A Tetrapyrrole Macrocycle Displaying a Multielectron Redox Chemistry and Tunable Absorbance Profile

Dicarbonyl[10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene]ruthenium(II): discovery of the first ruthenium tetrapyrrole cis-dicarbonyl complex by X-ray and electron diffraction

Dicarbonyl[10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene]ruthenium(II), [Ru(C33H16F10N4)(CO)2] or Ru(CO)2[DMBil1], is the first reported ruthenium(II) cis-dicarbonyl tetrapyrrole complex. The neutral complex sports two carbonyls and an oligotetrapyrrolic biladiene ligand. Notably, the biladiene adopts a coordination geometry that is well distorted from square planar and much more closely approximates a seesaw arrangement. Accordingly, Ru(CO)2[DMBil1] is not only the first ruthenium cis-dicarbonyl with a tetrapyrrole ligand, but also the first metal biladiene complex in which the tetrapyrrole does not adopt a (pseudo-)square-planar coordination geometry. Ru(CO)2[DMBil1] is weakly luminescent, displaying λem = 552 nm upon excitation at λex = 500 nm, supports two reversible 1 e- reductions at -1.45 and -1.73 V (versus Fc+/Fc), and has significant absorption features at 481 and 531 nm, suggesting suitability for photocatalytic and photosensitization applications. While the structure of Ru(CO)2[DMBil1] was initially determined by X-ray diffraction, a traditionally acceptable quality structure could not be obtained (despite multiple attempts) because of consistently poor crystal quality. An independent structure obtained from electron diffraction experiments corroborates the structure of this unusual biladiene complex.

Mapping the influence of ligand electronics on the spectroscopic and 1O2 sensitization characteristics of Pd(II) biladiene complexes bearing phenyl-alkynyl groups at the 2- and 18-positions

Photodynamic therapy (PDT) is a promising treatment for certain cancers that proceeds via sensitization of ground state 3O2 to generate reactive 1O2. Classic macrocyclic tetrapyrrole ligand scaffolds, such as porphyrins and phthalocyanines, have been studied in detail for their 1O2 photosensitization capabilities. Despite their compelling photophysics, these systems have been limited in PDT applications because of adverse biological side effects. Conversely, the development of non-traditional oligotetrapyrrole ligands metalated with palladium (Pd[DMBil1]) have established new candidates for PDT that display excellent biocompatibility. Herein, the synthesis, electrochemical, and photophysical characterization of a new family of 2,18-bis(phenylalkynyl)-substituted PdII 10,10-dimethyl-5,15-bis(pentafluorophenyl)-biladiene (Pd[DMBil2-R]) complexes is presented. These second generation biladienes feature extended conjugation relative to previously characterized PdII biladiene scaffolds (Pd[DMBil1]). We show that these new derivatives can be prepared in good yield and, that the electronic nature of the phenylalkynyl appendages dramatically influence the PdII biladiene photophysics. Extending the conjugation of the Pd[DMBil1] core through installation of phenylacetylene resulted in a ∼75 nm red-shift of the biladiene absorption spectrum into the phototherapeutic window (600-900 nm), while maintaining the PdII biladiene's steady-state spectroscopic 1O2 sensitization characteristics. Varying the electronics of the phenylalkyne groups via installation of electron donating or withdrawing groups dramatically influences the steady-state spectroscopic and photophysical properties of the resulting Pd[DMBil2-R] family of complexes. The most electron rich variants (Pd[DMBil2-N(CH3)2]) can absorb light as far red as ∼700 nm but suffer from significantly reduced ability to sensitize formation of 1O2. By contrast, Pd[DMBil2-R] derivatives bearing electron withdrawing functionalities (Pd[DMBil2-CN] and Pd[DMBil2-CF3]) display 1O2 quantum yields above 90%. The collection of results we report suggest that excited state charge transfer from more electron-rich phenyl-alkyne appendages to the electron deficient biladiene core circumvents triplet sensitization. The spectral and redox properties, as well as the triplet sensitization efficiency of each Pd[DMBil2-R] derivative is considered in relation to the Hammett value (σp) for each biladiene's R-group. More broadly, the results reported in this study clearly demonstrate that biladiene redox properties, spectral properties, and photophysics can be perturbed greatly by relatively minor alterations to biladiene structure.

Elucidation of complex triplet excited state dynamics in Pd(II) biladiene tetrapyrroles

Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks' sp³-hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectroscopy revealed that higher-energy excitation (λ_exc ∼ 350-500 nm) led to an additional lifetime (i.e., an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states (λ_exc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ∼20 μs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the 'extra' lifetime observed upon higher energy excitation was revealed using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin-orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

Advancements and Challenges in Reductive Conversion of Carbon Dioxide via Thermo-/Photocatalysis

Carbon dioxide (CO₂) is the major greenhouse gas and also an abundant and renewable carbon resource. Therefore, its chemical conversion and utilization are of great attraction for sustainable development. Especially, reductive conversion of CO₂ with energy input has become a current hotspot due to its ability to access fuels and various important chemicals. Nowadays, the controllable CO₂ hydrogenation to formic acid and alcohols using sustainable H₂ resources has been regarded as an appealing solution to hydrogen storage and CO₂ accumulation. In addition, photocatalytic CO₂ reduction to CO also provides a potential way to utilize this greenhouse gas efficiently. Besides direct CO₂ hydrogenation, CO₂ reductive functionalization integrates CO₂ reduction with subsequent C-X (X = N, S, C, O) bond formation and indirect transformation strategies, enlarging the diverse products derived from CO₂ and promoting CO₂ reductive conversion into a new stage. In this Perspective, the progress and challenges of CO₂ reductive conversion, including hydrogenation, reductive functionalization, photocatalytic reduction, and photocatalytic reductive functionalization are summarized and discussed along with the key issues and future trends/directions in this field. We hope this Perspective can evoke intense interest and inspire much innovation in the promise of CO₂ valorization.

An Easily Prepared Monomeric Cobalt(II) Tetrapyrrole Complex That Efficiently Promotes the 4e-/4H+ Peractivation of O2 to Water

The selective 4e-/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) or Co(III) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. However, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e-/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based on a nonaromatic isocorrole scaffold that is competent for 4e-/4H+ oxygen reduction reaction (ORR). This Co(II) 10,10-dimethyl isocorrole (Co[10-DMIC]) is obtained in just four simple steps and has excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co porphyrin (cobalt 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, [Co(TPFPP)]) and corrole (cobalt 5,10,15-tris(pentafluorophenyl)corrole triphenylphosphine, Co[TPFPC](PPh3)) homologues demonstrated that the spectroscopic and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, the investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ∼85% selective for the 4e-/4H+ reduction of O2 to H2O over the more kinetically facile 2e-/2H+ process that delivers H2O2. In contrast, the same ORR evaluations conducted for the Co porphyrin and corrole homologues demonstrated that these traditional aromatic systems catalyze the 2e-/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups or ditopic molecular clefts or that impose cofacially oriented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and shows a privileged performance for the 4e-/4H+ peractivation of O2 to water as compared to other simple cobalt containing tetrapyrroles.

Loading Phlorins with Fullerenes by a [4 + 2]-Cycloaddition Reaction: Regulation of the Regioselectivity by Pyrrole Linkage Modes

Sulfolenopyrrole-based normal and N-confused phlorins have been constructed to address the seldom touched phlorin functionalization and simultaneously explore the effect of the pyrrole linkage modes (αα, αβ) on the [4 + 2] cycloaddition reaction. The common sulfolenophlorin 1 contains two sulfolenopyrroles with the same reactivity upon tautomerization and undergoes stepwise [4 + 2]-cycloaddition with fullerene to furnish monoadduct 1-C60 and bisadduct 1-2C60 with a total yield up to 76%. By contrast, the presence of the confused pyrrole in 2 fixes the π-system owing to the low tendency to tautomerize and enables the two sulfolenopyrroles to exhibit in different fashions (i.e., normal NH-type and imino-type). Notably, under milder conditions (120 °C), the monofullerenoadduct 2-C60 forms rapidly and has been isolated from the [4 + 2] cycloaddition reaction of 2 and fullerene as the predominant fraction, accompanied by a trace amount of bisadduct 2-2C60. Raising the temperature to 140 °C did not improve the yield of 2-2C60. The structural analysis of 2-C60 indicates the attachment of fullerene at the iminopyrrole part. The high regioselectivity in the [4 + 2] cycloaddition of the imino-type sulfolenopyrrole unit has been rationalized thermodynamically by the DFT calculation on the relative energy of the two diene intermediates.

Synthesis, Electrochemistry, and Photophysics of Pd(II) Biladiene Complexes Bearing Varied Substituents at the sp3-Hybridized 10-Position

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp³-hybridized meso-carbon (the 10-position of the biladiene framework) was prepared and studied. In addition to a previously described Pd(II) biladiene complex bearing geminal dimethyl substituents a the 10-position (Pd[DMBil]), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents (Pd[MPBil1]) and geminal diphenyl groups(Pd[DPBil1]) were prepared and structurally characterized. Detailed electrochemical as well as steady-state and time-resolved spectroscopic experiments were undertaken to evaluate the influence of the substituents on the biladiene's tetrahedral meso-carbon. Although all three biladiene homologues are isostructural, Pd[MPBil1] and Pd[DPBil1] display more intense absorption profiles that shift slightly toward lower energies as geminal methyl groups are replaced by phenyl rings. All three biladiene homologues support a triplet photochemistry, and replacement of the geminal dimethyl substituents of Pd[DMBil1] (Φ_Δ = 54%) with phenyl groups improves the ability of Pd[MPBil1] (Φ_Δ = 76%) and Pd[DPBil1] (Φ_Δ = 66%) to sensitize ¹O₂. Analysis of the excited-state dynamics of the Pd(II) biladienes by transient absorption spectroscopy shows that each complex supports a long-lived triplet excited-state (i.e., τ > 15 μs for each homologue) but that the ISC quantum yields (Φ_T) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (Φ_Δ) across the biladiene series considered in this work. The results of this study provide new insights to guide future development of biladiene based agents for PDT and other photochemical applications.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pK_a values of the two PILs studied (i.e., trialkylammonium and [DBU-H]⁺-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]^- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}⁸ species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]⁺-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]^- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]⁺ does not protonate electrochemically generated [Fe(OEP)(NO)]^-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Synthesis, structure, electronic characterization, and halogenation of gold(III) phlorin complexes

The metalation chemistry of the phlorin, which is a non-aromatic tetrapyrrole macrocycle containing a single sp3-hybridized meso-carbon has remained underdeveloped, as compared to that of more traditional tetrapyrroles such as porphyrins, corroles and phthalocyanines. There have been few prior efforts to prepare metallophlorins, and those that have been reported have relied on either reduction or nucleophilic attack of parent metalloporphyrins, rather than direct metalation of freebase phlorin constructs. In this work, an alternate synthetic approach for preparation of gold(III) phlorin complexes that involves the first direct metalation of two different freebase phlorin derivatives (3H(Phl[Formula: see text] and 3H(Phl[Formula: see text] with AuBr3 to produce the stable and fully isolable gold(III) phlorin complexes Au(Phl[Formula: see text] and Au(Phl[Formula: see text] is reported. The first structural characterization of a metallophlorin bearing geminal dimethyl substituents at the sp3-hybridized meso-carbon via X-ray crystallography is also reported. In addition to the preparation of Au(Phl[Formula: see text] and Au(Phl[Formula: see text], the UV-vis absorption and redox properties of these two gold(III) phlorins in comparison to those of their freebase homologues is also detailed. Notably, the metallophlorins are characterized by panchromatic absorbance profiles and intense and broad bands that span the long-visible and into the near-IR regions, as well as two fully reversible oxidation and reduction waves as probed by cyclic voltammetry. Finally, the chlorination of Au(Phl[Formula: see text] using PhI(Cl[Formula: see text] was probed and it was found that this regioslective reaction generates monochlorinated (Au(Phl[Formula: see text]Cl)) and dichlorinated (Au(Phl[Formula: see text]Cl[Formula: see text] products, which were both structurally characterized by X-ray crystallography.

Fluoride ion promoted NH deprotonation of dioxaPhlorin with NIR absorption vs. acid induced facile conversion to aromatic dioxaSapphyrin: Synthesis, spectroscopic and theoretical characterization

Synthesis, spectroscopic and solid state structural proof of hitherto unknown highly stable meso-pyrrole appended expanded dioxaphlorin is reported. The macrocycle was susceptible to NH deprotonation upon TBAF titration leading to NIR absorption with vivid colour change to the naked eye paving the way towards the application as a fluoride ion sensor. Contrarily, efficient conversion to aromatic dioxasapphyrin has been anticipated upon trifluoroacetic acid reaction owing to facile elimination of meso-appended pyrrole ring. The conformational preorganzation and anion induced conformational reorganization owing to the base promoted NH deprotonation are thoroughly investigated by various spectroscopic techniques and DFT level theoretical calculations.

Doubly N-confused phlorin and phlorinone analogue

A doubly N-confused phlorin and phlorinone analogue were synthesized from a β,β'-linked dipyrromethane precursor and characterized by means of NMR and UV-Vis spectroscopies, X-ray crystallography, and electrochemistry. Solvents have a considerable impact on the optical absorption of the doubly N-confused phlorin so that it can differentiate simple alcohols such as methanol and ethanol.

Synthesis, Redox, and Spectroscopic Properties of Pd(II) 10,10-Dimethylisocorrole Complexes Prepared via Bromination of Dimethylbiladiene Oligotetrapyrroles

Two brominated 10,10-dimethylisocorrole (10-DMIC) derivatives containing Pd(II) centers have been prepared and characterized. These compounds were prepared via bromination of 10,10-dimethylbiladiene-based oligotetrapyrroles. Bromination of free base 10,10-dimethylbiladiene (DMBil1) followed by metalation with Pd(OAc)2, as well as bromination of the corresponding Pd(II) dimethylbiladiene complex (Pd[DMBil1]) provide routes to Pd(II) hexabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br6]) and Pd(II) octabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br8]). The solid-state structures of the two brominated isocorrole complexes are presented, as is that for a new decabrominated dimethylbiladiene derivative (DMBil-Br10). The electronic and spectroscopic properties of the brominated biladiene and isocorrole derivatives were probed using a combination of voltammetric methods and steady-state UV-vis absorption and emission experiments. Data obtained from these experiments allow the properties of the brominated biladiene and isocorrole derivatives to be compared to previously studied biladiene derivatives (i.e., DMBil1 and Pd[DMBil1]). CV and DPV experiments demonstrate that Pd[10-DMIC-Br6] and Pd[10-DMIC-Br8] support well-behaved multielectron redox chemistry, similar to that which has been observed for other nonaromatic tetrapyrroles containing sp3-hybridized meso-carbons. Spectroscopic experiments reveal that bromination of the dimethylbiladiene core shifts this system's UV-vis absorption profile to lower energy and that the dimethylisocorrole complexes support panchromatic absorption profiles that extend across the UV-vis and into the near-IR region. Photosensitization experiments demonstrate that unlike previously studied Pd(II) biladiene constructs, DMBil-Br10, Pd[10-DMIC-Br6], and Pd[10-DMIC-Br8] support limited triplet excited state chemistry with O2, indicating that the novel nonaromatic tetrapyrrole derivatives described in this work may be best suited for applications other than singlet oxygen sensitization.

Expanded N-Confused Phlorin: A Platform for a Multiply Fused Polycyclic Ring System via Oxidation within the Macrocycle

Novel interrupted π-conjugated macrocycles derived from expanded porphyrinoids were synthesized, and their unique reactivity was investigated in this work. The specific porphyrin analogs, so-called phlorins and isoporphyrins, possess a meso-sp³ methylene moiety, showing inner 3NH and 1NH pyrrolic cores, respectively, and extended near-infrared (NIR) absorption. Expanded N-confused pentapyrrolic phlorin analog 1 bears an interrupted cyclic π-conjugated system that is featured by a distinct higher HOMO and a lower LUMO. Oxidation of 1 allowed structural transformations through the expanded isoporphyrin-like species 2. One of the representative products is a spiro-carbon-bridged multiply N-fused product 3 comprising a fused [5.6.5.7.6.5]-hexacyclic ring obtained by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone. When magic blue was used as the oxidant, an aromatic N-confused pentaphyrin 4 was obtained via migration of one of the meso-phenyl groups to the β-position of the neighboring pyrrolic ring. By employing the flexible cavity of 1 for metal coordination, Pd(II) complexation occurred with a specific meso oxygenation to give a bimetallic complex 5. In contrast to the rich oxidation reactions, reduction of 1 with NaBH₄ resulted in the regioselective nucleophilic hydrogen substitution reaction at the para position of one of the meso-C₆F₅ groups. These results provide a practical approach for synthesizing novel interrupted or aromatic π-conjugated frameworks showing NIR absorptions.

Gold Nanoshell-Linear Tetrapyrrole Conjugates for Near Infrared-Activated Dual Photodynamic and Photothermal Therapies

Photodynamic therapy (PDT) is a treatment in which photoactive compounds delivered to cancerous tissues are excited with light and then transfer the absorbed energy to adjacent tissue oxygen molecules to generate toxic singlet oxygen (¹O₂). As ¹O₂ is produced only where light and photosensitizers (PSs) are combined, PDT holds promise as a minimally invasive, highly selective treatment for certain cancers. The practical application of PDT requires easily synthesized, water-soluble PSs that have low dark toxicities, high ¹O₂ quantum yields, and efficient absorption of 650-850 nm near-infrared (NIR) light, which deeply penetrates tissue. We recently developed a linear tetrapyrrole metal complex, Pd[DMBil1]-PEG₇₅₀, that meets most of these criteria. This complex is remarkably effective as a PS for PDT against triple-negative breast cancer (TNBC) cells but, critically, it does not absorb NIR light, which is necessary to treat deeper tumors. To enable NIR activation, we synthesized a new derivative, Pd[DMBil1]-PEG₅₀₀₀-SH, which bears a thiol functionality that facilitates conjugation to NIR-absorbing gold nanoshells (NSs). Upon excitation with pulsed 800 nm light, NSs emit two-photon-induced photoluminescence spanning 500-700 nm, which can sensitize the attached PSs to initiate PDT. Additionally, NSs produce heat upon 800 nm irradiation, endowing the NS-PS conjugates with an auxiliary photothermal therapeutic (PTT) capability. Here, we demonstrate that NS-PS conjugates are potent mediators of NIR-activated tandem PDT/PTT against TNBC cells in vitro. We show that Pd[DMBil1]-PEG₅₀₀₀-SH retains the photophysical properties of the parent Pd[DMBil1] complex, and that NS-PS generate ¹O₂ under pulsed 800 nm irradiation, confirming activation of the PSs by photoluminescence emitted from NSs. TNBC cells readily internalize NS PS conjugates, which generate reactive oxygen species in the cells upon pulsed NIR irradiation to damage DNA and induce apoptosis. Together, these findings demonstrate that exploiting photoluminescent NSs as carriers of efficient Pd[DMBil1] PSs is an effective strategy to enable NIR light-activated tandem PDT/PTT.

Water-Soluble Nitroxyl Porphyrin Complexes FeIITPPSHNO and FeIITPPSNO- Obtained from Isolated FeIITPPSNO•

The first biomimetic water-soluble Fe^II-porphyrin nitroxyl complexes were obtained and characterized by UV-vis in protonated and deprotonated forms by reduction of previously isolated and characterized Fe^IITPPSNO^•. The pK_a involved in the Fe^II-HNO ⇄ Fe^II-NO^- + H⁺ equilibrium was estimated to be around 9.7. The Fe^IITPPSHNO complex spontaneously reoxidizes to the nitrosyl form following a first-order kinetic decay with a measured kinetic constant of k = 0.017 s^-1. Experiments show that the HNO adduct undergoes unimolecular homolytic cleavage of the H-NO bond. DFT calculations suggest a phlorin radical intermediate for this reaction. The deprotonated NO^- complex resulted to be more stable, with a half-life of about 10 min.

Aromaticity versus regioisomeric effect of β-substituents in porphyrinoids

Maximizing the regioisomeric effect of β-substituents on photophysical properties of porphyrinoids through disruption of TT-conjugation and reducing the aromaticity.

Reversible π-system switching of thiophene-fused thiahexaphyrins by solvent and oxidation/reduction

The concept of chemical topology has generated considerable interest among chemists and one of the state-of-the-art topics is Möbius topology in cyclic π-conjugated molecules. In this regard, expanded porphyrins have been extensively studied because of their facile topological interconversions and attractive optoelectronic properties. A typical example involves [28]hexaphyrins: they show topological conversion between planar Hückel and twisted Möbius topologies owing to their flexible structure. With this in mind, we designed a [28]hexaphyrin where one dimethine pyrrole unit was replaced with dithieno[3,4-b:3',4'-d]thiophene (β-DTT), aiming at a reversible switching between macrocyclic and cross-conjugated π-systems by a change in molecular topologies. Considering that the β-DTT unit can offer both macrocyclic and cross-conjugated π-circuits, we envisioned that a combination of the topological interconversion of [28]hexaphyrin with the two π-circuits of the β-DTT unit would enable a reversible switching between macrocyclic and cross-conjugated π-circuits on Möbius and Hückel topologies, respectively, by a simple conformational change. Unexpectedly, the hexaphyrin revealed a unique, unprecedented π-system switching between a Möbius cross-conjugated π-system and a Hückel antiaromatic π-system, which was fully supported by both experimental and theoretical investigations. Meanwhile, the [28]hexaphyrin was also found to be redox interconvertible with the corresponding [26]hexaphyrin with a Hückel cross-conjugated π-system. These results demonstrate that the β-DTT unit is a new effective motif to realize π-system switching by changing molecular and π-system topologies. Importantly, external stimuli, i.e., solvent, as well as oxidation/reduction can be used to trigger the topological changes in expanded porphyrins with the help of the β-DTT unit.

Evaluating Nanoshells and a Potent Biladiene Photosensitizer for Dual Photothermal and Photodynamic Therapy of Triple Negative Breast Cancer Cells

Light-activated therapies are ideal for treating cancer because they are non-invasive and highly specific to the area of light application. Photothermal therapy (PTT) and photodynamic therapy (PDT) are two types of light-activated therapies that show great promise for treating solid tumors. In PTT, nanoparticles embedded within tumors emit heat in response to laser light that induces cancer cell death. In PDT, photosensitizers introduced to the diseased tissue transfer the absorbed light energy to nearby ground state molecular oxygen to produce singlet oxygen, which is a potent reactive oxygen species (ROS) that is toxic to cancer cells. Although PTT and PDT have been extensively evaluated as independent therapeutic strategies, they each face limitations that hinder their overall success. To overcome these limitations, we evaluated a dual PTT/PDT strategy for treatment of triple negative breast cancer (TNBC) cells mediated by a powerful combination of silica core/gold shell nanoshells (NSs) and palladium 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene-based (Pd[DMBil1]-PEG₇₅₀) photosensitizers (PSs), which enable PTT and PDT, respectively. We found that dual therapy works synergistically to induce more cell death than either therapy alone. Further, we determined that low doses of light can be applied in this approach to primarily induce apoptotic cell death, which is vastly preferred over necrotic cell death. Together, our results show that dual PTT/PDT using silica core/gold shell NSs and Pd[DMBil1]-PEG₇₅₀ PSs is a comprehensive therapeutic strategy to non-invasively induce apoptotic cancer cell death.

Photochemotherapeutic Properties of a Linear Tetrapyrrole Palladium(II) Complex displaying an Exceptionally High Phototoxicity Index

Photodynamic therapy (PDT) represents a minimally invasive and highly localized treatment strategy to ablate tumors with few side effects. In PDT, photosensitizers embedded within tumors are activated by light and undergo intersystem crossing, followed by energy transfer to molecular oxygen, resulting in the production of toxic singlet oxygen (¹O₂). Previously, we reported a robust, linear tetrapyrrole palladium(II) complex, Pd[DMBil1], characterized by its facile and modular synthesis, broad absorption profile, and efficient ¹O₂ quantum yield of Φ_Δ = 0.8 in organic media. However, the insolubility of this porphyrinoid derivative in aqueous solution prevents its use under biologically relevant conditions. In this work, we report the synthesis of Pd[DMBil1]-PEG₇₅₀, a water-soluble dimethylbiladiene derivative that is appended with a poly(ethylene) glycol (PEG) functionality. Characterization of this complex shows that this PEGylated biladiene architecture maintains the attractive photophysical properties of the parent complex under biologically relevant conditions. More specifically, the absorption profile of Pd[DMBil1]-PEG₇₅₀ closely matches that of Pd[DMBil1] and obeys the Beer-Lambert Law, suggesting that the complex does not aggregate under biologically relevant conditions. Additionally, the emission spectrum of Pd[DMBil1]-PEG₇₅₀ retains the fluorescence and phosphorescence features characteristic of Pd[DMBil1]. Importantly, the PEGylated photosensitizer generates ¹O₂ with Φ_Δ = 0.57, which is well within the range to warrant interrogation as a potential PDT agent for treatment of cancer cells. The Pd[DMBil1]-PEG₇₅₀ is biologically compatible, as it is taken up by MDA-MB-231 triple negative breast cancer (TNBC) cells and has an effective dose (ED₅₀) of only 0.354 μM when exposed to λ_ex > 500 nm light for 30 min. Impressively, the lethal dose (LD₅₀) of Pd[DMBil1]-PEG₇₅₀ without light exposure was measured to be 1.87 mM, leading to a remarkably high phototoxicity index of ∼5300, which is vastly superior to existing photosensitizers that form the basis for clinical PDT treatments. Finally, through flow cytometry experiments, we show that PDT with Pd[DMBil1]-PEG₇₅₀ induces primarily apoptotic cell death in MDA-MB-231 cells. Overall these results demonstrate that Pd[DMBil1]-PEG₇₅₀ is an easily prepared, biologically compatible, and well-tolerated photochemotherapeutic agent that can efficiently drive the photoinduced apoptotic death of TNBC cells.

Synthesis of a doubly SO2-fused phlorin: Tuning the structure and properties by the SO2 groups

A doubly SO2-fused phlorin 4 has been synthesized by the [2 + 2] condensation of dipyrromethanecarbinol 2 and SO2-fused dipyrromenthane 3 in the presence of TFA, followed by DDQ oxidation. The SO2-fused phlorin 4 has been characterized by absorption, fluorescence, mass and NMR spectra, as well as X-ray analysis. Compared to the [Formula: see text]-unsubstituted phlorin 5, the SO2-fused phlorin 4 exhibits a red-shifted absorption spectrum (around 12 nm), a more distorted molecular conformation, as well as nice photostability even with an electron-donating meso-3,5-di-tert-butylphenyl group. The titration of 4 and 5 with TBAF has been monitored by absorption spectroscopy. The deprotonated phlorin 4 shows a peak at 870 nm which is red shifted by 26 nm compared to that of deprotonated 5.

Calix[5]phyrin for Fluoride Ion Sensing with Visible and Near Infrared Optical Responses

Fluoride (F- ) ion sensing is an important topic due to its roles in health, medical, and environmental sciences. In this regard, colorimetric sensors with a near infrared (NIR) optical response are useful in biological systems because they can avoid interference from endogenous chromophores. Although calix[n]phyrins are highly attractive as sensors with the NIR optical response, studies on calix[n]phyrins are still limited owing to their intrinsic instability against ambient light and air. In this study, we report the synthesis and characterization of a new calix[5]phyrin bearing one sp3 -hybridized carbon atom as a π-expanded calix[n]phyrin. Upon addition of tetrabutylammonium fluoride, the calix[5]phyrin exhibited distinct NIR absorptions at 908 and 1064 nm as well as a visible color change. Importantly, it revealed an excellent selectivity for F- ion. These results demonstrate that calix[5]phyrins are promising colorimetric and NIR sensors of F- ion.

The Eponymous Cofactors in Cytochrome P460s from Ammonia-Oxidizing Bacteria Are Iron Porphyrinoids Whose Macrocycles Are Dibasic

The enzymes hydroxylamine oxidoreductase and cytochrome (cyt) P460 contain related unconventional "heme P460" cofactors. These cofactors are unusual in their inclusion of nonstandard cross-links between amino acid side chains and the heme macrocycle. Mutagenesis studies performed on the Nitrosomonas europaea cyt P460 that remove its lysine-heme cross-link show that the cross-link is key to defining the spectroscopic properties and kinetic competence of the enzyme. However, exactly how this cross-link confers these features remains unclear. Here we report the 1.45 Å crystal structure of cyt P460 from Nitrosomonas sp. AL212 and conclude that the cross-link does not lead to a change in hybridization of the heme carbon participating in the cross-link but rather enforces structural distortions to the macrocycle away from planarity. Time-dependent density functional theory coupled to experimental structural and spectroscopic analysis suggest that this geometric distortion is sufficient to define the spectroscopic properties of the heme P460 cofactor and provide clues toward establishing a relationship between heme P460 electronic structure and function.

Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting Pd^II or Pt^II centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single Pd^II and Pt^II center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV-visible region, especially in the 350-600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ∼500-700 nm and phosphorescence from ∼700-875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier Pt^II center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of ¹O₂ (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize ¹O₂ may make them attractive platforms for development of new agents for photodynamic therapy.

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

Ultrafast time-resolved measurements were performed on a novel pentafluorophenyl substituted 5,5-dimethyl phlorin derivative in solution and when attached to TiO2 colloidal films. The complex excited state dynamics of this porphyrinoid after S1 and S2 excitation was compared at different wavelengths and can be assigned to several subsequent relaxation mechanisms. The difference between excited state dynamics in the free molecule and when attached to an electron accepting electrode was measured. For both cases the dynamics was compared after excitation to the S1 and the S2 state. For the free molecule in solution an intermediate relaxation step was identified and assigned to a buckling motion of the tetrapyrrole ring. On the electrode, heterogeneous electron transfer (HET) times from both states were very similar and around 50 fs. Surprisingly, the large difference in the density of acceptor states that are resonant with the respective donor level of the molecule does not significantly influence HET dynamics. This result indicates that HET proceeds into intermediate transition states that are different from steady state surface states obtained from experiments or computations. The density of states (DOS) of these transient acceptor states appears not to be directly related to the corresponding surface or bulk DOS.

Unique prototropy of meso-alkylidenyl carbaporphyrinoid possessing one meso-exocyclic double bond

The unique prototropy ofmeso-alkylidenyl-thia(m-benzi)porphyrinoid containing one exocyclic double bond at themeso-position is presented.

Remarkably selective, non-linear allosteric regulation of anion binding by a tetracationic calix[4]pyrrole homodimer

A covalently coupled, dimeric tetra-cationic calix[4]pyrrole homodimer bearing anthracene linkers displayed distinctive cooperativity and fluoride selectivity with large positive allosterism.

Vegetable-based dye-sensitized solar cells

In this review we provide an overview of vegetable pigments in dye-sensitized solar cells, starting from main limitations of cell performance to cost analysis and scaling-up prospects.

Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn²⁺ and Cu²⁺ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of ¹O₂ at wavelengths longer than 500 nm was probed. Both the free base and Zn²⁺ 10,10-dimethylbiladiene architectures show modest efficiencies for ¹O₂ sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O₂ and other small molecules.

Factors Controlling the Spectroscopic Properties and Supramolecular Chemistry of an Electron Deficient 5,5-Dimethylphlorin Architecture

A new 5,5-dimethylphlorin derivative (3H(Phl^CF₃ )) was prepared and studied through a combination of redox, photophysical, and computational experiments. The phlorin macrocycle is significantly distorted from planarity compared to more traditional tetrapyrrole architectures and displays solvatochroism in the soret region of the UV-vis spectrum (∼370-420 nm). DFT calculations indicate that this solvatochromic behavior stems from the polarized nature of the frontier orbital (LUMO+1) that is most heavily involved in these transitions. Compound 3H(Phl^CF₃ ) also displays an intriguing supramolecular chemistry with certain anions; this phlorin can cooperatively hydrogen-bond two equivalents of fluoride to form 3H(Phl^CF₃ )·2F^- but does not bind larger halides such as Cl^- or Br^-. Analogous studies revealed that the phlorin can hydrogen-bond with carboxylate anions such as acetate to form 1:1 complexes such as 3H(Phl^CF₃ )·OAc^- . These supramolecular assemblies are robust and form even in relatively polar solvents such as MeCN. Hydrogen-bonding of fluoride and acetate anions to the phlorin N-H residues significantly attenuates the redox and photophysical properties of the phlorin. Moreover, The ability to independently vary the size and pK_a of a series of carboxylate hydrogen-bond acceptors has allowed us to probe how phlorin-anion association is controlled by the anion's size and/or basicity. These studies elucidate the physical properties and the electronic effects that shape the supramolecular chemistry displayed by the phlorin platform.

Reduction of CO2 using a Rhenium Bipyridine Complex Containing Ancillary BODIPY Moieties

The reduction of carbon dioxide to chemical fuels such as carbon monoxide is an important challenge in the field of renewable energy conversion. Given the thermodynamic stability of carbon dioxide, it is difficult to efficiently activate this substrate in a selective fashion and the development of new electrocatalysts for CO2 reduction is of prime importance. To this end, we have prepared and studied a new fac-ReI(CO)3 complex supported by a bipyridine ligand containing ancillary BODIPY moieties ([Re(BB2)(CO)3Cl]). Voltammetry experiments revealed that this system displays a rich redox chemistry under N2, as [Re(BB2)(CO)3Cl] can be reduced by up to four electrons at modest potentials. These redox events have been characterized as the ReI/0 couple, and three ligand based reductions - two of which are localized on the BODIPY units. The ability of the BB2 ligand to serve as a non-innocent redox reservoir is manifest in an enhanced electrocatalysis with CO2 as compared to an unsubstituted Re-bipyridine complex lacking BODIPY units ([Re(bpy)(CO)3Cl]). The second order rate constant for reduction of CO2 by [Re(BB2)(CO)3Cl] was measured to be k = 3400 M-1s-1 at an applied potential of -2.0 V versus SCE, which is roughly three times greater than the corresponding unsubstituted Re-bipyridine homologue. Photophysical and photochemical studies were also carried out to determine if [Re(BB2)(CO)3Cl] was a competent platform for CO2 reduction using visible light. These experiments showed that this complex supports unusual excited state dynamics that precludes efficient CO2 reduction and are distinct from those that are typically observed for fac-ReI(CO)3 complexes.

Photocatalytic Conversion of CO2 to CO using Rhenium Bipyridine Platforms Containing Ancillary Phenyl or BODIPY Moieties

Harnessing of solar energy to drive the reduction of carbon dioxide to fuels requires the development of efficient catalysts that absorb sunlight. In this work, we detail the synthesis, electrochemistry and photophysical properties of a set of homologous fac-ReI(CO)3 complexes containing either an ancillary phenyl (8) or BODIPY (12) substituent. These studies demonstrate that both the electronic properties of the rhenium center and BODIPY chromophore are maintained for these complexes. Photolysis studies demonstrate that both assemblies 8 and 12 are competent catalysts for the photochemical reduction of CO2 to CO in DMF using triethanolamine (TEOA) as a sacrificial reductant. Both compounds 8 and 12 display TOFs for photocatalytic CO production upon irradiation with light (λex ≥ 400 nm) of ~5 hr-1 with TON values of approximately 20. Although structural and photophysical measurements demonstrate that electronic coupling between the BODIPY and fac-ReI(CO)3 units is limited for complex 12, this work clearly shows that the photoactive BODIPY moiety is tolerated during catalysis and does not interfere with the observed photochemistry. When taken together, these results provide a clear roadmap for the development of advanced rhenium bipyridine complexes bearing ancillary BODIPY groups for the efficient photocatalytic reduction of CO2 using visible light.

Synthesis, electrochemistry, and photophysics of a family of phlorin macrocycles that display cooperative fluoride binding

A homologous set of 5,5-dimethylphlorin macrocycles in which the identity of one aryl ring is systematically varied has been prepared. These derivatives contain ancillary pentafluorophenyl (3H(Phl(F))), mesityl (3H(Phl(Mes))), 2,6-bismethoxyphenyl (3H(Phl(OMe))), 4-nitrophenyl (3H(Phl(NO2))), or 4-tert-butylcarboxyphenyl (3H(Phl(CO2tBu))) groups at the 15-meso-position. These porphyrinoids were prepared in good yields (35-50%) and display unusual multielectron redox and photochemical properties. Each phlorin can be oxidized up to three times at modest potentials and can be reduced twice. The electron-donating and electron-releasing properties of the ancillary aryl substituent attenuate the potentials of these redox events; phlorins containing electron-donating aryl groups are easier to oxidize and harder to reduce, while the opposite trend is observed for phlorins containing electron-withdrawing functionalities. Phlorin substitution also has a pronounced effect on the observed photophysics, as introduction of electron-releasing aryl groups on the periphery of the macrocycle is manifest in larger emission quantum yields and longer fluorescence lifetimes. Each phlorin displays an intriguing supramolecular chemistry and can bind 2 equiv of fluoride. This binding is allosteric in nature, and the strength of halide binding correlates with the ability of the phlorin to stabilize the buildup of charge. Moreover, fluoride binding to generate complexes of the form 3H(Phl(R))·2F(-) modulates the redox potentials of the parent phlorin. As such, titration of phlorin with a source of fluoride represents a facile method to tune the ability of this class of porphyrinoid to absorb light and engage in redox chemistry.