eV due to polaron pairs; there is also a distinctive band at 0.3 eV due to both, polarons and polaron pairs. Upon doping with C60- the 1.1 eV triplet band loses intensity, whereas the 0.3 eV feature dramatically increases and the typical signature of C60- HOMO to LUMO transition appears at 1.15 eV

Photoexcitations in disubstituted acetylene polymers

Journal ArticleWe have used a variety of optical techniques to study the photoexcitations dynamics in several disubstituted acetylene polymer thin films. We found that although these polymers have a degenerate ground state, as determined by iodine doping and C6Q photodoping, they show strong photoluminescence (PL) in the blue/green spectral range with high quantum efficiency (QE) in thin films. The cw photo modulation spectrum contains a single photoinduced absorption (PA) band at 1.7 eV with monomolecular recombination kinetics with a lifetime of 25 ms. PA detected magnetic resonance (PADMR) shows that this PA band is related to spin 1/2 excitations, which therefore identifys it as due to neutral soliton antisoliton pairs (S°, s° ), similar to the 1.35 eV PA band in t-(CH)x . No triplet PADMR signal was observed and we therefore conclude that triplet excitations are unstable in disubstituted acetylene polymers, even though singlet excitons relatively stable. We consider therefore a possible S° photogeneration mechanism via singlet to triplet intersystem crossing, followed by triplet exciton decomposition into S° S° pairs. Also we detected polarons in a degenerate ground state polymer for the first time

Electromodulation of charge-transfer photoexcitations in pristine and C60-doped conjugated polymers

Journal ArticleWe study the effect of an electric field of up to 100 kV/cm on photoexcitations in pristine and C60-doped p-conjugated polymers by electromodulated photoinduced absorption (EPA). In EPA, we measure changes in the photoinduced absorption of a polymer film induced by an electric field. The dominant effect is a change in the photoexcitation recombination kinetics, which leads to reduction or enhancement of the PA bands. We found that the electric field increases the decay rate of charge-transfer excitations; exciton dissociation near defects and impurities was also enhanced. A Stark shift of the photoexcitation energy levels for C60-doped poly(para-phenylene) was also detected. From the EPA we calculate that the polarizability of the charge-transfer excitations in this blend is of order 33107(Å )3. This is two orders of magnitude larger than the polarizability of the 1Bu exciton in most luminescent conjugated polymers

Pesticides reduce regional biodiversity of stream invertebrates

The biodiversity crisis is one of the greatest challenges facing humanity, but our understanding of the drivers remains limited. Thus, after decades of studies and regulation efforts, it remains unknown whether to what degree and at what concentrations modern agricultural pesticides cause regional-scale species losses. We analyzed the effects of pesticides on the regional taxa richness of stream invertebrates in Europe (Germany and France) and Australia (southern Victoria). Pesticides caused statistically significant effects on both the species and family richness in both regions, with losses in taxa up to 42% of the recorded taxonomic pools. Furthermore, the effects in Europe were detected at concentrations that current legislation considers environmentally protective. Thus, the current ecological risk assessment of pesticides falls short of protecting biodiversity, and new approaches linking ecology and ecotoxicology are needed

Relaxation energies and excited state structures of poly(para-phenylene)

We investigate the relaxation energies and excited state geometries of the light emitting polymer, poly(para-phenylene). We solve the Pariser-Parr-Pople-Peierls model using the density matrix renormalization group method. We find that the lattice relaxation of the dipole-active $1^1B_{1u}^-$ state is quite different from that of the $1^3B_{1u}^+$ state and the dipole-inactive $2^1A_g^+$ state. In particular, the $1^1B_{1u}^-$ state is rather weakly coupled to the lattice and has a rather small relaxation energy ca. 0.1 eV. In contrast, the $1^3B_{1u}^+$ and $2^1A_g^+$ states are strongly coupled with relaxation energies of ca. 0.5 and ca. 1.0 eV, respectively. By analogy to linear polyenes, we argue that this difference can be understood by the different kind of solitons present in the $1^1B_{1u}^-$, $1^3B_{1u}^+$ and $2^1A_g^+$ states. The difference in relaxation energies of the $1^1B_{1u}^-$ and $1^3B_{1u}^+$ states accounts for approximately one-third of the exchange gap in light-emitting polymers.Comment: Submitted to Physical Review

Trend analysis using non-stationary time series clustering based on the finite element method

In order to analyze low-frequency variability of climate, it is useful to model the climatic time series with multiple linear trends and locate the times of significant changes. In this paper, we have used non-stationary time series clustering to find change points in the trends. Clustering in a multi-dimensional non-stationary time series is challenging, since the problem is mathematically ill-posed. Clustering based on the finite element method (FEM) is one of the methods that can analyze multidimensional time series. One important attribute of this method is that it is not dependent on any statistical assumption and does not need local stationarity in the time series. In this paper, it is shown how the FEM-clustering method can be used to locate change points in the trend of temperature time series from in situ observations. This method is applied to the temperature time series of North Carolina (NC) and the results represent region-specific climate variability despite higher frequency harmonics in climatic time series. Next, we investigated the relationship between the climatic indices with the clusters/trends detected based on this clustering method. It appears that the natural variability of climate change in NC during 1950–2009 can be explained mostly by AMO and solar activity