Infoscience

Thesis

Investigations of the non-adiabatic photophysics of Cu(I)-phenanthroline complexes

Cu(I)-phenanthrolines are an important class of metal-organic molecules that exhibits much promise for solar energy harvesting and solar-driven catalysis applications. Although many experimental studies have been performed calling for high-level simulations to elucidate their photophysics, a complete picture is still missing. This is the goal of the present thesis. On the ultrafast (femtosecond) timescale we studied the non-adiabatic relaxation of a prototypical Cu(I)-phenanthroline, [Cu(dmp)2]+, by performing excited state simulations using two approaches: quantum dynamics and trajectory surface hopping. These simulations help to identify several mechanisms, internal conversion, pseudo Jahn-Teller distortion, intersystem crossing, occurring in the subpicosecond time scale. Surprisingly, we have found that intersystem crossing does not take place between the lowest singlet and triplet excited states, as previously proposed, but between the lowest singlet and higher triplet states. Moreover, we observed the initial stages (< 100 fs) of the solvent reorganization due to the electronic density changes in the excited state. This leads to an energy stabilization of the excited states that is associated with an increase of the non-radiative decay rate. The quantum dynamics simulations allowed us to provide indications for performing additional spectroscopy measurements by using the recently developed X-ray Free Electron Lasers (X-FELs). This technology can monitor both electronic and structural changes with an unprecedented time resolution of tens of femtoseconds and, therefore, is capable of revealing the aforementioned processes. In addition, we questioned the feasibility of such experiments and calculated the signal strengths for XAS and XES transient spectra. Finally, we analyzed the luminescence quenching, which has been observed for all Cu(I)-phenanthroline complexes when they are dissolved in strongly donating solvents. By performing Molecular Dynamics calculations we showed that, in contrast with the previously accepted model based on the formation of an exciplex (a species formed by two molecules, one in the excited state and one in the ground state), no stable exciplex is formed and that quenching is due to electrostatic solute-solvent interactions. In addition, we investigated how the geometry configuration can affect the luminescence lifetime in these molecules. We found a correlation between rigidity of the copper complex - inhibition of the pseudo Jahn-Teller distortion - and lifetime of the emission. The more the metal complex retains the ground state structure (large substituents), the longer its lifetime. This effect is attributed to a higher energy gap (excited state minus ground state energy) due to the reduction of relaxation. Our research reveals important insights into the relaxation mechanism and the complex interplay between geometry and electronic structure in Cu(I)-phenanthroline. These results can be exploited for guiding the synthesis of complexes with the desired physical properties.

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