Excited-state dynamics of transition metal complexes investigated by ultrafast vibrational spectroscopy

Abstract

Transition metal complexes often show distinct photophysical properties arising from the structural symmetries compared to other chromophores, and are widely adopted in many applications including photocatalysts, solar cells, organic light-emitting diodes, therapeutic agents, etc. Copper(Ⅰ) diimine complexes show a tetrahedral geometry in the ground state.Upon photoexcitation, the Cu+ ion oxidizes to Cu2+ with the subsequent structural changes into a so-called “flattened” geometry which is facilitated by the metal-to-ligand charge transfer (MLCT) state in the excited states. The flattening distortion of ligands has been studied by Xray absorption near-edge structure and femtosecond transient absorption (TA) spectroscopy. Flattened geometries of the copper(I) diamine complexes along the dihedral angle between two coordinating ligands were found optimal in the excited states by theoretical calculations. However, the excited state dynamics for the flattening distortion and the subsequent photophysical processes were somewhat inconsistent between several previous works on the complexes. In this thesis, the excited state dynamics and structural changes of the copper(I) diimine complexes, [Cu(phen)2]+ and [Cu(dmphen)2]+ (phen = 1,10-phenanthroline and dmphen = 2,9- dimethyl-1,10-phenanthroline) in dichloromethane, acetonitrile, and dimethyl sulfoxide are explored by TA and femtosecond stimulated Raman spectroscopy (FSRS). The flattening dynamics of 1,10-phenanthroline ligands takes place in hundred femtoseconds which is represented by the rising components in the excited state absorption (ESA) signals of TA results. The flattened complexes of [Cu(phen)2]+ in the excited states decay via multiple photochemical pathways including internal conversion (IC) and/or intersystem crossing (ISC), where the excited state dynamics show strong dependence on solvent, for example, on the coordinating ability of solvent molecules. The IC and ISC dynamics processes analyzed from the spectral changes in the ground state bleaching (GSB) and ESA bands have shown much faster dynamics in the strongly coordinating solvents (acetonitrile and dimethyl sulfoxide) than in the poorly coordination solvent (dichloromethane). Although the TA is powerful for the excited state electronic structures, it is inherently insensitive to the small structural changes of chromophores in the ultrafast time scales. FSRS was instead employed to investigate the structural dynamics of the copper(I) diimine complexes upon photoexcitation including the flattening distortions and IC/ISC. Two kinetic components within 1 ps have been observed from the changes in the frequency of ligands’ vibrational modes in the FSRS results of [Cu(phen)2]+ and [Cu(dmphen)2]+, which may represent the vibrational changes during the MLCT transition in ~100 fs and the flattening distortion of ligands in hundreds of femtoseconds. The effect of the steric hindrance of ligands on the flattening dynamics will be discussed on [Cu(phen)2]+ and [Cu(dmphen)2]+. The flattening dynamics appears slightly faster for [Cu(phen)2]+ where a slightly more flattened geometry is expected due to a smaller steric hindrance than [Cu(dmphen)2]+. The comprehensive investigations for the non-radiative and radiative processes and the related structural changes of the transition metal complexes may reveal the details of the photodynamic processes including the charge transfers, structural changes, and coordination changes in the excited states.