Pump—probe XAS investigation of the triplet state of an Ir photosensitizer with chromenopyridinone ligands

Abstract

The triplet excited state of a new Ir-based photosensitizer with two chromenopyridinone and one bipyridine-based ligands has been studied by pump-probe X-ray absorption near edge structure (XANES) spectroscopy coupled with DFT calculations. The excited state has a lifetime of 0.5 μs in acetonitrile and is characterized by very small changes of the local atomic structure with an average metal-ligand bond length change of less than 0.01 Å. DFT-based calculations allow the interpretation of the XANES in the energy range of ∼50 eV around the absorption edge. The observed transient XANES signal arises from an additional metal-centered Ir 5d vacancy in the excited state which appears as a result of electron transfer from the metal to the ligand. The overall energy shift of the excited state spectrum originates from the shift of 2p and unoccupied states induced by screening effects. The approach for the analysis of time-resolved spectra of 5d metal complexes is quite general and can also be used if excited and ground state structures are significantly different.

Site‐Selective Real‐Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L‐Edge X‐Ray Absorption Spectroscopy**

“…The present study is motivated by early pump‐probe X‐ray absorption spectroscopy (XAS) studies, in which the oxidation states [31,32] and local geometric structures [31–33] of liquid‐phase molecular systems have been investigated. Later, TR XAS has been used to investigate the triplet excited state of the Ir model system Ir(ppy) 3 [34] and the combination with DFT allowed the study of this excited state of a pure solvated Ir photosensitizer [35] . Picosecond‐ and femtosecond‐resolved X‐ray studies of a bimetallic donor‐acceptor Ru−Co complex (a prototypical photocatalytic system) have shown how ultrashort hard X‐ray spectroscopies can be efficiently used to follow both the intra‐ and intermolecular charge transfer processes at the optically silent sites of photocatalysts [36,37] .…”

Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays

“…This approach allows for the simultaneous acquisition of the kinetics ( Fig. 2b) and absorption spectra, which is optimal for experiments in the nanosecond-microsecond time range 34,35 . The absorption spectra at the Cu K-edge are sensitive to changes of the Cu oxidation state, but both non-equivalent Cu sites contribute to the measured XAS.…”

Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes

“…Even with cryo-cooling, soft X-rays induce radiation damage too rapidly, owing to their higher absorption cross section and the larger Auger electron yield. SR studies of metalloenzymes at room temperature are almost impossible in the soft X-ray energy range, although recent advances in sample delivery approaches allow some serial measurements at SR sources in the hard X-ray regime [6][7][8][9][10] .…”