Relaxation in Polymers

The following sections are included:

• PREFACE

• CONTENTS

By femtosecond pump-probe spectroscopy various ultrafast nonlinear optical processes were studied for many conjugated polymers, polydiacetylene (PDAs) such as poly[4,6-decadiyne-1,10-diol - bis([(n-butoxycarbonyl)methyl] urethane)](PDA-3BCMU)(blue-phase), poly[5,7-dodecadiyne-1,12-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-4BCMU) (red- and blue-phases), poly[1,4-bis([2,5-bis(trifluoromethyl]phenyl)-1,3-butadiyne] ( PDA-DFMP)(blue-phase), and polythiophenes (PTs) such as poly(3-methylthiophene (P3MT) and poly(3-dodecylthiophene)(P3DT), and polythienylenevinylene)(PTV). Picosecond luminescence spectroscopy was also studied for PDA-4BCMU in red phase.

Geometrical relaxation of free ¹Bu excitons photogenerated by 100-fs pump pulses to the quasithermal self-trapped (ST) excitons takes place with time constants of 100-150 fs in PDAs and 70-100 fs in PTs. The lifetimes of the ST excitons at 10 K (290 K) were 2.0±0.1 ps (1.5±0.1 ps) in PDA-3BCMU, 3.0±0.3 ps (2.1±0.2 ps) in PDA-4BCMU, and 800±100 fs (800±100 fs) in P3MT. The decay kinetics of the ST exciton were explained by potential crossing and tunneling between two potential curves of the ST exciton and the ground state. The extremely weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. The relaxation dynamics of fluorescence from the PDA-4BCMU film cannot be represented by a single exponential decay; however, it can be described in terms of a random walk in the fractal dimension. By applying the fractal dimension model, the spectral dimension was determined to be between 0.50 and 0.85. The spectral change due to several nonlinear optical processes, i.e. hole burning, Raman gain, and dynamic Stark effect, were also observed in femtosecond resolved spectra. The third-order susceptibility was determined for these nonlinear processes, such as absorption saturation at the exciton transition, Raman gain, and resonant Kerr effect from the observed spectral change. The third-order susceptibility corresponding to absorption saturation (hole burning) in blue-phase PDA-3BCMU cast film and in P3MT was obtained as Im[χ⁽³⁾₁₁₁₁(-ω;ω,-ω,ω)] = - 2.6×10^-9 esu and -3.5×10^-10 esu, respectively, for ħχ = 1.97 eV. The third-order susceptibility corresponding to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ⁽³⁾(-ω₂;ω₂,-ω₁,ω₁)] = -5.8×10^-10 esu and -1.5×10^-10 esu, respectively, for ħω₁=1.97 eV and ħω₂=1.79 eV. From the resonant Kerr experiment in the P3MT film, |Δχ⁽³⁾| = |Δχ⁽³⁾₁₁₁₁(-ω₂;ω₂,-ω₁,ω₁)-Δ⁽³⁾1122(-ω₂;ω₂,-ω₁, ω)|= 3.9×10^-11 esu was determined for ħω₁ = 1.97 eV and ħω₂ =1.88 eV.

The following sections are included:

• Introduction

• Excited States and Relaxation Channels in Polysilanes

• Hole burning spectroscopy

• Steady State and Time Resolved Emission Spectroscopy

• Transient Absorption Spectroscopy

• Coherent time resolved non-linear optical techniques

• Oligomeric polysilanes

• Summary

• Acknowledgement

• References

The following sections are included:

• Introduction

• Theoretical Approaches to σ-Conjugation

• Isolated Strands In Solution And Low-Temperature Glasses

• Solid-State Spectra of PDHS and PMPS

• Photoconductivity and Exciton-Exction Annihilation

• Concluding Remarks

• Acknowledgments

• References

We review the experimental results of fast transient photoconductivity studies of the two prototypical quasi-one-dimensional semiconducting polymers: trans-polyacetylene, trans - (CH)_X, and polydiacetylene(bis p-toluene sulfonate), PDA-TS. For both polymers, the measurements reveal two distinct photoconduction mechanisms: the initial transport is by carriers occuping extended band states; whereas at later times, the carriers are trapped on shallow defects, and transport is governed by trap-limited mobility. The quantum yield is independent of temperature and external field, demonstrating that the Onsager geminate recombination model, used extensively for analysis of the photoconductiivty in organic materials and molecular crystals, is not applicable to PDA-TS. Since the quantum yield is temperature independent, and since the de Broglie wavelength of the carriers is comparable to the width of the potential barrier which prevents the separation of bound geminate carriers, tunneling appears to play fundamental role in the geminate carrier dissociation process. Using carrier sweepout measurements, the mobility has been measured in PDA-TS in the nanosecond time regime; µ ≈ 5 cm²/Vs. The polarization dependence of the photoconductive response in both non-oriented and oriented trans -(CH)_X samples has been studied. As a consequence of the large anisotropy in the absorption coefficient, photoconduction in non-oriented samples is dominated by intrachain excitation and transport. In the case of oriented samples, it is possible to have interchain generation with relatively high quantum efficiency. Comparative studies of the properties of the photocurrent tail in polydiacetylene and in oriented and non-oriented trans -polyacetylene indicate that the magnitude of the tail provides a rough gauge for the quality of the material; in oriented polyacetylene, the enhanced photocurrent tail indicates the importance of shallow traps rather than deep sp³ defects.

We have measured photoexcitation dynamics in trans (t) and cis (c) isomers of polyacetylene ((CH)_x) and in polydiethynylsilanes, in a broad time interval from femtoseconds to miliseconds and spectral range from 0.1 eV to 2.4 eV, using a variety of experimental techniques. These include picosecond to milisecond transient and cw photomodulation, femtosecond degenerate four-wave mixing, picosecond transient photoinduced resonant Raman scattering and the newly developed technique of absorption detected magnetic resonance. In the degenerate ground state polymer t-(CH)_x we followed the evolution of charged and neutral photogenerated solitons, their geminate recombination in the form of t^-1/2 which dramatically slows down into t^-β (β < 0.5) due to disorder and inhomogeneity in the films, and their diffusion along the chains. In partially isomerized (CH)_x we identified tightly bound soliton pairs in short trans segments and a fast excitons intersystem crossing from cis chains to the more energetically favorable trans chains. In the almost degenerate ground state polydiethynylsilane we observed a subpicosecond geminate recombination with a very small long tail. We identified charged and neutral photogenerated solitons in long chains and bipolarons in short chains where the degeneracy can be easily lifted.

The following sections are included:

• Introduction

• Excitons in Conjugated Polymers

• Linear and Nonlinear Optical Susceptibilities

• Experiments in Polydiacetylene

• Influence of Disorder

• Exciton-Polarons

• Experiments in Other Systems

• Conclusions

• Acknowledgments

• References

In this chapter, a brief review of theoretical and experimental studies on the optical properties of halogen-bridged mixed-valent metal complexes is given, with special emphases on the following two topics. One is the nature of the charge transfer band, and the other is the intragap absorptions due to nonlinear excitations such as solitons or polarons. In relation to the first topics, we introduce a theory of spectral shapes of optical absorptions in various charge-density-wave states. In that theory, two important effects beyond the mean field theory are taken into account: the electron-hole correlation on the metal atoms and the classical fluctuation of the halogen-sublattice coordinates. It is found that the spectral shapes both of the linear absorption and of that in an electric field, i.e., the electroabsorption agree well with the experimental results if we assume that the electron-hole attraction is rather strong in real systems. Moreover, the intensity of the third-harmonic generation is also calculated, using the same assumption. In relation to the second topics, on the other hand, we show some examples of theoretical calculations in which the modifications of the absorption spectra due to a soliton or a polaron is obtained. It is clarified that the intragap-absorption bands also are drastically renormalized by the exciton effect.

The dynamics of soliton pairs and polaron pairs is numerically studied using the Su-Schrieffer-Heeger model extended to include the short-ranged Coulomb interactions. The unrestricted time-dependent Hartree-Fock approximation is used. As a trigger to put the solitons and polarons in motion, spatially uniform external electronic fields are introduced to the system through time-dependent vector potentials, which are included into the model as a phase factor of the transfer integral. It is found that the pair of charged solitons with opposite charges, that of neutral solitons with parallel spins and that of polarons with opposite charges and antiparallel spins are highly stable against collisions, while the pair of neutral solitons with antiparallel spins is unstable. The charge and/or spin transfer during the collisions is also discussed.

Strong electron-phonon coupling in the quasi-one-dimensional organic semiconductors leads to photoexcitations which are best described as lattice stabilized quasiparticles. We review the use of picosecond infrared spectroscopy to learn about the formation and decay dynamics of these solitons, polarons and one-dimensional excitons in conjugated polymers. The cases of degenerate and nondegenerate ground state systems, exemplified by trans-polyacetylene and poly(paraphenylenevinylene) respectively, are discussed.