Time-resolved Electron Paramagnetic Resonance Study Research Articles

(Invited) Non-Polymer Organic Solar Cells: Microscopic Phonon Control to Suppress Non-Radiative Voltage Loss Via Charge-Separated State

Recent remarkable developments on non-fullerene solar cells have reached photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this respect, development of the small donor molecule is essential. Although several methods have been applied to elucidate the elementary processes of charge carriers in BHJ-OSCs, mechanistic details of non-radiative voltage loss are poorly understood originating from interfacial heterogeneous electron-hole pairs.A recent time-resolved electron paramagnetic resonance (TREPR) study suggested an impact of the recombination process generating the triplet excitons in non-fullerene acceptor solar cells,[1] while geometries and motions of charge-separated (CS) states were elucidated in details by electron spin polarization (ESP).[2-4] Furthermore, although effects of low-frequency nuclear motions (phonon) on the photocarrier generations have been discussed in several literatures,[2,5,6] microscopic origin of the phonon playing a role on the OPV performance is unclear including impact of the phonon assist on primary recombination processes. Here we systematically investigated mechanisms of solar cell performance with diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates of C4-DPP–H2BP and C4-DPP–ZnBP where C4 represents butyl group substituted at the DPP unit as small p-type molecules in non-polymer organic solar cells (OSC) with an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester. We clarified microscopic origins of the photocarrier caused by phonon-assisted 1D electron-hole dissociations at donor:acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking, ensuring carrier transport through face-on molecular conformations to suppress non-radiative voltage loss via capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. While disordered lattice motions from the π-π stackings via zinc ligation are essential to enhance the entropy for charge-dissociations at the interface, too much ordered crystallinity causes backscattering phonon to reduce OSC performances.

Photoinduced energy and electron transfer in a cofacial aluminum(III) porphyrin – Phosphorus(V) porphyrin heterodimer

A cofacial ‘aluminum(III) porphyrin – phosphorus(V) porphyrin’ (AlPor-Ph-PPor.PF6) heterodimer has been synthesized to investigate the factors that govern the energy and electron transfer processes in a direction perpendicular to the porphyrin plane. The AlPor and PPor+ are bound in the cofacial arrangement through a benzoate spacer. The electron-rich aluminum(III) porphyrin acts as an electron and energy donor, whereas the electron-poor phosphorus(V) porphyrin performs the role of electron and energy acceptor. Steady-state absorption studies and DFT calculations indicate that the energy states and orbitals of the two porphyrins are not significantly perturbed in the heterodimer compared to the corresponding monomers. However, fluorescence, femtosecond transient absorption and time-resolved electron paramagnetic resonance studies reveal that the excited state behavior of the heterodimer is very different from that of its constituent monomeric porphyrins. Excitation of the AlPor unit in the heterodimer results in both energy and electron transfer from the 1AlPor* to the PPor+ unit. The measured rate constants for energy and electron transfer in o-dichlorobenzene were found to be 2.9 × 109 s–1 and 5 × 109 s–1, respectively. The relative yield of the energy and electron processes can be modulated by changing the solvent polarity with energy transfer being slightly favored in less polar solvents. In contrast, excitation of the PPor+ unit results exclusively in electron transfer from the AlPor unit to 1(PPor+)*. Regardless of the excitation wavelength, the resulting charge-separated state eventually decays to the triplet state of the PPor+ unit. Finally, the photophysical properties of the investigated heterodimer, AlPor-Ph-PPor.PF6 are compared with a previously reported μ-oxo-heterodimer, AlPor-O-PPor.PF6, in which the distance between the AlPor and PPor+ is much shorter. In AlPor-O-PPor.PF6 the strong exciton coupling between AlPor and PPor+ leads to a charge transfer (CT) absorption, while in AlPor-Ph-PPor.PF6 neither of these features is observed because of the larger distance between the porphyrins.

(Invited) Triplet Sensitizer Derived, Nanocarbon Bearing Donor-Acceptor Conjugates for High Potential, Long-Lived Charge Separation

Donor-acceptor conjugates capable of producing high potential, long-lived charge separated states are invaluable for artificial photosynthesis of converting light energy into either electricity or environmentally friendly fuels. The majority of the donor-acceptor conjugates designed for this purpose are derived mainly from singlet sensitizers.1,2 In such conjugates, charge separation from the singlet excited donor or acceptor due to rapid charge separation and recombination processes results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish.3-5 In the present contribution, we summarize our efforts to derive and witness donor-acceptor conjugates where excited state electron transfer exclusively occurs from the triplet excited state. To achieve the goal, two triplet sensitizers, viz., palladium porphyrin and platinum porphyrin are covalently linked to either fullerene, C60 3-5 or (6,5) or (7,6) enriched single wall carbon nanotubes (SWCNTs).6 Using transient absorption spectral studies covering wide temporal and spatial regions, and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1MP* (M = Pd or Pt) to C60 in a series of dyads, leading to the formation of MP•+-C60 •- charge separated state of triplet spin character will be demonstrated. Similarly, the role of triplet sensitizers in promoting charge separation in PdP-SWCNT(6,5) and PdP-SWCNT(7,6) in solvents of different polarity will be highlighted. 1. C. B. KC, F. D’Souza, ‘Design and photochemical study of supramolecular donor-acceptor systems assembled via metal-ligand axial coordiation’ Coord. Chem. Rev. 2016, 322, 104-141.2. F. D’Souza, and O. Ito ‘Photosensitized electron transfer processes of nanocarbons applicable to solar cells’ Chem. Soc. Rev. 2012, 41, 86-96.3. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185.4. C. Obondi, G. N. Lim, B. Churchill, P. K. Poddutoori, A. van der Est, and F. D’Souza, ‘Modulating the generation of long-lived charge separated states exclusively from the triplet excited states in palladium porphyrin–fullerene conjugates’, Nanoscale, 2016, 8, 8333-8344.5. P. K. Poddutoori, Y. E. Kandrashkin, C. O. Obondi, F. D’Souza and A. van der Est, Triplet Electron Transfer and Spin Polarization in a Palladium Porphyrin–Fullerene Conjugate, Phys. Chem. Chem. Phys. 2018, 20, 28223-28231.6. L. M. Arellano, H. B. Gozebe, M. J. Gómez-Escalonilla, J. L. G. Fierro, F. D’Souza and F. Langa, Photoinduced charge separation in PdP-SWCNT(6,5) and PdP-SWCNT(7,6) donor-acceptor hybrids, manuscript in preparation.

Time-Resolved Electron Paramagnetic Resonance Study of Photoexcited Fullerenes in Ionic Liquids.

Molecular-level properties of ionic liquids (ILs) draw an increasing interest. Several informative experimental approaches for investigation of nano/miscrostructuring phenomena and local viscosity/rigidity of ILs use probe molecules sensitive to microenvironment along with suitable detection techniques. In this work, we for the first time investigate capabilities of photoexcited triplet fullerenes to probe local properties of ILs, with time-resolved electron paramagnetic resonance (TR EPR) being a sensitive detection tool. We have selected C60 and its derivative phenyl-C61-butyric acid methyl ester (PCBM) as probes and ILs [Bmim]BF4 and [C10mim]BF4 as solvents. C60 and PCBM demonstrate different sensitivities to microenvironment in ILs. Spin dynamics of photoexcited C60 is strongly contributed by pseudorotation of the Jahn-Teller axis, making its use as a probe for microenvironment challenging. This behavior is strongly suppressed in PCBM, which, in addition, is more soluble in ILs than C60. The in-depth analysis of variable-temperature two-dimensional TR EPR data shows that spectral shapes are sensitive to the restricted mobility of PCBM in ILs. In this way, the information on local environment and heterogeneities in ILs can be obtained by TR EPR. PCBM usefully complements the other spin probes previously implemented for EPR studies in ILs. It is larger in size and in addition allows high-sensitivity TR EPR measurements up to a room temperature, which is an important improvement for characterization of heterogeneities in room-temperature ILs.

Continuous Wave and Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Radicals in Fluoroacrylic Porous Polymer Films

Fluoroacrylic polymers with inherent micro/nanoporosity are promising media for incorporation of fluorescent molecules and following application as pressure-sensitive paints (PSPs), and UV photostability of PSPs is critically important for their long-term performance. Although photodegradation mechanisms of fluoroacrylic polymers have been studied previously in solutions, they have never been addressed in practically relevant for PSPs solid-state porous films. In this work we combined continuous wave (CW) and time-resolved (TR) electron paramagnetic resonance (EPR) to study UV photodegradation of thin porous films of a few representative fluoroacrylic polymers. Different types of spectra were detected using CW and TR EPR and assigned to the species formed on the inner surface of the pores and in the bulk of the polymer, respectively. The radical pairs formed in the bulk are short-lived, as is evidenced by TR EPR, and most likely recombine back to the initial polymer. On the contrary, the radicals formed o...

(Invited) Long-Lived Charge Separated States Exclusively from the Triplet Excited States in Palladium Porphyrin-Fullerene Conjugates

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish. Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C60 dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1PdP* to C60 leading to the formation of PdP•+-C60 •- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1 1) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)3PdP]•+-C60 •- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of 3C60*, electron transfer from 3(TPA)3PdP* to C60 leading to [(TPA)3PdP]•+-C60 •- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185. Figure 1

Microwave multimode memory with an erbium spin ensemble

Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. In that respect, hybrid quantum systems combining circuit QED with ions doped into solids are an attractive platform. There, the ions serve as coherent memory elements and reversible conversion elements of microwave to optical qubits. Among many possible spin-doped solids, erbium ions offer the unique opportunity of a coherent conversion of microwave photons into the telecom C-band at $1.54\,\mu$m employed for long distance communication. In our work, we perform a time-resolved electron spin resonance study of an Er$^{3+}$:Y$_2$SiO$_5$ spin ensemble at milli-Kelvin temperatures and demonstrate multimode storage and retrieval of up to 16 coherent microwave pulses. The memory efficiency is measured to be 10$^{-4}$ at the coherence time of $T_2=5.6\,\mu$s.

Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Electron Transfer in Pd Porphyrin-Quinone and Zn Porphyrin-Quinone Dyads with a Cyclohexylene Spacer.

Peculiarities of the light induced intramolecular electron transfer processes in two ensembles where Pd porphyrin and Zn porphyrin donors with similar peripheral substituents are covalently linked via cyclohexylene spacer with a quinone acceptor, were studied by time-resolved electron paramagnetic resonance spectroscopy in different phases of the magnetically oriented nematic liquid crystal E-7. In the photoexcited PdP-Q the net absorptive signal was observed and ascribed to the thermally equilibrated spectrum of (3)*(PdP(•+)-Q(•-)). In ZnP-Q photoinduced intramolecular electron transfer was also found. It was demonstrated that the multiplet spectrum of the charge-separated state (3)*(ZnP(•+)-Q(•-)) consists of two signals with different widths and decay times. The signals were assigned to two spin-polarized triplets of the radical pairs formed in "stretched" and "folded" ensemble conformers, corresponding to different configurations of the cyclohexylene spacer. These findings were discussed in terms of differences in the properties of the porphyrin metal cores, macrocycle peripheral substituents and geometry of the donor-acceptor cyclohexylene spacer.

Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center

To elucidate how cofactor geometries after photoinduced primary charge-separations influence electronic couplings (VCR) for primary charge-recombination (CR) processes in the photosynthetic reaction center, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra both of the primary charge-separated (CS) state (P+•HA–•) and its charge-recombined triplet state (3P*) of the special pair in Rhodobacter sphaeroides R26. To determine the CS-state geometry, quantum mechanical modeling has been performed on the spin polarization of the 3P* generated by the spin dynamics due to the anisotropies of the hyperfine and the spin–spin dipolar interactions in the primary CS state. From transverse magnetizations of the primary CS state, we have also determined the VCR value leading to the singlet excited state (1P*) of the special pair. The above analyses have revealed that while the primary charge separation does not largely modulate the cofactor conformations, it leads to significant enhancement i...

Optical and Time-Resolved Electron Paramagnetic Resonance Studies of the Excited States of a UV-B Absorber (4-Methylbenzylidene)camphor

The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet-triplet (T-T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T(1)) state of MBC were determined. The energy level of the T(1) state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T-T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S(1)) molecules undergoes intersystem crossing to the T(1) state and the deactivation process from the T(1) state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)ππ* character in the T(1) state. The zero-field splitting parameters in the T(1) state are D = 0.0901 cm(-1) and E = -0.0498 cm(-1). The sublevel preferentially populated by intersystem crossing is T(y) (y close to in-plane short axis and to the C═O direction).

Time-Resolved Electron Paramagnetic Resonance and Phosphorescence Studies of the Lowest Excited Triplet States of Rh(III) Corrole Complexes

The lowest excited triplet (T(1)) ππ* states of gallium (Ga) and various rhodium (Rh) 5,10,15-trispentafluorophenyl corroles (Cors) were studied in the liquid crystal (LC) E-7 and in rigid glasses by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The triplet sublevel energies were experimentally determined by the alignment of the molecules in the LC and by magnetophotoselection in the glass. The sublevel scheme of GaCor was determined by calculating the zero field splitting (ZFS) parameters. Axial ligand effects and quantum chemical calculations were used for the sublevel assignment of RhCors. The anisotropic EPR parameters were used to determine the important higher excited states and the magnitudes of their spin-orbit coupling (SOC) contributions were evaluated. On the basis of these results and analyses, the EPR parameters and triplet lifetime were discussed for each RhCor complex.

W-Band Time-Resolved Electron Paramagnetic Resonance Study of Light-Induced Spin Dynamics in Copper–Nitroxide-Based Switchable Molecular Magnets

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

Exploring the Electron Transfer Pathways in Photosystem I by High-Time-Resolution Electron Paramagnetic Resonance: Observation of the B-Side Radical Pair P700+A1B– in Whole Cells of the Deuterated Green Alga Chlamydomonas reinhardtii at Cryogenic Temperatures

Crystallographic models of photosystem I (PS I) highlight a symmetrical arrangement of the electron transfer cofactors which are organized in two parallel branches (A, B) relative to a pseudo-C2 symmetry axis that is perpendicular to the membrane plane. Here, we explore the electron transfer pathways of PS I in whole cells of the deuterated green alga Chlamydomonas reinhardtii using high-time-resolution electron paramagnetic resonance (EPR) at cryogenic temperatures. Particular emphasis is given to quantum oscillations detectable in the tertiary radical pairs P700(+)A1A(-) and P700(+)A1B(-) of the electron transfer chain. Results are presented first for the deuterated site-directed mutant PsaA-M684H in which electron transfer beyond the primary electron acceptor A0A on the PsaA branch of electron transfer is impaired. Analysis of the quantum oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the geometry of the B-side radical pair. The orientation of the g tensor of P700(+) in an external reference system is adapted from a time-resolved multifrequency EPR study of deuterated and 15N-substituted cyanobacteria (Link, G.; Berthold, T.; Bechtold, M.; Weidner, J.-U.; Ohmes, E.; Tang, J.; Poluektov, O.; Utschig, L.; Schlesselman, S. L.; Thurnauer, M. C.; Kothe, G. J. Am. Chem. Soc. 2001, 123, 4211-4222). Thus, we obtain the three-dimensional structure of the B-side radical pair following photoexcitation of PS I in its native membrane. The new structure describes the position and orientation of the reduced B-side quinone A1B(-) on a nanosecond time scale after light-induced charge separation. Furthermore, we present results for deuterated wild-type cells of C. reinhardtii demonstrating that both radical pairs P700(+)A1A(-) and P700(+)A1B(-) participate in the electron transfer process according to a mole ratio of 0.71/0.29 in favor of P700(+)A1A(-). A detailed comparison reveals different orientations of A1A(-) and A1B(-) in their respective binding sites such that formation of a strong hydrogen bond from A1(-) to the protein backbone is possible only in the case of A1A(-). We suggest that this is relevant to the rates of forward electron transfer from A1A(-) or A1B(-) to the iron-sulfur center F(X), which differ by a factor of 10. Thus, the present study sheds new light on the orientation of the phylloquinone acceptors in their binding pockets in PS I and the effect this has on function.

Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in β-Cyclodextrin

The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated β-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in β-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in β-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T1 and T2 of triplet BP at 30 K measured by pulse EPR are found to be different in β-CD compared to nonpolar toluene glass. The observed increase of T2 by up to a factor of four in β-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in β-CD. The influence of β-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.

ChemInform Abstract: The Excited Multiplet States of Metalloporphyrins and Metallophthalocyanines Coordinated or Linked to Nitroxide Radicals

Abstract We have reviewed several time-resolved electron paramagnetic resonance (TREPR) studies focusing on influences of nitroxide radicals on the photophysical properties of metalloporphyrins and metallophthalocyanines, which are coordinated or linked to nitroxide radicals. This paper summarizes TREPR studies on the excited multiplet states of metalloporphyrins and metallophthalocyanines coordinated by several kinds of nitroxide radicals, where the excited doublet and quartet states generated by the interaction between the excited triplet chromophore and doublet radical have been analyzed in detail. Selective population from the excited multiplet states to the triplet ground state is also discussed for phthalocyaninatosilicon covalently linked to two TEMPO radicals in terms of a new concept for controlling magnetic properties by photoexcitation.

Time-Resolved Electron Paramagnetic Resonance Study of Rhodium(III) Corrole Excited States

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.

Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine−Anthracene Molecules with Perpendicular π Systems

Time-resolved electron paramagnetic resonance studies show that the primary mechanism of triplet formation following photoexcitation of julolidine-anthracene molecules linked by a single bond and having perpendicular pi systems is a spin-orbit, charge-transfer intersystem crossing mechanism (SOCT-ISC). This mechanism depends on the degree of charge transfer from julolidine to anthracene, the dihedral angle (theta1) between their pi systems, and the magnitude of the electronic coupling between julolidine and anthracene. We compare 4-(9-anthracenyl)-julolidine with the more sterically encumbered 4-(9-anthracenyl)-3,5-dimethyljulolidine and find that fixing theta1 congruent with 90 degrees serves to enhance SOCT-ISC by increasing the change in orbital angular momentum accompanying charge transfer. Given that the requirements for the SOCT-ISC mechanism are quite general, we expect it to occur in a variety of electron donor-acceptor systems.

Structure-function relationship in antimony corrole photosensitizers: Time-resolved electron paramagnetic resonance and optical study

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.

An EPR study of the pH dependence of formate effects on Photosystem II

Effects of formate on rates of O 2 evolution and electron paramagnetic resonance (EPR) signals were observed in the oxygen evolving PS II membranes as a function of pH. In formate treated PS II membranes, decrease in pH value resulted in the inhibition of the O 2 evolving activity, a decrease in the intensity of S 2 state multiline signal but an increase in the intensity of the Q A –Fe 2+ EPR signal. Time-resolved EPR study of the Y Z ● decay kinetics showed that the light-induced intensity of Y Z ● EPR signal was proportional to the formate concentration. The change in the pH affected both the light-induced intensities and the decay rates of Y Z ●, which was found to be faster at lower pH. At 253 K, t 1/e value of Y Z ● decay kinetics was found to be 8-10 s at pH 6.0 and 18-21 s at pH 5.0. The results presented here indicate that the extent of inhibition at the donor and the acceptor side of PS II due to formate is pH dependent, being more effective at lower pH.

Hydrogen abstraction from solvents by the triplet state of p-benzoquinone: a time-resolved electron paramagnetic resonance and laser flash photolysis study

The reaction of triplet p-benzoquinone in several solvents, such as ethanol, 2-propanol, ethylene glycol, t-butanol, 1,4-dioxane, tetrahydrofuran and cyclohexane, has been studied. The primary photochemical event was shown to be a hydrogen atom abstraction from the solvent and not an electron transfer. In the time-resolved EPR experiments, in all cases, except t-butanol and cyclohexane, the radicals from the solvent were recorded along with p-benzosemiquinone radical and the spectra were dominated by triplet mechanism of spin polarization, giving totally emissive EPR signals. The sites of abstracted hydrogen atom have been identified. In particular, whereas the methine hydrogen atom is abstracted from 2-propanol, both the methylene and hydroxyl hydrogen atoms are abstracted from ethanol. From laser flash photolysis experiments, the rate constants of the hydrogen abstraction in all solvents, except t-butanol and cyclohexane, were found to be (1–5) × 108 M−1 s−1.