Light-induced Electron Spin Resonance Research Articles

Terahertz emission from diamond nitrogen-vacancy centers.

Coherent light sources emitting in the terahertz range are highly sought after for fundamental research and applications. Terahertz lasers rely on achieving population inversion. We demonstrate the generation of terahertz radiation using nitrogen-vacancy centers in a diamond single crystal. Population inversion is achieved through the Zeeman splitting of the S = 1 state in 15 tesla, resulting in a splitting of 0.42 terahertz, where the middle Sz = 0 sublevel is selectively pumped by visible light. To detect the terahertz radiation, we use a phase-sensitive terahertz setup, optimized for electron spin resonance (ESR) measurements. We determine the spin-lattice relaxation time up to 15 tesla using the light-induced ESR measurement, which shows the dominance of phonon-mediated relaxation and the high efficacy of the population inversion. The terahertz radiation is tunable by the magnetic field, thus these findings may lead to the next generation of tunable coherent terahertz sources.

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

Photoinduced electron spin polarization (ESP) is reported in the electronic ground states of three Pt(II) complexes comprised of two S = 1/2 nitronyl nitroxide (NN) radicals attached through different length para-phenylethynyl bridges to the 3,6 positions of a catecholate (CAT, donor) and 4,4'-di-tert-butyl-2,2'-bipyridine (bpy, acceptor). Complexes 1-3 have from 17 to 41 bonds separating NN radicals and display cw-EPR spectra consistent with |JNN-NN| ≫ |aN|, |JNN-NN| ≥ |aN|, and |JNN-NN| < |aN|, respectively, where JNN-NN is the magnetic exchange coupling between NN radicals in the electronic ground state, and aN is the isotropic 14N hyperfine coupling constant. Light-induced transient EPR spectra characterized as enhanced ground-state absorption were observed for all three complexes using 532 nm pulsed laser excitation into the ligand-to-ligand charge transfer (LL'CT) band of the (CAT)Pt(bpy) chromophore. The magnitude of the observed ESP increases in the order 1 < 2 < 3 and is inversely correlated with the magnitude of ground-state JNN-NN. In addition to the experimental observation of net absorptive polarization in 1-3, light excitation also produces multiplet polarization in 2. Since the weak dipolar coupling leads to a strong spectral overlap of the absorptive and emissive components, the multiplet polarization is not observed in 1 and 3 and is very weak in 2. The ability to spin-polarize multiple radical spins with a single photon is anticipated to advance new photoinduced multi qubit/qudit ESP protocols for quantum information science applications.

Engineered protein-iron and/or gold-protein-iron nanocomposites in aqueous solutions upon UVA light: Photo-induced electron transfer possibilities and limitations

Occurrence/suppression of photo-induced electron transfer (PI-ET) process in solutions of nanocomposites represents a very hot topic in several photo-related research domains. In this work, this phenomenon is investigated in aqueous solutions using iron-containing mono-/bi-metallic nanocomposites that have been engineered through protein-templated syntheses, with the aim to potentially create new biocompatible imaging contrast agents for medical diagnostics. Two types of iron-containing nanocomposites, mono-metallic (protein-Fe) and bi-metallic (Au-protein-Fe), have been synthesized and characterized with the aid of several experimental techniques, such as high-resolution transmission electron microscopy (HR-TEM), steady-state fluorescence, electron paramagnetic resonance (EPR) and light-induced EPR (LEPR). Theoretical model of the N-terminal region of the bovine serum albumin protein interacting with Fe cations and simulations of the EPR spectral features upon UVA light irradiation complemented the material’s analysis. Two iron forms within the mono-/bi-metallic nanocomposites have been detected experimentally: (i)complexed iron cations (giving EPR signal at g = 4.29) and (ii)superparamagnetic iron oxide nanoparticles (SPIONs; expressing EPR resonances at g// = 2.21 and g⊥ = 2.09). Upon UVA-light irradiation (325 nm), PI-ET between the two iron forms have been observed in the protein-Fe nanocomposite; however, this electronic communication is suppressed in the Au-protein-Fe system. The presence of luminescent Au nanoclusters and dissimilar sizes of SPIONs in bi-metallic nanocomposites (around 5 nm vs. approx. 2 nm in the monometallic protein-Fe system) represent the two possible reasons underneath suppression of the PI-ET process in the former. Moreover, OH radicals were detected in aqueous solutions in both iron-containing nanocomposites (Fe and Au/Fe systems) when irradiated at 325 nm for 5 min at r.t. The reported evidence of PI-ET in iron-containing nanocomposite aqueous solutions can thus have a large impact on their potential medical and/or environmental applications.

Electron paramagnetic resonance of VN–VGa complex in BGaN

A metastable photoinduced electron paramagnetic resonance (EPR) signal at low temperatures is reported for GaN alloyed with boron (i.e., BxGa1−xN) epitaxial layers grown at temperatures ranging from 840 to 1090° C. An isotropic EPR line with g = 2.004 is observed with an intensity depending on the growth temperature for all the samples with boron content between 0.73% and 2.51%. The temperature dependence of EPR intensities is compared with the results of high-resolution photoinduced transient spectroscopy. This enables a link between particular traps and the EPR signal. The activation energies of these traps are consistent with the theoretical position of the VN–VGa complex. Thermal annihilation of the EPR signal with 30 meV activation energy corresponds to shallow donor ionization. A model explaining the light-induced EPR signal that involves redistribution of electrons between deep and shallow donors, mediated by photoionization to the conduction band, is proposed.

Carbon Dots Enabling Parts‐Per‐Billion Sensitive and Ultraselective Photoluminescence Lifetime‐Based Sensing of Inorganic Mercury

AbstractOne of the UN Sustainable Development Goals is to ensure universal access to clean drinking water. Among the various types of water contaminants, mercury (Hg) is considered to be one of the most dangerous ones. It is mostly its immense toxicity and vast environmental impact that stand out. To tackle the issue of monitoring water quality, a nanosensor based on carbon dots (CDs) is developed, whose surface is functionalized with carboxylic groups. CDs show Hg2+ concentration‐dependent photoluminescence (PL) lifetimes along with an ultrahigh sensitivity and selectivity. The selectivity of PL quenching by Hg2+ is rationalized by performing light‐induced electron paramagnetic resonance (LEPR) spectroscopy showing significant perturbation of the CD photoexcited state upon Hg2+ binding. The experimental findings are supported by time‐dependent density functional theory (TD‐DFT) calculations. These unveiled the emergence of a low‐lying charge transfer state involving a vacant 6s orbital of Hg2+ stabilized by relativistic effects.

Titanium dioxide nanoparticles heavily doped with niobium: A light-induced electron paramagnetic resonance study

Light-induced electron paramagnetic resonance (EPR) spectra of titanium dioxide nanoparticles heavily doped with niobium(v) are studied. At low temperatures, the EPR signal caused by interband illumination is associated with paramagnetic Ti3+ sites in anatase, which, as the temperature rises above 210 K, are discharged to the non-paramagnetic Ti4+ state due to the escape of trapped photoelectrons and their recombination with holes. The temperature dependence of the integral EPR signal has a non-Curie character, especially in the temperature range where the discharge of Ti3+ centers is already significant.

Magnetic Polaron States in Photoluminescent Carbon Dots Enable Hydrogen Peroxide Photoproduction.

Photoactivation of aspartic acid-based carbon dots (Asp-CDs) induces the generation of spin-separated species, including electron/hole (e- /h+ ) polarons and spin-coupled triplet states, as uniquely confirmed by the light-induced electron paramagnetic resonance spectroscopy. The relative population of the e- /h+ pairs and triplet species depends on the solvent polarity, featuring a substantial stabilization of the triplet state in a non-polar environment (benzene). The electronic properties of the photoexcited Asp-CDs emerge from their spatial organization being interpreted as multi-layer assemblies containing a hydrophobic carbonaceous core and a hydrophilic oxygen and nitrogen functionalized surface. The system properties are dissected theoretically by density functional theory in combination with molecular dynamics simulations on quasi-spherical assemblies of size-variant flakelike model systems, revealing the importance of size dependence and interlayer effects. The formation of the spin-separated states in Asp-CDs enables the photoproduction of hydrogen peroxide (H2 O2 ) from water and water/2-propanol mixture via a water oxidation reaction.

The effect of the zero-field splitting in light-induced pulsed dipolar electron paramagnetic resonance (EPR) spectroscopy.

Laser-induced magnetic dipole (LaserIMD) spectroscopy and light-induced double electron-electron resonance (LiDEER) spectroscopy are important techniques in the emerging field of light-induced pulsed dipolar electron paramagnetic resonance (EPR) spectroscopy (light-induced PDS). These techniques use the photoexcitation of a chromophore to the triplet state and measure its dipolar coupling to a neighboring electron spin, which allows the determination of distance restraints. To date, LaserIMD and LiDEER have been analyzed with software tools that were developed for a pair of two spins and that neglected the zero-field splitting (ZFS) of the excited triplet. Here, we explore the limits of this assumption and show that the ZFS can have a significant effect on the shape of the dipolar trace. For a detailed understanding of the effect of the ZFS, a theoretical description for LaserIMD and LiDEER is derived, taking into account the non-secular terms of the ZFS. Simulations based on this model show that the effect of the ZFS is not that pronounced in LiDEER for experimentally relevant conditions. However, the ZFS leads to an additional decay in the dipolar trace in LaserIMD. This decay is not that pronounced in Q-band but can be quite noticeable for lower magnetic field strengths in X-band. Experimentally recorded LiDEER and LaserIMD data confirm these findings. It is shown that ignoring the ZFS in the data analysis of LaserIMD traces can lead to errors in the obtained modulation depths and background decays. In X-band, it is additionally possible that the obtained distance distribution is plagued by long distance artifacts.

Erythrosin B as a New Photoswitchable Spin Label for Light-Induced Pulsed EPR Dipolar Spectroscopy.

We present a new photoswitchable spin label for light-induced pulsed electron paramagnetic resonance dipolar spectroscopy (LiPDS), the photoexcited triplet state of erythrosin B (EB), which is ideal for biological applications. With this label, we perform an in-depth study of the orientational effects in dipolar traces acquired using the refocused laser-induced magnetic dipole technique to obtain information on the distance and relative orientation between the EB and nitroxide labels in a rigid model peptide, in good agreement with density functional theory predictions. Additionally, we show that these orientational effects can be averaged to enable an orientation-independent analysis to determine the distance distribution. Furthermore, we demonstrate the feasibility of these experiments above liquid nitrogen temperatures, removing the need for expensive liquid helium or cryogen-free cryostats. The variety of choices in photoswitchable spin labels and the affordability of the experiments are critical for LiPDS to become a widespread methodology in structural biology.

Photoinduced spin-orbital coupling effect at donor: acceptor interface in non-fullerene organic solar cells

Non-fullerene based organic solar cells show excellent optoelectronic properties. In this study, firstly, we measured the magneto-photocurrent (MPC) effect to monitor the spin orbit coupling (SOC) action of electron-hole pairs at the donor: acceptor (D:A) interfaces, introduced by the dipole-dipole interaction in non-fullerene ITIC molecules. The MPC of non-fullerene based bulk heterojunctions show a broader line-shape, which implies a stronger SOC in electron-hole pairs at D:A interface as compared to the fullerene based bulk heterojunctions under photo-excitation. Secondly, the light-induced electron paramagnetic resonance (LEPR) results further give a direct evidence for stronger SOC in electron-hole pairs at D:A interface in non-fullerene bulk-heterojunction. Theoretically, increasing SOC provides a pathway for singlet electron-hole pairs to convert into triplet electron-hole pairs, leading to more spin states ready for dissociation. Therefore, the increased SOC will increase the magneto-photocurrent, and improve the performance of non-fullerene solar cells, we consider that photoexcitation can present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. Non-fullerene based organic solar cells show excellent optoelectronic properties and device performance. We think these excellent properties come from a stronger spin orbit coupling effect in electron-hole pairs at D: A interface under photoexcitation as compared to the fullerene bulk-heterojunctions, leading to increase the magneto-photocurrent. In addition, the capacitance-frequency is also reveal that the stronger SOC for non-fullerene bulk-heterojunctions originates from the larger electron cloud deformation electron-hole pairs caused by photoinduced dipole-dipole interactions. Therefore, we think the photoexcitation present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. • Non-fullerene based solar cell of ITO/ZnO/Active layer/MoO 3 /Ag shows a higher PCE of 11.30%. • A strong SOC exists in electron-hole pairs at D:A interface in the non-fullerene under lighting. • The strong SOC comes from the larger electron cloud deformation. • Photo-excitation can present a new approach to strengthen the SOC effect in organic solar cells.

Evidence of Au(II) and Au(0) States in Bovine Serum Albumin-Au Nanoclusters Revealed by CW-EPR/LEPR and Peculiarities in HR-TEM/STEM Imaging.

Bovine serum albumin-embedded Au nanoclusters (BSA-AuNCs) are thoroughly probed by continuous wave electron paramagnetic resonance (CW-EPR), light-induced EPR (LEPR), and sequences of microscopic investigations performed via high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray analysis (EDS). To the best of our knowledge, this is the first report analyzing the BSA-AuNCs by CW-EPR/LEPR technique. Besides the presence of Au(0) and Au(I) oxidation states in BSA-AuNCs, the authors observe a significant amount of Au(II), which may result from a disproportionation event occurring within NCs: 2Au(I) → Au(II) + Au(0). Based on the LEPR experiments, and by comparing the behavior of BSA versus BSA-AuNCs under UV light irradiation (at 325 nm) during light off-on-off cycles, any energy and/or charge transfer event occurring between BSA and AuNCs during photoexcitation can be excluded. According to CW-EPR results, the Au nano assemblies within BSA-AuNCs are estimated to contain 6–8 Au units per fluorescent cluster. Direct observation of BSA-AuNCs by STEM and HR-TEM techniques confirms the presence of such diameters of gold nanoclusters in BSA-AuNCs. Moreover, in situ formation and migration of Au nanostructures are observed and evidenced after application of either a focused electron beam from HR-TEM, or an X-ray from EDS experiments.

Fast and selective reduction of nitroarenes under visible light with an earth-abundant plasmonic photocatalyst

Reduction of nitroaromatics to the corresponding amines is a key process in the fine and bulk chemicals industry to produce polymers, pharmaceuticals, agrochemicals and dyes. However, their effective and selective reduction requires high temperatures and pressurized hydrogen and involves noble metal-based catalysts. Here we report on an earth-abundant, plasmonic nano-photocatalyst, with an excellent reaction rate towards the selective hydrogenation of nitroaromatics. With solar light as the only energy input, the chalcopyrite catalyst operates through the combined action of hot holes and photothermal effects. Ultrafast laser transient absorption and light-induced electron paramagnetic resonance spectroscopies have unveiled the energy matching of the hot holes in the valence band of the catalyst with the frontier orbitals of the hydrogen and electron donor, via a transient coordination intermediate. Consequently, the reusable and sustainable copper-iron-sulfide (CuFeS2) catalyst delivers previously unattainable turnover frequencies, even in large-scale reactions, while the cost-normalized production rate stands an order of magnitude above the state of the art.

Light-induced pulsed dipolar EPR spectroscopy for distance and orientation analysis.

Measuring distances in biology at the molecular level is of great importance for understanding the structure and function of proteins, nucleic acids and other biological molecules and their complexes. Pulsed Dipolar Spectroscopy (PDS) offers advantages with respect to other methods as it is uniquely sensitive and specific to electronic spin centers and allows measurements in near-native conditions, comprising the in-cell environment. PDS methods measure the electron spin-spin dipolar interaction, therefore they require the presence of at least two paramagnetic centers, which are often stable radicals. Recent developments have introduced transient triplet states, photo-activated by a laser pulse, as spin labels and probes, thereby establishing a new family of techniques-Light-induced PDS (LiPDS). In this chapter, an overview of these methods is provided, looking at the chromophores that can be used for LiPDS and some of the technical aspects of the experiments. A guide to the choice of technique that can yield the best results, depending on the type of system studied and the information required, is provided. Examples of previous LiPDS studies of model systems and proteins are given. Characterization data for the chromophores used in these studies is tabulated to help selection of appropriate triplet state probes in future studies.

Light-Induced Migration of Spin Defects in TiO2 Nanosystems and their Contribution to the H2 Evolution Catalysis from Water.

The photocatalytic activity for H2 production from water, without presence of hole scavengers, of thermally reduced TiO2 nanoparticles (H-500, H-700) and neat anatase were followed by in-situ continuous-wave light-induced electron paramagnetic resonance technique (CW-LEPR), in order to correlate the H2 evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e- +h+ ) recombination and were the crucial electronic factor affecting catalysis. In the best-performant system (H-500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti3+ sites, including interstitial Ti3+ , and singly ionized electrons trapped in oxygen vacancies (VO . ). The interplay of these species fed successfully surface exposed Ti4+ sites, which became a catalytically active, fast reacting Ti4+ ⇄Ti3+ state that was key for the H2 evolution process.

Magnetic-field-controlled charge transport in organic polymer composites

Spin properties of charge carriers photoexcited in the P3HT:PC61BM and PCDTBT:PC61BM composites were studied at X-band EPR spectroscopy. Light-induced EPR spectra were deconvoluted and the contributions of all spin charge carriers were determined. The main magnetic resonance parameters of charge carriers obtained were compared with those determined at millimeter EPR wavebands and analyzed as a function of electron precession frequency. The linewidth of spin charge carriers was established to originated mainly due to the weak anisotropy of their g-factors. The external magnetic field significantly affects dynamics, electron relaxation and recombination of spin charge carriers.

Light-Induced Electron Paramagnetic Resonance Study of Charge Transport in Fullerene and Nonfullerene PBDB-T-Based Solar Cells

Light-Induced Electron Paramagnetic Resonance Study of Charge Transport in Fullerene and Nonfullerene PBDB-T-Based Solar Cells

Mode of action of the third component in ternary organic photovoltaic blend PBDB-T/ITIC:PC70BM revealed by EPR spectroscopy

Light-induced EPR (LEPR) and out-of-phase electron spin echo (ESE) techniques were applied to trace the difference of charge generation and recombination processes between highly efficient ternary OPV blend PBDB-T/ITIC:PC70BM and the corresponding binary blends PBDB-T/ITIC and PBDB-T/PC70BM. The average charge trap depth measured from LEPR decay traces decreases in the series PBDB-T/ITIC, PBDB-T/PC70BM, PBDB-T/ITIC:PC70BM. As it is evident from the out-of-phase ESE, the initial electron-hole distance within the charge-transfer state upon its formation increases in this series. The former effect facilitates charge transport, in accordance with increase of the electron mobility for the ternary blend revealed previously. The latter effect leads to reduced geminate recombination. Both effects are suggested to improve the power conversion efficiency of the ternary blends. Presumably, they originate from modification of the PBDB-T/ITIC blend morphology by addition of small amount of PC70BM.

Carbon Dots Detect Water-to-Ice Phase Transition and Act as Alcohol Sensors via Fluorescence Turn-Off/On Mechanism.

Highly fluorescent carbon nanoparticles called carbon dots (CDs) have been the focus of intense research due to their simple chemical synthesis, nontoxic nature, and broad application potential including optoelectronics, photocatalysis, biomedicine, and energy-related technologies. Although a detailed elucidation of the mechanism of their photoluminescence (PL) remains an unmet challenge, the CDs exhibit robust, reproducible, and environment-sensitive PL signals, enabling us to monitor selected chemical phenomena including phase transitions or detection of ultralow concentrations of molecular species in solution. Herein, we report the PL turn-off/on behavior of aqueous CDs allowing the reversible monitoring of the water-ice phase transition. The bright PL attributable to molecular fluorophores present on the CD surface was quenched by changing the liquid aqueous environment to solid phase (ice). Based on light-induced electron paramagnetic resonance (LEPR) measurements and density functional theory (DFT) calculations, the proposed kinetic model assuming the presence of charge-separated trap states rationalized the observed sensitivity of PL lifetimes to the environment. Importantly, the PL quenching induced by freezing could be suppressed by adding a small amount of alcohols. This was attributed to a high tendency of alcohol to increase its concentration at the CD/solvent interface, as revealed by all-atom molecular dynamics simulations. Based on this behavior, a fluorescence "turn-on" alcohol sensor for exhaled breath condensate (EBC) analysis has been developed. This provided an easy method to detect alcohols among other common interferents in EBC with a low detection limit (100 ppm), which has a potential to become an inexpensive and noninvasive clinically useful diagnostic tool for early stage lung cancer screening.

Origin of poor photovoltaic performance of bis(tetracyanoantrathiophene) non-fullerene acceptor

Abstract Light-induced EPR and out-of-phase electron spin echo techniques were applied to identify the origin of low photovoltaic performance of novel non-fullerene antrathiophene-based acceptor 2,2′-[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitrile (AT1), compared to conventional fullerene acceptors. These techniques revealed that average trapping energy of electrons in PCDTBT/AT1 blend is higher than that in the blends of PCDTBT with fullerene acceptors, while the distance of the initial photo-induced electron transfer is decreased. These two effects are suggested to enhance the efficiency of geminate recombination in PCDTBT/AT1 blend and reduce the yield of free charges, which is supposed to be the main reason for poor OPV performance of AT1-containing blends. Low electron mobility μe = 3.5 *10-6 cm2V-1s−1 is estimated for AT1 from photo-CELIV experiment. Strong localization of the electrons on terminal acceptor units of AT1 consistently explains both efficient geminate recombination in blends of antrathiophene-based acceptor with polymer donors and low electron mobility in AT1.

Light-Induced Triplet-Triplet Electron Resonance Spectroscopy.

We present a new technique, light-induced triplet–triplet electron resonance spectroscopy (LITTER), which measures the dipolar interaction between two photoexcited triplet states, enabling both the distance and angular distributions between the two triplet moieties to be determined on a nanometer scale. This is demonstrated for a model bis-porphyrin peptide that renders dipolar traces with strong orientation selection effects. Using simulations and density functional theory calculations, we extract distance distributions and relative orientations of the porphyrin moieties, allowing the dominant conformation of the peptide in a frozen solution to be identified. LITTER removes the requirement of current light-induced electron spin resonance pulse dipolar spectroscopy techniques to have a permanent paramagnetic moiety, becoming more suitable for in-cell applications and facilitating access to distance determination in unmodified macromolecular systems containing photoexcitable moieties. LITTER also has the potential to enable direct comparison with Förster resonance energy transfer and combination with microscopy inside cells.