Disorder-Order Transitions in p-Conjugated Polymers

Presented on December 2, 2016 at 4:00 p.m. in the Engineered Biosystems Building (EBB), Room 1005, Georgia Tech.Professor Anna Köhler holds the Chair of Experimental Physics II at the University of Bayreuth. Her research is concerned with photophysical processes in organic semiconductors. Professor Köhler's research group focuses in particular on the processes of energy and charge transfer in singlet and triplet excited states, the exciton dissociation mechanism and intermolecular/interchain interactions. She has published more than 100 papers, several book chapter and one textbook. Professor Köhler received her PhD in 1996 from the University of Cambridge, UK, where she continued her research funded through Research Fellowships by Peterhouse and by the Royal Society. In 2003 she was appointed Professor at the University of Potsdam, Germany, from where she moved to the Department of Physics at the University of Bayreuth, Germany, in 2007.Runtime: 65:44 minutesThe aggregation of p-conjugated materials significantly impacts on the photophysics, and thus on the performance of optoelectronic devices. Nevertheless, we know comparatively little about the laws governing aggregate formation of p-conjugated materials from solution. In this talk, I shall compare, discuss and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor-acceptor type polymers and low molecular weight compounds. To understand how aggregates form, we employ temperature dependent optical spectroscopy, which is a simple yet powerful tool for such investigations. I shall discuss how optical spectra can be analysed to identify distinct conformational states and to obtain quantitative information on changes in the inter-chain coupling, the conjugation length and the oscillator strength upon aggregate formation. We find aggregate formation to proceed alike in all these compounds by a coil-to-globule like first order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. I will address the role of side chains and the impact of changes in environmental polarization