CP-SMS · Understanding conjugated polymers via single-molecule spectroscopy

Final Activity Report Summary - CP-SMS (Understanding conjugated polymers via single-molecule spectroscopy)

Nanoscale organisation is the key issue for operation of molecular opto-electronic devices based on supramolecular structures, nanoparticles and polymers. Properties of these complex systems are determined not only by their chemical composition but mostly by their organization and environment at nano-meter scale. Single Molecule Spectroscopy (SMS) allows unravelling individual properties of nano-objects normally hidden by sample heterogeneity. Conjugated polymers, as charge conducting and light emitting organic semiconductors, are very important materials for nano-electronics. They have applications in light emitting diodes, field-effect transistors, solar-cells and in biodetection. Their unique properties are determined by energy and electron transfer in the polymer nanoparticles, which are of 5-10 nm in size. These properties are however strongly dependent on the particle size and topology. During the past decade, SMS has been intensively applied to study the complex dynamics in multi-chromophoric systems including conjugated polymers (CPs). However, some fundamental photophysics such as energy migration, excited state dynamics, triplet and charged state generation and influence of internal organisation of chromophores and environment on these processes have not been clearly understood for single CP chains. Understanding such issues is of a great importance for applications of these materials. In this project, we applied SMS to multi-chromophoric nano-objects such as single chains and single aggregates of conjugated polymers, and J-aggregates of organic dyes. A novel approach called '2D polarisation imaging' to monitor topology and energy transfer in a multi-chromophoric system has been developed in our group [See details in our published paper A1]. This new method and time-resolved SMS allowing for fluorescence decay and fluorescence fluctuations measurements are the primary techniques used in the project. We have investigated the evolution of fluorescence properties from bulk materials (pristine films), nano-aggregates to isolated chains for MEH-PPV (poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene), a widely studied conjugated polymer. It was found that the MEH-PPV chains experience great changes in fluorescence brightness, emission spectra, exciton decay dynamics, and polarisation anisotropy upon isolation in an inert polymer matrix. We found that, in many published SMS studies on MEH-PPV and its analogues, single aggregates contributed together with truly isolated molecules to the statistics of fluorescence properties, resulting in ambiguous understandings about these polymers at the single chain level. The absorption cross-section and fluorescence quantum yield of single isolated MEH-PPV chains were evaluated by comparing their fluorescence intensities to those of single-chromophoric dyes. We concluded that in such an isolated MEH-PPV chain potentially consisting of hundreds of chromophores, only a few chromophores are active, while the other chomophores are non-emissive at all. These observations will have important consequence to all the SMS studies on CPs published so far. Another type of multi-chomophoric system, J-aggregates, was also studied at the single aggregate level using fluorescence microscopy. Relatively large fluorescence intensity fluctuation depths (tens of monomer units) were observed, suggesting the occurrence of collective quenching of many monomer units.

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