Intramolecular Charge Transfer and Spin-Orbit Coupled Intersystem Crossing in Hypervalent Phosphorus(V) and Antimony(V) Porphyrin Black Dyes.

Porphyrin dyes with strong push-pull type intramolecular charge transfer (ICT) character and broad absorption across the visible spectrum are reported. This combination of properties has been achieved by functionalizing the periphery of hypervalent and highly electron-deficient phosphorus(V) and antimony(V) centered porphyrins with electron-rich triphenylamine (TPA) groups. As a result of the large difference in electronegativity between the porphyrin ring and the peripheral groups, their absorption profiles show several strong charge transfer transitions, which in addition to the porphyrin-centered π → π* transitions, make them panchromatic black dyes with high absorption coefficients between 200 and 800 nm.

Photoinduced Magnetic Exchange-Jump Promotes Ground State Biradical Electron Spin Polarization.

Photoinduced electron spin polarization (ESP) is reported in the electronic ground states of three Pt(II) complexes comprised of two S = 1/2 nitronyl nitroxide (NN) radicals attached through different length -phenylethynyl bridges to the 3,6 positions of a catecholate (CAT, donor) and 4,4'-di--butyl-2,2'-bipyridine (bpy, acceptor). Complexes - have from 17 to 41 bonds separating NN radicals and display cw-EPR spectra consistent with || ≫ ||, || ≥ ||, and || < ||, respectively, where is the magnetic exchange coupling between NN radicals in the electronic ground state, and is the isotropic N hyperfine coupling constant. Light-induced transient EPR spectra characterized as enhanced ground-state absorption were observed for all three complexes using 532 nm pulsed laser excitation into the ligand-to-ligand charge transfer (LL'CT) band of the (CAT)Pt(bpy) chromophore.

Electronic structure and energetics of a heterodimeric BChl '/Chl ' special pair generated by exposure of to dioxygen.

Heliobacteria are anoxygenic phototrophs that have a Type I homodimeric reaction center containing bacteriochlorophyll (BChl ). Previous experimental studies have shown that in the presence of light and dioxygen, BChl is converted into 8-OH-chlorophyll (hereafter Chl ), with an accompanying loss of light-driven charge separation. These studies suggest that the reaction center only loses the ability to transfer electrons once both BChl ' molecules of the P special pair have been converted to Chl '.

Competitive reversed quartet mechanisms for photogenerated ground state electron spin polarization.

Photoinduced electron spin polarization (ESP) of a spin-½ organic radical (nitronyl nitroxide, NN) in a series of Pt(ii) complexes comprised of 4,4'-di--butyl-2,2'-bipyridine (bpy) and 3--butylcatecholate (CAT) ligands, where the CAT ligand is substituted with (CH)--phenyl-NN (bridge-NN) groups, is presented and discussed. We show the importance of attenuating the energy gap between localized NN radical and chromophoric excited states to control both the magnitude and sign of the optically-generated ESP, and to provide deeper insight into the details of the ESP mechanism. Understanding electronic structure contributions to optically generated ESP will enhance our ability to control the nature of prepared states for a variety of quantum information science applications, where strong ESP facilitates enhanced sensitivity and readout capabilities at low applied magnetic fields and higher temperatures.

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer.

The germylone dimNHCGe (dimNHC=diimino N-heterocyclic carbene) reacts with azides N R (R=SiMe or p-tolyl) to furnish the first examples of germanium π-complexes, i. e. guanidine-ligated compounds (dimNHI-SiMe )Ge (NHI=N-heterocyclic imine, R=SiMe ) and (dimNHI-Tol)Ge (R=p-tolyl).

Optically detected magnetic resonance and mutational analysis reveal significant differences in the photochemistry and structure of chlorophyll f synthase and photosystem II.

In cyanobacteria that undergo far red light photoacclimation (FaRLiP), chlorophyll (Chl) f is produced by the ChlF synthase enzyme, probably by photo-oxidation of Chl a. The enzyme forms homodimeric complexes and the primary amino acid sequence of ChlF shows a high degree of homology with the D1 subunit of photosystem II (PSII). However, few details of the photochemistry of ChlF are known.

A two-site triplet exciton hopping model: Application to P.

A model is presented describing the effect on spin-polarized transient EPR signals caused by incoherent state hopping between two sites. It is shown that the size of the spin state space can be reduced by half to the subspace described by the site-average Hamiltonian and that the dynamics of the system results in a redistribution of the population between its eigenstates. Analytical expressions for the rates of population redistribution and the line shape are derived for the general case in which the back-and-forth rates are unequal.

Correction: Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer.

Correction for 'Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer' by Susanna Ciuti , , 2022, https://doi.org/10.1039/D2CP04291F.

Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer.

The excited triplet state of a cofacial aluminum(III) porphyrin-phosphorus(V) porphyrin heterodimer is investigated using transient EPR spectroscopy and quantum chemical calculations. In the dimer, the two porphyrins are bound covalently to each other a μ-oxo bond between the Al and P centres, which results in strong electronic interaction between the porphyrin rings. The spin polarized transient EPR spectrum of the dimer is narrower than the spectra of the constituent monomers and the magnitude of the zero-field splitting parameter is solvent dependent, decreasing as the polarity of the solvent increases.

Single-Photon-Induced Electron Spin Polarization of Two Exchange-Coupled Stable Radicals.

Transient electron paramagnetic resonance spectroscopy has been used to probe photoinduced electron spin polarization of a stable exchange-coupled organic biradical in a Pt(II) complex comprising 4,4'-di--butyl-2,2'-bipyridine (bpy) and 3,6-bis(ethynyl--phenyl-nitronyl nitroxide)--catecholate (CAT(-C≡C-Ph-NN)). Photoexcitation results in four unpaired spins in excited states of this complex, with spins being localized on each of the two radicals, CAT and bpy. The four spins are all exchange-coupled in these excited states, and an off-diagonal matrix element in the CAT-NN exchange allows for exchange-enhanced intersystem crossing to the T state, which possesses (bpy)Pt(CAT) chromophoric triplet character.

Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization.

Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via -phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy, CAT (= semiquinone, SQ) and NN], charge-separated doublet S (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a T (T = chromophore excited spin triplet configuration) state. The T state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy.

EPR of Type I photosynthetic reaction centers.

Light-induced reactions in photosynthetic reaction centers are initiated by the absorption of a photon, which results in the transfer of a single electron and the generation of radical ions in the donor and acceptor molecules involved in the charge-separated state. Electron paramagnetic resonance (EPR) spectroscopy is the ideal method for the study of such reactions. In addition to measuring spectra of the electron transfer cofactors in continuous light, reactions can be initiated by brief flashes of light, thereby allowing the kinetics of forward electron transfer as well as recombination reactions to be obtained.

Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States.

Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT--MePhMe-NN) (bpy = 5,5'-di--butyl-2,2'-bipyridine, CAT = 3--butylcatecholate, -Ph = -phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet S (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a T (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction (), which determines the T-T energy gap.

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization.

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor-acceptor chromophore two different -phenylene bridges in () and () (bpy = 5,5'-di--butyl-2,2'-bipyridine, CAT = 3--butylcatecholate, -Ph = -phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙, CAT˙ = semiquinone (SQ), and NN), charge-separated doublet S (S = chromophore excited spin singlet configuration) excited state. Following excitation, the S state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversion the T (T = chromophore excited spin triplet configuration) state.

Enhanced Intersystem Crossing due to Resonant Energy Transfer to a Remote Spin.

A new mechanism for enhanced intersystem crossing in coupled three-spin systems consisting of a chromophore and an attached radical is proposed. It is shown that if the unpaired electron of the radical experiences spin-orbit coupling and different exchange interactions with the two unpaired electron spins of the chromophore, energy transfer from the chromophore to the radical can occur together with singlet-triplet intersystem crossing in the chromophore. The efficiency of this process increases dramatically when the electronic excitation of the radical is resonant with the S-T energy gap of the chromophore.

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States.

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (S/D) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a -phenylene bridge, (bpy)M(CAT--Ph-NN) () (bpy = 4,4'-di--butyl-2,2'-bipyridine, M = Pd () or Pt (), CAT = 3--butylcatecholate, -Ph = -phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy, CAT = semiquinone (SQ), and NN), charge-separated doublet S (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a T (T = chromophore excited spin triplet configuration) state.

Differential sensitivity to oxygen among the bacteriochlorophylls g in the type-I reaction centers of Heliobacterium modesticaldum.

The type-I, homodimeric photosynthetic reaction center (RC) of Heliobacteria (HbRC) is the only known RC in which bacteriochlorophyll g (BChl g) is found. It is also simpler than other RCs, having the smallest number of protein subunits and bound chromophores of any type-I RC. In the presence of oxygen, BChl g isomerizes to 8-hydroxychlorophyll a (Chl a).

A charge transfer state induced by strong exciton coupling in a cofacial μ-oxo-bridged porphyrin heterodimer.

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, μ-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(iii) and octaethylporphyrinatophosphorus(v), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(iii) and P(v) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ∼130 ps.

Reversible triplet energy hopping in photo-excited molecules: A two-site model for the spin polarization.

The effect of reversible energy hopping between different local environments on the properties of spin-polarized excited states is investigated theoretically using a two-site model. The kinetic equations for the populations of the spin sublevels of the excited state are derived and then used to obtain analytical expressions for the evolution of the spin polarization of excited triplet states under specific conditions. The time dependence of the triplet state polarization patterns is also obtained by numerical solution of the kinetic equations.

Electron spin polarization transfer induced by triplet-radical interactions in the weakly coupled regime.

We report the observation of electron spin polarization transfer from the triplet state of a porphyrin to a weakly coupled nitroxide radical in a mutant of human neuroglobin (NGB). The native iron-containing heme substrate of NGB has been substituted with Zn(ii) protoporphyrin IX and the nitroxide has been attached via site-directed spin labeling to the Cys120 residue. A reference synthetic polypeptide with free base tetraphenylporphyrin and a nitroxide bound to it is also studied.

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles.

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr] , Ar=2,6-iPr C H , 1) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C-H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene.

Exclusive triplet electron transfer leading to long-lived radical ion-pair formation in an electron rich platinum porphyrin covalently linked to fullerene dyad.

The formation of a high-energy, long-lived radical ion-pair by electron transfer exclusively from the triplet excited state, is demonstrated in a newly synthesized platinum porphyrin-fullerene dyad, in which the porphyrin ring is modified with three electron rich triphenylamine entities. The spin selectivity of the electron transfer leading to the formation of the radical ion-pair is demonstrated using time-resolved optical and EPR spectroscopic techniques.

Decelerating Charge Recombination Using Fluorinated Porphyrins in -Bis(3,4,5-trimethoxyphenyl)aniline-Aluminum(III) Porphyrin-Fullerene Reaction Center Models.

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors.

Two-dimensional Zn HYSCORE spectroscopy reveals that a Zn-bacteriochlorophyll a' dimer is the primary donor (P) in the type-1 reaction centers of Chloracidobacterium thermophilum.

Chloracidobacterium (C.) thermophilum is a microaerophilic, chlorophototrophic species in the phylum Acidobacteria that uses homodimeric type-1 reaction centers (RC) to convert light energy into chemical energy using (bacterio)chlorophyll ((B)Chl) cofactors. Pigment analyses show that these RCs contain BChl a, Chl a, and Zn-BChl a' in the approximate ratio 7.

Synthesis and structural studies of copper(ii) complex with NS based N-substituted pendant phosphonic acid arms.

The synthesis of a heteromacrocyclic bifunctional chelator with phosphonic acid pendent arms is presented along copper(ii) complexation. Ligand NS-POH featuring N,N'-bis-substituted phosphonate pendent arms was isolated in respectable yields, characterized, and chelated to copper(ii). Implementation of both Moedritzer-Irani and Kabachnik-Fields conditions using aza-thia macrocycle 1,8-dithia-4,11-diazacyclotetradecane afforded 1,8-dithia-4,11-diazacyclotetradecane-4,11-diyl-bis-(methylene)-bis-(phosphonic acid) (NS-POH).