Imidazo-Phenanthroline Ligands as a Convenient Modular Platform for the Preparation of Heteroleptic Cu(I) Photosensitizers

Abstract

The capture and storage of solar energy is a promising option to overcome current energy issues. To put such systems into practice, molecular photosensitizers should be based on abundant metals and possess a strong absorption capability for visible light. Therefore, a systematic series of four novel heteroleptic Cu(I) complexes of the type [(P^P)Cu(N^N)]+ (with P^P = xantphos and N^N = different diimine ligands) has been prepared. As an essential feature, these copper photosensitizers contain an imidazole moiety at the backbone of the diimine ligand, which increases the aromatic π-system compared to phenanthroline type ligands. Moreover, 2-(4-bromophenyl)-1-phenyl-1H-imidazo-[4,5-f][1,10]phenanthroline was used as a starting point and modular platform for gradually extended diimine ligands. Suzuki cross-coupling was applied to introduce different kind of substituents in the back of this ligand. Afterwards, a combination of NMR spectroscopy, mass spectrometry, X-ray analysis, cyclic voltammetry, UV/vis and emission spectroscopy was used to investigate the structural, electrochemical and photophysical properties of these compounds. As a result, a reversible reduction, strongly increased extinction coefficients and significantly redshifted absorption maxima (>20 nm) were found compared to traditional Cu(I) photosensitizers without an imidazo moiety. Moreover, these compounds show a bright emission in the solid state.

Modulating the Excited-State Decay Pathways of Cu(I) 4H-Imidazolate Complexes by Excitation Wavelength and Ligand Backbone

“…Upon anodic scanning, two irreversible peaks appear for the four Cu­(I) 4 H -imidazolate complexes in DCM (0.50, 0.95 V ( Phe ); 0.40, 0.86 V ( Tol ); 0.46, 0.97 V ( Mes ), 0.59, >1 V ( COOEt ) vs Fc/Fc + ) and in ACN (0.61, 0.78 V ( Phe ); 0.57, 1.01 V ( Tol ); 0.52, 0.84 V ( Mes ), 0.93, 1.21 V ( COOEt ) vs Fc/Fc + ). These oxidation events are assigned to a Cu­(I)-centered oxidation ( E ox,1 ) as well as a xantphos oxidation ( E ox,2 ). ,,− ,,,, The observed trend of the copper-based oxidation potentials of COOEt , Phe , Tol , and Mes in both DCM and ACN also reflects an increasing electron density on the copper center as a consequence of the N -aryl +I effect (Table and SI Figure S11).…”

“…One of the current greatest challenges in science and technology is the conversion of solar energy into chemical energy. − Among the many contemporary concepts, artificial photosynthesis holds great promise. − In artificial photosynthesis, photocatalysts are employed to produce fuels and chemicals by harvesting solar energy. In this context, transition-metal complexes are of interest as photosensitizers due to their versatile optical and electronic properties. − , Especially noble-metal-free systems are currently investigated as molecular components in photocatalytic schemes to produce hydrogen from water. ,, To this end, the requirements for an ideal photosensitizer to capture sunlight and initiate a charge transfer cascade are: a broad and intense absorption in the visible region, reversible electrochemical behavior, high stability under reaction conditions, and a long-lived excited state. ,,,, Cu­(I) complexes meet these requirements and present an emerging class of molecular photosensitizers due to their straightforward and low-cost synthesis. ,, …”

“…10,17,19 To this end, the requirements for an ideal photosensitizer to capture sunlight and initiate a charge transfer cascade are: a broad and intense absorption in the visible region, reversible electrochemical behavior, high stability under reaction conditions, and a longlived excited state. 1,2,9,15,19 Cu(I) complexes meet these requirements and present an emerging class of molecular photosensitizers due to their straightforward and low-cost synthesis. 14,19,20 Various homoleptic Cu(I)diimine complexes 11,21−33 and heteroleptic Cu(I)phosphine-diimine complexes 15,18,34−40 have been investigated.…”

Photosensitization Ability of 1,7-Phenanthroline Based Bis−BODIPYs: Perplexing Role of Intramolecular Rotation on Photophysical Properties

“…Most of the TPSs are based on expensive transition metal complexes of [Pt­(II), − Ir­(III), − or Ru­(II) − ] or bromo- or iodo-containing organic chromophores. , Many examples of TPSs have been reported to be based on 1,10-phenanthroline. Among these, it has been observed that substitutions at the 2- and 9-position of the phenanthroline-based ligands sterically restrict excited-state distortion, thereby lengthening the excited-state lifetime. − Unfortunately, heavy metal based complexes present severe problems due to enhanced dark toxicity . Further, heavy atom free organic triplet photosensitizers are rather scarce, largely due to the low ISC efficiency of common organic compounds.…”

Role of MLCT States in the Franck–Condon Region of Neutral, Heteroleptic Cu(I)–4H-imidazolate Complexes: A Spectroscopic and Theoretical Study

“…Cu­(I) complexes, which can be photoactivated by visible light, are considered promising earth-abundant alternatives to commonly employed ruthenium, platinum, iridium, or rhodium complexes in light-driven processes, such as photosensitizers in artificial photosynthesis, photoredoxcatalysis, or dye-sensitized solar cells. − Heteroleptic Cu­(I) complexes of the type [Cu­(I)­(N ∧ N)­(P ∧ P)] + (with P ∧ P as diphosphine and N ∧ N as a diimine ligand) exhibit strong absorption of ultraviolet light and moderate absorption in the visible spectrum, often alongside with long microsecond excited-state lifetimes . However, while the redox properties and excited-state lifetimes can be optimized by tuning the electronic structure of the ligands, ,,, the tuning of the light-harvesting ability toward broad absorption in the visible spectrum is challenging. ,,− An exception to that are heteroleptic [Cu­(I)­(Xantphos)­(4 H -imidazolate)] complexes (cf. Figure ), reported by us. , They exhibit unusually broad and intense absorption bands that span the visible spectrum with extinction coefficients up to about 2 × 10 4 M –1 cm –1 .…”