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Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for studying photoinduced intramolecular electron transfer (ET) and energy transfer (EnT) processes in three mechanically interlocked molecules in which zinc(II)porphyrin (ZnP) and C60 fullerene moieties are arrayed around a central Cu(I) bisphenanthroline core used to assemble these donor–acceptor (D–A) systems. The specific molecules studied include a “long” (ZnP)2–Cu(I)(phen)2–C60 rotaxane as well as ZnP–Cu(I)(phen)2 and ZnP–Cu(I)(phen)2–C60 catenanes, embedded in different phases of nematic liquid crystal and frozen isotropic solvents. It was demonstrated that the routes and rates of the transfer processes in these supramolecules strongly depend on the characteristics of their microenvironment and the molecular entity which was selectively photoexcited. This is reflected by formation of distinct long-lived charge-separated species such as the radical ion pair (ZnP)2•+–Cu(I)(phen)2•– and the metal-to-ligand charge-separated sta...

The effect of the corrole macrocycle bromination on its photoexcited triplet state parameters was examined in a comparison study of brominated and nonbrominated Ga(III) 5,10,15-tris(pentafluorophenyl)corroles, employing X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. It is demonstrated that the spectrum of the brominated Ga-corrole is characterized by an opposite polarization pattern and a larger zero-field splitting (ZFS) parameter |D|, compared to nonbrominated Ga-corrole. With the assignment of a negative sign for the ZFS parameter D, the dominant intersystem crossing (ISC) pathways are evaluated. Spectral line shape analysis reveals that in the brominated Ga-corrole,the out-of-plane triplet sublevel is overpopulated (A(Z) > A(X), A(Y)), while in nonbrominated Ga-corrole, the in-plane triplet sublevels are preferentially populated (A(X), A(Y) ≫ A(Z)). The differences in the photophysical properties of the corroles are attributed to the heavy atom effect upon corrole skeleton bromination, which enhances the spin-orbit coupling strength in the brominated complex, thus affecting its ISC and ZFS parameters.

Photoinduced intramolecular electron-transfer (ET) and energy-transfer (EnT) processes in two rotaxanes, one containing both zinc porphyrin and C(60) fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)(2)-Cu(I)(phen)(2)-C(60)] and a second complex lacking the fullerene [(ZnP)(2)-Cu(I)(phen)(2)], were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) combined with a selective photoexcitation of the rotaxane moieties. The experiments were carried out in isotropic toluene and ethanol and in anisotropic nematic liquid-crystal (E-7) media over a wide range of temperatures corresponding to the different states of the solvents. The TREPR results are compared with those obtained previously by optical methods in dichloromethane at room temperature. It is demonstrated that the efficiencies and pathways of the light-driven ET and EnT processes in both rotaxanes strongly depend on the properties of their microenvironment, resulting in the formation of distinct charge-separated states under different experimental conditions. The complementary results revealed by the optical and TREPR techniques are attributed to the relatively high conformational mobility of the mechanically interlocked rotaxane systems. Because of the solute-solvent interactions, the rotaxanes are able to change conformation in different microenvironments, which affects the parameters of the photoinduced processes occurring in these systems.

Three photosensitizers based on tris-(pentafluorophenyl)antimony corroles that differ in oxidation state and axial ligands, namely, (pyridine) Sb (III)-, (oxo) Sb (V)- and (difluoro) Sb (V) complexes, were studied by time-resolved electron paramagnetic resonance spectroscopy and laser flash photolysis. The magnetic and orientational parameters of the corroles oriented in a nematic liquid crystal as well as their triplet lifetimes in liquid toluene were determined and interpreted in terms of their structure and geometry. The negative zero-field splitting parameter D assigned to all studied corroles is explained by the asymmetric π-electron withdrawal effect caused by perfluorinated peripheral aryl groups, which force the triplet electron spins to align in head-to-tail configuration. The effect of the axial ligands on the photoexcited triplet state properties of the corroles is correlated with their different efficiency to perform photoassisted aerobic oxygenation of some organic molecules. This is explained by the dependence of the main parameters of the photoexcited complexes on the interaction between the central ion and corrole π-system. This interaction is strongly influenced by axial ligands coordination, affecting the macrocycle symmetry, planarity, and rigidity.

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc) and Al(pyr)2(tpfc-Br8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD AX, AY, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.

Light-driven multi-step intramolecular electron transfer in a rod-like triad, in which two of the three redox components are linked by three hydrogen bonds, was studied by time-resolved electron paramagnetic resonance (TREPR) and optical spectroscopies. One part of the molecule consists of a p-methoxyaniline primary electron donor (MeOAn) covalently linked to a 4-aminonaphthalene-1, 8-dicarboximide (6ANI) chromophoric electron acceptor (MeOAn-6ANI). The unsubstituted dicarboximide of 6ANI serves as one half of a hydrogen bonding receptor pair. The other half of the receptor pair consists of a melamine linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) secondary electron acceptor (MEL-NI). TREPR spectroscopy is used to probe the electronic interaction between the radicals within the photogenerated, spin-correlated radical ion pair MeOAn +-6ANI/MEL-NI −. The results are compared to those obtained in earlier studies in which MeOAn-6ANI is covalently linked to NI through a 2,5-dimethylphenyl group (MeOAn-6ANI-Ph-NI). We show that the electronic coupling between the oxidized donor and reduced acceptor in the hydrogen-bonded radical ion pair MeOAn +-6ANI/MEL-NI − is very similar to that of MeOAn +-6ANI-Ph-NI −.

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1 → 2CT → 2S0) or internal conversion, 2T1 → 2S0.

Light-induced spin-polarized transient EPR spectra are reported for several water-soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge-transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.

Electron paramagnetic resonance (EPR) spectroscopy was used to study the interactions between stable free radicals and gold nanoparticles. The nitroxyl free radicals used were TEMPO, TEMPAMINE, and TEMPONE. Two sizes of Au particles, 15 and 2.5 nm in diameter, were synthesized to investigate the interactions with the metallic particles. We find that the EPR signal is reduced upon adsorption of the radicals onto the 15 nm Au particle surface. Despite the strong adsorption of TEMPAMINE on the particles, the signal intensity recovers upon the introduction of a high concentration of ethanolamine to the solution. The signal reduction was proportional to the concentration of Au particles, and the signal totally disappeared at high concentrations of Au particles. Possible explanations of the signal reduction are discussed in this Article. We propose that the reduction in signal intensity arises from exchange interactions between the unpaired electrons of the adsorbed radicals and conduction-band electrons of the metallic particles. In addition, in the presence of oxygen, the adsorbed TEMPAMINE radicals are catalytically oxidized to the carbonyl derivative, TEMPONE. A mechanism for this unexpected catalytic reaction is proposed.

The photoexcited triplet state of the two new porphyrin isomers, octaethylhemiporphycene (OEHPC) and octaethylcorrphycene (OECPC), embedded in isotropic (toluene) and an anisotropic liquid crystal matrices was studied by time-resolved electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 150–300 K. The magnetic, kinetic and orientation parameters were determined and interpreted in terms of structure, symmetry and dynamic. Analysis of the results suggests that at T ⩾250 K, the orientation and packing of the chromophores result from a discrete solid-like jumps mechanism, which is more efficient for OECPC. This difference is rationalized in terms of differences in the symmetry of the two chromophores.

We report on an unusual intramolecular electron transfer following the photoexcitation of the ground, charge separated, state of the zwitterionic compound, tetraphenylhexaazaanthracene, which results in a neutral biradical product. The reaction, studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy, could be carried out only when liquid crystals were used as the orienting solvents. On the basis of the analysis of the magnetic, kinetic, and polarization parameters of the TREPR spectra, the radical pair genesis is attributed to an intramolecular electron transfer from the singlet excited state.

We report on intramolecular electron and/or energy transfer (IET and IEnT), in two photoexcited electrostatically bound dimers: (1) zinc(II) tetrakis(4-sulfonatophenyl)porphyrin and magnesium(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin, [ZnTSPP−MgTTAP]; and (2) zinc(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin and magnesium(II) tetrakis(4-sulfonatophenyl)porphyrin, [ZnTTAP−MgTSPP]. The photochemical properties of both dimers, dissolved in dimethylsulfoxide−glycerol (DMSO−Glyc, 140 K), were investigated by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy. It is demonstrated that by changing the peripheral substitution of the particular metalloporphyrin in the dimer, an IET route, parallel to the singlet IEnT, can be triggered. Thus, singlet IEnT from the Zn to the Mg constituent is the only process in [ZnTSPP−MgTTAP], while IET to produce a charge-separated state, [ZnTTAP•+−MgTSPP•-] accompanies the IEnT in [ZnTTAP−MgTSPP].

Light-driven intramolecular electron transfer (ET) and energy transfer (EnT) processes in two rotaxanes, the first containing two free base porphyrins and C(60) fullerene moieties incorporated around a Cu(I)bisphenanthroline core ((H(2)P)(2)-Cu(I)(phen)(2)-C(60)) and a second lacking the fullerene moiety ((H(2)P)(2)-Cu(I)(phen)(2)), were studied by X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The experiments were performed in frozen toluene and ethanol and different phases of the nematic liquid crystal (E-7). It is demonstrated that the ET and EnT processes in the (H(2)P)(2)-Cu(I)(phen)(2)-C(60) rotaxane in different media result in the formation of the same charge-separated state, namely (H(2)P)(2)(•+)-Cu(I)(phen)(2)(•-)-C(60), while photoexcitation of the (H(2)P)(2)-Cu(I)(phen)(2) rotaxane does not induce noticeable transfer processes in these matrices. The results are discussed in terms of the high conformational mobility of the rotaxanes, which enables changes in the molecular topography and resultant modification of the rates and routes of photoinduced processes occurring in these systems. The parameters of the transfer processes are compared with those obtained in our previous study of (ZnP)(2)-Cu(I)(phen)(2)-C(60) and (ZnP)(2)-Cu(I)(phen)(2) rotaxanes under the same experimental conditions.

Photoexcited states of three Rh(III) 5,10,15-tris(pentafluorophenyl)corroles coordinated by different axial ligands; namely, triphenylphosphine P(C(6)H(5))(3) group (1), pyridine C(6)H(5)N group (2), and two pyridine groups (3) were studied by X- and Q-band time-resolved electron paramagnetic resonance (TREPR) in frozen toluene and liquid crystal E-7. Transient mutations were utilized to identify multiplicity of the detected paramagnetic species. The spectra of 1 and 2 were assigned to triplet ((3)pipi*) states, while contributions of triplet ((3)dd and charge transfer (3)CT) and quintet ((5)dd) states were revealed in the spectrum of 3. The results are interpreted in terms of a peculiar nature of transition metal complexes with the unfilled d-shell, where close lying electronic states of different multiplicities may be mixed through configurational, spin-orbit, and vibronic coupling. From the EPR spectra, the spin-orbit coupling constant was estimated to be about 25 cm(-1). It is shown that different axial ligation of complexes shifts the relative energy of the excited states and, consequently, leads to population of different states. Plausible explanations of the effects governing unusual spectral and dynamic parameters of the photoexcited Rh corrole complexes are presented.

The photoexcited triplet state of the thiouredopyrenetrisulfonate (TUPS) covalently linked to (1) α-acetyl lysine (3*TUPS-L) and (2) lysine residue 122 of azurin (3*TUPS-A) were studied by time-resolved EPR spectroscopy. The triplet spectra of 3*TUPS-L and 3*TUPS-A dissolved in water−glycerol (1:1) mixture, at freezing temperatures, show different temperature dependence. While in the case of 3*TUPS-L, no apparent dynamic effects were noticed, 3*TUPS-A exhibits a strong effect at 230 K. The dynamic effects were treated by line shape analysis with the discrete jumps model, suggesting that the environment in which 3*TUPS-A is embedded is not frozen, allowing for fast molecular motion with a correlation time of ∼1.8 × 10-10 s. These results are explained by the unique properties of the protein-bound water layers, which are kept in the liquid state even at relatively low temperatures, where the bulk water is frozen.

Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state.

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.