Phenylene‐Coated Magnetic Nanoparticles that Boost Aqueous Asymmetric Transfer Hydrogenation Reactions

Phenylene‐coated organorhodium‐functionalized magnetic nanoparticles are developed through co‐condensation of chiral 4‐(trimethoxysilyl)ethyl)phenylsulfonyl‐1,2‐diphenylethylene‐diamine and 1,4‐bis(triethyoxysilyl)benzene onto Fe₃O₄ followed complexation with [{Cp*RhCl₂}₂]. This magnetic catalyst exhibits excellent catalytic activity and high enantioselectivity in asymmetric transfer hydrogenation in aqueous medium. Such activity is attributed to the high hydrophobicity and the confined nature of the chiral organorhodium catalyst. The magnetic catalyst can be easily recovered by using a small external magnet and it can be reused for at least 10 times without loss of its catalytic activity. This characteristic makes it an attractive catalyst for environmentally friendly organic syntheses.     

Phenothiazine–BODIPY–Fullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Recombination

Novel photosynthetic reaction center model compounds of the type donor₂–donor₁–acceptor, composed of phenothiazine, BF₂‐chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X‐ray structures of three of the phenothiazine‐BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N‐substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C‐substituted analogue. The geometry and electronic structures were obtained by B3LYP/6‐31G(dp) calculations (for H, B, N, and O) and B3LYP/6‐31G(df) calculations (for S) in vacuum, followed by a single‐point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO?1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy‐level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ–BODIPY^.+–C₆₀^.?; subsequent hole shift resulted in PTZ^.+–BODIPY–C₆₀^.? charge‐separated species. The return of the charge‐separated species was found to be solvent dependent. In nonpolar solvents the PTZ^.+–BODIPY–C₆₀^.? species populated the ³C₆₀* prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The ¹BODIPY* generated radical ion‐pair in these triads persisted for few nanoseconds due to electron transfer/hole‐shift mechanism.     

Photodynamic Therapy Efficacy Enhanced by Dynamics: The Role of Charge Transfer and Photostability in the Selection of Photosensitizers

Progress in the photodynamic therapy (PDT) of cancer should benefit from a rationale to predict the most efficient of a series of photosensitizers that strongly absorb light in the phototherapeutic window (650–800 nm) and efficiently generate reactive oxygen species (ROS=singlet oxygen and oxygen‐centered radicals). We show that the ratios between the triplet photosensitizer–O₂ interaction rate constant (k_D) and the photosensitizer decomposition rate constant (k_d), k_D/k_d, determine the relative photodynamic activities of photosensitizers against various cancer cells. The same efficacy trend is observed in vivo with DBA/2 mice bearing S91 melanoma tumors. The PDT efficacy intimately depends on the dynamics of photosensitizer–oxygen interactions: charge transfer to molecular oxygen with generation of both singlet oxygen and superoxide ion (high k_D) must be tempered by photostability (low k_d). These properties depend on the oxidation potential of the photosensitizer and are suitably combined in a new fluorinated sulfonamide bacteriochlorin, motivated by the rationale.     

On the Control of Magnetic Anisotropy through an External Electric Field

The effect of an external electric field on the magnetic anisotropy of a single‐molecule magnet has been investigated, for the first time, with the help of DFT. The application of an electric field can alter the magnetic anisotropy from “easy‐plane” to “easy‐axis” type. Excitation analysis performed through time‐dependent DFT predicts that the external electric field facilitates metal to π‐acceptor ligand charge transfer, leading to uniaxial magnetic anisotropy and concomitant spin Hall effect in a single molecule.     

Tuning the Electronic Coupling in Cyclometalated Diruthenium Complexes through Substituent Effects: A Correlation between the Experimental and Calculated Results

A common bridging ligand, 3,3′,5,5′‐tetrakis(N‐methylbenzimidazol‐2‐yl)biphenyl, and four terpyridine terminal ligands with various substituents (amine, tolyl, nitro, and ester groups) have been used to synthesize ten cyclometalated diruthenium complexes 1 ²⁺– 10 ²⁺. Among them, compounds 1 ²⁺– 6 ²⁺ are redox nonsymmetric, and others are symmetric. These complexes show two Ru^III/II processes and an intervalence charge transfer (IVCT) transition in the one‐electron oxidized state. The potential separation (ΔE) of 1 ²⁺– 10 ²⁺ has been correlated to the energy difference ΔG⁰, the energy of the IVCT band E_op, and the ground‐state delocalization coefficient α². Time‐dependent (TD)DFT calculations suggest that the absorptions in the visible region of 1 ²⁺– 6 ²⁺ are mainly associated with the metal‐to‐ligand charge‐transfer transitions from both ruthenium ions and to both terminal ligands and the bridging ligand. However, the energies of these transitions vary significantly. DFT calculations have been performed on 1 ²⁺– 6 ²⁺ and 1 ³⁺– 6 ³⁺ to give information on the electronic structures and spin populations of the mixed‐valent compounds. The TDDFT‐predicted IVCT excitations reproduce well the experimental trends in transition energies. In addition, three monoruthenium complexes have been synthesized for a comparison study.     

Luminescent Hybrid Materials Based on Laponite Clay

The spectroscopic behavior of ionic Eu³⁺ or Tb³⁺ complexes of an aromatic carboxyl‐functionalized organic salt as well as those of the hybrid materials derived from adsorption of the ionic complexes on Laponite clay are reported. X‐ray diffraction (XRD) patterns suggest that the complexes are mainly adsorbed on the outer surfaces of the Laponite disks rather than intercalated within the interlayer spaces. Photophysical data showed that the energy‐transfer efficiency from the ligand to Eu³⁺ ions in the hybrid material is increased remarkably with respect to the corresponding ionic complex. The hybrid material containing the Eu³⁺ complex shows bright red emission from the prominent ⁵D₀→⁷F₂ transition of Eu³⁺ ions, and that containing the Tb³⁺ complex exhibits bright green emission due to the dominant ⁵D₄→⁷F₅ transition of Tb³⁺ ions.     

Synthesis and Photophysical Properties of Multichromophoric Carbonyl‐Bridged Triarylamines

The synthesis and photophysical properties of two novel multichromophoric compounds is presented. Their molecular design comprises a carbonyl‐bridged triarylamine core and either naphthalimides or 4‐(5‐hexyl‐2,2′‐bithiophene)naphthalimides as second chromophore in the periphery. The lateral chromophores are attached to the core via an amide linkage and a short alkyl spacer. The synthetic approach demonstrates a straightforward functionalization strategy for carbonyl‐bridged triarylamines. Steady‐state and time‐resolved spectroscopic investigations of these compounds, in combination with three reference compounds, provide clear evidence for energy transfer in both multichromophoric compounds. The direction of the energy transfer depends on the lateral chromophore used. Furthermore, the compound bearing the lateral 4‐(bithiophene)naphthaimides is capable of forming fluorescent gels at very low concentrations in the sub‐millimolar regime whilst retaining its energy transfer properties.     

Experimental and Computational Studies of a Multi‐Electron Donor–Acceptor Ligand Containing the Thiazolo[5,4‐d]thiazole Core and its Incorporation into a Metal–Organic Framework

A ligand containing the thiazolo[5,4‐d]thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N′‐(thiazolo[5,4‐d]thiazole‐2,5‐diylbis(4,1‐phenylene))bis(N‐(pyridine‐4‐yl)pyridin‐4‐amine ( 1 ), was designed as a donor–acceptor system for incorporation into electronically active metal–organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis‐near‐infrared spectroelectrochemistry (UV/Vis‐NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge‐transfer interactions between the terminal triarylamine moieties upon oxidation, whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi‐electron donor–acceptor ligand into a new two‐dimensional MOF, [Zn(NO₃)₂( 1 )] ( 2 ), resulted in an inhibition of the oxidation processes, but retention of the reduction capability of 1 . Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.     

Versatile Iridicycle Catalysts for Highly Efficient and Chemoselective Transfer Hydrogenation of Carbonyl Compounds in Water

Cyclometalated iridium complexes are shown to be highly efficient and chemoselective catalysts for the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include α‐substituted ketones (α‐ether, α‐halo, α‐hydroxy, α‐amino, α‐nitrile or α‐ester), α‐keto esters, β‐keto esters and α,β‐unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50 000 at pH 4.5 and required no organic solvent. The protocol provides a practical, easy and efficient way for the synthesis of β‐functionalised secondary alcohols, such as β‐hydroxyethers, β‐hydroxyamines and β‐hydroxyhalo compounds, which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis.     

On the Reaction Mechanism of the Complete Intermolecular O2 Transfer between Mononuclear Nickel and Manganese Complexes with Macrocyclic Ligands

The recently described intermolecular O₂ transfer between the side‐on Ni‐O₂ complex [(12‐TMC)Ni‐O₂]⁺ and the manganese complex [(14‐TMC)Mn]²⁺, where 12‐TMC and 14‐TMC are 12‐ and 14‐membered macrocyclic ligands, 12‐TMC=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane and 14‐TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long‐range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two‐step reaction, with a first rate‐determining bimolecular step and predict the exothermic character of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a μ‐η¹:η¹‐O₂ coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction.