Single-molecule spectroscopy of conjugated polymers in organic solvents

Abstract

This thesis presents the development of an organic-solvent-based single-molecule technique for the study of the relation between conformation and photophysics of conjugated polymers (CPs). These solution-processable materials combine the mechanical advantages of plastics with the electronic properties of semiconductors. However, our current understanding of their photophysical properties is limited by our capability of studying the several different processes that take place in their highly heterogeneous structures.

Single-molecule fluorescence microscopy and spectroscopy (SMM&S) techniques are ideal for unravelling the nature of the photophysical processes in these intrinsically photoluminescent materials. Currently available techniques are based on trapping single CP chains in transparent matrices. This approach can limit both the range of conformations that can be adopted by these materials and further studies on their dynamics.

Here, a surface-anchoring strategy was developed using silane coupling agents, thereby eliminating the need for immobilising single CP chains in transparent matrices. Additionally, water-based methods for imaging single molecules in solution were adapted for their compatibility with organic solvents. This new methodology was tested on the model polymer poly(3-hexylthiophene) in SMM&S experiments. A clear correlation was observed between the photoluminescence of single chains and their solubility in different solvents. Surprisingly, peaks in the single-chain spectra were observed at higher energies in poor solvents (dimethyl sulfoxide and isopropanol) than in a good solvent (o-dichlorobenzene). Different explanations in terms of possible polymer conformations were proposed.

Finally, a new kind of experiment was developed for inducing conformational changes in single CP chains by rapidly switching the solvent environment while measuring their photoluminescence (PL) intensity in real time. Abrupt changes in the PL intensity were i measured in a sub-second time-scale and associated with conformational changes in the chains. These results show the unprecedented capability of this technique for conformational and real-time manipulation studies on CPs.

Date of Award	24 Jun 2019
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ifor Samuel (Supervisor) & Carlos Penedo (Supervisor)