Tracking dynamics of metallophilic interactions in the excited state using ultrafast structural probes

Abstract

Metallophilic interactions in closed-shell d10 metal centres play a significant role in governing the photophysical and photochemical properties of their complexes. These complexes have formed a wide range of applications, ranging from luminescent materials for energy-efficient optoelectronics and photocatalysts for solar energy conversion to sensors for environmental monitoring and fluorescent markers for microscopy imaging. The relevant photophysical properties are defined by initial processes right after photoexcitation on the way to the formation of the long-lived luminescent excited state. Thus, a detailed understanding of their photoinduced structural dynamics is crucial, as these early processes define the relaxation pathways leading to the formation of long-lived emissive or reactive excited states. Elucidating these dynamics provides fundamental insights necessary for the rational design and optimization of metallophilic systems with tailored photophysical performance and enhanced functional stability.

This thesis focuses on the investigation of metallophilic dinuclear complexes. It includes a detailed description of the synthesis and characterization of five different bimetallic complexes. Among them, two gold(I) dimer complexes, [Au2(dcpm)2](PF6)2, and [Au2(pnnp)2]Cl2, were investigated in detail with the combination of steady-state extended X-ray absorption fine structure (EXAFS), and time-resolved wide-angle X-ray scattering (WAXS) measurements with optical excitation at 266 and 320 nm to enable the structural reorganization and Au-Au bond distance change over time ranges spanning from femtoseconds to picoseconds. Three other metal dimer complexes, [Au2(pnnp)Cl2], [Ag2(dcpm)2](PF6)2 and [Cu2(dcpm)2](PF6)2 were studied, but only the preliminary results are included in this thesis.

The experiments were conducted at large-scale synchrotron and XFEL facilities, including PETRA III (P64), ESRF (ID09), and European XFEL (FXE), exploiting high-brilliance X-ray sources. The thesis includes the respective theoretical background and experimental setup for these techniques. These studies address the fundamental interplay between metallophilic interactions, ligand environment, and solvent reorganization driving the excited-state structural evolution. This thesis will present the experimental results and analysis of the gold(I) dimer complexes in the excited state, having Au-Au bond contraction. The ultrafast structural dynamics of [Au2(dcpm)2](PF6)2 complex exhibits an Au–Au contraction of approximately -0.11 Å whereas [Au2(pnnp)2]Cl2 complex undergoes a larger contraction of about -0.52 Å at the same 100 fs delay. Analysis of the solution-phase scattering data incorporated both solute and solvent contributions, while initial classical molecular dynamics (MD) simulations of the solvation shell structure were employed to support the experimental findings.