Paramagnetism and Fluorescence of Zinc(II) Tripyrrindione: A Luminescent Radical Based on a Redox-Active Biopyrrin

Gold Tripyrrindione: Redox Chemistry and Reactivity with Dichloromethane

The identification of ligands that stabilize Au(III) centers has led to the isolation of complexes for applications in catalysis, gold-based therapeutics, and functional materials. Herein, we report the coordination of gold by tripyrrin-1,14-dione, a linear tripyrrole with the scaffold of naturally occurring metabolites of porphyrin-based protein cofactors (e.g., heme). Tripyrrindione H3TD2 binds Au(III) as a trianionic tridentate ligand to form square planar complex [Au(TD2)(H2O)], which features an adventitious aqua ligand. Two reversible ligand-based oxidations of this complex allow access to the other known redox states of the tripyrrindione framework. Conversely, (spectro)electrochemical measurements and DFT analysis indicate that the reduction of the complex is likely metal-based. The chemical reduction of [Au(TD2)(H2O)] leads to a reactive species that utilizes dichloromethane in the formation of a cyclometalated organo-Au(III) complex. Both the aqua and the organometallic Au(III) complexes were characterized in the solid state by microcrystal electron diffraction (MicroED) methods, which were critical for the analysis of the microcrystalline sample of the organo-gold species. Overall, this study illustrates the synthesis of Au(III) tripyrrindione as well as its redox profile and reactivity leading to gold alkylation chemistry.

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Multicenter interactions and ligand field effects in platinum(II) tripyrrindione radicals

The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum(II) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert-butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt(II) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand (i.e., aqua vs. isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt(II)-Pt(II) contact in the case of the aqua complex. Overall, the properties of Pt(II) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.

Doublet-emissive materials for organic light-emitting diodes: exciton formation and emission processes

Doublet-emission is mainly discovered in stable radicals, lanthanide-metal complexes with an f¹ electron configuration and transition-metal complexes with a low-spin d⁵ electron configuration, and has a distinct radiation mechanism from closed-shell luminescent molecules and thus technology opportunities. There exists an unpaired electron in the frontier molecular orbitals which enables efficient nanosecond-scale luminescence in these materials due to the spin-allowed transitions between doublet-spin states. In this review, we summarize recent advances in these materials and their application in organic light emitting diodes (OLEDs). The photoluminescence and electroluminescence mechanisms of different doublet-emissive molecular systems are discussed, in addition to the photophysical phenomena arising from doublet states. We also outline the current challenges faced by each molecular system, and the potential outlook on the future research trends in this field.

Time-resolved dynamics of stable open- and closed-shell neutral radical and oxidized tripyrrindione complexes

Stable open- and closed-shell Pd(II) and Cu(II) complexes of hexaethyl tripyrrin-1,14-dione (TD1) produce triplet, doublet or singlet states depending on the metal center and the redox state of the ligand. Pd(II) and Cu(II) form neutral TD1 complexes featuring ligand-based radicals, thus resulting in doublet and triplet states, respectively. The reversible one-electron oxidation of the complexes removes an unpaired electron from the ligand, generating singlet and doublet states. The optical properties and time-resolved dynamics of these systems are studied here using steady-state and ultrafast transient absorption (pump-probe) measurements. Fast relaxation with recovery of the ground state in tens of picoseconds is observed for the copper neutral radical and oxidized complexes as well as for the palladium neutral radical complex. Significantly longer timescales are observed for the oxidized palladium complex. The ability to tune the overall spin state of the complexes through their stable open-shell configurations as well as the reversible redox activity of the tripyrrolic systems makes them particularly interesting for catalytic applications as well as exploring magnetism and conductivity properties.

Biopyrrin Pigments: From Heme Metabolites to Redox-Active Ligands and Luminescent Radicals

Redox-active ligands in coordination chemistry not only modulate the reactivity of the bound metal center but also serve as electron reservoirs to store redox equivalents. Among many applications in contemporary chemistry, the scope of redox-active ligands in biology is exemplified by the porphyrin radicals in the catalytic cycles of multiple heme enzymes (e.g., cytochrome P450, catalase) and the chlorophyll radicals in photosynthetic systems. This Account reviews the discovery of two redox-active ligands inspired by oligopyrrolic fragments found in biological settings as products of heme metabolism.Linear oligopyrroles, in which pyrrole heterocycles are linked by methylene or methine bridges, are ubiquitous in nature as part of the complex, multistep biosynthesis and degradation of hemes and chlorophylls. Bile pigments, such as biliverdin and bilirubin, are common and well-studied tetrapyrroles with characteristic pyrrolin-2-one rings at both terminal positions. The coordination chemistry of these open-chain pigments is less developed than that of porphyrins and other macrocyclic oligopyrroles; nevertheless, complexes of biliverdin and its synthetic analogs have been reported, along with fluorescent zinc complexes of phytobilins employed as bioanalytical tools. Notably, linear conjugated tetrapyrroles inherit from porphyrins the ability to stabilize unpaired electrons within their π system. The isolated complexes, however, present helical structures and generally limited stability.Smaller biopyrrins, which feature three or two pyrrole rings and the characteristic oxidized termini, have been known for several decades following their initial isolation as urinary pigments and heme metabolites. Although their coordination chemistry has remained largely unexplored, these compounds are structurally similar to the well-established tripyrrin and dipyrrin ligands employed in a broad variety of metal complexes. In this context, our study of the coordination chemistry of tripyrrin-1,14-dione and dipyrrin-1,9-dione was motivated by the potential to retain on these compact, versatile platforms the reversible ligand-based redox chemistry of larger tetrapyrrolic systems.The tripyrrindione ligand coordinates several divalent transition metals (i.e., Pd(II), Ni(II) Cu(II), Zn(II)) to form neutral complexes in which an unpaired electron is delocalized over the conjugated π system. These compounds, which are stable at room temperature and exposed to air, undergo reversible one-electron processes to access different redox states of the ligand system without affecting the oxidation state and coordination geometry of the metal center. We also characterized ligand-based radicals on the dipyrrindione platform in both homoleptic and heteroleptic complexes. In addition, this study documented noncovalent interactions (e.g., interligand hydrogen bonds with the pyrrolinone carbonyls, π-stacking of ligand-centered radicals) as important aspects of this coordination chemistry. Furthermore, the fluorescence of the zinc-bound tripyrrindione radical and the redox-switchable emission of a dipyrrindione BODIPY-type fluorophore showcased the potential interplay of redox chemistry and luminescence in these compounds. Supported by computational analyses, the portfolio of properties revealed by this investigation takes the tripyrrindione and dipyrrindione motifs of heme metabolites to the field of redox-active ligands, where they are positioned to offer new opportunities for catalysis, sensing, supramolecular systems, and functional materials.

Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic (J ≈ −2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.

The coordination and redox chemistry of the dipyrrindione scaffold, which is found in several heme metabolites, is investigated in heteroleptic palladium(II) complexes. The bis(aqua) complex undergoes a deprotonation-driven dimerization to form a hydroxo-bridged binuclear species. Crystallographic, electrochemical, and spectroscopic data, as well as computational analysis, demonstrate that a two-electron reduction of the binuclear complex leads to a diradical dianion with spin density delocalized over the two dipyrrindione ligands.

Heteroleptic palladium(II) complexes of dipyrrin-1,9-dione supported by intramolecular hydrogen bonding

The dipyrrin-1,9-dione framework, which is characteristic of the propentdyopent pigments deriving from heme metabolism, coordinates metal ions as monoanionic bidentate donors. The resulting analogs of dipyrrinato complexes undergo reversible ligand-based reductions, thus showcasing the ability of the dipyrrindione scaffold to act as an electron reservoir. Herein we report the synthesis and characterization of three heteroleptic palladium complexes of the redox-active dipyrrindione ligand. Primary amines were chosen as additional ligands so as to assemble complexes of planar geometries with complementary interligand hydrogen-bonding. Full chemical characterization confirms the hydrogen bonding interactions between the primary amine ligands and the acceptor carbonyl groups on the dipyrrolic ligand. The resulting heteroleptic compounds display reversible one-electron reduction events that are centered on the dipyrrindione ligand as revealed by voltammetry and spectroelectrochemistry data. Within these planar Pd(II) complexes, the propentdyopent motif therefore combines reversible ligand-based redox chemistry with interligand hydrogen bonding in the primary coordination sphere of the metal center.

Propentdyopents: Brief history of a family of dipyrrolic pigments

Propentdyopents are naturally occurring dipyrroles deriving from the metabolism of heme and characterized by a dipyrrin-1,9-dione motif. The unusual name propentdyopent is due to the first colorimetric method (the Stokvis reaction) for the detection of these compounds, which were initially isolated from urine samples. Upon reduction in alkaline solutions, they produced red species that were termed pentdyopents to describe with Greek numerals their absorption maximum (525 nm) in the visible range. The precursors to the red pentdyopents were thus indicated as propentdyopents.Over the course of several decades, these dipyrrolic compounds have appeared in several studies of human physiology, typically associated to conditions of abnormal heme metabolism and/or oxidative stress. Concurrently, synthetic investigations have confirmed their chemical structure, reactivity, and ability to coordinate metals as bidentate monoanionic ligands. Notably, the planar dipyrrindione platform can undergo reversible one-electron redox processes and thereby act as an electron reservoir in metal complexes. In combination with the documented ability of the carbonyl groups to act as hydrogen-bonding acceptors, the coordination chemistry of propentdyopents could lead to new applications for this old class of pigments. Furthermore, the observation of these pigments in several clinical contexts could potentially delineate a role of propentdyopents as diagnostic biomarkers. This mini-review summarizes both the chemistry and biology of propentdyopents while highlighting the ample space for new discoveries.

DFT/MRCI-R2018 study of the photophysics of the zinc(ii) tripyrrindione radical: non-Kasha emission?

The fluorescence of a radical-based emitter has been theoretically investigated after measurements had shown absorption bands to lie below the emission energy. The results of the all-multiplicity DFT/MRCI-R2018 study indicate D₃ emission.