Proceedings Volume 3145

Optical Probes of Conjugated Polymers

Z. Valy Vardeny, Lewis J. Rothberg
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Proceedings Volume 3145

Optical Probes of Conjugated Polymers

Z. Valy Vardeny, Lewis J. Rothberg
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 1 December 1997
Contents: 13 Sessions, 55 Papers, 0 Presentations
Conference: Optical Science, Engineering and Instrumentation '97 1997
Volume Number: 3145

Table of Contents

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Table of Contents

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  • Spectral Narrowing I: Cooperative Emission
  • Spectral Narrowing II: Laser Action
  • Spectroscopy at High Intensity
  • MX Chains
  • Photoexcitations I: Magnetic Resonance
  • Photoexcitations II: Spectroscopy
  • Optical Characterization I
  • Optical Characterization II: Structure
  • Theory
  • Photoexcitations III
  • Oligomers
  • Photoexcitations IV
  • Poster Session
  • Optical Characterization I
  • Poster Session
Spectral Narrowing I: Cooperative Emission
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Laser action in conducting polymers
Sergey V. Frolov, Maxim N. Shkunov, Z. Valy Vardeny, et al.
We discuss both cooperative radiation and stimulated emission and consider their role in spectral narrowing of luminescent conducting polymers. We argue that cooperative radiation is favored in films with poor optical confinement. On the other hand, directional stimulated emission can be observed in dilute solution and thin films with superior optical confinement. Spectral narrowing in this case can be achieved by increasing either the excitation length or excitation intensity. The optical gain and loss coefficients are measured. Narrow line (approximately 1.5 cm-1) laser emission is observed in cylindrical microcavities formed by thin polymer films coated around glass fibers in the red and green spectral ranges. The cavity quality factors of these plastic lasers are mainly determined by selfabsorption and estimated to be about 5000.
Cooperative emission from a disordered system: a classical model
The effect of disorder on the collective emission from a system of classical oscillators is studied theoretically. Three types of disorder are considered: random orientation of dipole moments, finite spread in frequencies of the individual oscillators (diagonal disorder), and dipole- dipole interaction (off-diagonal disorder). We found that for sufficiently high concentration of oscillators the diagonal disorder does not completely destroy the collective character of the emission. We show that the eigenmodes of a cooperative system with disorder comprise a large number of oscillators and, due to cooperative character of the emission, their lifetime is much longer than the lifetime of an individual oscillator. Consequently, the cooperative emission spectrum of the system is not simply broadened by the disorder, but represents a superposition of relatively narrow peaks.
Spectral Narrowing II: Laser Action
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Stimulated emission, lasing, and line narrowing in conjugated polymers
Graham J. Denton, Nir Tessler, Mark A. Stevens, et al.
We present time-resolved and time-integrated measurements of optical gain and its effects in films and microcavities made from poly(p-phenylenevinylene) (PPV). We demonstrate that PPV films yield net gain due to stimulated emission with a lifetime of about 20 ps when excited with 200 fs pulses. The short duration of this gain is shown to result from exciton- exciton annihilation. We present results showing how this gain leads to spectral narrowing of the emission from PPV films and discuss why it is unlikely cooperative phenomena play a role in this phenomenon. We present evidence that it results from waveguiding of amplified spontaneous emission. Newly developed high-finesse microcavities are described, for which it is possible to obtain lasing thresholds below the onset of exciton-exciton annihilation. We present measurements of the far-field emission patterns from these structures and also demonstrate a measurement of the coherence time of their emission.
Highly photoluminescent conjugated polymers: stimulated emission and device applications
Fumitomo Hide, Benjamin J. Schwartz, Maria Angeles Diaz-Garcia, et al.
The recent emergence of semiconducting (pi) -conjugated polymers as solid-state laser materials has been made possible by the availability of numerous high photoluminescence efficiency polymers with emission wavelengths spanning the visible spectrum. As the first important step in exploring the feasibility of electrically pumped polymer laser diodes, we have demonstrated photopumped gain narrowing in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. The dramatic collapse of the emission line width (to as little as 7 nm) occurs at very low pump energy thresholds (approximately 10 (mu) J/cm2 per 10 ns pulse). Gain narrowing is found in over a dozen different polymers representing a variety of molecular structures and is explained in terms of amplified spontaneous emission in planar waveguides which allow the distance traveled by emitted photons to exceed the short intrinsic gain lengths. The short gain lengths arise from the high density of chromophores, the large density of states associated with the (pi) -(pi) * transition in quasi-one-dimensional systems, and the Stokes shift which minimizes self- absorption. Prospects for producing electrically pumped solid-state polymer diode lasers using this class of materials are discussed in the context of the low-threshold gain narrowing in submicron thick films.
Cooperative radiation and stimulated emission at high excitation densities in poly(para-phenylene)-type ladder polymer
Gerald Kranzelbinder, Wilhelm Graupner, Klaus Muellen, et al.
We have studied the radiative recombination processes in poly (para-phenylene)-type ladderpolymer films using ultrafast spectroscopy techniques at excitation densities up to approximately 5 X 1020 photons/cm3 per pulse. In addition to the vibronic manifold of singlet exciton emission the photoluminescence spectrum at such elevated conditions is characterized by an optical transition, which is not present at low pump fluence. This spectral new component is identified as biexcitonic radiative recombination, an assignment that is supported by the hyperlinearity of its total emission as a function of pump intensities and the kinetics revealing a thermal equilibrium state between excitonic and biexcitonic populations. The biexciton binding energy is evaluated to Eb,xx equals 120 meV. Stimulated emission in the spectral blue region is investigated by femtosecond pump and probe experiments.
Spectroscopy at High Intensity
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Exciton and biexciton signatures in femtosecond transient absorption of pi-conjugated oligomers
Victor I. Klimov, Duncan W. McBranch, Nikolay N. Barashkov, et al.
We report femtosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photochemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long- wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, we see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. We attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.
Biexcitons in mixed-stack charge transfer solids, conjugated polymers, and molecular aggregates
Sumitendra Mazumdar, Michael E. Chandross, Y. Shimoi, et al.
Ultrafast spectroscopic studies of organic solids reveal features unexpected within simple noninteracting models for these systems. We consider: (1) organic mixed-stack charge- transfer solids, (2) conjugated polymers, and (3) aggregates of metal-halogen phthalocyanines, and show that in all cases the photophysics is dominated by excitons and bound multiexciton states. Theoretical modeling is simplest for charge-transfer solids, where stable multiexcitons are verified by femtosecond pump-probe spectroscopy as well as two-photon absorption. In conjugated polymers, pump-probe spectroscopy reveals features due to biexcitons as well as low energy charge-transfer exciton. In both charge-transfer solids and conjugated polymers the biexcitons are bound by electron-electron Coulomb interactions. In contrast, the exciton-exciton binding in molecular aggregates can originate from various sources and the magnitude of the biexciton binding energy in these systems is considerably smaller. No evidence for a biexciton in molecular aggregates have found to date. We present the first experimental evidence for a biexciton in the H-aggregate of a metal- halogen-phthalocyanine. Biexcitons, well-established in conventional semiconductors, are therefore characteristic elementary excitations of several different classes of organic solids.
Ultrafast conformational readjustment in hexamethylsexithiophene oligomer studied by femtosecond time-resolved photoluminescence
Kam Sing Wong, H. Wang, Guglielmo Lanzani
The ultrafast excited state dynamics of Hexamethylsexithiophene are investigated by femtosecond time-resolved photoluminescence. The spontaneous recombination lifetime of the singlet excited state is measured to be in the order of 400 ps. A photoluminescence (PL) risetime of approximately 1 ps and temporal spectra redshift is observed. These effects are attributed to planarization of the oligomer backbone in the excited state via vibrational and torsional relaxation which increases the effective conjugation length of the oligomer. The rapid vibrational and torsional relaxation gave rise to the PL risetime. The change in oligomer bandgap energy due to a longer effective conjugation length which is manifested in the transient spectral redshift.
Raman scattering gain in DOO-PPV films
Maxim N. Shkunov, Werner Gellermann, Akihiko Fujii, et al.
We have investigated the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV) thin films under high intensity ps pulsed laser excitation (Ipump > 1 MW/cm2). We observed that the emission spectrum consist of a spectrally narrowed emission (SNE) band (FWHM approximately 10 nm) and superimposed sharp spectral lines. High resolution spectral measurements conducted at room and liquid nitrogen temperatures show that the spectral width of these lines is less than 1 nm. The lines peak positions are independent of film thickness, however, they shift upon changing the excitation wavelength. Also, the excitation intensity dependence of these narrow lines is found to be different from that of the SNE band. We argue that the narrow lines are due to Raman scattering gain of the 1st, 2nd and 3d order Raman active modes of DOO-PPV.
MX Chains
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Quantum spin fluctuations in quasi-one-dimensional chlorine-bridged platinum complexes
Xing Wei, Robert J. Donohoe, Wen Zheng Wang, et al.
We report experimental and theoretical studies of spin dynamic process in the quasi-one-dimensional chlorine- bridged platinum complex, [PtII(en)2][PtIV(en)2Cl2](ClO4)4, where en equals ethylenediamine, C2N2H8. The process manifests itself in collapsing of the hyperfine and superhyperfine structures in the electron spin resonance spectrum and non-statistical distribution of spectral weight of the Pt isotopes. More surprisingly, it is activated only at temperatures below 6 K. We interpret the phenomenon in terms of quantum tunneling of the electronic spin in a strong electron-electron and electron-phonon coupling regime. This is modeled using a non-adiabatic many-body approach, in which polarons and solitons represent local spin-Peierls regions in a strongly disproportional charge- density-wave background and display intriguing spin-charge separation in the form of pinned charge and tunneling spin fluctuations.
Competition between electron correlation of NiIII states and electron-phonon interaction of PdII-PdIV mixed-valence states in quasi-one-dimensional halogen-bridged mixed-metal complexes, Ni1-xPdX(ch
Toshio Manabe, Masahiro Yamashita, Takuya Kawashima, et al.
A series of single crystals of quasi-one-dimensional halogen-bridged Ni-Pd mixed-metal compounds, Ni1-xPdx(chxn)2Br3 (chxn equals 1R, 2R- cyclohexanediamine) have been obtained by electrochemical oxidation methods. In order to investigate the competition between the electron-correlation in the NiIII states (SDW states) and the electron-phonon interaction in the PdII-PdIV mixed-valence states (CDW states) in the Ni-Pd mixed-metal compounds, the single-crystal electrical conductivities, IR, resonance Raman spectra, ESR, magnetic susceptibilities, and XPS, have been measured. The electrical conductivities of these compounds show semiconducting behavior with the conductivities maximized and the activation energies minimized at x equals ca. 0.5. The IR, resonance Raman spectra, and XPS show that the PdII- PdIV mixed-valence states are influenced and approaching to the PdIII states with the increase of the NiIII components.
Photoexcitations I: Magnetic Resonance
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Long-lived polarons and triplet excitons in ladder-type poly(p-phenylenes) and poly(p-phenylene ethynylenes)
Jonathan Partee, Joseph Shinar, Wilhelm Graupner, et al.
Recent x-band optically detected magnetic resonance studies of spin 1/2 polaron and spin 1 triplet exciton dynamics in ladder-type poly(p-phenylene) and poly(p-phenylene ethynylene) (PPE) films and solutions are described. The results suggest that the polaron resonance is due to interchain polaron pairs with lifetimes 10 microsecond(s) <EQ (tau) <EQ 4 ms range. The absence of delayed fluorescence at such times as well as other observations lead to the conclusion that the mechanism responsible for this resonance is the nonradiative quenching of singlet excitons by the polarons, whose population decreases at the field-for- resonance. The full-field triplet pattern is 700 - 1200 G wide, consistent with a triplet localized on a phenylene ring. In frozen toluene solutions of PPE, the amplitudes (Delta) I/I of both the polaron and half-field triplet resonances decrease similarly with concentration, and increase similarly with laser power P as P(alpha ), where 1/3 <EQ (alpha) <EQ 1/2. Hence, the triplet resonance is believed to be due to triplet intrinsically trapped and stabilized at similar interchain sites. Possible scenarios which link the processes yielding the resonances are also discussed.
Spin and spectral signatures of polaron pairs in pi-conjugated polymers
Paul A. Lane, Xing Wei, Z. Valy Vardeny
We have studied polaron pair photoexcitations in a variety of (pi) -conjugated polymer films by photoinduced absorption (PA) and optically-detected magnetic resonance (ODMR). An exchange interaction between polarons broadens the ODMR spectrum, permitting the unambiguous identification of the polaron pair species. Both isotropic and anisotropic exchange interactions have been observed. An isotropic exchange interaction broadens the spin-1/2 resonance associated with polarons, whereas an anisotropic exchange interaction gives rise to both full-field and half-field powder patterns. We have successfully modeled the observed spin-1/2 ODMR spectra and estimate the strength of the isotropic exchange interaction J. Both polarons and polaron pairs are characterized by two absorption bands. The high energy PA band of polaron pairs is blueshifted with respect to that of isolated polarons.
Time-resolved microwave measurements of the polarizability of photoexcitons on conjugated polymer chains
John M. Warman, Gerwin H. Gelinck, Jacob J. Piet, et al.
Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.
Photoexcitations II: Spectroscopy
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Franz-Keldysh effect of band states in polydiacetylene
Gerhard Weiser, A. Horvath, H. J. Kolbe
Electroabsorption spectra of polydioacetylene single crystals with well ordered chains show 0.5 eV above the excitonic absorption edge a strong signal from states which contribute little to absorption. While the strongly absorbing excitons respond to an external field by a quadratic Stark effect this particular signal is attributed to the Franz-Keldysh effect, the response of a free carrier band. The extended band states are very sensitive to an external field but also to disorder which destroys the coherence of free motion. Experimental spectra of the Franz- Keldysh effect of well ordered chains will be compared with those of semiconductors and with model calculations for a 1- and a 3-D electron gas. These calculations reveal that 1-D bands respond much more sensitive to electric fields than 3D band states. The comparison of the spectral lineshapes points to a forbidden parity state very close to the band edge which enhances some spectral features. The Franz- Keldysh effect deteriorates rapidly with disorder which is interpreted as due to localization of bandstates by disorder. The Stark effect of such near degenerate states localized by boundary conditions can lead to similar spectra as the Franz-Keldysh effect.
Photoluminescence in poly(3-hexylthiophene)
Ifor D. W. Samuel, Laura Magnani, Garry Rumbles, et al.
We report a study of the photophysics of solutions and films of the conjugated polymer poly(3-hexylthiophene) (P3HT). We have performed measurements of absorption, photoluminescence (PL), PL quantum yield, and time-resolved PL. For P3HT in a good solvent the PL decays with a time constant of 510 +/- 20 ps, and the PL quantum yield is 0.42 +/- 0.04, we assign the PL to the intra-chain singlet exciton, and deduce a natural radiative lifetime for this species of 1.2 +/- 0.1 ns. We explore the use of solvent mixture as a way of controlling the degree of intermolecular interaction. We find that there is a red-shift of absorption and PL in poor solvent mixtures, and the spectra and PL quantum yield become similar to the film. The PL decay becomes faster and non-exponential in poor solvent mixtures and the thin film.
Optical properties of disubstituted acetylene polymers
Kazuya Tada, Rahmat Hidayat, Masaharu Hirohata, et al.
Optical properties and electronic energy structures of di- substituted acetylene polymers have been studied. Strong blue and green photoluminescence and electroluminescence have been observed in the poly(diphenylacetylene) derivatives and poly(1-alkyl-2-phenylacetylene) derivatives, contrary to trans-polyacetylene, in which only very weak photoluminescence are observed at infrared region. The dependence of optical properties of the di-substituted polyacetylene derivatives such as photoluminescence intensity and electroluminescence efficiency on the molecular structure of substituents and the change of optical properties upon electrochemical doping and their interpretation by soliton model have also bee discussed. Spectral narrowing in photoluminescence of some di- substituted polyacetylenes have also been studied.
Photoexcitations in disubstituted acetylene polymers
Martin Liess, Ilarie I. Gontia, Toshio Masuda, et al.
We have used a variety of optical techniques to study the photoexcitation 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 C60 photodoping, they show strong photoluminescence in the blue/green spectral range with high quantum efficiency 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 identifies it as due to neutral soliton antisoliton pairs (So, So), 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 So photogeneration mechanism via singlet to triplet intersystem crossing, followed by triplet exciton decomposition into So So pairs. Also we detected polarons in a degenerate ground state polymer for the first time.
Optical Characterization I
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Optical properties of poly[(disilanylene) oligophenylenes]
Katsumi Yoshino, Masaharu Hirohata, Kazuya Tada, et al.
Optical properties of poly[(disilanylene)oligophenylenes] such as poly[(disilanylene) terphenylene] (PDSiTP) and poly[(disilanylene) quarterphenylene] (PDSiQP) have been studied and compared with those of poly(methylpropylsilane), p-terphenyl and p-quarterphenyl. Band-gap energies of PDSiTP and PDSiQP were 3.5 eV and 3.3 eV, respectively, which are similar to those of p-terphenyl and p-quarterphenyl. Among various poly[(disilanylene)oligophenylenes], influence of short alkyl substituents such as methyl and ethyl substituents on their properties are relatively small. However, slightly stronger photoluminescence (PL) is observed in ethyl substituted PDSiTP than in methyl substituted PDSiTP. Blue electroluminescence has been observed in diode of MgIn/PDSiTP/ITO and MgIn/PDSiQP/ITO structures. Spectral narrowing of PL of blue in color due to stimulated emission has been observed both PDSiTP and PDSiQP. Poly[(disilanylene)oligophenylenes] are much more stable than poly(methylpropylsilane) under ultraviolet light irradiation.
Spectroscopic optimization of all-solid-state electrochromic devices using PANI
Anne Hugot-Le Goff, Marie-Claude Bernard, Vu Thi Bich, et al.
The interesting optical properties of polyaniline (PANI) allowed its utilization in all solid-state electrochromic devices. Using a sulfonic acid polymer as solid electrolyte gave to PANI an unusual optical behavior leading to electrochromic properties very superior to the properties that it has in any liquid inorganic electrolyte. The improved conductivity of PANI doped with AMP-sulfonate is displayed by the presence of a free-carriers tail even at pH as high as 4.5. The free-carriers tail is studied using UV/vis/near IR spectroscopy, and the kinetics of coloration/bleaching are studied using Optical Multichannel Analysis in the 1.5 - 3 eV range. The modifications of the PANI optical features by solid-state doping are examined. The possibility to still improve the performances of these devices--in particular their rate of color change--by using `secondarily doped' PANI is investigated, which requires a preliminary spectrochemical analysis of PANI films doped with camphorsulfonic acid and treated in m-cresol during their electrochemical polarization.
Two-photon absorption spectra of luminescent conducting polymers measured over wide spectral range
Ron K. Meyer, Martin Liess, Robert E. Benner, et al.
We report the two-photon absorption (TPA) spectra of poly(2,5-dibutoxy-p-phenylene acetylene) (PPA-DBO), poly(2,5-dioctyloxy-p-phenylene vinylene) (PPV-DOO), and poly(3-hexylthiophene) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution, we measured a strong two-photon allowed transition in all three materials which we attribute to the mAg essential state. In the case of PPA-DBO and PPV-DOO, TPA peaks were coincident with dispersion in the nonlinear refractive indices as detected by reduced aperture Z scan. In all three polymers this peak occurs at approximately 1.3 the bandgap energy. The excitonic nature of the excited electronic states in PPA-DBO is indicated by the lack of a TPA band at or near the 1Bu exciton position. Saturation was observed in the nonlinear index of refraction near spectral peaks, as well as an apparent reverse Kramers- Kronig effect.
Dynamics of triplet excitons in MEH-PPV measured by two-photon photoemission
We have used two-photon photoemission to probe the dynamics of the long-lived triplet excitons in pristine and photo- oxidized films of poly[2-methoxy,5-(2'-ethylhexoxy)-1,4- phenylenevinylene] (MEH-PPV). We use a rate equation model to explain the intensity dependence of the total photoelectron yield in terms of the relaxation dynamics of the triplet excitons. Observations of the pristine films indicate that triplet-triplet annihilation is an important deexcitation mechanism for photoexcited triplet excitons in MEH-PPV. Upon photo-oxidation, the measured triplet exciton energy is shifted downward and the triplet exciton-derived photoemission linewidth is dramatically narrowed. Both of these effects are attributed to modification of the physical and electronic structure of the films arising from the formation of carbonyl defects on the polymer chains. We also observe significant oxygen-induced lifetime quenching of the triplet exciton.
Optical Characterization II: Structure
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Tuning of fluorescence in films and nanoparticles of oligo-phenylenevinylenes
Dieter Oelkrug, Alfred Tompert, Johannes Gierschner, et al.
Oligo-phenylenevinylenes (OPV) with a series of distance controlling, electron donating, and/or electron withdrawing substituents are deposited from vapor phase and solution as ultrathin films or nanoparticles with diameters of 20 - 200 nm. In some cases the systems are doped at levels of 10-5 - 10-3 with energy accepting OPV's of longer chainlengths. Absorption and fluorescence spectra, steady-state and time-resolved anisotropies, radiative and nonradiative deactivation rates of these systems are investigated and compared to the corresponding properties in dilute solutions. Fluorescence yields of the parent oligomers and their alkyl or oxyalkyl derivatives are high in solution with an `infinite' chain limit of (Phi) F approximately equals 0.5 and an upper radiative rate constant limit of kr(infinity ) equals (1 +/- 0.3) (DOT) 109 s-1. Yields and kr decrease strongly in films and nanoparticles because of H-aggregate formation. However, doping with fluorescent acceptors can increase the yields up to (Phi) F yields 0.7. Introduction of electron withdrawing -CN and -SO2CF3 substituents reduces (Phi) F in low viscous dilute solutions almost to zero. High viscosities and condensation to solid phases will rise the yields up to (Phi) F yields 0.6 because of suppression of nonradiative torsional deactivation and formation of J- aggregates with high kr.
Influence of aggregation on the optical properties of a polyfluorene
Donal D. C. Bradley, Martin Grell, Xiao Long, et al.
We report on the photophysical properties of a dioctyl substituted polyfluorene, with a focus on the effects of intermolecular interactions on absorption and emission spectra. We discuss these results in context of the strong current interest in the nature of aggregation phenomena and their role in controlling the emissive properties of conjugated polymers.
Copolymers containing meta-pyridylvinylene and para-arylenevinylene fragments: synthesis, quaternization reaction, and photophysical properties
Nikolay N. Barashkov, Hernando J. Olivos, John P. Ferraris
We report the Wittig reaction of the diphosphonium salt of 2,6-bis(chloromethyl)pyridine with terephthaldehyde, 2.5- dioctyloxyterephthaldehyde, or 9,10-anthracenedicarbaldehyde to form conjugated copolymers with fragments of 2,6 - pyridylene and para-arylenevinylene. The protonation reaction of poly(para-arylenevinylene)-co-(meta- pyridylvinylene) with hydrochloric acid and the quaternization reaction of poly(2,5-dioctyloxy-1,4- phenylenevinylene)-co-(2,6-pyridylvinylene) with methyl triflate have been investigated by spectrophotometric and fluorescent methods. The absorption, excitation and fluorescence spectra of these copolymers as well as their corresponding model compounds were studied and compared. The photophysical properties of the investigated polymers suggest that these materials could be good candidates for the fabrication of efficient blue and green light-emitting diodes.
Theory
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Dynamic dielectric screening and exciton binding energies in conjugated polymers
Jason Weibel, David Yaron
Semi-empirical quantum chemistry predicts exciton binding energies for a single chain of a conjugated polymer that are significantly larger than those measured in the solid state. This paper continues our development of an electronic- polaron model that explicitly includes Coulomb screening from adjacent chains. The original model used Huckel theory to describe the polarization induced in the surrounding solvent chains. Here, the use of Pariser-Parr-Pople theory instead of Huckel theory is shown to have little effect on the solvation energies obtained from the electronic-polaron model. The results confirm that the dynamic dielectric response plays a crucial role in establishing the exciton binding energy. The free electron-hole pair states are well solvated because the charge fluctuations arising from electron-hole motion are slow and the polarization of the surrounding chains can nearly follow the electron-hole motion. HOwever, the exciton state is weakly solvated because the charge-fluctuations are fast, and the surrounding chains cannot follow the electron-hole motion. The electron and hole must then shed their solvation cloud when they join to form an exciton. This consequence of dynamic dielectric screening enhances the differential solvation of the free electron-hole pair versus the exciton, and leads to a large reduction in the exciton binding energy.
Symmetrized DMRG studies of low-energy electronic states of poly-paraphenylene (PPP) and poly-para-phenylenevinylene (PPV) systems
S. Ramasesha, Kunj S. Tandon, Y. Anusooya, et al.
Symmetrized DMRG calculations on long oligomers of poly- para-phenylene (PPP) and poly-para-phenylene vinylene (PPV) systems within a `U-V' model have been carried out to obtain the one-photon, two-photon and singlet-triplet gaps in these systems. The extrapolated gaps (in eV) are 2.89, 3.76 and 2.72 in PPP and 3.01, 3.61 and 2.23 in PPV for the one- photon, two-photon and spin gaps respectively. By studying doped systems, we have obtained the exciton binding energies. The larger exciton binding energies, compared to strongly dimerized linear chains emphasizes the role of topology in these polymers. Bond orders, charge and spin correlations in the low-lying states bring out the similarities between the lowest one-photon, the lowest triplet and the lowest bipolaronic states in these systems. The two-photon state bond orders show evidence for strong localization of this excitation in both PPP and PPV systems.
Quantum confinement effects on the ordering of the lowest-lying excited states in conjugated polymers
Zhigang Shuai, Jean-Luc Bredas, Swapan K. Pati, et al.
The symmetrized density matrix renormalization group approach is applied within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond alternation (delta) ) to study the ordering of the lowest one-photon (11Bu-) and two-photon (21Ag+) states as well as the lowest-lying triplet states in 1D conjugated systems with chain lengths, N, up to N equals 80 sites. Three different types of crossovers are studied, as a function of U/t, (delta) , and N. The `U-crossover' and `(delta) -crossover' are examined for long chains, which provide a sharp contrast to the situation found previously for short chains. The `N-crossover', which only occurs for realistic intermediate correlation strength, illustrates the more localized nature of the 2Ag excitation relative to the 1Bu excitation. We also apply the quantum-chemical Pariser-Parr-Pople model with long-range Ohno potential to the polyene molecules. We find that the 2Ag state is always below the 1Bu state for chain lengths N equals 4 to 40. Most interestingly, the gap between 2Ag and 1Bu first increases for N up to 16, then levels off, and finally starts to decrease. Thus, extrapolations to long chain based on short polyene data are hazardous.
Variational studies on charge transfer in a molecularly doped polymer
Zhi Gang Yu, Avadh B. Saxena, Alan R. Bishop
The photoinduced charge transfer between a conjugated polymer chain and an adjacent dopant molecule is studied by constructing a variational wavefunction for the whole system including the chain and dopant. The dopant is modeled by assuming it has an acceptor level for the electron and interacts with the polymer chain only via a nearest neighbor hopping. The initial exciton state in the polymer is calculated from a recently developed exciton theory for conjugated polymers. The variational wavefunction contains two terms: the first term describes the exciton as confined to the chain while the other describes the electron, with different electron momenta corresponding to different probability of transfer to the dopant. Minimizing the energy of the variational state we obtain an equation from which the energy of the variation state and associated probability of charge transfer can be calculated. It is shown that a crossover occurs when the acceptor level approaches the exciton energy. When the energy of the acceptor level is higher than a critical value, the electron mainly remains on the chain, otherwise the electron will transfer to the dopant molecule with high probability. The hopping strength and the dopant concentration influence charge transfer by controlling the width of the crossover energy regime. Our results also indicate that the symmetry of the exciton wavefunction is important to charge transfer. For the Ag exciton, there exists a threshold for the acceptor level, below this value the electron will transfer to the dopant completely. In contrast, for the Bu exciton there is a long tail below the critical value, implying that the electron can still be found on the polymer chain with some likelihood.
Photoexcitations III
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Low-energy photophysics of phenylene-based strapped copolymers
Arthur J. Epstein, N. V. Piskun, Ilia V. Lebedenko, et al.
We present the absorption and emission properties of the phenylene-based copolymers, poly(pyridyl vinylene-phenylene vinylene) (PPyVPV), poly(thienylene p-phenylene) (PTpP) and poly(dithienylene p-phenylene) (PDTpP), which incorporate `straps' to bridge the 2 and 5 positions of the phenyl rings. The absorption and luminescence properties of the PPyVPV polymers are morphology dependent. The absorption maxima are at approximately 3.0 eV, with films having absorption edges that tail into the IR. The photoluminescence (PL) spectra are red shifted for films compared to solutions and powders, with the presence of smaller red shifted photoluminescence for the PPyVPV polymers with straps. A low lying absorption maximum occurs at 2.7 eV for both solution and film forms of PTpP. The (PL) spectra also are similar with features at 2.1 eV and 2.3 eV assigned to the 0-1 and 0-0 vibronic transitions, respectively. Solution and film photoluminescence excitation (PLE) profiles qualitatively follow the absorption spectra. Two low energy absorption maxima are seen for PDTpP at 2.8 eV and 3.2 eV with a single feature observed at 2.3 eV for the PL. The PLE spectra in PDTpP also follow the corresponding absorption features in both solution and film samples. Time resolved PL studies (ps to ns) show that there is smaller spectral shift for the films of polymers with straps, showing the importance of reduced aggregation. Steady state photoinduced absorption studies (ms) indicate the roles of both polarons and triplet excitons in these systems.
Phonon spectroscopy in pi-conjugated polymers: the role of the excited electronic states
Eitan A. Ehrenfreund, Z. Valy Vardeny
The Raman active vibrational modes in (pi) -conjugated polymers are known to have different frequencies in different types of experiments. The best known examples are the resonant Raman scattering (RRS) and doping or photoinduced absorption infrared active vibrations (IRAV) measurements, in which the same modes appear at different energies. Other examples are the phonon side bands, accompanying the polarons, bipolarons, and solitons absorption bands, in which the same modes give rise to different side band energies. In this work we discuss RRS and IRAV measurements and correlate their experimentally observed frequencies with the relevant electronic state that is active in this type of experiment. We show that the IRAV frequencies are lower than those of RRS since the relevant energy levels of charged solitons and/or polarons are closer to the ground state than the optically allowed 1Bu state of the neutral polymer. Thus a new insight into the meaning of the `pinning' potential, which is associated with the lowest frequency IRAV mode is obtained.
Photocurrent imaging of poly(p-phenylenevinylene) thin film light-emitting devices with a laser scanning microscope
Fu-Jen Kao, L. J. Chang, Sheng-Lung Huang, et al.
We have observed the poly(p-phenylene vinylene) (PPV) thin film based light emitting devices with a multifunction laser scanning microscope. The spatial distribution of PPV thin film's reflectivity, photoluminescence, and photocurrent are imaged. In addition, site dependent optical density and photocurrent spectra are also measured, which reveal the nature of optical excitations within PPV. The contrast between photoluminescence and photocurrent images is attributed to field induced luminescence quenching under exciton model.
Oligomers
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Stimulated emission in sexithiophene single crystals
Francis Garnier, Gilles Horowitz, Pierre Valat, et al.
Temperature dependent steady state and transient photoluminescence (PL) measurements were performed on sexithiophene single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The lowest energy set is attributed to transitions from the lower Au excited state, resulting from the Davydov splitting of the 1Bu excited state of the isolated molecules. The highest energy set of peaks is ascribed to an excited Ag level. At low excitation levels, the transient PL follows a single exponential decay, with a time constant of 1.8 ns. When the excitation energy is increased beyond to 35 (mu) J/cm2, a second much faster transient is superimposed, and the PL spectrum narrows to a single line, centered at the 0-1 transition of the low energy set of the steady PL spectrum. The width of the emission line is 13 cm-1. These observations are explained in terms of a stimulated emission process.
Photoexcitations IV
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Photoexcitations of polycarbazolyldiacetylenes in different time domains
Davide Comoretto, I. Moggio, Carla Cuniberti, et al.
We report on the photoinduced absorption spectra of polycarbazolydiacetylenes polyDCHD and its soluble analogous polyDCHD-S in a broad time domain ranging from femtosecond to millisecond. Both the blue and red forms of polyDCHD-S are investigated and the results are compared with those of blue polyDCHD. In the blue form of the two polymers charged states are detected in the ms domain in addition to triplet excitons. In the red form of polyDCHD-S only triplets are detected. Femtosecond pump and probe measurements allow us to interpret the generation mechanism of the triplet states as due to singlet excitons fission.
Femtosecond study of exciton dynamics in polyfluorene statistical copolymers in solutions and thin films
Jin Z. Zhang, Melissa A. Kreger, Gerrit Klaerner, et al.
The formation and decay dynamics of photogenerated excitons in polyfluorene statistical co-polymers in solutions and in thin films have been studied using femtosecond transient absorption spectroscopy. In solution photoexcitation of the polymer generates primarily intrachain singlet excitons which are initially hot and then relax quickly (< 200 fs) towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 30 ps and 330 ps in toluene. The fast decay is attributable to vibrational relaxation, spectral diffusion, or internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and non-radiative pathways. In films the exciton dynamics are found to depend strongly on excitation intensity. At low intensity, the dynamics are similar to that in solutions, with a double exponential decay with time constants of 15 ps and 300 ps. At high intensities, a fast decay component with a time constant of 0.8 ps appears, which becomes more dominant at higher intensities. This fast decay is attributed to exciton- exciton annihilation due to high density of excitons created. The signal in films at both low and high excitation intensities is attributable to intrachain singlet excitons, as in solution. There is no evidence for formation of interchain bound polaron pairs in films at low intensities. At high intensities, the possibility cannot be ruled out completely, especially in relation to the fast decay. If bound polaron pairs are formed as indicated by the fast decay, they must be generated as a result of interaction between excitons on different chains since they are absent at low power, an they must be created and then decay within about 1 ps.
Poster Session
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Raman resonances in sulfonate or Cl-doped PANI films
Marie-Claude Bernard, Vu Thi Bich, Susana Ines Cordoba de Torresi, et al.
The Raman spectra of polyaniline (PANI) doped by functionalized sulfonic acids present very few differences with the spectra obtained after doping by inorganic anions, in spite of the noticeable differences displayed elsewhere by their optical spectra: the 3 eV (localized polaron) as well as 1.5 eV bands are stronger and sharper in presence of sulfonate. In fact, during the study of the different PANI forms, it was observed that the connections between Raman and optical features are far from being straight; a simple analysis in terms of the resonance in either benzoid or quinoid units is not enough. To arrive to a better understanding of these connections, we have compared the Raman spectra obtained with blue (localized polaron excitation), red (excitonic resonance) and near-IR (intrachain excitations resonance) lines; with the green laser line, PANI is out of resonance. Spectra were also obtained on samples treated to present a `secondary doping', and therefore a strong IR absorption. The use of red and IR lights allows to study the splitting of the C equals N peak, which can clearly be decomposed in two sub-bands, as well as the splitting of the 1330 cm-1 peak (assumed to be due to the stretching of C - N+$CCL).
Electroabsorption spectroscopy of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and related materials
Helen Mellor, Andreas Bleyer, Donal D. C. Bradley, et al.
We report experimental studies of poly(m-phenylenevinylene- co-2,5-dioctoxy-p-phenylenevinylene) (PmPV-co-DOctOPV). Model oligomers based on this polymer have been synthesized and results on these materials are also presented. Electroabsorption measurements have been performed on spin coated films between 1.5 eV and 5 eV with electric fields of up to 200 KV/cm. Using Stark shift analysis the electroabsorption spectra are compared with the first derivative of the corresponding low temperature absorption spectra. We also investigate the effect of changes in chemical structure by studying the variation between the electroabsorption spectra of related materials. Samples of these polymers and oligomers that differ by the proportion of trans and cis vinylene bonds are also compared to see what effect the trans or cis bond has on the electronic structure. Increased cis content enhances emission efficiency but little alters the electroabsorption spectra. The results for the different materials are compared and several features occurring at the onset and at the minima of the electroabsorption spectra are considered. These features do not depend on the trans/cis ratio and we present evidence that suggests aggregation effects play an important role in these samples.
Time-of-flight measurements in Langmuir-Blodgett films of poly(3-hexylthiophene)
Ronald Oesterbacka, Gytas Juska, Kestutis Arlauskas, et al.
Time-of-Flight (TOF) measurements in the voltage mode in thin films of poly(3-hexylthiophene) (PHT) have been studied. Thin films of PHT were fabricated using Langmuir- Blodgett (LB) technique giving unique possibility of controlling the thickness of sandwich type samples. We have used a method developed for the study of subnanosecond transients in thin films of amorphous semiconductors. The TOF signals have been measured in LB films for the first time, and a hole mobility of the order of 8 X 10-3 cm2/Vs in PHT LB films has been estimated.
Spectroscopic characterization of polymer photodiodes
C. S. Sulaiman, Carsten Giebeler, Robert N. Marks, et al.
We have studied the influence of alkoxy side-chain length (methoxy to hexadecyloxy) on photodiodes made from a series of poly(p-phenylene-co-2,5-dialkoxy phenylene vinylene)s [PPV-co-DAOPV]. The current-voltage of unilluminated devices indicate that the conductivity of the polymer drops as the side-chain length increases. We interpret this as a drop in the hole mobility in the polymer, due to an increase in the average separation of transport-active sites. The quantum efficiency of the short-circuit photocurrent under 0.25 mW/cm2 illumination at 2.48 eV drops by an order of magnitude from about 0.3% for the polymer with the shortest side chains to about 0.03% for the polymer with the longest side chains. We consider that this is primarily a reduction in the efficiency of exciton dissociation. We have also studied poly(p-phenylene-2, 3' bis(3, 2' diphenyl) quinoxaline-7-7'-diyl), which is of interest as an electron transport material. We find that it is indeed primarily an electron transporting material, but that the mobility of both carrier species is low. The quantum efficiency of the short-circuit photocurrent is very small--only 1.7 X 10-5% under 10 mW illumination at 420 nm. Doping the polymer with tetra(1-dimethylamino-phenyl)-ethynylene (TDPE) increases the quantum efficiency of the short-circuit current to 4 X 10-4%, probably due to enhanced exciton dissociation. Even with the TDPE dopant, the limiting factor for the photocurrent appears to be charge transport to the contacts.
Thermal and disorder broadening of excited states in polydiacetylene: 4BCMU
Gerhard Weiser, S. Moeller, A. Horvath, et al.
We present a study of the temperature dependence of absorption and electroabsorption spectra of a thin polydiacetylene single crystal 4BCMU poly[1,7 dodecadiyine- 1,12 diol-bis (4-butoxy carbonyl methyl urethane)] up to 450 K. Comparison with spectra of single chains diluted in a monomer single crystal reveals that the polymerization induces strong inhomogeneous broadening leading to two types of chains with exciton transition energies at 2 eV and 2.3 eV, respectively. Chains with transition energy at 2 eV transform into the other species near 380 K. The Stark effect of the excitons remains unchanged until the crystal is heated to 450 K where the spectra transform into the shape observed in spin cast films. These changes of the spectra are discussed in relation to the increasing disorder in the sample.
Optical probes of pristine and C60-doped silicon-bridged PT polymer: a novel highly luminescent low-bandgap polymer
Martin Liess, Z. Valy Vardeny, W. Chen, et al.
Silicon 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, photo induced absorption (PA), electromodulated PA, photoluminescence (PL) and photo induced absorption detected magnetic resonance 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 of 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.
Theory of excited states in polyphenylenes
Aparna Chakrabarti, Michael E. Chandross, Sumitendra Mazumdar
Exact and high order configuration interaction calculations are done on oligomers of poly(paraphenylene) within a molecular exciton basis to investigate high energy excited states relevant in linear and nonlinear optical absorptions. Two different sets of calculations are done. In the first of these, the exciton basis is constructed out of both delocalized and localized highest bonding and lowest antibonding molecular orbitals of each benzene unit. In the second, only the delocalized frontier orbitals of benzene are retained. Fully pictorial descriptions of all correlated eigenstates are obtained. Comparison of the two sets of results indicates that while the lowest energy eigenstates of polyphenylenes can be obtained within effective linear chain models with very large bond alternation, this may not be true for high energy states of the real materials with finite chain lengths. Low energy photoinduced absorption within the model is to an even parity charge-transfer exciton with analog of which occurs also in linear chain polymers. High energy photoinduced absorption, at least in the short oligomers, is to a new kind of two-exciton state that is absent in linear chain systems.
Aggregated luminescence from light-emitting polymer in dilute solution
Jui-Hung Hsu, Wunshain S. Fann, Kuen-Ru Chuang, et al.
The photo-physical properties of a PPV derivative. Poly(dioctyloxy phenylene vinylene) (PdOPV) in various states good solvent, poor solvent and thin films were investigated. The solubility of PdOPV in toluene exhibits dramatic variation at different temperature. Below room temperature, the solubility is decreased, and polymer forms aggregates, even in dilute phase. This decreasing of solubility also introduces a quenching effect. From compared the absorption and fluorescence of the film and the poor solution, it was concluded that the films are comprised of individual polymer and interchain aggregates, but the fluorescence only exhibits the behavior of aggregates. In addition, at low temperature, the fluorescence dynamics of the films exhibits a wavelength dependent behavior, which excludes the vibronic origin for the feature of fluorescence. In the films, the aggregates suffer additional quenching, which reduces the fluorescence efficiency.
Electroabsorption spectroscopy of self-assembled (PPV/SPS) films: size-dependent blue shift
Ofer Levi, Jorge M. Poplawski, Moeen Tarabia, et al.
We have studied the electroabsorption (EA) of self-assembled organic heterostructures (PPV/SPS)n consisting of alternate layers of poly(p-phenylenevinylene) (PPV) and `spacer' layers of poly(styrene-4-sulfonate), SPS. The heterostructures were prepared utilizing a layer-by-layer self-assembly technique using the precursor pre-polymer, followed by heat treatment to convert the pre-polymer into the conjugated polymer. For comparison, we have also studied the electroabsorption of a device made of spin-coated PPV. The thickness of the spacer layer, as determined by X-ray reflectivity, is varied from approximately equals 5 angstroms to 45 angstroms where the average thickness of the active PPV layers are maintained coarsely constant, 16 +/- 3 angstroms for the ten layers of each device. In all our measurements here the applied electric field is perpendicular to the plane of the films. The EA spectra of self-assembled films is significantly blue-shifted with increasing thickness of the spacer in the bilayers. The spectral width of the main peak in the EA spectrum for all the heterostructures is significantly broadened with respect to that of the spin- coated PPV, but it is almost independent of the spacer thickness. It exhibits a quadratic dependence on the applied electric field but no spectral shift with increasing field strength. The blue shift of the EA spectra is consistent with the blue shift in the absorption spectra as demonstrated in this work and in previous studies. The EA results, and particularly the blue-shifted spectra in the self-assembled films, are tentatively attributed to confinement effects, reduction of the PPV centrosymmetry and to an increase of the in-plane disorder of the active PPV layers.
Near-infrared ps transient photoinduced absorption in conjugated systems
Sergey V. Frolov, Werner Gellermann, Z. Valy Vardeny, et al.
We demonstrate the use of a color center laser in a pump- and-probe correlation technique which enables us to extend the probe spectral range into the infrared. As a result, we find strong photoinduced infrared absorption in several (pi) -conjugated polymers and oligomers. We associate this absorption with optical transitions in the singlet exciton manifold.
Dynamics of photoinduced gap states and self-trapped excitons in MX chain compounds with degenerate and nondegenerate CDW ground states
Hiroshi Okamoto, Yusei Kaga, Yasuo Oka, et al.
Excitation profiles and time characteristics of luminescence from the self-trapped exciton (STE) have been measured on the quasi-1D halogen-bridged metal complexes having degenerate and nondegenerate CDW ground states. From the comparison of the excitation profiles of the STE luminescence with those of the gap states, it was demonstrated that excitons are converted to spin-soliton pairs. This conversion occurs from the STE through a finite potential barrier, magnitude of which strongly depends on the degeneracy of CDW.
Photoinduced absorption in polyaniline solutions
L. J. Hartwell, Andrew P. Monkman
Photoinduced absorption experiments have been performed on solutions of emeraldine base in N-methyl-2-pyrrolidinone at various concentrations. In the energy range 0.5 eV - 1.7 eV photoinduced peak at 1.45 eV and photobleaching starting at 1.7 eV are observed, as is the case for solid films. The additional photoinduced peak at ca 0.9 eV which is seen in emeraldine thin films is not observed in solution, but a clearly resolvable induced feature is seen at 1.2 eV in the solutions. The 0.9 eV feature in films has been ascribed to an intra chain polaronic excitation, and the 1.45 eV feature to a long lived inter chain excitation. However the intensity of the 1.45 eV peak seen in the solutions scales linearly with concentration, clearly implying intra chain origin. The low energy feature of the thin films is either missing in the solution spectra implying it is an inter chain excitation or it substantially blue shifts in solution implying it too is an intra chain excitation but with a more highly dipolar nature. These results are suggestive of a strong role being played here by triplet excitons as well as residual polaronic species.
Optical studies of ordered and disordered alpha-sexithiophene films
Paul A. Lane, Martin Liess, Z. Valy Vardeny, et al.
We have studied (alpha) -sexithiophene ((alpha) -6T) films by photoinduced absorption (PA) and optically-detected magnetic resonance (ODMR). The PA of the disordered film is composed of two polaron PA bands and one bipolaron PA band, each with its own vibronic side bands. Measurements of an ordered film indicate that charge conjugation symmetry is broken and the polaron energy bands are shifted. The spin-1/2 ODMR of this film has a Gaussian lineshape with an 8 Gauss width. The PA of the ordered film shows no evidence for polarons. The bipolaron band is clearly split, indicating violation of charge conjugation symmetry. The ODMR of this film is significantly broader than that of the disordered film, which may be evidence for spin-spin interaction between paired charged excitations.
Two-photon absorption measured in the presence of strong one-photon saturation in cumulene-containing polymer
Ron K. Meyer, Maxim N. Shkunov, Robert E. Benner, et al.
The Z-scan technique is used to extract the real and imaginary third order nonlinear susceptibilities, Re(chi )(3) and Im(chi )(3), respectively. A series of Z scans were conducted at 590 nm (near one-photon resonance) on a cumulene-containing polymer, poly(p- phenylene-1,4-diphenyl-1,2,3-butatriene) or PPC3, prepared in solution. At very high peak focal point intensities two- photon absorption is seen superimposed on a one-photon saturation signature in open aperture Z scans. A negative real third-order nonlinear susceptibility is also observed in closed aperture Z scans. We describe a procedure to extract the desired third-order nonlinear susceptibilities by conducting Z scans at various peak focal point intensities and then fitting the results by adjusting Re(chi )(3) and saturation intensity. Molecular second hyperpolarizability, is calculated to be (-1.4 + 2.2i) X 10-29 esu which is almost 40 times larger than that measured in a monomer equivalent, and scaling nonlinearly with chain length. Z scans were also conducted at 780 nm, (below one-photon resonance) on PPC3 and also gives a molecular second hyperpolarizability that is greater than that seen in the monomer. The higher value of molecular second hyperpolarizability measured near one-photon resonance is attributed to the existence of a real state at the first transition in a two-photon process.
Ultrafast femtosecond relaxation processes in luminescent and nonluminescent conducting polymers
Jiandong D. Huang, Sergey V. Frolov, Z. Valy Vardeny, et al.
We have employed the time-resolved photomodulation (PM) technique to study the photoexcitation dynamics of a luminescent (Si-PT) and nonluminescent (s-(CH)x) conducting polymers in the low signal limit. In each polymer, we identify two exponential decay processes in the PM decay, with characteristic time constants T1 and T2, where T1 is of order 1 ps and T2 depends on the specific polymer; T2 in Si-PT is about 50 ps, whereas T2 in s-(CH)x is about 5 ps. The difference in T2 is tentatively attributed to radiative and non-radiative recombination kinetics, respectively. We also found that the PM decay does not show any obvious temperature and intensity dependences, whereas the polarization memory decay is longer at low temperatures.
Optical Characterization I
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Photophysical characterization of poly(p-pyridine)
Andrew P. Monkman, Mounir Halim, Stuart Dailey, et al.
In this paper we report measurements of the photophysical behavior of poly(p-pyridine) (PPY), including thin film absolute photoluminescence quantum yield, photoluminescence properties of solid, solution and solid-solution states, electroluminescence and nanosecond time-resolved luminescence. The influence of vacuum and air on the quantum yield of the PPY is also reported. We find a significant decay of the efficiency under vacuum whereas it is very stable in air. The role of aggregate versus excimer will be discussed and also it will be demonstrated that with appropriate fabrication conditions, good single layer PPY light emitting diodes can be fabricated.
Poster Session
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Electrochemically and photo-induced infrared bands in PPV: a comparative study
Helmut Neugebauer, Shankaran Srinivasan, Stefan Tasch, et al.
Results of in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy during electrochemical oxidation processes (electrochemically induced doping) as well as by photoinduced infrared absorption spectroscopy (photoinduced doping) of polyparaphenylenevinylene (PPV) are presented. Infrared active vibrational bands in the lower energy part of the infrared spectrum and infrared absorption due to electronic transitions at higher energies are observed and compared. The electrochemical doping of PPV occurs in two potential regions. In the `low doping' region, the difference spectra obtained by in situ FTIR-ATR spectroscopy are similar to spectra obtained by chemical doping. In the `high doping' region, the spectral behavior is different to the `low damping' region and shows a higher similarity to the photoinduced absorption spectrum.
Photoexcitations of conjugated polymer-fullerene composites in conventional polymers
Conjugated polymeric semiconductors are established to be effective donors upon photoexcitation. In blends of these polymers with Buckminsterfullerene, C60, the latter acts as a strong electron acceptor upon illumination. Lately the research on semiconducting polymers has put enhanced emphasis on the development of optoelectronic devices made from these polymer blends. Especially the device production demands good processability of the working material. A complete new approach to improve the processability of semiconducting polymers in their incorporation into host matrices formed by conventional polymers such as polyethylene (PE) or polystyrene (PS). We have characterized the linear optical properties of several guest-host systems by absorption and luminescence measurements and probed the excited states by photoinduced absorption measurements. The composition of the blend strongly influences the interaction between the conjugated polymer and the C60. In blends with PS as matrix and a soluble polyphenylenevinylene derivative as conjugated polymer we found strong luminescence quenching upon adding C60, in contrast to a system with a PE matrix.
Charge transport mechanism in transpolyacetylene
G. P. Das, A. Todd Yeates, Douglas S. Dudis
Both theoretical and experimental studies in the past have indicated that the charge transport in lightly doped polyacetylene is due primarily to traveling charged solitonic waves along the polymer chain backbone accompanied by hopping from one chain to another. The conductivity in this model is still dictated by a bandgap. The nature of the ground and excited states of the doped system, however, is not fully understood. Previous ab initio calculations on polyenes doped by a single iodine atom have brought out the interesting feature that, while calculations at the Hartree- Fock level lead to the charge-transfer state as the ground state, a correlated calculation, on the other hand, shows it to be an excited state with the ground state showing little charge transfer. Since, however, only polyiodide anions I3-, I5-, etc. are found in solution rather than neutral radicals such as I, I3 etc., inferences based on the calculation employing a single iodine atom are not conclusive. We present here a systematic ab initio study in which the nature of the ground and excited states of polyenes, doped with iodine, are investigated.
Studies of Raman scattering in novel disubstituted acetylene polymers
Akihiko Fujii, Maxim N. Shkunov, Z. Valy Vardeny, et al.
We have studied resonant and non-resonant Raman scattering spectra in thin films of novel disubstituted acetylene polymers such as poly(1-ethyl-2-phenylacetylene) (PEtPA), poly(1-n-hexyl-2-phenylacetylene) (PHxPA) and poly(1-phenyl- 2-p-n-butylphenylacetylene) (PDPA-nBu), which possess high photoluminescence (PL) quantum efficiency. We found that the Raman scattering frequency dispersion is smaller in disubstituted acetylene polymers than in other acetylene polymers, in agreement with many other strongly luminescent polymers. Assuming the model of short polyene conjugation length in these acetylene polymers, we can obtain the conjugation length (N) for each polymer from the respective phonon frequency of the carbon-carbon double bond; we obtained N equals 7 for PDPA-nBu, and N equals 5 or 6 for PHxPA and PEtPA. The related energies of 11Bu and 21Ag can be estimated from these N and are in good agreement with the respective absorption and PL spectra of the various disubstituted polymers.