Copper Photosensitizers Research Articles

Photo-catalyzedα-arylation of enol acetate using recyclable silica-supported heteroleptic and homoleptic copper(I) photosensitizers.

Earth-abundant photosensitizers are highly sought after for light-mediated applications, such as photoredox catalysis, depollution and energy conversion schemes. Homoleptic and heteroleptic copper(I) complexes are promising candidates in this field, as copper is abundant and the corresponding complexes are easily obtained in smooth conditions. However, some heteroleptic copper(I) complexes suffer from low (photo)stability that leads to the gradual formation of the corresponding homoleptic complex. Such degradation pathways are detrimental, especially when recyclability is desired. In here, we report a novel approach for the heterogenization of homoleptic and heteroleptic Cu complexes on silica nanoparticles. In both cases, the photophysical properties upon surface immobilization were only slightly affected. Excited-state quenching with aryl diazonium derivatives occurred efficiently (108-1010M-1s-1) with heterogeneous and homogeneous photosensitizers. Moderate but almost identical yields were obtained for thea-arylation of enol acetate using the homoleptic complex in homogeneous or heterogeneous conditions. Importantly, the silica-supported photocatalysts were recycled with moderate loss in photoactivity over multiple experiments. Transient absorption spectroscopy confirmed that excited-state electron transfer occurred from the homogeneous and heterogeneous homoleptic copper(I) complexes to aryl diazonium derivatives, generating the corresponding copper(II) center that persisted for several hundreds of microseconds, compatible with photoredox catalysis applications.

Sterically Encumbered Heteroleptic Copper(I) β-Diketiminate Complexes with Extended Excited-State Lifetimes.

One of the main challenges in developing effective copper(I) photosensitizers is their short excited-state lifetimes, usually attributed to structural distortion upon light excitation. We have previously introduced copper(I) charge-transfer chromophores of the general formula Cu(N^N)(ArNacNac), where N^N is a conjugated diimine ligand and ArNacNac is a substituted β-diketiminate ligand. These chromophores were promising regarding their tunable redox potentials and intense visible absorption but were ineffective as photosensitizers, presumably due to short excited-state lifetimes. Here, we introduce sterically crowded analogues of these heteroleptic chromophores with bulky alkyl substituents on the N^N and/or ArNacNac ligand. Structural analysis was combined with electrochemical and photophysical characterization, including ultrafast transient absorption (UFTA) spectroscopy to investigate the effects of the alkyl groups on the excited-state lifetimes of the complexes. The molecular structures determined by single-crystal X-ray diffraction display more distortion in the ground state as alkyl substituents are introduced into the phenanthroline or the NacNac ligand, showing smaller τ4 values due to the steric hindrance. UFTA measurements were carried out to determine the excited-state dynamics. Sterically encumbered Cu5 and Cu6 display excited-state lifetimes 15-20 times longer than unsubstituted complex Cu1, likely indicating that the incorporation of bulky alkyl substituents inhibits the pseudo-Jahn-Teller (PJT) flattening distortion in the excited state. This work suggests that the steric properties of these heteroleptic copper(I) charge-transfer chromophores can be readily modified and that the excited-state dynamics are strongly responsive to these modifications.

Bright or Dark: Unexpected Photocatalytic Behavior of Closely Related Phenanthroline‐based Copper(I) Photosensitizers

Bright or Dark: Unexpected Photocatalytic Behavior of Closely Related Phenanthroline‐based Copper(I) Photosensitizers

Bright or Dark: Unexpected Photocatalytic Behavior of Closely Related Phenanthroline‐based Copper(I) Photosensitizers

AbstractIn search of a better understanding of the structure‐property relationship of heteroleptic CuI photosensitizers, two structurally closely related complexes are investigated in detail. Although both compounds bear the same 4‐methoxyphenyl substituents at the phenanthroline moiety (i. e. at 5,6‐position in Cu1 and at 4,7‐position in Cu2), substitution at the 4,7‐position increases the attenuation coefficient in the 300–500 nm range by a factor of more than two. Even more drastic is the difference in emission lifetime (11 vs. 628 ns), which rises by a factor of almost 60 when positions 4 and 7 (i. e. Cu2) are chosen. A combination of solid‐state structural analysis, TDDFT calculations, transient absorption spectroscopy and quenching studies is then applied to elucidate the reasons for the superior photophysical properties of Cu2. Subsequently, a non‐emissive and long‐lived excited state (715 ns) has been revealed for Cu1. In a next step, both complexes were successfully used in the photocatalytic splitting of water and the reduction of CO2 to CO to test the influence of this behavior on the activity. Most importantly, Cu2 not only performs significantly better than Cu1, but also than the benchmark system and the noble‐metal‐based complex [Ir(dFppy)3].

Metal-Organic Framework Supported Copper Photoredox Catalysts for Iminyl Radical-Mediated Reactions.

Visible-light copper photocatalysis has recently emerged as a viable technology for building sustainable synthetic processes. To broaden the applications of phosphine-ligated copper(I) complexes, we describe herein an effective metal-organic framework (MOF)-supported copper(I) photocatalyst for multiple iminyl radical-mediated reactions. Due to site isolation, the heterogenized copper photosensitizer has a significantly higher catalytic activity than its homogeneous counterpart. Using a hydroxamic acid linker to immobilize copper species on MOF supports affords the heterogeneous catalysts with high recyclability. The post-synthetic modification sequence on MOF surfaces allows for the preparation of previously unavailable monomeric copper species. Our findings highlight the potential of using MOF-based heterogeneous catalytic systems to address fundamental challenges in the development of synthetic methodologies and mechanistic investigations of transition-metal photoredox catalysis.

Metal‐Organic Framework Supported Copper Photoredox Catalysts for Iminyl Radical‐Mediated Reactions

AbstractVisible‐light copper photocatalysis has recently emerged as a viable technology for building sustainable synthetic processes. To broaden the applications of phosphine‐ligated copper(I) complexes, we describe herein an effective metal‐organic framework (MOF)‐supported copper(I) photocatalyst for multiple iminyl radical‐mediated reactions. Due to site isolation, the heterogenized copper photosensitizer has a significantly higher catalytic activity than its homogeneous counterpart. Using a hydroxamic acid linker to immobilize copper species on MOF supports affords the heterogeneous catalysts with high recyclability. The post‐synthetic modification sequence on MOF surfaces allows for the preparation of previously unavailable monomeric copper species. Our findings highlight the potential of using MOF‐based heterogeneous catalytic systems to address fundamental challenges in the development of synthetic methodologies and mechanistic investigations of transition‐metal photoredox catalysis.

Controlling the Photofunctionality of a Polyanionic Heteroleptic Copper(I) Photosensitizer for CO2 Reduction Using Its Ion-pair Formation with Polycationic Ammonium in Aqueous Media.

A water-soluble trianionic heteroleptic copper(I) photosensitizer having four sulfonate groups (CuPS3- ) was found to afford the 1 : 2 ion-pair adduct with dicationic alkylammonium (hexamethonium) cations (HM2+ ) in aqueous media, leading to exhibit excellent photophysical and photocatalytic performances owing to the substantial suppression of water-derived non-radiative decay of the photoexcited state.

Controlling the Photofunctionality of a Polyanionic Heteroleptic Copper(I) Photosensitizer for CO2 Reduction Using Its Ion‐pair Formation with Polycationic Ammonium in Aqueous Media

AbstractA water‐soluble trianionic heteroleptic copper(I) photosensitizer having four sulfonate groups (CuPS3−) was found to afford the 1 : 2 ion‐pair adduct with dicationic alkylammonium (hexamethonium) cations (HM2+) in aqueous media, leading to exhibit excellent photophysical and photocatalytic performances owing to the substantial suppression of water‐derived non‐radiative decay of the photoexcited state.

Sensitizer-controlled photochemical reactivity via upconversion of red light.

By combining the energy input from two red photons, chemical reactions that would normally require blue or ultraviolet irradiation become accessible. Key advantages of this biphotonic excitation strategy are that red light usually penetrates deeper into complex reaction mixtures and causes less photo-damage than direct illumination in the blue or ultraviolet. Here, we demonstrate that the primary light-absorber of a dual photocatalytic system comprised of a transition metal-based photosensitizer and an organic co-catalyst can completely alter the reaction outcome. Photochemical reductions are achieved with a copper(i) complex in the presence of a sacrificial electron donor, whereas oxidative substrate activation occurs with an osmium(ii) photosensitizer. Based on time-resolved laser spectroscopy, this changeover in photochemical reactivity is due to different underlying biphotonic mechanisms. Following triplet energy transfer from the osmium(ii) photosensitizer to 9,10-dicyanoanthracene (DCA) and subsequent triplet-triplet annihilation upconversion, the fluorescent singlet excited state of DCA triggers oxidative substrate activation, which initiates the cis to trans isomerization of an olefin, a [2 + 2] cycloaddition, an aryl ether to ester rearrangement, and a Newman-Kwart rearrangement. This oxidative substrate activation stands in contrast to the reactivity with a copper(i) photosensitizer, where photoinduced electron transfer generates the DCA radical anion, which upon further excitation triggers reductive dehalogenations and detosylations. Our study provides the proof-of-concept for controlling the outcome of a red-light driven biphotonic reaction by altering the photosensitizer, and this seems relevant in the greater context of tailoring photochemical reactivities.

Photocatalytic CO2 reduction with a TiO2-supported copper photosensitizer and an iron-based CO2 reduction catalyst

Selective photocatalytic carbon dioxide reduction to carbon monoxide is achieved employing a supported photosensitizer in combination with an iron catalyst. The composite material showed a better performance than the purely homogeneous system.

Mechanism of Photocatalytic CO2 Reduction by Iron Spin-Crossover Complex with Copper Photosensitizer

Mechanism of Photocatalytic CO₂ Reduction by Iron Spin-Crossover Complex with Copper Photosensitizer

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore

CO2 reduction through artificial photosynthesis represents a prominent strategy toward the conversion of solar energy into fuels or useful chemical feedstocks. In such configuration, designing highly efficient chromophores comprising earth-abundant elements is essential for both light harvesting and electron transfer. Herein, we report that a copper purpurin complex bearing an additional redox-active center in natural organic chromophores is capable to shift the reduction potential 540 mV more negative than its organic dye component. When this copper photosensitizer is employed with an iron porphyrin as the catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant, the system achieves over 16100 turnover number of CO from CO2 with a 95% selectivity (CO vs H2) under visible-light irradiation, which is among the highest reported for a homogeneous noble metal-free system. This work may open up an effective approach for the rational design of highly efficient chromophores in artificial photosynthesis.

Structure and excited-state dynamics of dimeric copper(i) photosensitizers investigated by time-resolved X-ray and optical transient absorption spectroscopy.

Time-resolved X-ray (tr-XAS) and optical transient absorption (OTA) spectroscopy in the picosecond time scale coupled with Density Functional theory (DFT) and X-ray absorption near-edge structure (XANES) calculations are applied to study three homoleptic Cu(i) dimeric chromophores with ethyl and longer propyl spacers, denoted as [Cu2(mphenet)2]Cl2 (C1), [Cu2(mphenet)2](ClO4)2 (C2) and [Cu2(mphenpr)2](ClO4)2 (C3) (where mphenet = 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane and mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane). Tr-XAS analysis after light illumination at ∼ 100 ps illustrate the formation of a flattened triplet excited state in all 3 complexes. Optical transient absorption (OTA) analysis for C1 monitored in water and C2 and C3 measured in acetonitrile reveals distinct excited-state lifetimes of 169 ps, 670 ps and 1600 ps respectively. These differences are associated to changes in the solvent (comparing C1 and C2) and the flexibility of the ligand to adapt after Cu flattening upon excitation (C2 and C3). Our results are important for the improved structural dynamics of these types of Cu-based dimeric compounds, and can guide the integration of these chromophores into more complex solar energy conversion schemes.

Node-Dependent Photoinduced Electron Transfer in Third-Generation 2D MOFs Containing Earth-Abundant Metal Ions.

Five isostructural 2D metal-organic frameworks (MOFs), based on a photoactive CuI metallolinker and mixed mono-/dinuclear secondary building units (SBUs), are reported. The MOFs 1(M) (M = Mn, Co, Cu, Zn, and Cd) exhibit broad absorption across the visible-light spectrum and emission centered at ca. 730 nm. Upon photoexcitation, the rigidity of the framework hinders the pseudo-Jahn-Teller distortion of the metallolinker's excited state, providing efficient intersystem crossing into the triplet state. Rapid luminescence quenching in 1(Cu) and 1(Co) suggests photoinduced electron transfer (PET) to the SBUs, whereas lifetimes of up to 22.2 ns are observed in 1(Zn). The quantum yields relative to the parent photosensitizer (PS) decrease for metal nodes containing transition metal ions with partially occupied d-orbitals but increase for the d10 systems CdII and ZnII by a factor of up to 6. Importantly, the excited state decay rates directly correlate with the occupancy of the [MII(OH2)]x moieties in the MOFs providing nonradiative decay pathways via O-H oscillators. Cyclovoltammetry reveals minor changes in CuI/II oxidation potential, with excited-state reduction potentials for 1(M) rivalling Ru analogues. These results establish bis(diimine)copper(I) photosensitizers as viable metallolinkers for MOFs and present a rare example of an isostructural series obtained from a photosensitive metallolinker.

Molecular Copper(I)-Copper(II) Photosensitizer-Catalyst Photoelectrode for Water Oxidation.

Copper(II)-based electrocatalysts for water oxidation in aqueous solution have been studied previously, but photodriving these systems still remains a challenge. In this work, a bis(diimine)copper(I)-based donor-chromophore-acceptor system is synthesized and applied as the light-harvesting component of a photoanode. This molecular assembly was integrated onto a zinc oxide nanowire surface, and upon photoexcitation, chronoamperometric studies reveal that the integrated triad can inject electrons directly into the conduction band of zinc oxide, generating oxidizing equivalents that are then transferred to a copper(II) water oxidation catalyst in aqueous solution, yielding O2 from water with a Faradaic efficiency of 76%.

Cu(I) photosensitizers with alkylated diphosphines: Towards enhancing photostability and architecture extension

Homogeneous copper(I) photosensitizers (PSs) have garnered attention as viable alternatives to the use of noble metal-containing complexes in photoredox processes. More specifically, the heteroleptic architecture [CuI(PP)(NN)]+ (PP = diphosphine; NN = diimine) is attractive as it permits directional charge transfer, an important consideration when integrating PSs into heterogeneous structures. Towards that goal, three new Cu(I) PSs were synthesized and characterized, each containing an alkylated-diphosphine. The diphosphine ligands possess a Nixantphos backbone (Nixantphos = 4,6-bis(diphenylphosphino)-10H-phenoxazine) that has been alkylated at the phenoxazine’s nitrogen atom using ethyl, n-butyl, and n-hexyl groups (labeled Et-Nixantphos, Bu-Nixantphos, and Hex-Nixantphos, respectively). While not capable of direct surface attachment, these alkylated-diphosphines serve as structural and electronic controls for future Cu(I) PSs with anchoring groups. Similar to the [CuI(Nixantphos)(biq)]+ (biq = 2,2′-biquinoline) parent complex, all three alkyl-Nixantphos Cu(I) complexes absorb visible light and populate redox-active excited states (ESs) of sufficient lifetime to undergo bimolecular electron transfer. In the presence of an electron donor and H2-evolving catalyst, 447 ± 10 nm irradiation results in H2 gas production, albeit to a lesser degree than the previously reported PP = Xantphos analogue (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). The results highlight that alkylated-Nixantphos Cu(I) PSs maintain favorable properties for visible-light-induced electron transfer; however, long-term photostability remains an issue to prevent formation of the inactive, homoleptic [CuI(biq)2]+ species.

Metal-Organic Frameworks Significantly Enhance Photocatalytic Hydrogen Evolution and CO2 Reduction with Earth-Abundant Copper Photosensitizers.

We report here the design of two multifunctional metal-organic frameworks (MOFs), mPT-Cu/Co and mPT-Cu/Re, comprising cuprous photosensitizers (Cu-PSs) and molecular Co or Re catalysts for photocatalytic hydrogen evolution (HER) and CO2 reduction (CO2RR), respectively. Hierarchical organization of Cu-PSs and Co/Re catalysts in these MOFs facilitates multielectron transfer to drive HER and CO2RR under visible light with an HER turnover number (TON) of 18 700 for mPT-Cu/Co and a CO2RR TON of 1328 for mPT-Cu/Re, which represent a 95-fold enhancement over their homogeneous controls. Photophysical and electrochemical investigations revealed the reductive quenching pathway in HER and CO2RR catalytic cycles and attributed the significantly improved performances of MOFs over their homogeneous counterparts to enhanced electron transfer due to close proximity between Cu-PSs and active catalysts and stabilization of Cu-PSs and molecular catalysts by the MOF framework.

On the interplay of solvent and conformational effects in simulated excited-state dynamics of a copper phenanthroline photosensitizer.

Copper(i) bis-phenanthroline complexes represent Earth-abundant alternatives to ruthenium-based sensitizers for solar energy conversion and photocatalysis. Improved understanding of the solvent-mediated excited-state structural dynamics can help optimize their photoconversion efficiency. Through direct dynamics simulations in acetonitrile and excited-state minimum energy path calculations in vacuum, we uncover the mechanism of the photoinduced flattening motion of the prototypical system [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline). We find that the ligand distortion is a two-step process in acetonitrile. The fast component (∼110 fs) is due to spontaneous pseudo Jahn-Teller instability and is largely solvent independent, while the slow component (∼1.2 ps) arises from the mutual interplay between solvent molecules closely approaching the metal center and rotation of the methyl substituents. These results shed new light on the influence of a donor solvent such as acetonitrile and methyl substituents on the flattening dynamics of [Cu(dmphen)2]+.

Quantifying Entatic States in Photophysical Processes: Applications to Copper Photosensitizers.

The entatic or rack-induced state is a core concept in bioinorganic chemistry. In its simplest form, it is present when a protein scaffold places a transition metal ion and its first coordination sphere into an energized geometric and electronic structure that differs significantly from that of the relaxed form. This energized complex can exhibit special properties. Under this purview, however, entatic states are hardly unique to bioinorganic chemistry, and their effects can be found throughout a variety of important chemistries and materials science applications. Despite this broad influence, there are only a few examples where entatic effects have been quantified. Here we extend the entatic concept more generally to photophysical processes by developing a combined experimental and computational methodology to quantify entatic states across an entire class of functional molecules, e.g., Cu-based photosensitizers. These metal complexes have a broad range of applications, including solar electricity generation, solar fuels synthesis, organic light emitting diodes (OLEDs), and photoredox catalysis. As a direct consequence of quantifying entatic states, this methodology allows the disentanglement of steric and electronic contributions to excited state dynamics. Thus, before embarking on the syntheses of new Cu-based photosensitizers, the correlations described herein can be used as an estimate of entatic and electronic contributions and thus guide ligand design and the development of next-generation transition metal complexes with improved or tailored excited state dynamics. Lastly, entatic energies in some Cu photosensitizers are the largest yet quantified and are found here to approach 20 kcal/mol relative to the conformationally flexible [Cu(phen)2]+. These energetics are significant relative to typical chemical driving forces and barriers, highlighting the utility in extending entatic state descriptors to new classes of molecules and materials with interesting functional properties involving the coupling between electron and vibrational dynamics.

Photochemical CO2 Reduction Driven by Water-Soluble Copper(I) Photosensitizer with the Catalysis Accelerated by Multi-Electron Chargeable Cobalt Porphyrin

Without using precious elements, a highly efficient and selective molecular-based photocatalytic system for CO2-to-CO conversion in fully aqueous media has been developed. Our copper(I)-based water...