The Coordination Behaviour of Cu^I Photosensitizers Bearing Multidentate Ligands Investigated by X‐ray Absorption Spectroscopy

Abstract

A systematic series of four novel homo‐ and heteroleptic Cu I photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.

Ligand‐Induced Donor State Destabilisation – A New Route to Panchromatically Absorbing Cu(I) Complexes

“…However, the Cu−N bonds of the cuprates are about 0.13 Å shorter, than in the respective [Cu(I)(xantphos)(4 H ‐imidazolato)] complexes [37] . This observation is in agreement with reports on Cu(I)‐phenanthroline complexes [19,38,39] where similar bond length differences of about 0.1 Å were observed. These differences mainly attributed to the steric bulk of the xantphos ligand.…”

Time‐Resolved X‐Ray Spectroscopy to Study Luminophores with Relevance for OLEDs

“…[24,90] The structure of this complex in the triplet excited state was investigated in pioneering XAS experiments, [52,91] later [Cu-(dmp) 2 ] + and its derivatives were further investigated utilizing pump-probe X-ray spectroscopy. [75,[92][93][94][95][96] A previous review [97] summarizes early progress in this field and we will focus on the findings reported in recent articles. [75,94,98,99] For homoleptic diimine Cu(I) complexes, the photoexcitation induces Jahn-Teller distortions around the metal center and the initially tetrahedral configuration flattens.…”

“…The copper site can be additionally coordinated in the excited state not only by the solvent molecule but also intramolecularly by suitable coordinating moieties present in the ligand scaffold. For example, for related derivatives of [Cu(dmp) 2 ] + with −CH 2 OCH 3 or −CH 2 SCH 3 in the 2 and 9 position of the phenanthroline ligand, five‐coordinated excited states were observed (Figure 6a) [94] . Naturally, a −OMe or −SMe donor, respectively, of one of the substituents is pre‐aligned closely to the Cu center already in the ground state and after photoexcitation the corresponding distances became shorter: Cu−O 2.29 Å and Cu−S 2.47 Å.…”

“…The homoleptic Cu complex [Cu(dmp) 2 ] + (dmp=2,9‐dimethyl‐1,10‐phenanthroline) is one of the first materials in which the TADF effect was discovered, [90] but according to the current standards, this complex is not so efficient (the emission quantum yield is ∼2*10 −4 in CH 2 Cl 2 at room temperature) [24,90] . The structure of this complex in the triplet excited state was investigated in pioneering XAS experiments, [52,91] later [Cu(dmp) 2 ] + and its derivatives were further investigated utilizing pump‐probe X‐ray spectroscopy [75,92–96] . A previous review [97] summarizes early progress in this field and we will focus on the findings reported in recent articles [75,94,98,99] .…”

Time‐Resolved X‐Ray Absorption Spectroscopy: Visualizing the Time Evolution of Photophysics and Photochemistry in Photocatalytic Solar Energy Conversion

“…Furthermore, Moonshiram and coworkers investigated the influence of two additional donor moieties (SMe or OMe at the 2,9‐position of the 1,10‐phenanthroline) on the excited states of the homoleptic Cu I photosensitizers by picosecond TR‐XAS. [ 52 ] By replacing one of the diimine donors with a bulky diphosphine xantphos, four novel homo‐ and heteroleptic copper photosensitizers (C1–C4) were compared to systematically elucidate the impact of steric and electronic effects on the excited states. Figure a shows the transient spectra (laser on–off) at an averaged time delay of 80 ps.…”