Geometric and electronic properties in a series of phosphorescent heteroleptic Cu(I) complexes: Crystallographic and computational studies

Ultrafast electronic, infrared, and X-ray absorption spectroscopy study of Cu(I) phosphine diimine complexes

The study aims to understand the role of the transient bonding in the interplay between the structural and electronic changes in heteroleptic Cu(I) diimine diphosphine complexes. This is an emerging class of photosensitisers which absorb in the red region of the spectrum, whilst retaining a sufficiently long excited state lifetime. Here, the dynamics of these complexes are explored by transient absorption (TA) and time-resolved infrared (TRIR) spectroscopy, which reveal ultrafast intersystem crossing and structural distortion occurring. Two potential mechanisms affecting excited state decay in these complexes involve a transient formation of a solvent adduct, made possible by the opening up of the Cu coordination centre in the excited state due to structural distortion, and by a transient coordination of the O-atom of the phosphine ligand to the copper center. X-ray absorption studies of the ground electronic state have been conducted as a prerequisite for the upcoming X-ray spectroscopy studies which will directly determine structural dynamics. The potential for these complexes to be used in bimolecular applications is confirmed by a significant yield of singlet oxygen production.

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes

A series of six new Cu(i) complexes with ([Cu(N-{4-R}pyridine-2-yl-methanimine)(PPh₃)Br]) formulation, where R corresponds to a donor or acceptor p-substituent, have been synthesized and were used to study self-association effects on their structural and electrochemical properties. X-ray diffraction results showed that in all complexes the packing is organized from a dimer generated by supramolecular π stacking and hydrogen bonding. ¹H-NMR experiments at several concentrations showed that all complexes undergo a fast-self-association monomer-dimer equilibrium in solution, while changes in resonance frequency towards the high or low field in specific protons of the imine ligand allow establishing that dimers have similar structures to those found in the crystal. The thermodynamic parameters for this self-association process were calculated from dimerization constants determined by VT-¹H-NMR experiments for several concentrations at different temperatures. The values for K _D (4.0 to 70.0 M^-1 range), ΔH (-1.4 to -2.6 kcal mol^-1 range), ΔS (-0.2 to 2.1 cal mol^-1 K^-1 range), and ΔG ₂₉₈ (-0.8 to -2.0 kcal mol^-1 range) are of the same order and indicate that the self-dimerization process is enthalpically driven for all complexes. The electrochemical profile of the complexes shows two redox Cu(ii)/Cu(i) processes whose relative intensities are sensitive to concentration changes, indicating that both species are in chemical equilibrium, with the monomer and the dimer having different electrochemical characteristics. We associate this behaviour with the structural lability of the Cu(i) centre that allows the monomeric molecules to reorder conformationally to achieve a more adequate assembly in the non-covalent dimer. As expected, structural properties in the solid and in solution, as well as their electrochemical properties, are not correlated with the electronic parameters usually used to evaluate R substituent effects. This confirms that the properties of the Cu(i) complexes are usually more influenced by steric effects than by the inductive effects of substituents of the ligands. In fact, the results obtained showed the importance of non-covalent intermolecular interactions in the structuring of the coordination geometry around the Cu centre and in the coordinative stability to avoid dissociative equilibria.

Unexpected Boost in Activity of a Cu(I) Photosensitizer by Stabilizing a Transient Excited State

In this study, we present a slight but surprisingly successful structural modification of the previously reported heteroleptic Cu(I) photosensitizer Cubiipo ([(xantphos)Cu(biipo)]PF₆; biipo = 16H-benzo-[4',5']-isoquinolino-[2',1':1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one). As a key feature, biipo bears a naphthalimide unit at the back, which is directly fused to a phenanthroline moiety to extend the conjugated π-system. This ligand was now altered to include two additional methyl groups at the 2,9-positions at the phenanthroline scaffold. Comparing the novel Cudmbiipo complex to its predecessor, ultrafast transient absorption spectroscopy reveals the efficient suppression of a major deactivation pathway by stabilization of a transient triplet state. Furthermore, quantitative measurements of singlet oxygen evolution in solution confirmed that a larger fraction of the excited-state population is transferred to the photocatalytically active ligand-centered triplet ³LC state with a much longer lifetime of ∼30 μs compared to Cubiipo (2.6 μs). In addition, Cudmbiipo was compared with the well-established reference complex Cubcp ([(xantphos)Cu(bathocuproine)]PF₆) in terms of its photophysical and photocatalytic properties by applying time-resolved femto- and nanosecond absorption, step-scan Fourier transform infrared (FTIR), and emission spectroscopies. Superior light-harvesting properties and a greatly enhanced excited-state lifetime with respect to Cubcp enable Cudmbiipo to be more active in exemplary photocatalytic applications, i.e., in the formation of singlet oxygen and the isomerization of (E)-stilbene.

Quasi-Restricted Orbital Description of the Copper(I) Photoredox Catalytic Cycle

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis[2-(diphenylphosphino)phenyl]ether) are scrutinized and characterized using quasi-restricted orbitals (QRO), electron density differences and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet 3MLCT (d9L-1), Dox (d9L0) and Dred (d10L-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Amongst others, we reveal the molecular-scale delocalization of the ligand-centered radical in the a 3MLCT (d9L-1) and Dred (d9L-1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S₁) and excited triplet (T₁) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]⁺ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)₂(N^N)]⁺ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N₃)]⁺, [Cu(P)(N^N)(N)]⁺ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]⁺ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]⁺ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]⁺ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]⁺ and two-coordinate [Cu(N)(NHC)]⁺ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

The surprising effects of sulfur: achieving long excited-state lifetimes in heteroleptic copper(i) emitters

A series of heteroleptic [Cu(N^N)(P^P)][PF₆] complexes is reported in which N^N is a di(methylsulfanyl)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeS)₂phen) or di(methoxy)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeO)₂phen) and P^P is bis(2-(diphenylphosphano)phenyl)ether (POP) or 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos). The effects of the different substituents are investigated through structural, electrochemical and photophysical studies and by using DFT and TD-DFT calculations. Introducing methylsulfanyl groups in the 2,9-, 3,8- or 4,7-positions of the phen domain alters the composition of the frontier molecular orbitals of the [Cu(N^N)(P^P)]⁺ complexes significantly, so that ligand-centred (LC) transitions become photophysically relevant with respect to metal-to-ligand charge transfer (MLCT). Within this series, [Cu(2,9-(MeS)₂phen)(POP)][PF₆] exhibits the highest photoluminescence quantum yield of 15% and the longest excited-state lifetime of 8.3 μs in solution. In the solid state and in frozen matrices at 77 K, the electronic effects of the methylsulfanyl or methoxy substituents are highlighted, thus resulting in luminescence lifetimes of 2 to 4.2 ms at 77 K with predominantly LC character for both the 3,8- and 4,7-(MeS)₂phen containing complexes. The results of the investigation give new guidelines on how to influence the luminescence properties in [Cu(N^N)(P^P)]⁺ complexes which will aid in the development of new sustainable and efficient copper(i) emitters.

Dependence between luminescence properties of Cu(i) complexes and electronic/structural parameters derived from steric effects

Quantification of steric effects induced by bulky N^N ligands and their relationship with the luminescence properties of Cu(i) complexes.

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

Cu(I) 4H-imidazolato complexes are excellent photosensitizers with broad and intense light absorption properties, based on an earth-abundant metal, and hold great promise as photosensitizers in artificial photosynthesis and for accumulation of redox equivalents. In this study, the excited-state relaxation dynamics of three novel heteroleptic Cu(I) 4H-imidazolato complexes with phenyl, tolyl, and mesityl side groups are systematically investigated by femtosecond and nanosecond time-resolved transient absorption spectroscopy and theoretical methods, complemented by steady-state absorption spectroscopy and (spectro)electrochemistry. After photoexcitation into the metal-to-ligand charge transfer (MLCT) and intraligand charge transfer absorption band, fast (0.6-1 ps) intersystem crossing occurs into the triplet MLCT manifold. The triplet-state population relaxes via the geometrical planarization of the N-aryl rings on the Cu(I) 4H-imidazolato complexes. Depending on the initial Franck-Condon state, the remaining small singlet state population relaxes into two geometrically distinct minima geometries with similar energy, S_1/2,relax and S_3/4,relax. Subsequent ground-state recovery from S_1/2,relax and internal conversion from S_3/4,relax to S_1/2,relax take place on a 100 ps time scale. The internal conversion can be understood as hole transfer from a d_yz-orbital to a d_xz-orbital, which is accompanied with the structural reorganization of the coordination environment. Generally, the photophysical processes are determined by the steric hindrance of the side groups on the ligands. And the excited singlet-state pathways are dependent on the excitation wavelength.

Wide-Bite-Angle Diphosphine Ligands in Thermally Activated Delayed Fluorescent Copper(I) Complexes: Impact on the Performance of Electroluminescence Applications

We report a series of seven cationic heteroleptic copper(I) complexes of the form [Cu(P^P)(dmphen)]BF₄, where dmphen is 2,9-dimethyl-1,10-phenanthroline and P^P is a diphosphine chelate, in which the effect of the bite angle of the diphosphine ligand on the photophysical properties of the complexes was studied. Several of the complexes exhibit moderately high photoluminescence quantum yields in the solid state, with Φ_PL of up to 35%, and in solution, with Φ_PL of up to 98%. We were able to correlate the powder photoluminescence quantum yields with the % V_bur of the P^P ligand. The most emissive complexes were used to fabricate both organic light-emitting diodes and light-emitting electrochemical cells (LECs), both of which showed moderate performance. Compared to the benchmark copper(I)-based LECs, [Cu(dnbp)(DPEPhos)]⁺ (maximum external quantum efficiency, EQE_max = 16%), complex 3 (EQE_max = 1.85%) showed a much longer device lifetime (t_1/2 = 1.25 h and >16.5 h for [Cu(dnbp)(DPEPhos)]⁺ and complex 3, respectively). The electrochemiluminescence (ECL) properties of several complexes were also studied, which, to the best of our knowledge, constitutes the first ECL study for heteroleptic copper(I) complexes. Notably, complexes exhibiting more reversible electrochemistry were associated with higher annihilation ECL as well as better performance in a LEC.

Heteroleptic [Cu(P^P)(N^N)][PF6] Complexes: Effects of Isomer Switching from 2,2′-biquinoline to 1,1′-biisoquinoline

The preparation and characterization of [Cu(POP)(biq)][PF6] and [Cu(xantphos)(biq)][PF6] are reported (biq = 1,1′-biisoquinoline, POP = bis(2-(diphenylphosphanyl)phenyl)ether, and xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane). The single crystal structure of [Cu(POP)(biq)][PF6] 0.5Et2O was determined and compared to that in three salts of [Cu(POP)(bq)]+ in which bq = 2,2′-biquinoline. The P–C–P angle is 114.456(19)o in [Cu(POP)(biq)]+ compared to a range of 118.29(3)–119.60(3)o [Cu(POP)(bq)]+. There is a change from an intra-POP PPh2-phenyl/(C6H4)2O-arene π-stacking in [Cu(POP)(biq)]+ to a π-stacking contact between the POP and bq ligands in [Cu(POP)(bq)]+. In solution and at ambient temperatures, the [Cu(POP)(biq)][PF6]+ and [Cu(xantphos)(biq)]+ cations undergo several concurrent dynamic processes, as evidenced in their multinuclear NMR spectra. The photophysical and electrochemical behaviors of the heteroleptic copper (I) complexes were investigated, and the effects of changing from bq to biq are described. Short Cu···O distances within the [Cu(POP)(biq)]+ and [Cu(xantphos)(biq)]+ cations may contribute to their very low photoluminescent quantum yields.

Extended π-Systems in Diimine Ligands in [Cu(P^P)(N^N)][PF6] Complexes: From 2,2′-Bipyridine to 2-(Pyridin-2-yl)Quinoline

We describe the synthesis and characterization of [Cu(POP)(1)][PF6], [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] in which ligands 1 and 2 are 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, respectively. With 2,2'-bipyridine (bpy) as a benchmark, we assess the impact of the extended π-system on structural and solid-state photophysical properties. The single crystal structures of [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] were determined and confirmed a distorted tetrahedral copper(I) coordination environment in each [Cu(P^P)(N^N)]+ cation. The xanthene unit in [Cu(xantphos)(1)][PF6] and [Cu(xantphos)(2)][PF6] hosts the quinoline unit of 1, and the 6-methylpyridine group of 2. 1H NMR spectroscopic data indicate that these different ligand orientations are also observed in acetone solution. In their crystal structures, the [Cu(POP)(2)]+, [Cu(xantphos)(1)]+, and [Cu(xantphos)(2)]+ cations exhibit different edge-to-face and face-to-face π-interactions, but in all cases, the copper(I) centre is effectively protected by a ligand sheath. In [Cu(POP)(2)][PF6], pairs of cations engage in an efficient face-to-face π-stacking embrace, and we suggest that this may contribute to this compound having the highest photoluminescence quantum yield (PLQY = 21%) of the series. With reference to data from the Cambridge Structural Database, we compare packing effects and PLQY data for the complexes incorporating 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, with those of the benchmark bpy-containing compounds. We also assess the effect that Cu⋯O distances in the {Cu(POP)} and {Cu(xantphos)} domains of [Cu(P^P)(N^N)][X] compounds have on solid-state PLQY values.

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.

Excited State Properties of Heteroleptic Cu(I) 4H-Imidazolate Complexes

The excited state properties of three heteroleptic copper(I) xantphos 4H-imidazolate complexes are investigated by means of femtosecond and nanosecond time-resolved transient absorption spectroscopy in dichloromethane solution. The subpicosecond spectral changes observed after excitation into the MLCT absorption band are interpreted as intersystem crossing from the singlet to the triplet manifold. This interpretation is corroborated by DFT and TD-DFT results, indicating a comparable molecular geometry in the ground state (and hence the nonrelaxed singlet state) and the excited triplet state. Population of the triplet state is followed by planarization of the N-aryl rings of the 4H-imidazolate ligand on a 10 ps time scale. The planarization strongly depends on the substitution pattern of the N-aryls and correlates with the reduced moment of inertia for the planarization motion. The triplet state subsequently decays to the ground state in about 100 ns. These results demonstrate that the excited state processes of copper(I) complexes depend on the specific ligand(s) and their substitution pattern. Thus, the work presented points to a possibility to design copper(I) complexes with specific photophysical properties.

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.

Ground-State Gas-Phase Structures of Inorganic Molecules Predicted by Density Functional Theory Methods

We tested a battery of density functional theory (DFT) methods ranging from generalized gradient approximation (GGA) via meta-GGA to hybrid meta-GGA schemes as well as Møller-Plesset perturbation theory of the second order and a single and double excitation coupled-cluster (CCSD) theory for their ability to reproduce accurate gas-phase structures of di- and triatomic molecules derived from microwave spectroscopy. We obtained the most accurate molecular structures using the hybrid and hybrid meta-GGA approximations with B3PW91, APF, TPSSh, mPW1PW91, PBE0, mPW1PBE, B972, and B98 functionals, resulting in lowest errors. We recommend using these methods to predict accurate three-dimensional structures of inorganic molecules when intramolecular dispersion interactions play an insignificant role. The structures that the CCSD method predicts are of similar quality although at considerably larger computational cost. The structures that GGA and meta-GGA schemes predict are less accurate with the largest absolute errors detected with BLYP and M11-L, suggesting that these methods should not be used if accurate three-dimensional molecular structures are required. Because of numerical problems related to the integration of the exchange-correlation part of the functional and large scattering of errors, most of the Minnesota models tested, particularly MN12-L, M11, M06-L, SOGGA11, and VSXC, are also not recommended for geometry optimization. When maintaining a low computational budget is essential, the nonseparable gradient functional N12 might work within an acceptable range of error. As expected, the DFT-D3 dispersion correction had a negligible effect on the internuclear distances when combined with the functionals tested on nonweakly bonded di- and triatomic inorganic molecules. By contrast, the dispersion correction for the APF-D functional has been found to shorten the bonds significantly, up to 0.064 Å (AgI), in Ag halides, BaO, BaS, BaF, BaCl, Cu halides, and Li and Na halides and hydrides. These results do not agree well with very accurate structures derived from microwave spectroscopy; we therefore believe that the dispersion correction in the APF-D method should be reconsidered. Finally, we found that inaccurate structures can easily lead to errors of few kcal/mol in single-point energies.