Christoph Riehn

Time-resolved photoelectron spectroscopy of a dinuclear Pt(II) complex: Tunneling autodetachment from both singlet and triplet excited states of a molecular dianion

Time-resolved pump-probe photoelectron spectroscopy has been used to study the relaxation dynamics of gaseous [Pt2(μ-P2O5H2)4 + 2H](2-) after population of its first singlet excited state by 388 nm femtosecond laser irradiation. In contrast to the fluorescence and phosphorescence observed in condensed phase, a significant fraction of the photoexcited isolated dianions decays by electron loss to form the corresponding monoanions. Our transient photoelectron data reveal an ultrafast decay of the initially excited singlet (1)A2u state and concomitant rise in population of the triplet (3)A2u state, via sub-picosecond intersystem crossing (ISC). We find that both of the electronically excited states are metastably bound behind a repulsive Coulomb barrier and can decay via delayed autodetachment to yield electrons with characteristic kinetic energies. While excited state tunneling detachment (ESETD) from the singlet (1)A2u state takes only a few picoseconds, ESETD from the triplet (3)A2u state is much slower and proceeds on a time scale of hundreds of nanoseconds. The ISC rate in the gas phase is significantly higher than in solution, which can be rationalized in terms of changes to the energy dissipation mechanism in the absence of solvent molecules. [Pt2(μ-P2O5H2)4 + 2H](2-) is the first example of a photoexcited multianion for which ESETD has been observed following ISC.

Vibrational Coherence Controls Molecular Fragmentation: Ultrafast Photodynamics of the [Ag2Cl]+ Scaffold

The recently introduced pump-probe fragmentation action spectroscopy reveals a unique observation of excited state vibrational coherence (430-460 fs) in the isolated metal complex [Ag2(Cl)(dcpm)2)]+ (dcpm = bis(dicyclohexylphosphino)methane) containing the [Ag2Cl]+ scaffold. After photoexcitation by an 1XMCT transition (260 nm) in an ion trap, an unexpected correlation between specific fragment ions (loss of HCl/Cl- vs loss of dcpm) and the phase of the wave packet is probed (1150 nm). Based on ab initio calculations, we assign the primary electronically excited state and ascribe the observed coherences (72-78 cm-1) to contain predominantly Ag-Ag stretch character. We propose specific probe absorption and vibronic coupling at the classical turning points to switch remarkably early on between the different fragmentation pathways. The overall excited state dynamics are fitted to a multiexponential decay with time constants: 0.2-0.4/3-4/19-26/104-161 ps. These findings open new perspectives for further dynamics investigations and possible applications in photocatalysis.

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)(AuDppy)6Ag2·(BF4) x](4- x)+ with x = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal-organic frameworks. The monocationic species ( x = 3) is found to be less stable than the dicationic one ( x = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation.

Rational in situ tuning of a supramolecular photocatalyst for hydrogen evolution

A new heterodinuclear ruthenium platinum complex containing an N-heterocyclic carbene bridging ligand was synthesised and used for photocatalytic hydrogen evolution. The molecular integrity of the unit was proven by mercury poisoning and the lability of terminal halido ligands verified with ESI-MS. In situ ligand exchange enhances the catalytic performance leading to an almost 500-fold increase in the turnover frequency and a significantly improved overall turnover number.

Excited-State Dynamics of Catalytically Active Transition Metal Complexes Studied by Transient Photofragmentation in Gas Phase and Transient Absorption in Solution

Femtosecond photofragmentation (gas phase) and transient absorption (solution) revealed ultrafast electronic coupling (0.1–3 ps) and energy transfer (7–12 ps) in a Ru(II)-complex and ultrafast formation (~0.4 ps) of a long-lived triplet state in a Pd3-complex.