Heteroleptic bis-diimine copper(I) complexes for applications in solar energy conversion

Towards efficient sustainable full-copper dye-sensitized solar cells

Two new heteroleptic copper(i) sensitizers bearing 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid, to anchor the dye on the titania surface, and a π-delocalized 2-(R-phenyl)-1H-phenanthro[9,10-d]imidazole (R = NPh2 or O-hexyl) ancillary ligand were prepared and well characterized. Their performance as dyes in DSSCs is quite similar to that of the related complex bearing 2,9-dimesityl-1,10-phenanthroline as an ancillary ligand, when using the common I-/I3- redox couple or homoleptic copper complexes as electron shuttles. The experimental results along with theoretical calculations confirm the great potential of full-copper DSSCs.

Improving the efficiency of copper-dye-sensitized solar cells by manipulating the electrolyte solution

The use of a copper(i) dye, bearing a 2,9-dimesityl-1,10-phenanthroline and a 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid, was investigated in DSSCs with various electrolyte solutions based on two different redox mediators, namely the common I-/I3- couple and an interesting copper electron shuttle. The experimental results provide evidence of the importance of the redox mediator concentration and the crucial role of additives such as 4-tert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide in the performance of sustainable "full-copper" DSSCs, consolidating the way to DSSCs with Earth-abundant components.

Electrolyte tuning in dye-sensitized solar cells with N-heterocyclic carbene (NHC) iron(II) sensitizers

We demonstrate that the performances of dye-sensitized solar cells (DSCs) sensitized with a previously reported N-heterocyclic carbene iron(II) dye in the presence of chenodeoxycholic acid co-adsorbant, can be considerably improved by altering the composition of the electrolyte while retaining an I^-/I₃ ^- redox shuttle. Critical factors are the solvent, presence of ionic liquid, and the use of the additives 1-methylbenzimidazole (MBI) and 4-tert-butylpyridine (TBP). For the electrolyte solvent, 3-methoxypropionitrile (MPN) is preferable to acetonitrile, leading to a higher short-circuit current density (J _SC) with little change in the open-circuit voltage (V _OC). For electrolytes containing MPN, an ionic liquid and MBI (0.5 M), DSC performance depended on the ionic liquid with 1-ethyl-3-methylimidazolium hexafluoridophosphate (EMIMPF) > 1,2-dimethyl-3-propylimidazolium iodide (DMPII) > 1-butyl-3-methylimidazolium iodide (BMII) ≈ 1-butyl-3-methylimidazolium hexafluoridophosphate (BMIMPF). Omitting the MBI leads to a significant improvement in J _SC when the ionic liquid is DMPII, BMII or BMIMPF, but with EMIMPF the removal of the MBI additive results in a dramatic decrease in V _OC (542 to 42 mV). For electrolytes containing MPN and DMPII, the effects of altering the MBI concentration have also been investigated. Although the addition of TBP improves V _OC, it causes significant decreases in J _SC. The best performing DSCs with the NHC-iron(II) dye employ an I^-/I₃ ^--based electrolyte with MPN as solvent, DMPII ionic liquid (0.6 M) with no or 0.01 M MBI; values of J _SC = 2.31 to 2.78 mA cm^-2, V _OC = 292 to 374 mV have been achieved giving η in the range of 0.47 to 0.57% which represents 7.8 to 9.3% relative to an N719 reference DSC set at 100%. Electrochemical impedance spectroscopy has been used to understand the role of the MBI additive in the electrolytes.

Heteroleptic Copper(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands: Rationalization of the Photophysical and Electrochemical Properties

The electronic and structural properties of ten heteroleptic [Cu(NN)(PP)]⁺ complexes have been investigated. NN indicates 1,10-phenanthroline (phen) or 4,7-diphenyl-1,10-phenanthroline (Bphen); each of these ligands is combined with five PP bis-phosphine chelators, i.e., bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)benzene (dppb), and bis[(2-diphenylphosphino)phenyl] ether (POP). All complexes are mononuclear, apart from those based on dppm, which are dinuclear. Experimental data-also taken from the literature and including electrochemical properties, X-ray crystal structures, UV-vis absorption spectra in CH₂Cl₂, luminescence spectra and lifetimes in solution, in PMMA, and as powders-have been rationalized with the support of density functional theory calculations. Temperature dependent studies (78-358 K) have been performed for selected complexes to assess thermally activated delayed fluorescence. The main findings are (i) dependence of the ground-state geometry on the crystallization conditions, with the same complex often yielding different crystal structures; (ii) simple model compounds with imposed C_{2 v} symmetry ([Cu(phen)(PX₃)₂]⁺; X = H or CH₃) are capable of modeling structural parameters as a function of the P-Cu-P bite angle, which plays a key role in dictating the overall structure of [Cu(NN)(PP)]⁺ complexes; (iii) as the P-Cu-P angle increases, the energy of the metal-to-ligand charge transfer absorption bands linearly increases; (iv) the former correlation does not hold for emission spectra, which are red-shifted for the weaker luminophores; (v) the larger the number of intramolecular π-interactions within the complex in the ground state, the higher the luminescence quantum yield, underpinning a geometry locking effect that limits the structural flattening of the excited state. This work provides a general framework to rationalize the structure-property relationships of [Cu(NN)(PP)]⁺, a class of compounds of increasing relevance for electroluminescent devices, photoredox catalysis, and solar-to-fuels conversion, which so far have been investigated in an unsystematic fashion, eluding a comprehensive understanding.

Photoactive Complexes with Earth-Abundant Metals

In this invited Perspective, recent developments and possible future directions of research on photoactive coordination compounds made from nonprecious transition metal elements will be discussed. The focus is on conceptually new, structurally well-characterized complexes with excited-state lifetimes between 10 ps and 1 ms in fluid solution for possible applications in photosensitizing, light-harvesting, luminescence and catalysis. The key metal elements considered herein are Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W and Ce in various oxidation states equipped with diverse ligands, giving access to long-lived excited states via a range of fundamentally different types of electronic transitions. Research performed in this area over the past five years demonstrated that a much broader spectrum of metal complexes than what was long considered relevant exhibits useful photophysics and photochemistry.

Probing Cancer Cells through Intracellular Aggregation-Induced Emission Kinetic Rate of Copper Nanoclusters

pH-responsive luminescent copper nanoclusters (Cu NCs) with aggregation-induced emission (AIE) characteristics have been synthesized. Upon internalization into living cells, the NCs displayed a cellular pH environment-dependent luminescence change with orange-red emission at pH_i 4.5, whereas bright green emission was observed over time at pH_i 7.4, through their AIE attributes. Furthermore, the intracellular AIE kinetics of the NC probe was measured in MCF-7 cells and compared to that of HEK-293 cells. Intriguingly, the intracellular rate constant value derived for AIE kinetics in MCF-7 cells was found to be 3-fold higher than that in HEK-293 cell lines, whereas the value was 2-fold higher than that observed in aqueous medium. This provided a new platform to study different cell lines based on intracellular AIE in living cells, with additional potential for future applications in cellular imaging, diagnostics, and disease detection.

Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore-Acceptor Dyads

In this work, we present the photoinduced charge separation dynamics of four molecular dyads composed of heteroleptic Cu(I)bis(phenanthroline) chromophores linked directly to the common electron acceptor naphthalene diimide. The dyads were designed to allow us to (1) detect any kinetic preference for directionality during photoinduced electron transfer across the heteroleptic complex and (2) probe the influence of excited-state flattening on intramolecular charge separation. Singular value decomposition of ultrafast optical transient absorption spectra demonstrates that charge transfer occurs with strong directional preference, and charge separation occurs up to 35 times faster when the acceptor is linked to the sterically blocking ligand. Further, the charge-separated state in these dyads is stabilized by polar solvents, resulting in dramatically longer lifetimes for dyads with minimal substitution about the Cu(I) center. This unexpected but exciting observation suggests a new approach to the design of Cu(I)bis(phenanthroline) chromophores that can support long-lived vectorial charge separation.

Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H2

The synthesis of two new heteroleptic Cu(I) photosensitizers (PS), [Cu(Xantphos)(NN)]PF₆ (NN = biq = 2,2'-biquinoline, dmebiq = 2,2'-biquinoline-4,4'-dimethyl ester; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), along with the associated structural, photophysical, and electrochemical properties, are described. The biquinoline diimine ligand extends the PS light absorbing properties into the visible with a maximum absorption at 455 and 505 nm for NN = biq and dmebiq, respectively, in CH₂Cl₂ solvent. Following photoexcitation, both Cu(I) PS are emissive at low energy, albeit displaying stark differences in their excited state lifetimes (τ_MLCT = 410 ± 5 (biq) and 44 ± 4 ns (dmebiq)). Cyclic voltammetry indicates a Cu-based HOMO and NN-based LUMO for both complexes, whereby the methyl ester substituents stabilize the LUMO within [Cu(Xantphos)(dmebiq)]⁺ by ∼0.37 V compared to the unsubstituted analogue. When combined with H₂O, N,N-dimethylaniline (DMA) electron donor, and cis-[Rh(NN)₂Cl₂]PF₆ (NN = Me₂bpy = 4,4'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, dmebpy = 2,2'-bipyridine-4,4'-dimethyl ester) water reduction catalysts (WRC), photocatalytic H₂ evolution is only observed using the [Cu(Xantphos)(biq)]⁺ PS. Furthermore, the choice of cis-[Rh(NN)₂Cl₂]⁺ WRC strongly affects the catalytic activity with turnover numbers (TON_Rh = mol H₂ per mol Rh catalyst) of 25 ± 3, 22 ± 1, and 43 ± 3 for NN = Me₂bpy, bpy, and dmebpy, respectively. This work illustrates how ligand modification to carefully tune the PS light absorbing, excited state, and redox-active properties, along with the WRC redox potentials, can have a profound impact on the photoinduced intermolecular electron transfer between components and the subsequent catalytic activity.

Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and X-ray transient absorption spectroscopy

The kinetics of photoinduced electron and energy transfer in a family of tetrapyridophenazine-bridged heteroleptic homo- and heterodinuclear copper(i) bis(phenanthroline)/ruthenium(ii) polypyridyl complexes were studied using ultrafast optical and multi-edge X-ray transient absorption spectroscopies. This work combines the synthesis of heterodinuclear Cu(i)-Ru(ii) analogs of the homodinuclear Cu(i)-Cu(i) targets with spectroscopic analysis and electronic structure calculations to first disentangle the dynamics at individual metal sites by taking advantage of the element and site specificity of X-ray absorption and theoretical methods. The excited state dynamical models developed for the heterodinuclear complexes are then applied to model the more challenging homodinuclear complexes. These results suggest that both intermetallic charge and energy transfer can be observed in an asymmetric dinuclear copper complex in which the ground state redox potentials of the copper sites are offset by only 310 meV. We also demonstrate the ability of several of these complexes to effectively and unidirectionally shuttle energy between different metal centers, a property that could be of great use in the design of broadly absorbing and multifunctional multimetallic photocatalysts. This work provides an important step toward developing both a fundamental conceptual picture and a practical experimental handle with which synthetic chemists, spectroscopists, and theoreticians may collaborate to engineer cheap and efficient photocatalytic materials capable of performing coulombically demanding chemical transformations.

Synthesis, structure, and excited state kinetics of heteroleptic Cu(i) complexes with a new sterically demanding phenanthroline ligand

In this report we describe the synthesis of a new phenanthroline ligand, 2,9-di(2,4,6-tri-isopropyl-phenyl)-1,10-phenanthroline (bL2) and its use as the blocking ligand in the preparation of two new heteroleptic Cu(i)diimine complexes. Analysis of the CuHETPHEN single crystal structures shows a distinct distortion from an ideal tetrahedral geometry around the Cu(i) center, forced by the secondary phenanthroline ligand rotating to accommodate the isopropyl groups of bL2. The increased steric bulk of bL2 as compared to the more commonly used 2,9-dimesityl-1,10-phenanthroline blocking ligand prohibits intramolecular ligand-ligand interaction, which is unique among CuHETPHEN complexes. The ground state optical and redox properties of CuHETPHEN complexes are responsive to the substitution on the blocking ligand even though the differences in structure are far removed from the Cu(i) center. Transient optical spectroscopy was used to understand the excited state kinetics in both coordinating and non-coordinating solvents following visible excitation. Substitution of the blocking phenanthroline ligand has a significant impact on the ³MLCT decay and can be used to increase the excited state lifetime by 50%. Electronic structure calculations established relationships between ground and excited state properties, and general entatic state concepts are discussed for copper photosensitizers. This work contributes to the growing library of CuHETPHEN complexes and broadens the fundamental understanding of their ground and excited state properties.

Determination of side products in the photocatalytic generation of hydrogen with copper photosensitizers by resonance Raman spectroelectrochemistry

A combination of UV-Vis, resonance Raman spectroscopy and electrochemistry is employed to reveal the nature of a side product when using heteroleptic Cu(i)-photosensitizers for photocatalytic hydrogen production.