Photochemistry of a butylene linked porphyrin-quinone donor-acceptor system studied by time-resolved and steady-state EPR spectroscopy

Radicals generated photochemically from a covalently linked porphyrin-quinone donor-acceptor system dissolved in reversed micelles and isotropic solution have been studied by steady-state and time-resolved EPR spectroscopy. In these systems photoinducedintramolecular as well asintermolecular electron transfer processes occur which result in the formation of semiquinone radical anions and porphyrin radical cations. Disproportionation of the semiquinone leads to the formation of porphyrin-hydroquinone (and porphyrin-quinone). The porphyrin-hydroquinone is itself photoactive and reacts through the photoexcited triplet state of the porphyrin. Reduction of the porphyrin to the dihydro from — probablyvia hydrogen abstraction by the photoexcited porphyrin from the hydroquinone — appears to be the dominant reaction. Once formed the dihydrophyrin undergoes further similar photochemistry. Emissively polarized spectra are observed from these systems in steady-state EPR experiments. Timeresolved EPR indicates that this polarization is essentially due to the radical triplet pair mechanism.     

Time-Resolved EPR Spectra of Photoexcited Copper Porphyrin

Photoinduced spin-polarized transient electron paramagnetic resonance (EPR) spectra of copper 5,10,15,20-tetrakis(3-pyridyl)porphyrin (3PyNCu) in the frozen solution have been observed in the X-band. The time evolution and the temperature dependence of the spectra have been studied. The effect of molecular oxygen in the frozen solution on the polarization pattern has also been examined. The magnetic resonance parameters of the ground state of 3PyNCu have been obtained by comparing the experimental continuous-wave and echo-detected EPR spectra with the numerical computations. The magnetic resonance parameters of the excited states and the photoinduced polarizations have been investigated by time-resolved EPR (TREPR) spectroscopy and numerical analysis. The experimental spectra have been considered as a sum of the polarized spectra of the ground and excited states. Our analysis confirmed that the TREPR spectra consisted of two main patterns: the enhanced signal from the ground state and the multiplet contribution belonging to the excited quartet state.     

Interaction between the end groups and the main chain of conjugated polymers by time-resolved EPR and fluorescence spectroscopy

ABSTRACT

Interaction between a zinc porphyrin (ZnPor) as the end-group and poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) as the main chain in a porphyrin end-modified fluorescent conjugated polymer, ZnPFV, was studied by time-resolved electron paramagnetic resonance (EPR) and fluorescence spectroscopy. While fluorescence from the PFV part of ZnPFV showed a spectral profile almost identical to that of a PFV oligomer without end-modification, the emission spectrum of the ZnPor part exhibited a much broader profile compared to that of the reference zinc porphyrin monomer. Based on the analysis of lifetimes and quantum yields, it was found that radiative rate constant of the ZnPor part was enhanced by nearly three times. The observed unusual enhancement in the radiative rate constant was rationalised in terms of a partial π-conjugation between the end group and the main chain, as a result of co-planarisation in fluid solution. On the other hand, the time-resolved EPR spectrum of ZnPFV at 100?K basically showed a similar spectral pattern to that of the reference zinc porphyrin, but with significant differences in zero-field spitting parameters and initial population ratios. The π-system of the excited triplet state is deduced to deviate from D_4h symmetry in the end zinc porphyrin groups. The obtained results show that interaction of the porphyrin end group with the main chain of the polymer significantly influences the excited singlet state properties of the porphyrin, while its triplet state properties were affected to a lesser extent.     

Influence of C60 Aggregation on Pseudorotation in the Excited Triplet State Probed by Multifrequency Time-Resolved EPR

The influence of C₆₀ aggregation on time-resolved (TR) electron paramagnetic resonance (EPR) of C₆₀ in the excited triplet state was investigated by multifrequency EPR techniques. Temperature-independent X-band (9.7 GHz) TR-EPR spectra were observed in a fresh toluene solution, while temperature-dependent ones were reported in literatures. The experimental spectra in this study indicated that the pseudorotation of pristine C₆₀ in frozen toluene solution is not frozen out even at lower temperatures. Careful investigations of TR-EPR and its decay kinetics demonstrated that the pseudorotation can be affected by C₆₀ aggregation. A comparison between X- and W-band (94.9 GHz) results indicated that the aggregation can be accelerated by a capillary effect. Three decay constants were extracted from the analysis of the decay kinetics. The fastest component was ascribed to the pseudorotation, which was independent of temperature in the range of 10–40 K. The temperature dependences of the decay kinetics showed that the pseudorotation is not affected by C₆₀ aggregation at higher temperatures.     

EPR spectra of gable-type copper(II) porphyrin dimers in fluid solution: extraction of exchange interaction in weakly coupled doublet pairs

A comparison between EPR spectra of rigidly linked dicopper porphyrin dimers and those of the corresponding monocopper dimers (copper porphyrin-free base porphyrin dimers) in fluid solution reveals a very weak exchange interaction between the two copper spins. In these dimers, two porphyrin moieties are linked via an aromatic spacer such as benzene, naphthalene or phenanthrene in a gable-type geometry, with a distance of 10–13 Å. Although essentially all the spectra from the monocopper dimers are the same, exhibiting hyperfine (hf) structure due to the copper and nitrogen nuclei, the EPR spectral patterns of the dicopper dimers depend on the spacer molecule. Differences in hf patterns among the dicopper porphyrin dimers are ascribed to isotropic spin—spin coupling, i.e., exchange coupling between the two copper spins. This is because the anisotropic dipole—dipole interaction is averaged out due to random tumbling of the solute molecules in fluid solution. From the line shape analysis, the absolute value of the exchange interaction (|J|) is found to be 4 × 10^?4 cm^?1 ≦|J| < 3 × 10^?3 cm^?1 for the benzene linked dicopper dimer (Cu—Bz—Cu) whereas |J| ～ 1 × 10^?4cm^?1 for the other two dimers (Cu—Np—Cu and Cu—Pn—Cu). These values are comparable with or much smaller than the dipole—dipole coupling, which is estimated as about 1–3 × 10^?3 cm^?1 from the centre-to-centre distance. Since Cu—Bz—Cu shows a significantly larger |J| than Cu—Pn—Cu, despite a slightly longer centre-to-centre distance, and since no correlation could be obtained between |J| and the separation of the two copper atoms, it is likely that the interaction via spacer molecules is dominant between the two halves.     

Resolving Conformational and Rotameric Exchange in Spin-Labeled Proteins Using Saturation Recovery EPR

The function of many proteins involves equilibria between conformational substates, and to elucidate mechanisms of function it is essential to have experimental tools to detect the presence of conformational substates and to determine the time scale of exchange between them. Site-directed spin labeling (SDSL) has the potential to serve this purpose. In proteins containing a nitroxide side chain (R1), multicomponent electron paramagnetic resonance (EPR) spectra can arise either from equilibria involving different conformational substates or rotamers of R1. To employ SDSL to uniquely identify conformational equilibria, it is thus essential to distinguish between these origins of multicomponent spectra. Here we show that this is possible based on the time scale for exchange of the nitroxide between distinct environments that give rise to multicomponent EPR spectra; rotamer exchange for R1 lies in the ≈0.1–1 μs range, while conformational exchange is at least an order of magnitude slower. The time scales of exchange events are determined by saturation recovery EPR, and in favorable cases, the exchange rate constants between substates with lifetimes of approximately 1–70 μs can be estimated by the approach.     

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

The time-resolved electron paramagnetic resonance (EPR) spectra are studied in the temperature range of 110–300 K for two mixed solutions of porphyrins, ZnTPP and H₂TPP, in toluene and the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO varied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as 1 mM. The EPR spectra of triplet-state porphyrins and free radicals manifest the chemically induced spin polarization. For the relatively short-lived radical-triplet pairs, with the perturbation theory up to the fourth order, the theoretical expressions are obtained for the triplet and radical spin polarization induced by the enhanced intersystem crossing (ISC) due to the interaction of excited singlet-state porphyrins with free radicals and by the triplet quenching by free radicals. The time-dependent EPR spectra of the triplets are simulated taking into account the spin-lattice relaxation. It is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattice relaxation kinetics. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetic scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the rate of the ISC and the lifetime of the excited singlet state were estimated by fitting the kinetic curves for the triplet EPR spectra intensity. For the mixed porphyrin-TEMPO solutions, a possible set of the rate constants of important bimolecular processes were determined. For this set of parameters, it turns out that the spin polarization transfer has a smaller rate constant than the rate constant of the diffusion collisions of the triplet and radical. It appears that the rate constant of the ISC catalyzed by radicals is relatively high in the solutions close to the melting point of the solvent and in the soft-glassy state. In the triplet porphyrins the initial spin polarization induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.     

Time-resolved EPR studies on magnetic interactions between excited triplet (tetraphenylporphinato) zinc and doublet nitroxide radical

By time-resolved electron paramagnetic resonance (TREPR), four (tetraphenylporphinato) zinc (ZnTPP) complexes coordinated by an axial ligand containing a nitroxide radical (NRX; X=4, 5, 8, and 10, denotes the bond number from zinc to nitroxide nitrogen) have been studied in terms of magnetic interactions between the photoexcited triplet state of ZnTPP and NRX. The TREPR spectrum of ZnTPP coordinated by NR10 is almost the same as the one of ZnTPP coordinated by pyridine, indicating that the electron exchange interaction,J, between ZnTPP and the doublet nitroxide is negligibly small. On the other hand, TREPR spectra of the NR4 and NR5 complexes are assigned to the Q₁ state constituted by the ZnTPP and the nitroxide radical. In the case of the ZnTPP-NR8 complex, both T₁ and Q₁ TREPR signals are seen, which may originate from two conformations or degenerate T₁ states of ZnTPP. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores.     

Probing the orientation of porphyrin oligomers in a liquid crystal solvent – a triplet state electron paramagnetic resonance study

ABSTRACT

Linear porphyrin oligomers have found various applications as synthetic molecular wires in the context of light harvesting, solar energy conversion and molecular electronics. In many of these applications a partial ordering of the molecules helps to improve the reaction efficiency or device performance. In this work we study the orientational properties of the building blocks of such porphyrin-based molecular wires, namely a porphyrin monomer and the corresponding butadiyne-bridged dimer. The porphyrins have been embedded in the nematic liquid crystal solvent 4-cyano-4'-pentylbiphenyl (5CB) and the anisotropic properties of their photogenerated triplet states were characterised by transient electron paramagnetic resonance (EPR) spectroscopy. When aligned in strong magnetic fields, the liquid crystal molecules impose their orientational anisotropy onto the solute guest molecules whose orientation-dependent magnetic properties can then be explored. The line shape analysis of the porphyrin triplet state EPR spectra – highly sensitive to small conformational changes – confirms the orientation of the zero-field-splitting (ZFS) tensors previously determined for these molecules by magnetophotoselection experiments. A biaxial distribution function is shown to be necessary to simulate the experimental EPR data. The biaxial behaviour, in conjunction with symmetry considerations, allows an unambiguous assignment of the three ZFS tensor axes to the molecular axes. From the determined orientational distributions of the porphyrins in 5CB, the biaxial order parameters for both molecules were calculated.     

Theory of time-resolved two-photon photoemission from Cu(111): effect of Coulomb interactions among electrons

The effect of Coulomb interactions among electrons on the time-resolved two-photon photoemission spectra of Cu(111) is investigated by nonequilibrium perturbation theory. It is demonstrated that there are contributions to photoemission via an image state on the surface from processes involving scattering of photoexcited electrons and holes due to Coulomb interactions. As a result, the correlation trace (photoelectron intensity as a function of the pump-probe delay time) is affected by the lifetimes of the electron in the image state, the photoexcited electron and hole.     

EPR of Spin Transitions in Complexes of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

Polymer chain complexes [Cu(hfac)₂L^R] _n exhibit thermally and light-induced magnetic anomalies in many aspects similar to a spin crossover. These compounds attracted significant attention in the field of molecular magnetism and have been extensively studied by electron paramagnetic resonance (EPR) during the last several years. All compounds studied so far were based on copper(II) ions bridged by pyrazolyl-substituted nitronylnitroxides. The present work reports the first EPR study of complexes of Cu(hfac)₂ with tert-butylpyrazolylnitroxides—a new type of nitroxide ligand expected to modify exchange interaction pathways and physical properties of the crystals. The Q-band EPR spectra of three representative novel compounds are principally different from those studied previously, supporting the assumption that the magnetic motif of the compound has changed. Dominant intercluster exchange interactions are now found along the structural polymer chains. This complicates the EPR detection of phase transitions to some extent; however, theoretical modeling of the observed spectral changes allows for unambiguous assignment of different spin states and transitions between them. The magnitudes of intercluster exchange interaction were estimated to be ca. 0.1–1.5 cm^?1 for the studied compounds.     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

Variable Temperature Single Crystal EPR Studies of Cu(II) in Dipotassium Diaquabis(Malonato-<Emphasis Type="Italic">κ</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">O,O</Emphasis>′) Nickelate Dihydrate: An Example of Contaminated Ground State

Single crystal X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in dipotassium diaquabis(malonato-κ ² O,O′) nickelate dihydrate have been performed at 300, 123 and 77 K to understand the nature of Jahn–Teller distortion in the paramagnetic host lattice. The angular variation of the EPR spectra reveals the presence of two sites, with one site not showing hyperfine resolution even at 77 K. The spin-Hamiltonian parameters of this six-coordinated Cu(II) ion, evaluated from EPR spectra at various temperatures, are:

• 300 K: g ₁₁ = 2.125, g ₂₂ = 2.118, g ₃₃ = 2.290, no copper hyperfine resolution
• 123 K: g ₁₁ = 2.229, g ₂₂ = 2.113, g ₃₃ = 2.319 and A ₁₁ = 5.02, A ₂₂ = 3.82, A ₃₃ = 6.87 mT
• 77 K: g ₁₁ = 2.224, g ₂₂ = 2.114, g ₃₃ = 2.324 and A ₁₁ = 5.32, A ₂₂ = 3.90, A ₃₃ = 7.06 mT

respectively. The low value observed for A ₃₃ at 123 and 77 K has been explained by assuming a ground state \({\text{d}}_{{x^{2} - y^{2} }}\) wave function for Cu(II) ions, contaminated with the excited state \({\text{d}}_{{z^{2} }}\). From the temperature dependence of the EPR spectra, the Cu(II) ions can be considered as a static Jahn–Teller system, with contaminated ground state. The admixture coefficients and bonding parameters have also been calculated by combining EPR and optical data. The EPR spectrum of powder sample confirms single crystal data.

     

Transient EPR at 240 GHz of the excited triplet state of free-base tetra-phenyl porphyrin

The electron paramagnetic resonance (EPR) spectrum measured at 240 GHz of the lowest excited triplet state of the free-base tetra-phenyl porphyrin (H₂-TPP) molecule is presented. The high frequency that is employed allows for the full resolution of theg-anisotropy (2.00354(2), 2.00272(15), and 2.00194(5) forx, y, andz zero-field axes respectively), and the obtained values are compared to those of other porphyrin-based systems. These results are one of the first addressing theg-anisotropy in the photoexcited state of porphyrins. The usefulness of high-field transient EPR in elucidating not only the principal values of theg-tensor but also the orientation of theg-tensor principal axes with respect to those of the zero-field splitting tensor in porphyrin disordered systems is also discussed.     

An Electron Paramagnetic Resonance Study of the Orbital Ordering Compound MgTi<Subscript>2</Subscript>O<Subscript>4</Subscript>

We report the electron paramagnetic resonance (EPR) studies of MgTi₂O₄ in the 300–140 K range. Above the transition temperature T _t (~258 K), the EPR results indicate that MgTi₂O₄ is paramagnetic. The parameters of the EPR spectra show an anomalous change at T _t. The clear EPR lines can be observed in temperature between T _t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi₂O₄ system are formed gradually with decreasing temperature at low temperature range.     

Pulse EPR, ENDOR, and ELDOR Study of Anionic Flavin Radicals in Na+-Translocating NADH:Quinone Oxidoreductase

The Na⁺-translocating nicotinamide adenine dinucleotide (NADH):quinine oxidoreductase (Na⁺–NQR) is a component of respiratory chain of various bacteria and it generates a redox-driven transmembrane electrochemical Na⁺ potential. It contains four different flavin prosthetic groups, including two flavin mononucleotide (FMN) residues covalently bound to the subunits NqrB and NqrC. Na⁺–NQR from Vibrio harveyi was poised at different redox potentials to prepare two samples, containing either both FMN_NqrB and FMN_NqrC or only FMN_NqrB in a paramagnetic state. These two samples were comparatively studied using pulse electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectroscopy. The echo-detected EPR spectra and electron spin relaxation properties were very similar for flavin radicals in both samples. The splitting of the outer peaks in the proton ENDOR spectra, assigned to the C(8α) methyl protons, allows to identify both radicals as anionic flavosemiquinones. The mean interspin distance of 20.7 Å between these radicals was determined by pulse ELDOR experiment, which allows to estimate the edge-to-edge distance (r _e) between these flavin centers as: 11.7 Å < r _e < 20.7 Å. The direct electron transfer between FMN_NqrB and FMN_NqrC during the physiological turnover of the Na⁺–NQR complex is suggested.     

Structures and electronic states of photoexcited states in a system of two nitroxide radicals linked to fullerene studied by two-dimensional pulsed nutation and time-resolved electron paramagnetic resonance spectroscopy

Magnetic properties of fulleropyrrolidine adducts with two stable nitroxide radicals (2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO) were studied in toluene solution by continuous-wave time-resolved (TR) and pulsed electron paramagnetic resonance (EPR) spectroscopy in the ground and photoexcited states. Four isomers of the bisadduct,trans-1,trans-2,trans-3, and equatorial forms, having the second pyrrolidine ring at different [6-6] bonds were synthesized. In the ground states, the exchange interaction between two TEMPOs is so small that the spin state of the bisadduct is a doublet in nature. By means of spectral simulations of the EPR spectra in frozen solution at 70 K, the upper limit of the exchange interaction was estimated to be 5 MHz for thetrans-1 andtrans-2 and 10 MHz for thetrans-3 and equatorial isomers. The simulation was also made to determine relative positions of the two TEMPO groups with respect to the pyrrolidine ring. Photoexcited states of the bisadducts with excitation of the 532 nm laser pulse were studied in frozen toluene solution at 5–100 K by using two-dimensional (2-D) pulsed nutation EPR and TREPR. The spin multiplicity of the excited state was determined by the nutation frequency. All of the four bisadducts showed strong exchange couplings between two TEMPOs and fullerene triplet³C ₆₀ ^* , resulting in the generation of the excited quintet and triplet states. The excited triplet states have been observed and assigned for the first time in strongly coupled triplet-radical systems. The zero-field splittings of the quintet state determined from the 2-D nutation EPR spectra were analyzed as the sum of the spinspin interactions among the three paramagnetic centers, two TEMPOs and³C ₆₀ ^* . On the basis of these analyses, the spin distribution on the³C ₆₀ ^* part and the geometry of two TEMPOs are discussed.     

Time-Resolved ESR Study on Complex Radical Pairs Formed in the Photolysis of Methylene Blue Included in Water-Soluble Sulfonated Calixarenes

The photochemical reactions of methylene blue (MB) included in water-soluble sulfonated calix[n]arenes (n = 4, 6, 8) are studied using a time-resolved electron spin resonance method. The chemically induced dynamic electron polarization (CIDEP) spectra show the formation of the complex radical pair composed of the MB monocation radical and calixarene (phenoxyl-type) radical. The lifetime and broadened spectral shape are dependent on the size of the calixarene and are due to the longitudinal and transverse relaxation mainly induced by the tumbling motion of the radical pair with the spin dipole–dipole interaction. The pair dissociates in a few hundreds of nanoseconds in cases of n = 6 and 8.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.