FOR 5781 - P07: Pressure-induced luminescence shift in coordination compounds

Basic data for this project

Description

Luminescent organometallic complexes hold great technological promise for numerous applications such as organic light emitting diodes (OLEDs), bioimaging, and sensing. This project investigates how their luminescence can be systematically tuned by external pressure. The emission of square-planar Pt(II) complexes has been found to red-shift upon aggregation making it sensitive to pressure. Cr(III) based "molecular rubies", on the other hand, exhibit a pressure-induced red-shift due to changes in intramolecular bond lengths. By contrast, Ni(II) complexes are usually non-emissive at ambient conditions, but theoretical predictions suggest that they can be "switched on" at high pressure. This project aims to study of a large number of systems (i.e. different complexes in various environments for different p,T combinations) by theoretical means. We will develop a multiscale simulation approach which combines quantum chemistry with classical molecular mechanics (MM). Condensed phases are represented as infinitely periodic systems whose unit cell together with the atomic positions are simultaneously optimized for a given pressure. To determine the emission wavelength, this needs to be done both in the ground state and in the emissive excited state. In principle, these tasks can be achieved straightforwardly using density functional theory (DFT). However, there are several challenges in practice, which this project aims to tackle. Firstly, for large unit cells, the optimization is computationally very demanding as convergence is typically slow, prohibiting the study of a sufficient number of systems and pressures. Secondly, more fundamentally, only the total spin multiplicity of the entire unit cell can be specified. This is problematic when the unit cell is comprised of several complexes, as it is not possible to control the spin state of each individual complex. Thirdly, charge transfer excitations between different complexes require the use of range-separated hybrid functionals, which are computationally too costly in periodic DFT implementations. We will therefore perform QM/MM calculations, in which only selected metal complexes are treated quantum-mechanically, as well as pure MM calculations. However, accurate force fields for novel coordination compounds are not readily available, let alone for excited electronic states. We therefore intend to develop suitable state-dependent force fields based on ab initio reference data using a genetic algorithm technique. The newly developed force fields will also be employed to study metal complexes in solution. In particular, they will allow us to perform the necessary configurational sampling at a given p,T condition by performing molecular dynamics (MD) simulations.