Experimental investigations of excited-state dynamics in conjugated polymers using time-resolved laser spectroscopy

Abstract

In this thesis, time-resolved laser spectroscopy has been used to advance the understanding of the excited-state dynamics in a variety of conjugated polymers and related systems.

The properties of electron-phonon coupling in the conjugated polymer ladder-type methyl-substituted poly(para-phenylene) have been investigated using steady-state and picosecond fluorescence spectroscopy, with results demonstrating the thermal population of low energy stretching modes of the polymer backbone in the excited-state after thermalisation. Polarised measurements are used to demonstrate a fundamental relationship between the vibrational mode of the polymer through which fluorescence occurs and the corresponding fluorescence polarisation. Time-resolved measurements confirmed that the electron-phonon coupling in the polymer does not influence the fluorescence lifetime, with observations to the contrary attributed to the presence of an underlying defect state.

Picosecond fluorescence spectroscopy has also been used to investigate the properties of intramolecular energy transfer between different monomer subunits in the conjugated copolymer Super Yellow. The photoluminescence quantum yield of the copolymer was measured and its relatively high value was attributed to the effect of the aforementioned energy transfer.

Femtosecond pump-probe spectroscopy has been used to identify and characterise the properties of interchain exciton formation in ladder-type methyl-substituted
poly(para-phenylene). This process was observed to occur with a time period of 1ps, which was attributed to the time required for a singlet exciton to become delocalised over adjacent polymer chains.

Finally, delocalised polaron generation in blends of the regio-regular polythiophene poly(3-dodecylthiophene-2,5-diyl) with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester was investigated using femtosecond pump-probe spectroscopy. Both the rate and yield of delocalised polaron formation were shown to be dependent on excitation energy, with the dissociation of geminate polaron-pairs making a substantial contribution to the yield. This latter observation is significant, as the geminate recombination of polaron-pairs constitutes a fundamental and inherent loss of potential delocalised polarons in this system.