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The aim of this project is to study strain in organic semiconducting oligomers and polymers. We have applied first principles electronic structure methods to investigate the impact of mechanically induced strain on molecular configuration, and consequently on the vertical transition energies. This is a new research direction for the PI who has expertise in the influence of strain in inorganic nanostructures, and the PRF grant has been used to fund a postdoc with a quantum chemistry background. This enables us to determine how materials engineering on the nanoscale can be applied to organics. We have taken polyfluorenes for model systems as these have two well defined phases – a randomly twisted glassy phase and a planar ordered β-phase – which have distinct optical properties, and strain is thought to play a role in converting between the two. The two configurations have also been observed on the single molecule level and we apply density functional theory based methods to both strain in isolated molecules and adsorption systems, thus applying the principle of semiconductor strain engineering to organics.

We initially investigated torsional strain between monomers principally for comparison with previous studies. We determined that the defining feature of the β-phase is the planarity of the molecules. For instance a twist of 180° causes a bend in linear oligomers, but the vertical transition energies are unaffected. Slight shifts of the vertical transition energy can occur when the linearity of the molecule is distorted although order is retained. There are therefore several morphologies which are classed as β-phase. Looking at uniaxial strain in β-phase oligomers, we found that compression of the linear molecule can lead to two degenerate morphologies; one bent within the plane and the other bent out of the plane of the molecule. The vertical transition energies indicate that these structures have the same properties as the unstrained molecule, but the shape changes result in small shifts in opposite directions. On elongation of glassy phase molecules we did not observe planarization as anticipated. However, we did determine that twisted oligomers will planarize on excitation if they are stretched.

We have simulated the adsorption of a fluorene oligomer onto a clean Si(100) substrate, with the molecular axis oriented along the silicon dimer rows in order to mechanically induce a transition between the two phases. Using first principles molecular dynamics we found that a twisted oligomer was planarized on adsorption. We also did calculations to investigate the adsorption geometry of an oligomer and polymer in more detail. We determined that the molecules can be uniaxially compressed or extended depending on their alignment with the substrate, but the distribution of strain the backbone is dependent on the bonding configuration and subsequently the electronic structure is subject to localized perturbations. This work has been submitted for publication and we are currently preparing two more papers presenting the work on isolated molecules.

Our results demonstrate that when considering the impact of uniaxial strain on optoelectronic properties, not only the absolute molecular length, but also structural changes, which can differ in the extent to which structural order is retained, must be taken into account. Specific adsorption of organic semiconducting polymers onto a silicon substrate is of interest in so far as whether control can be exercised over the molecular structure. Using polyfluorenes on Si(100) as a model system, we have shown that this can be used as a means for straining the backbone in a defined way. These results demonstrate how important structural control is for utilizing the optoelectronic properties of organics; relevant for applications in organic electronics and photovoltaics. These molecules can indeed be mechanically manipulated to alter the HOMO – LUMO gap, but our study demonstrates the importance of identifying exactly how shape and bond parameters respond to external constraints.

This work has opened up interesting areas for research which we will investigate further such as whether tethering side groups can be used to ensure a more even distribution of strain and decoupling of the molecular electronic structure from the surface on adsorption, and whether the structure of these molecules can influence the configuration of subsequent organic layers.