Single Molecule Spectroscopy

Emma Hooley, XiaoTao Hao

Conjugated polymers are finding increasing application as materials for organic light emitting diodes, photovoltaic devices and lasers. However, polymers are inherently heterogeneous (e.g. molecular weight distribution, conformation) and understanding the light induced processes in these materials remains a challenge.

Single molecule fluorescence spectroscopy (SMS) allows isolated polymer chains to be studied, removing the averaging inherent to bulk fluorescence measurements. Intrachain relaxation processes otherwise obscured can be identified and characterised.

Widefield fluorescence microscopy is emerging as a powerful single molecule spectroscopy technique, as it allows a wide area to be illuminated and the behaviour of the molecules to be observed in real time. In addition, defocused widefield images can provide information on the properties and dynamics of the emission transition dipole, this allowing an insight into the molecular orientation and diffusion properties of the polymer chromophores.

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Figure 1: Fluorescence trajectory of a single molecule showing both “on” and “off” times, known as fluorescence intermittency or “blinking”.

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Figure 2: A defocused widefield image of a single emitter. The fringe patter can be used to determine the orientation of the emission dipole.