TR-EPR Spectra Research Articles

Electron Spin Dynamics of the Photoexcited Triplet States of Bromoperylenebisimide Compounds: Computation and Analysis of the Electron Spin Polarization Evolution in TREPR Spectra

Electron Spin Dynamics of the Photoexcited Triplet States of Bromoperylenebisimide Compounds: Computation and Analysis of the Electron Spin Polarization Evolution in TREPR Spectra

Time-Resolved EPR and Theoretical Investigations of Naphthalene Diimide Spin Dynamics in the Excited State

Abstract Naphthalene diimides (NDIs) are a promising material for n-type polymer acceptors in thin-film devices such as organic light-emitting diodes. The intersystem crossing (ISC) pathway of NDIs has been extensively studied, relying heavily on the accuracy of quantum chemical calculations. However, the theoretical results are inconsistent with the experimental findings. In this work, time-resolved electron paramagnetic resonance (tr-EPR) was applied to NDIs for the first time in order to elucidate the ISC pathway and assign their electronic structures. With the help of quantum chemical calculations, tr-EPR clearly demonstrated that ISC occurs via an upper excited triplet state (T4) after photoexcitation. The zero-field splitting (ZFS) constants of the NDIs, which strongly correlate with their spin density distributions, were experimentally determined by tr-EPR. We demonstrate that an ab initio complete active space self-consistent field is required to interpret the ZFS constants of the NDIs. From the theoretical analyses of the constants and phosphorescence measurements, the spin state observed in the tr-EPR spectra was identified as the lowest excited triplet state, illustrating that quick internal conversion occurs after the ISC process.

Measurements for Addition Reaction Rate Constants of Organic Free Radicals to Maleic Anhydride by Means of Pulsed EPR Spectroscopy with Laser Excitation

Addition reaction of photo-generated organic radicals to maleic anhydride (MA), that is a common monomer in polymerization, was studied by time-resolved (TR)- and pulsed electron paramagnetic resonance (EPR) spectroscopic methods. The radicals examined are 2-hydroxypropyl (2Hy-Pr), diphenylphosphine oxide (DPPO) and hydroxycyclohexyl (Hy-CyH). Analysis of TR-EPR spectra gave the structures of adduct radicals. The addition rate constants of the radicals generated by the photolysis of initiators were measured by electron spin echo detection method. It was found that the addition reaction rate constants of 2Hy-Pr, DPPO and Hy-CyH radicals to MA show exceptionally smaller values than those estimated by the theoretical reaction model based on enthalpy and polar effects.

Pulsed EPR measurements on reaction rate constants for addition of photo-generated radicals to double bonds of diethyl fumarate and diethyl maleate

ABSTRACTAddition reaction of photo-generated radicals to double bonds of diethyl fumarate (deF) and diethyl maleate (deM), which are geometrical isomers, was studied by means of time-resolved- (TR-) and pulsed-electron paramagnetic resonance (EPR). Analysis of TR-EPR spectra indicates that adduct radicals from deF and deM should have the same structure. The double bonds of these monomers are converted to single ones by addition reaction, which allows hindered internal rotation to give the same structure of adduct radical. The rate constants for addition reaction of photo-generated radicals were determined by Stern–Volmer analysis of the decay time of electron spin-echo intensity of these radicals measured by the pulsed EPR method. Rate constants for deF were found to be larger than those for deM. This relation is in good consistent with efficiency of polymerisation of deF and deM. Experimentally determined rate constants were evaluated by introducing the addition reaction model on the basis of two important factors enthalpy and polar effects.

Addition Rate Constants of Phosphorus- and Carbon-Centered Radicals to Double Bond of Monomers as Studied by a Pulsed Electron Paramagnetic Resonance Method.

Addition reaction of a radical to the double bond of a monomer, which is important at early stage of photopolymerization, has been studied by time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) spectroscopic methods. Reactions of one phosphorus-centered and three carbon-centered radicals attacking to several monomers have been employed. Intermediate radicals were identified by analyzing the recorded TR-EPR spectra, and the reaction rate constants were determined by the electron spin-echo detection method proposed by Weber and Turro [J. Phys. Chem. A, 2003, 107 (18), 3326 - 3334]. The quantum chemical calculation shows that the rate constants for the addition reactions are well-explained by introducing two factors of "enthalpy effect" and "polar effect" to control the activation barrier height. It was observed that the rate constants of the phosphorus-centered radical were larger than those of carbon-centered radicals for some monomers. The difference in the rate constants was argued on the basis of frequency factor and activation barrier both of which are influenced by an atom of radical center.

Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical.

Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only ∼2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.

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.

TREPR spectra of micelle-confined spin correlated radical pairs: II. Spectral decomposition and asymmetric line shapes

In the second paper, spectral decomposition is used to explain the origin of the asymmetry of the anti-phase structure (APS) and its temperature dependence in dynamic spin correlated radical pairs (SCRPs) created via the photoreduction of benzophenone (BP) in sodium dodecyl sulfate (SDS) micelles. It is shown that the main parameters defining the spectral shape of the TREPR spectra are the effectiveness of the electron spin exchange in contact pairs, and the ratio of the frequency of enforced encounters (Z) to the frequency of singlet-triplet mixing (q) in the separated radical pairs. The Z/q ratio is particularly important for the creation of the APS asymmetry. The existence of different q values in the same TREPR spectrum in this system affords the observation of SCRPs in both regimes: exchange broadening (large |q|/Z) and exchange narrowing (small |q|/Z). An important observation, supported by the successful simulation of the TREPR spectra, is that the S-component of the APS can be shifted in a direction opposite to that predicted by the earlier Closs-Forbes-Norris (CFN) model. This result is naturally explained in terms of a spectral exchange approach. Dispersion-like components in the spectra further amplify the asymmetry of the APS.

Electron spin polarisation in the photoreduction of xanthone in alcohol: effect of concentration and temperature

Time-resolved EPR studies of the hydrogen abstraction reaction of photoexcited xanthone in 2-propanol were carried out as a function of the concentration of xanthone and the sample temperature. The temperature was varied from 22°C to about −30°C, and the concentration from about 0.2 to 4.0 mM. At low temperature or concentration, the observed spectra of the xanthone ketyl radical and the propan-2-olyl radical could be simulated as a superposition of a hyperfine-independent component due to the emissive triplet mechanism and a hyperfine-dependent component due to the S–T0 radical pair mechanism. However, with an increase in the concentration of xanthone, the relative contribution of TM decreases, and, concomitantly, the net absorptive component of only the xanthone ketyl radical increases. As the spin polarisation mechanisms do not predict any concentration dependence, this unusual behaviour is explained by invoking the enhancement of the spin–lattice relaxation rates due to Heisenberg spin exchange occurring at high local concentrations of the radicals. The net absorptive signal is attributed to thermally equilibrated radicals. The observed temperature dependence of the spin polarisation behaviour is similarly explained. The origin of the net absorptive signal in the TREPR spectra of the acetone−2-propanol system is also attributed to thermally equilibrated radicals. The self-quenching mechanism of xanthone is proposed to be an electron-transfer reaction from an excited xanthone molecule to another xanthone in the ground state.

Photoreactions and Molecular Dynamics of Radical Pairs in a Reversed Micelle Studied by Time-Resolved Measurements of EPR and Magnetic Field Effect

Photoreaction of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) in an aerosol OT (AOT) reversed micelle (RM) is studied by time-resolved EPR (tr-EPR) and the transient absorption detected magnetic field effect (MFE). Tr-EPR and transient absorption spectra indicate electron transfer from a highly excited triplet state of TMPD to the AOT headgroup regardless of W = [H(2)O]/[AOT] values from 0 to 40. Noticeable MFEs on the yield of TMPD cation radical (TMPD(+)) are observed at W > 0 and maximized at W ∼ 10. The dynamics of TMPD(+) in the bound water region of the RM has been precisely analyzed by theoretical analysis of time-resolved magnetically affected reaction yield (MARY) spectra. The simulation of the MARY spectra indicates that two kinds of radical pairs exist, both of which are composed of an AOT alkyl radical and TMPD(+). One system has TMPD(+) strongly bound to the anionic interface, where the radical pair shows very slow relaxation and recombination. Another system has TMPD(+) diffusing in the bound water, which shows a smaller diffusion coefficient than that in bulk water by 1 order of magnitude. In the larger water pool (W > 15), the spin correlated radical pair of the hydrated electron and TMPD(+) generated by photoionization is observed by tr-EPR. The ionization reaction is followed by electron attachment to the AOT headgroup and generation of the sulfite radical. However, these radical pairs are not thought to contribute significantly to the observed MFEs. Spin multiplicities of the precursor state and recombination products have been discussed from the different sign of J values for the radical pairs at larger W.

Solvent- and Wavelength-Dependent Photochemistry of 1,3-Dihydroxyacetone Studied by TREPR: Implications for Commercial Sunless Tanning Lotions

1,3-Dihydroxyacetone (DHA) is the active ingredient in commercial personal care products for the production of a “sunless” tan on human skin. The DHA molecule is the simplest ketose sugar, and it reacts with amino acids on the surface of the skin in a biochemical pathway known as the Maillard reaction. However, DHA is known to be photochemically active and will produce free radicals with high quantum yield under ultraviolet (UV) excitation. We report here a detailed study of the free radicals produced by DHA photolysis as a function of wavelength and solvent. In contrary to the previous optical and steady-state electron paramagnetic resonance (EPR) studies, X-band time-resolved (TR)-EPR spectra reveal a complex reactivity pattern: Norrish I α-cleavage is observed in almost all cases, but when good H-atom donors are present, H-atom abstraction by the solvent is observed. Changing the wavelength of excitation from 308 to 248 nm can reverse this observation. Comparison of TREPR spectra obtained from commercially available sunless tanning lotions shows that many free radicals, including those from DHA, can be detected using direct detection TREPR upon photolysis of the lotions themselves. The results suggest that caution should be observed in the use of these products in conjunction with UV exposure.

Time-resolved EPR observation of synthetic eumelanin–superoxide radical pairs

TR-EPR spectra of UV irradiated synthetic eumelanin in equilibrium with air oxygen, reveal the formation of melanin-superoxide radical pairs and allow to determine both the electron dipolar interaction D and the exchange interaction J between the two radicals.

Photoredox Chemistry of AOT: Electron Transfer and Hydrogen Abstraction in Microemulsions Involving the Surfactant

Time-resolved magnetic resonance experiments (TREPR and CIDNP) are used to investigate previously unobserved redox chemistry of the surfactant dioctyl sulfosuccinate ester (AOT) using the photoexcited triplet state of anthraquinone 2,6-disulfonate (3AQDS*). Several different free radicals resulting from two independent oxidation pathways (electron transfer and hydrogen abstraction) are observed. These include the radical ions of AQDS and sulfite from electron-transfer processes, carbon-centered radicals from H-atom abstraction reactions, and an additional carbon-centered radical formed by electron transfer from the AOT sulfonate head group followed by the loss of SO3. The radicals exhibit intense chemically induced dynamic electron spin polarization (CIDEP) in their TREPR spectra. The intensity ratios of the observed TREPR signals for each radical depend on the water pool size and temperature, which in turn affect the predominant CIDEP mechanism. All signal carriers are accounted for by simulation, and CIDNP results provide strong supporting evidence for the assignments.

TR-EPR of single and double spin labelled C60 derivatives: observation of quartet and quintet excited states in solution

Time-resolved EPR spectra recorded after pulsed laser photoexcitation of two fullerene nitroxide radicals 1a and 1b and two fullerene nitroxide biradicals 2a and 2b in solution have been investigated at room temperature. The TR-EPR spectra of all the radicals are interpreted as arising from spin-polarized ground and excited states, the latter being obtained by the spin coupling of the nitroxide unpaired spins with the fullerene excited triplet state. The TR-EPR spectra of the monoradicals are consistent with strong (1a) or weak (1b) spin coupling, giving rise to pure quartet/doublet excited states (1a) or mixed states (1b). In the latter, the spectral simulation allowed the determination of the magnitude of the exchange coupling constant J TR between the triplet and the radical. The spin polarization of the excited and ground states is attributed to CIDEP effect induced by the radical triplet pair mechanism (RTPM). The sign of J TR is inferred from the sign of the spin polarization. In the case of biradicals 2a and 2b, the TR-EPR spectra are assigned, on the basis of hyperfine pattern and g-values, to quintet states obtained by spin coupling of the fullerene triplet photoexcited state with the two radicals. The spin polarization of the ground state is attributed to the triplet–triplet annihilation process and the sign of the polarization allows the assignment of the radical–radical exchange constant J RR.

Alternative source of emissive CIDEP in the TREPR spectra of benzophenone in alcohols

The flash photolysis EPR spectra of benzophenone (BP) in alcoholic solvents differ in terms of their chemically induced dynamic electron polarization (CIDEP) signatures when BP is photo-excited at different laser wavelengths. The difference is strongly dependent upon the concentration of BP and is attributed to the reaction of two BP triplet states to form a BP radical anion, BP radical cation pair which occurs on a timescale sufficiently rapid relative to relaxation in the BP triplet to transfer polarization into the resulting radicals. This polarized signal dominates the spectrum obtained by 266 nm photo-excitation due to the much larger extinction coefficient of BP at this wavelength.

A new series of radical–triplet pairs: modulation of radical–triplet interaction by a counter ion

Bis-(quinoline-TEMPO)-yttrium-tetraphenylporphyrin complexes having potassium and silver counter cations have been studied by cw-EPR in their ground states and by time-resolved (TR) EPR and two-dimensional (2D) nutation spectroscopy in the excited states after laser excitation. Simulations of the TR-EPR spectra at 30 K and at room temperature were made, yielding zero-field splitting parameters D and E, and g values. The parameters obtained together with the nutation frequencies clearly assigned the excited states of the bis-radical complexes having counter cations of potassium and silver as quartet and quintet states, respectively. This result is explained by modulation of radical–triplet interactions by the counter ion due to different sizes of metal ions.

Photo-oxidation of diglycine in confined media: Relaxation of longitudinal magnetization in spin correlated radical pairs

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.

Time-Resolved EPR Study of the Photophysics and Photochemistry of 1-(3-(Methoxycarbonyl)propyl)-1-phenyl[6.6]C61

Time-resolved (TR) EPR was used to study the photophysics and photochemistry of 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6.6]C61 (M1). The CW TREPR spectra of M1 in the photoexcited triplet state, frozen in a rigid matrix and in liquid solution at room temperature, were compared with those of 3C60. The introduction of the substituent on C60 has a striking effect on the spectra of the triplets, which is attributed to the lifting of the orbital degeneracy by the reduction in symmetry. Fourier transform (FT) EPR was used in an investigation of electron-transfer reactions in liquid solutions mediated by 3M1. Of particular interest was the system of M1/chloranil (CA)/perylene (Pe). Photoexcitation of M1 is found to lead to the formation of the chloranil anion radical and the perylene cation radical. From the chemically induced dynamic electron polarization (CIDEP) patterns in the FTEPR spectra and the dependence of the reaction kinetics on reactant concentrations, it was deduced that CA- is formed by two competing pathways following photoexcitation of M1: (1) direct electron transfer from 3M1 to CA followed by electron transfer from Pe to M1+ and (2) energy transfer from 3M1 to Pe followed by oxidative quenching of 3Pe by CA. In both pathways, M1 acts as a light-energy harvester and mediator of electron-transfer reactions from Pe to CA without itself being consumed in the process, that is, as a photocatalyst. It is found that the functionalization of C60 makes its triplet state a worse electron donor and acceptor, but it has no significant effect on the triplet energy transfer reaction.

Study of PhotoinducedN-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).

Theoretical and experimental studies of chemically induced electron-nuclear polarization in low magnetic fields

Chemically induced electron-nuclear polarization at low magnetic fields is considered theoretically for a radical with one magnetic nucleus. It is shown that large non-equilibrium populations of the radical spin levels are expected to exist under low magnetic fields. This large electron-nuclear polarization has been observed experimentally during the photolysis of two phosphine oxides using a modified L-band TREPR setup. The TREPR spectra and kinetics of dimethoxyphosphonyl and diphenylphosphonyl radicals have been measured and analysed in low magnetic fields, and excellent agreement between experiment and theory has been achieved.