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Molecular Mesoscopics for Organic Nano-Optoelectronics

Final Report Summary - MOLMESON (Molecular Mesoscopics for Organic Nano-Optoelectronics)

This project explored the boundary between the individual molecule and the bulk solid in the context of polymeric organic semiconductors by constructing and studying molecular aggregates from the single-molecule level upwards. We coined the phrase “molecular mesoscopics” to describe this intermediate region of molecular aggregates. We developed a range of techniques to grow and manipulate well-defined molecular aggregates by combining solid-state and vapour-phase processing conditions under an optical microscope. Using time-resolved and steady-state spectroscopies, the interaction of individual molecular units could be revealed, both in van-der-Waals bound aggregates and in custom-designed synthetic structures with intramolecularized intermolecular interactions. The crucial observation made is that many coupled molecules, even when they are large molecules such as conjugated polymers, can behave as one single entity in terms of the statistics of fluorescence photons emitted. This deterministic photon emission arises from the strong coupling between molecular entities within the aggregate. However, while photon emission is deterministic, the excited state dynamics within the molecule are not. In many regards, the excited state dynamics of large molecules and molecular aggregates resemble the attempt of balancing a pencil on its tip. Every time the experiment is performed, a different result occurs. One way to visualize this effect is through the spontaneous fluctuations in single photon polarization from large molecular complexes. We demonstrated that such fluctuations also arise in commercial fluorophores employed in organic electronics, in particular in organic light-emitting diodes (OLEDs), and pointed out the potential impact of such spontaneous symmetry breaking in the excited state on overall device efficiency.

The approach of the work programme was threefold. First, we developed a range of photon correlation techniques in the context of molecular complexes from organic electronics to study excited state dynamics. Second, we designed a range of new molecular model structures to mimic particular excited-state localization phenomena taking place in real materials. Third, we also opened the project to molecular materials in the broadest sense, probing new kinds of molecular structures. In particular, we investigated the origin of light generation in tiny metal nanoparticles and showed that, under certain conditions, light arises from the heating of the electrons within the solid. With a trick, we were able to demonstrate that this incoherent excitation energy of hot electrons can be harvested to a molecular-like structure, a two-dimensional semiconductor crystal. Such light harvesting originates from extremely efficient excited-state coupling between two very different materials and was shown to have application potential in nanoscale colour-tunable LED-like structures.