Single-­molecule spectroscopy of π-­conjugated polymers

Sammanfattning: When applied to condensed-matter samples of large organic molecules, optical spectroscopy faces the problem of inhomogeneous broadening caused by the diversity of environments and conformations that sample molecules are subjected to. This severely limits the information that can be extracted from optical spectra. Single-molecule spectroscopy offers the most natural way to overcome this problem. Π-conjugated polymers are a class of photoluminescent materials whose electronic structure is similar to that of semiconductors, which makes them promising materials for a range of applications. Spectroscopically, they are multicromophoric systems showing individual photophysical properties due to efficient excitation energy transfer to a small number of fluorescent exciton traps. This makes them a natural object to study with single-molecule spectroscopy. This thesis presents a series of results obtained with a range of single-molecule spectroscopy-based techniques on the model conjugated polymer MEH-PPV. Single-chain fluorescence intensity fluctuations (blinking), fluorescence spectral diffusion, fluorescence kinetics and polarization properties have been observed and investigated. Apart from the pristine single-molecule spectroscopy, the range of techniques included: single-molecule imaging (low temporal resolution), time-correlated single photon counting (high temporal resolution) and a novel technique – 2D polarization single-molecule imaging – which is being reported in detail for the first time. Most of the techniques have been combined with temperature-dependent measurements (from 15 K on). Other types of samples (e.g. chain aggregates and submonolayer films) have been studied for comparison, too. In addition, a technique for simulating the photophysical properties of the polymer chains by simple and computationally affordable means is presented.