Chromium complexes for luminescence, solar cells, photoredox catalysis, upconversion, and phototriggered NO release

Luminescent polypyridyl heteroleptic CrIII complexes with high quantum yields and long excited state lifetimes

Implementing high quantum yields and long-lived excited state lifetimes within heteroleptic luminescent CrIII complexes is a keystone for the design of supramolecular energy-converting devices exploiting this cheap metal. In this contribution, we discuss the stepwise and rational optimization of these two limiting factors within a series of heteroleptic CrIII complexes.

Quantifying Triplet State Formation in Zinc Dipyrrin Complexes

Photocatalysis is a promising method to harness solar energy and use it to form fuels and other high-value chemicals, but most sensitizers used in photocatalytic reactions are complexes of rare and expensive metals such as ruthenium and iridium. Zinc dipyrromethene complexes have potential to be a more earth-abundant alternative, but their photophysical properties are largely unexplored. In this study, triplet state formation was quantified in two zinc dipyrromethene complexes, with and without heavy atoms, by transient absorption spectroscopy. Without heavy atoms, the triplet quantum yield was 16% in toluene and 27% in THF. With the addition of heavy I atoms, the triplet quantum yield increased to 62-63% and was insensitive to solvent polarity. The fact that in the absence of heavy atoms the triplet yield is affected by solvent polarity and in the presence of heavy atoms it is not suggests that triplet formation occurs through different pathways in the two complexes. These triplet yields meet or exceed those of successful organic photosensitizers, illustrating the potential for zinc dipyrromethene complexes as photosensitizers.

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising Fe^II donors and Ti^IV acceptors with the general formula ^RCp₂Ti(C₂Fc)₂ (where ^RCp = Cp*, Cp, and ^MeOOCCp) and ^TMSCp₂Ti(C₂Fc)(C₂R) (where R = Ph or CF₃) are reported. The transient absorption spectra are consistent with an Fe^III/Ti^III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (¹MMCT) excited state for the Fe^II/Ti^IV complexes is not purely MMCT in nature but is contaminated with the higher-energy ¹Fc (d-d) state. For the diferrocenyl complexes, ^RCp₂Ti(C₂Fc)₂, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the ^MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased ¹Fc character. The effect of Cu^I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp₂Ti(C₂Fc)₂·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti^IV-³Fe^II-Cu^I.

Electronic structure in the transition metal block and its implications for light harvesting

Transition metal-based chromophores play a central role in a variety of light-enabled chemical processes ranging from artificial solar energy conversion to photoredox catalysis. The most commonly used compounds include elements from the second and third transition series (e.g., ruthenium and iridium), but their Earth-abundant first-row analogs fail to engage in photoinduced electron transfer chemistry despite having virtually identical absorptive properties. This disparate behavior stems from fundamental differences in the nature of 3d versus 4d and 5d orbitals, resulting in an inversion in the compounds' excited-state electronic structure and undermining the ability of compounds with first-row elements to engage in photoinduced electron transfer. This Review will survey the key experimental observations establishing this difference in behavior, discuss the underlying reasons for this phenomenon, and briefly summarize efforts that are currently under way to alter this paradigm and open the door to new opportunities for using Earth-abundant materials for photoinduced electron transfer chemistries.

Strong circularly polarized luminescence of an octahedral chromium(iii) complex

The chiral spin–flip luminophore [Cr(ddpd)₂]³⁺ can be resolved into enantiopure material by chiral HPLC. The pure enantiomers display strong CPL activity for the corresponding near-IR phosphorescence.

Photoredox catalysts based on earth-abundant metal complexes

Visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. We critically review Earth-abundant metal complexes photocatalysts including Cu(i), Zn(ii), Ni(0), V(v), Zr(iv), W(0), W(vi), Mo(0), Cr(iii), Co(iii) and Fe(ii).

A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor-Acceptor Cyanoarenes

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor-acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors' molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts' redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely organic photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts, that outperform and complement prevailing typical best photocatalysts.

Metal ion coordination enhancing quantum efficiency of ligand phosphorescence in a double-stranded helical chain coordination polymer of Pb2+ with nicotinic acid

A new 1D phosphorescence coordination polymer (CP) [Pb2O(C6H4NO2)2]n (1; C6H4NO2 = nicotinate) was synthesized by a solvothermal reaction and PbO was used as a Pb(ii) source instead of traditional Pb(ii) salts. This remarkably thermal-stable CP crystallizes in the space group I41/a. In the crystal structure of 1, two different Pb(ii) ions show a five-coordinated and hemidirected coordination geometry, two nonequivalent nicotinate ligands link to Pb(ii) ions in μ2-η1:η1 and μ4-η2:η2 modes, and the hemidirected coordination polyhedra of Pb(ii) form a helical lead-oxide chain via an edge-sharing fashion along the c-axis. Under ambient conditions, 1 emits cyan ligand-based phosphorescence with an absolute quantum yield as high as 59.4% and a lifetime of 9.86 ms under UV-light irradiation. Under the same conditions, nicotinic acid emits simultaneously fluorescence and phosphorescence with a total absolute quantum yield of 4.8%. The great enhancement of phosphorescence quantum yield in 1, regarding nicotinic acid, is assigned to the heavy atom effect of Pb(ii) and negligible ππ interaction between pyridyl rings. Noticeably, the vibronic fine structure is observed in the emission spectrum of 1 at room temperature. Additionally, 1 shows thermochromic behavior, and such functionality probably has realistic application in the field of temperature detection.

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.

Macrophage-mediated delivery of light activated nitric oxide prodrugs with spatial, temporal and concentration control

Nitric oxide (NO) holds great promise as a treatment for cancer hypoxia, if its concentration and localization can be precisely controlled. Here, we report a "Trojan Horse" strategy to provide the necessary spatial, temporal, and dosage control of such drug-delivery therapies at targeted tissues. Described is a unique package consisting of (1) a manganese-nitrosyl complex, which is a photoactivated NO-releasing moiety (photoNORM), plus Nd3+-doped upconverting nanoparticles (Nd-UCNPs) incorporated into (2) biodegradable polymer microparticles that are taken up by (3) bone-marrow derived murine macrophages. Both the photoNORM [Mn(NO)dpaqNO2 ]BPh4(dpaqNO2 = 2-[N,N-bis(pyridin-2-yl-methyl)]-amino-N'-5-nitro-quinolin-8-yl-acetamido) and the Nd-UCNPs are activated by tissue-penetrating near-infrared (NIR) light at ∼800 nm. Thus, simultaneous therapeutic NO delivery and photoluminescence (PL) imaging can be achieved with a NIR diode laser source. The loaded microparticles are non-toxic to their macrophage hosts in the absence of light. The microparticle-carrying macrophages deeply penetrate into NIH-3T3/4T1 tumor spheroid models, and when the infiltrated spheroids are irradiated with NIR light, NO is released in quantifiable amounts while emission from the Nd-UCNPs provides images of microparticle location. Furthermore, varying the intensity of the NIR excitation allows photochemical control over NO release. Low doses reduce levels of hypoxia inducible factor 1 alpha (HIF-1α) in the tumor cells, while high doses are cytotoxic. The use of macrophages to carry microparticles with a NIR photo-activated theranostic payload into a tumor overcomes challenges often faced with therapeutic administration of NO and offers the potential of multiple treatment strategies with a single system.

Electronic Energy Transduction from {Ru(py)4} Chromophores to Cr(III) Luminophores

Despite the large body of work on {Ru(bpy)₂} sensitizer fragments, the same attention has not been devoted to their {Ru(py)₄} analogues. In this context, we explored the donor-acceptor trans-[Ru(L)₄{(μ-NC)Cr(CN)₅}₂]^4-, where L = pyridine, 4-methoxypyridine, 4-dimethylaminopyridine. We report on the synthesis and the crystal structure as well as the electrochemical, spectroscopical, and photophysical properties of these trimetallic complexes, including transient absorption measurements. We observed emission from chromium-centered d-d states upon illuminating into either MLCT or MM'CT absorptions of {Ru(L)₄} or {Ru-Cr}, respectively. The underlying energy transfer is as fast as 600 fs with quantum efficiencies ranging from 10% to 100%. These results document that {Ru(py)₄} sensitizer fragments are as efficient as {Ru(bpy)₂} in short-range energy transfer scenarios.

Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes

Known stable [Cr(bpy)₂(Ph)₂](BPh₄) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)₂(Ar)₂](X) complexes (R = H or CMe₃; Ar = Ph, C₆H₄-CMe₃, or C₆H₄-OMe; X = I, BPh₄, or PF₆) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV-visible spectra and electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture of two different bis(aryl) complexes gave only the homocoupled biaryl products by ¹H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)₂(Ar)₂](PF₆) was trapped with bipyridine to generate [Cr(bpy)₃](PF₆) and with benzoyl peroxide to form [Cr(bpy)₂(O₂CPh)₂](PF₆). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCO₂H to Cp₂Cr, followed by air oxidation. The neutral Cr(bpy)(S₂CNMe₂)Ph₂ complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(^tBu-bpy)(dpm)Cl₂ [dpm = (OC^tBu)₂CH] with AgO₂CPh gave trans-Cr(^tBu-bpy)(dpm)(O₂CPh)₂, reaction of the dichloro precursor with PhMgCl produced anionic [Cr(^tBu-bpy)Ph₃]^- with [Mg(dpm)(THF)₄]⁺ as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph₃(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph₂, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin)₂ complex was synthesized by protonolysis of Cr(bpy)[N(SiMe₃)₂]₂ and characterized by single-crystal X-ray diffraction.

Luminescent complexes made from chelating isocyanide ligands and earth-abundant metals

In this invited frontier article, recently discovered d⁶ and d¹⁰ complexes with long-lived metal-to-ligand charge transfer (MLCT) excited states are highlighted. Chelating diisocyanide ligands give access to emissive Mo(0) and Cr(0) complexes with d⁶ electron configuration exhibiting photophysical properties similar to those of Ru(ii) polypyridines or cyclometalated Ir(iii) complexes. With Ni(0), these ligands yield luminescent tetrahedral d¹⁰ complexes similar to isoelectronic Cu(i) bis(diimine) compounds.

Versatile heteroleptic bis-terdentate Cr(iii) chromophores displaying room temperature millisecond excited state lifetimes

Cr–Br bonds can be specifically labilized for producing phosphorescent and tuneable heteroleptic bis-terdentate Cr(iii) long-lived sensitizers.

Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements

Photoredox chemistry with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5 years, important progress towards the use of complexes made from earth-abundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive Cr^III , Fe^II , Cu^I , Zn^II , Zr^IV , Mo⁰ , and U^VI complexes in the context of synthetic organic photoredox chemistry using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earth-abundant elements.