Heteroleptic Copper Photosensitizers: Why an Extended π‐System Does Not Automatically Lead to Enhanced Hydrogen Production

Abstract

A series of heteroleptic copper(I) photosensitizers of the type [(P^P)Cu(N^N)] with an extended π-system in the backbone of the diimine ligand has been prepared. The structures of all complexes are completely characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. These novel photosensitizers were assessed with respect to the photocatalytic reduction of protons in the presence of triethylamine and [Fe (CO) ]. Although the solid-state structures and computational results show no significant impact of the π-extension on the structural properties, decreased activities were observed. To explain this drop, a combination of electrochemical and photophysical measurements including time-resolved emission as well as transient absorption spectroscopy in the femto- to nanosecond time regime was used. Consequently, shortened excited state lifetimes caused by the rapid depopulation of the excited states located at the diimine ligand are identified as a major reason for the low photocatalytic performance.

Synthesis, Characterization, and Luminescent Properties of Silver(I) Complexes based on Diphosphine Ligands and 6,7‐Dicyanodipyridoquinoxaline

“…On the other hand, the development of chelate N‐donor ligands, including 1,10‐phenanthroline (phen) and the derivatives of phen, is rapid. The introduction of N‐donor ligands may cause the enhancement of fluorescence properties and the structural diversity of complexes . Moreover, the complexes composed of two ligands containing N and P exhibit supramolecular , self‐assembly through hydrogen bonds, π–π interactions, and C–H ··· π interactions .…”

Designing Highly Efficient CuI Photosensitizers for Photocatalytic H2 Evolution from Water

“…It is notable that the tert-amine-containing complexes,C u-TPAPhen and Cu-DMAPhen, showedp eculiar photophysical properties totally different from the other Cu I complexes.T hese two complexes showedo ne order of magnitude higher molar absorptivities (e)i nt he visible region and significantly higherP LQYs compared to the other complexes (see Figure 1a nd Ta ble 1). [7][8][9][12][13][14] In contrast, the HOMO of Cu-TPAPheni sl ocalizedi nt he amine moieties, and the electronic transition can be assigned to intra-ligand charge-transfer (ILCT). Although such long PL lifetimes of [Cu I (P^P)(N^N)] + complexes have been attributed to the thermally activated delayedf luorescence( TADF), [11] the significantly different absorption and emission behaviors of tert- amine-containing complexes compared to those of other Cu I complexes should be rationalized.…”

Copper(I) Phosphinooxazoline Complexes: Impact of the Ligand Substitution and Steric Demand on the Electrochemical and Photophysical Properties

“…The much lower yield of C3 significantly differs from all other complexes and might be caused by steric constraints (two adjacent phenyl groups) and hence kinetic instability in solution. Such instability, for example, substitution reactions in solution, is well known from sterically congested homo‐ and heteroleptic Cu I complexes …”

“…Unfortunately, a limited stability under operating conditions still hampers their large‐scale application in molecular solar energy conversion schemes . It is known, that in particular heteroleptic Cu I complexes of the type [(P^P)Cu(N^N)] + (with P^P representing a diphosphine and N^N a diimine ligand) can undergo ligand exchange reactions in solution upon light irradiation . This is mainly caused by the formation of thermodynamically more favored homoleptic bisdiimine complex [Cu(N^N) 2 ] + .…”

“…It is known, that in particular heteroleptic Cu I complexes of the type [(P^P)Cu(N^N)] + (with P^P representing a diphosphine and N^N a diimine ligand) can undergo ligand exchange reactions in solution upon light irradiation . This is mainly caused by the formation of thermodynamically more favored homoleptic bisdiimine complex [Cu(N^N) 2 ] + . Hence, this drawback drove the search and development of novel Cu I complexes with an increased stability, but also having other desired properties like a broad absorption in the visible and a reversible redoxchemistry …”