Conformational and Dynamical Aspects of Conjugated Polymers and Oligomers of Interest for Photoluminescence
Sammanfattning: Transient absorption laser spectroscopy and resonance Raman spectrscopy have been used to study conjugated polymers and oligomers. To extract more information the measurements have been compared to computer simulations and calculations. In order to obtain a better understanding of the role of conformational disorder in the photophysics of light-emitting, conjugated polymers the ultrafast transient absorption anisotropy of a substituted polythiophene, PDOPT, in solution, partially deconjugated MEH-PPV in solution and an irregular substituted PTOPT in a thin amorphous film have been measured. The experimental results have been compared to calculated anisotropy decays of computer simulation generated polymer chains. Estimates of the energy hopping time and, for the polymers in sollution, energy migration distances for the samples have been obtained. The Polymer chains are generated by a partially correlated random walk with a hard sphere repulsion. The randomness, i.e. the angle between one segment and the next is constrained by a Gaussian distribution. The width of this Gaussian is denoted as the disorder parameter and determines the disorder of the polymer. A large disorder parameter will give a wide distribution of angles between segments, called spectroscopic units, which will result in a chain that contains more abrupt changes of direction. A Förster type of hopping mechanism is used in the simulated energy transfer. From comparisons between the calculated anisotropy of the simulated polymer chains and the experimental anisotropy we can obtain (i) the disorder parameter, (ii) the time the excitation takes to transfer between segments (pair-wise hopping time) and (iii) (for polymers in solution) the distance that the excitation travels along the chain before it is trapped on a local energy minimum. To better understand the properties of the conjugated polymers two oligothiophenes have been studied. The formation of the terthiophene radical cation (3T+?) in an electron acceptor kind of TiO2-SiO2 hybrid polymer matrix was followed by transient absorption laser spectroscopy with ~100 fs time resolution. The electron transfer took place from the lowest-excited singlet state.The formation of the radical cation can be described by as a biexponential rise with time constants of ~1 ps (62 %) and ~8 ps (38%). The bithiophene radical cation was created via a photoinduced electron transfer between the excited bithiophene and fumaronitrile as an acceptor, and by ?-irradiátion of bithiophene in a glassy matrix at 77 K. The resonance Raman spectra of the bithiophene radical cation were measured and compared to DFT and ab inito calculations to yield the vibrational frequencies and the absorption spectrum of the bithiophene radical cation.
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