Conjugated polymers such as polyacetylene form a group of model semiconductors in which the 'one-dimensional' character of the polymer strongly modifies the behaviour of charges added to the chains from that usually observed in three-dimensionally bonded semiconductors. First, the polymer chain can undergo local reorganisation of the pi -electron bonding in the vicinity of a charge added to it. This has the effect of localising the charge in a state that can be described as a soliton for the particular case of trans-polyacetylene, and more generally as a polaron. Secondly, the motion of charged excitations is highly anisotropic, with strong confinement to the chain through the 'polaronic' structural relaxation of the chain. This anisotropy strongly affects charge transport and charge separation following photo-excitation. The authors review some of the recent work in this area and present polarisation-dependent measurements of photoluminescence (PL) and photo-induced absorption (PA) on highly oriented films of polyacetylene and poly(phenylenevinylene). They find that charge separation, detected through PA from photogenerated solitons or bipolarons, is the result of inter-chain electron transfer, whereas intra-chain excitation leads to charge confinement, and decay by PL (for poly(phenylenevinylene)) and by non-radiative channels. They find furthermore that these non-radiative channels become more efficient as the length of the conjugated polymer chain increases, and consider that this occurs through rapid motion of the excited state to recombination centres. They have identified both monomolecular and bimolecular kinetics for these processes.

https://iopscience.iop.org/article/10.1088/0022-3727/20/11/005/pdf

Photo-excitation in conjugated polymers

The relaxation dynamics of photoexcitations in cast films of the polydiacetylenes, poly[4,6-decadiyne-1,10-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-3BCMU) and poly[5,7-dodecadiyne-1,12-diol bis([n-butoxycarbonyl)methyl]urethane)] (PDA-4BCMU), and in electrochemically prepared films of poly(3-methylthiophene) (P3MT) were investigated by femtosecond absorption and picosecond luminescence spectroscopies. 1Bu free excitons, generated by 100-fsec pump pulses, relaxed to the self-trapped (ST) state with time constants of 150 ± 50 fsec in PDA-3BCMU and 70 ± 50 fsec in P3MT. The difference was explained in terms of the relative sizes of the side chains. PDA-3BCMU has bulky side chains that are linked by hydrogen bonds, while P3MT has a repeating unit that contains a ring structure with a small methyl group. The lifetimes of the ST excitons at 10 K (290 K) were 2.0 ± 0.1 (1.5 ± 0.1) psec in PDA-3BCMU, 3.0 ± 0.3 (2.1 ± 0.2) psec in PDA-4BCMU, and 800 ± 100 (800 ± 100) fsec 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 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 changes that are due to several nonlinear-optical processes, i.e., hole burning, Raman gain, and the 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 the resonant Kerr effect from the observed spectral change. The third-order susceptibility that corresponds to absorption saturation in PDA-3BCMU and in P3MT was obtained as Im[χ1111(3)(−ω;ω,ω,−ω)] equal to −2.6 × 10−9 and −3.5 × 10−10 esu, respectively, for ħω = 1.97 eV. The third-order susceptibility that corresponds to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ1111(3)(−ω2;ω2,ω1,−ω1)] equal to 5.8 × 10−10 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)|≡|χ1111(3)(−ω2;ω2,ω1,−ω1)−χ1122(3)(−ω2;ω2,ω1,−ω1)|=3.9×10−11 esu was determined for ħω1 = 1.97 eV and ħω2 = 1.88 eV.

Relaxation dynamics of photoexcitations in polydiacetylenes and polythiophene

Resonance enhancement in cubic susceptibility of Langmuir-Blodgett multilayers of polydiacetylene

We present the results of time‐resolved luminescence studies of poly[2‐methoxy, 5‐(2’‐ethyl‐hexyloxy)‐p‐phenylene‐vinylene] (MEH‐PPV), as a pure film, in solution, in a gel formed by a network of ultrahigh molecular weight polyethylene (UHMW‐PE), and in a blend with UHMW‐PE. The luminescence has a characteristic lifetime of 200–300 ps at room temperature, increasing to 500–700 ps when the materials are cooled to 80 K; the decay time is approximately the same for all the physical forms of the material (solution, film, gel, blend). The relatively short lifetimes, compared to intrinsic values calculated from absorption and emission spectra, and the observed temperature dependence indicate that the luminescence decay is quenched by nonradiative processes. The time decay of the photoluminescence deviates from a single exponential for most forms of the MEH‐PPV. Best fits to stretched exponential and double exponential expressions are presented. Steady state photoluminescence spectra and integrated luminescence vs pump intensity data are presented in order to establish which of the possible mechanisms are most important.

Time‐resolved photoluminescence from poly[2‐methoxy, 5‐(2’‐ethyl‐hexyloxy)‐p‐phenylene‐vinylene]: Solutions, gels, films, and blends

Measurements of the third-order optical nonlinear susceptibility χ(3) have shown that nonlinearity around the single-photon absorption region is dominated by the saturation absorption in polythiophene thin films. The sign and the size of the real and the imaginary part of χ(3) have been measured at 532 nm to be −6.1 × 10−9 and −1.9 × 10−9 esu, respectively, by the Z-scan method.

Excited-state nonlinearity in polythiophene thin films investigated by the Z-scan technique

Studies of photoinduced absorption of crystalline toluene sulphonate polydiacetylene (PDA-TS) and 1-hydroxyhexa-2,4-diyne polydiacetylene (PDA-1OH) show absorption peaks near 1.35 eV. This absorption is compared with photoinduced absorption in polyacetylene and polythiophene and its origin is discussed.

Photoinduced absorption and luminescence in polydiacetylenes

Near-band-edge luminescence in polythiophene has a short lifetime of <or approximately=9 ps at 4K owing, it is suggested, to competition by fast nonradiative processes. The decay of luminescence in polydiacetylene (1OH) at 4K is somewhat slower, and much more complex.

Picosecond studies of luminescence in polythiophene and polydiacetylene

Photo-induced electronic and vibrational absorption of polydiacetylene (10H) in the infrared (400-4000 cm-1) is found to be consistent with photo-generation of bipolarons Overlap of electronic and vibrational transitions occurs due to strong electron correlation effects which reduce the electronic transition energies Electron-phonon interaction produces anti-resonances in the bipolaron electronic absorption

http://iopscience.iop.org/article/10.1088/0022-3719/20/3/004/pdf

Infrared photo-induced absorption in polydiacetylene

Photo-induced absorption (PA) is observed for the polydiacetylene PDA-10H in the infrared which is found to be consistent with the photogeneration of charged bipolarons. No comparable PA is seen in PDA-TS, a result that is consistent with photo-excitation leading to the formation of triplet excitonic bipolarons. Persistent effects are observed for PA in PDA-10H indicating a long carrier lifetime due to trapping. Electron correlation is significant and shifts the electronic transitions to lower energy in PDA-10H so that overlap of electronic and vibrational transitions occurs. This overlap leads to strong anti-resonances in the PA due to electron-phonon interaction.

Infrared studies of optically excited polydiacetylene

Photoinduced absorption (PA) is a valuable technique for studying the excitations of conjugated polymer systems. Both vibrational and electronic properties and the interaction between them may be observed for the photoinduced states. Polydiacetylene (PDA) is a particularly good material for such a study, as highly aligned crystalline films are available. Electron correlation effects are generally agreed to be significant in conducting polymers and are particularly strong for PDA. We report here infrared PA measurements on the polydiacetylene PDA-1OH.

D Bloor

Papers

Photoinduced absorption and luminescence in polydiacetylenes

Picosecond studies of luminescence in polythiophene and polydiacetylene

Infrared photo-induced absorption in polydiacetylene

Infrared studies of optically excited polydiacetylene

Photoinduced Infrared Absorption in Polydiacetylene