Spinning-Driven Dynamic Nuclear Polarization with Optical Pumping.

We propose a new, more efficient, and potentially cost effective, solid-state nuclear spin hyperpolarization method combining the cross-effect mechanism and electron spin optical hyperpolarization in rotating solids. We first demonstrate optical hyperpolarization in the solid state at low temperatures and low field and then investigate its field dependence to obtain the optimal condition for high-field electron spin hyperpolarization. The results are then incorporated into advanced magic-angle spinning dynamic nuclear polarization (MAS-DNP) numerical simulations that show that optically pumped MAS-DNP could yield breakthrough enhancements at very high magnetic fields. Based on these investigations, enhancements greater than the ratio of electron to nucleus magnetic moments (>658 for 1H) are possible without microwave irradiation. This could solve at once the MAS-DNP performance decrease with increasing field and the high cost of MAS-DNP instruments at very high fields.

Harnessing Electron Spin Hyperpolarization in Chromophore-Radical Spin Probes for Subcellular Resolution in Electron Paramagnetic Resonance Imaging: Concept and Feasibility.

Obtaining a subcellular resolution for biological samples doped with stable radicals at room temperature (RT) is a long-sought goal in electron paramagnetic resonance imaging (EPRI). The spatial resolution in current EPRI methods is constrained either because of low electron spin polarization at RT or the experimental limitations associated with the field gradients and the radical linewidth. Inspired by the recent demonstration of a large electron spin hyperpolarization in chromophore-nitroxyl spin probe molecules, the present work proposes a novel optically hyperpolarized EPR imaging (OH-EPRI) method, which combines the optical method of two-photon confocal microscopy for hyperpolarization generation and the rapid scan (RS) EPR method for signal detection. An important aspect of OH-EPRI is that it is not limited by the abovementioned restrictions of conventional EPRI since the large hyperpolarization in the spin probes overcomes the poor thermal spin polarization at RT, and the use of two-photon optical excitation of the chromophore naturally generates the required spatial resolution, without the need for any magnetic field gradient. Simulations based on time-dependent Bloch equations, which took into account both the RS field modulation and the hyperpolarization generation by optical means, were performed to examine the feasibility of OH-EPRI. The simulation results revealed that a spatial resolution of up to 2 fL can be achieved in OH-EPRI at RT under in vitro conditions. Notably, the majority of the requirements for an OH-EPRI experiment can be fulfilled by the currently available technologies, thereby paving the way for its easy implementation. Thus, the proposed method could potentially bridge the sensitivity gap between the optical and magnetic imaging techniques.

Achieving Maximal Enhancement of Electron Spin Polarization in Stable Nitroxyl Radicals at Room Temperature.

Enhancing the polarization of spin levels at room temperature is one of the active research areas in magnetic resonance. Generation of electron spin hyperpolarization involves a complex interplay of electronic and spin processes. In this work, the optimization of crucial electron spin polarization (ESP) generating parameters and synthesis of a radical-chromophore adduct are described. The ESP of the synthesized adduct is about 550 times the equilibrium polarization at room temperature, which is possibly the maximal value for a chromophore-nitroxyl system. The present work highlights the crucial role of the photophysical quenching process toward the generation of a large ESP. Additionally, a chromophore-diradical adduct is synthesized, and the effects of the additional radical in the ESP generation process are discussed. Enhanced photochemical stability is demonstrated for the diradical adducts, thereby suggesting a potential route toward the generation of photostable radical-chromophore adducts for future studies. The large ESP in these molecules should enable a wide range of applications, such as in DNP, spintronics, and magnetometers.

Design and Photo-Induced Dynamics of Radical-Chromophore Adducts with One- or Two-Atom Separation: Toward Potential Probes for High Field Optical DNP Experiments.

Breaking the maximum enhancement barrier of 660 at room temperature in a conventional dynamic nuclear polarization (DNP) experiment has the immense potential of practical applications. Optical DNP experiments with radical-chromophore (RC) adducts, which harnesses hyperpolarized radicals, instead of thermalized radicals, offers a powerful way to achieve this. Typical DNP and NMR experiments, however, are carried out at high magnetic fields of about 5-10 T, whereas the large electron spin hyperpolarization (ESP) demonstrated in the RC adducts so far are at a much low field of 0.3 T. Thus, in order to realize a successful optical DNP experiment, it is imperative to ask whether the RC adducts, which are currently available, can achieve a large ESP even at high fields. The present work poses this question and shows that the current RC adducts would not generate a large ESP at high fields unless the separation between the chromophore and nitroxyl moiety is reduced to less than four bonds. Two serious bottlenecks in this direction are the near impossibility of synthesizing such RC adducts using the common nitroxyl radicals and the absence of any photophysical studies on RC adducts with such short spacer groups. In this regard, the present work exploits the spin trapping methodology to synthesize one- and two-atom separated naphthalene-nitroxyl RC adducts. Good yields and excellent stability of the adducts have been demonstrated. Furthermore, the present work presents their detailed photophysical and photochemical studies by transient optical and time-resolved EPR studies. On the basis of the present results, a potential RC adduct is proposed for the high field optical DNP experiments. Finally, the prospect of exploiting the large EPR signal enhancement due to ESP in the field of spin trapping studies has been discussed.

A phenomenological scheme for reversed quartet mechanism of electron spin polarization in covalently linked systems of chromophore and free radical: Determination of magnitude of polarization and application to pyrene-TEMPO linked molecules.

Generation of electron spin polarization (ESP) during the bimolecular quenching of an excited chromophore by a free radical is generally explained by the radical-triplet pair mechanism, which is capable of giving the magnitudes of ESP arising from the quenching of the singlet or the triplet excited chromophore. When the chromophore and the free radical are covalently linked, although there are several mechanisms to explain the observed spin-polarized electron paramagnetic resonance signals under a variety of experimental conditions and in different chromophore-radical systems, there are no schemes that allow quantitative determination of the magnitude of ESP. In this work, we present a phenomenological scheme with this objective. In this scheme, we have incorporated several concepts of the reversed quartet mechanism of Rozenshtein et al. [J. Phys. Chem. A 109, 11144 (2005)] to our phenomenological sequential quenching scheme [V. Rane and R. Das, J. Phys. Chem. A 119, 5515 (2015)] of ESP in covalently linked chromophore-radical systems. This phenomenological reversed quartet scheme is able to explain the observed inversion of ESP with time and can also give a quantitative measure of the absorptive and emissive ESP in such systems. We have applied this scheme to the photophysical quenching of a series of newly synthesized pyrene-TEMPO molecules, where a spacer group of different lengths covalently links the pyrene chromophore and the TEMPO free radical. Given the simplicity of our scheme, reasonable estimates of the magnitudes of the ESP have been obtained in all cases.

Toward Achieving the Theoretical Limit of Electron Spin Polarization in Covalently Linked Radical-Chromophore Dyads.

Electron spin systems with non-Boltzmann spin distributions are commonly observed in photophysical and photochemical processes involving free radicals. Understanding the origin of electron spin polarization (ESP) reveals detailed insight into the spin-dependent interactions occurring in these processes. The ability to transfer ESP to the nuclear spins of the solvent to produce large nuclear polarization is itself an active area of research in the field called dynamic nuclear polarization (DNP). Harnessing ESP in such fields demands a large magnitude of ESP and importantly on free radicals that are stable. In this work, we explored various factors that could play a prominent role in generation of ESP on the stable nitroxyl radical. By exploiting the dependence of ESP on the zero-field splitting parameter DT of the chromophore triplets and a careful choice of the chromophores for efficient quenching of their excited states by a free radical, we present here our strategies for designing covalently linked chromophore-free radical dyads, which could generate high magnitudes of ESP. Syntheses and EPR studies on a series of such dyads to demonstrate successful realization of our strategy are reported here. Our work reveals that, along with the DT value and the triplet quenching efficiency, the relative orientational dynamics of the radical and chromophore moiety and an efficient intersystem crossing of the chromophore primarily govern the ESP of the chromophore-radical dyads. One such molecule, an anthraquinone moiety linked to a TEMPO free radical that optimally satisfies the above criteria, produces a very large value of ESP-about 300 times its Boltzmann value-reaching almost half of the theoretical maximum of the radical-triplet pair mechanism of ESP. Coupled with its excellent photostability in benzene and the use of ambient conditions, this molecule should prove to be highly desirable in photoinduced DNP studies and other applications such as highly sensitive magnetometers, which require generation of large nuclear spin polarization in the solvent by transferring the electron spin polarization.

Efficient Radical-Enhanced Intersystem Crossing in an NDI-TEMPO Dyad: Photophysics, Electron Spin Polarization, and Application in Photodynamic Therapy.

A compact naphthalenediimide (NDI)-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) dyad has been prepared with the aim of studying radical-enhanced intersystem crossing (EISC) and the formation of high spin states as well as electron spin polarization (ESP) dynamics. Compared with the previously reported radical-chromophore dyads, the present system shows a very high triplet state quantum yield (ΦT =74 %), a long-lived triplet state (τT =8.7 µs), fast EISC (1/kEISC =338 ps), and absorption in the red spectral region. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy showed that, upon photoexcitation in fluid solution at room temperature, the D0 state of the TEMPO moiety produces strong emissive (E) polarization owing to the quenching of the excited singlet state of NDI by the radical moiety (electron exchange J>0). The emissive polarization then inverts into absorptive (A) polarization within about 3 µs, and then relaxes to a thermal equilibrium while quenching the triplet state of NDI. The formation and decay of the quartet state were also observed. The dyad was used as a three-spin triplet photosensitizer for triplet-triplet annihilation upconversion (quantum yield ΦUC =2.6 %). Remarkably, when encapsulated into liposomes, the red-light-absorbing dyad-liposomes show good biocompatibility and excellent photodynamic therapy efficiency (phototoxicity EC50 =3.22 µm), and therefore is a promising candidate for future less toxic and multifunctional photodynamic therapeutic reagents.

EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a Self-Confinement Effect?

Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidence. Recently, electron paramagnetic resonance (EPR) has been used to study deeply supercooled water in a rapidly frozen polycrystalline ice. The same technique can also be used to probe the presence of LW in polycrystalline ice that has formed through a more conventional, slow cooling one. In this context, the present study aims to emphasize that in case of an external probe involving techniques such as EPR, the results are influenced by the binary phase (BP) diagram of the probe-water system, which also predicts the existence of LW domains in ice, up to the eutectic point. Here we report the results of our such EPR spin-probe studies on water, which demonstrate that smaller the concentration of the probe stronger is the EPR evidence of liquid domains in polycrystalline ice. We used computer simulations based on stochastic Liouville theory to analyze the lineshapes of the EPR spectra. We show that the presence of the spin probe modifies the BP diagram of water, at very low concentrations of the spin probe. The spin probe thus acts, not like a passive reporter of the behavior of the solvent and its environment, but as an active impurity to influence the solvent. We show that there exists a lower critical concentration, below which BP diagram needs to be modified, by incorporating the effect of confinement of the spin probe. With this approach, we demonstrate that the observed EPR evidence of LW domains in ice can be accounted for by the modified BP diagram of the probe-water system. The present work highlights the importance of taking cognizance of the possibility of spin probes affecting the host systems, when interpreting the EPR (or any other probe based spectroscopic) results of phase transitions of host, as its ignorance may lead to serious misinterpretations.

Subtle Structural Changes in (CuIIL)2MnII Complexes To Induce Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic Substrates (H2L = Salen Type Unsymmetrical Schiff Base).

A new Cu(II) complex of an asymmetrically dicondensed Schiff base (H2L = N-(2-hydroxyacetophenylidene)-N'-salicylidene-1,3-propanediamine) derived from 1,3-propanediamine, salicylaldehyde, and o-hydroxyacetophenone has been synthesized. Using this complex, [CuL] (1), as a metalloligand, two new trinuclear Cu-Mn complexes, [(CuL)2Mn(N3)(H2O)](ClO4)·H2O (2) and [(CuL)2Mn(NCS)2] (3), have been prepared. Single-crystal structural analyses reveal that complexes 2 and 3 both have the same bent trinuclear {(CuL)2Mn}2+ structural unit in which two terminal bidentate square-planar (CuL) units are chelated to the central octahedral Mn(II) ion. This structural similarity is also evident from the variable-temperature magnetic susceptibility measurements, which suggest that compounds 2 and 3 are both antiferromagnetically coupled with comparable exchange coupling constants (-21.8 and -22.3 cm-1, respectively). The only difference between 2 and 3 lies in the coordination around the central Mn(II) ion; in 3, two SCN- groups are coordinated to the Mn(II), leaving a neutral complex, but in 2, one N3- group and one H2O molecule are coordinated to give a positively charged species. The presence of such a labile H2O coligand makes 2 catalytically active in mimicking two well-known polynuclear copper proteins, catecholase and phenoxazinone synthase. The turnover numbers (kcat) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 1118 and 6581 h-1, respectively, values which reflect the facility of the heterometallic catalyst in terms of both efficiency and catalytic promiscuity for aerial dioxygen activation. The mechanisms of these biomimetic oxidase reactions are proposed for the first time involving any heterometallic catalyst on the basis of mass spectral analysis, EPR spectroscopy, and cyclic voltammetry. The evidence of the intermediates indicates possible heterometallic cooperative activity where the substrates bind to a Mn(II) center and Cu(II) plays the role of an electron carrier for transformation of the phenolic substrates to their respective products with the reduction of aerial dioxygen.

The first examples of multiply bonded dirhenium(iii,ii) paramagnetic complexes containing nitrobenzoate ligands: spectroscopic, structural, cytotoxicity and computational studies.

4-Nitrobenzoic acid, 3-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid react with the multiply bonded paramagnetic dirhenium(iii,ii) complex Re2(µ-O2CCH3)Cl4(µ-Ph2PCH2PPh2)2 (1) in refluxing ethanol to afford the paramagnetic substitution products of the type Re2(µ-L)Cl4(µ-Ph2PCH2PPh2)2, where L represents the nitrobenzoate ligands [L = 4-nitrobenzoate, 2; 3-nitrobenzoate, 3; 4'-nitro[1,1'-biphenyl]-4-carboxylate, 4]. These are the first examples of paramagnetic dirhenium complexes containing nitrobenzoate ligands. The spectral (UV-vis, IR, and EPR) and electrochemical properties of the complexes are described. The identity of 4 has been established by single-crystal X-ray structure determination (Re-Re distance of 2.2967(4) Å). The electronic structures of the complexes were scrutinized by density functional theory (DFT) calculations. X-band EPR spectral measurements along with the DFT analysis show that the unpaired electron resides in the metal-metal δ* antibonding orbital. The complexes were also screened in vitro for their antiproliferative properties against the human breast cancer cell line MCF-7 by the MTT assay. Flow cytometry analysis showed that the complexes arrested the sub-G0/G1 phase.

Photophysical Studies on Covalently-linked Naphthalene and TEMPO Free Radical Systems: Observation of a Charge Transfer State in the Ground State.

A series of molecules containing a naphthalene chromophore and a stable free radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) covalently linked by a spacer group of different lengths have been synthesized. In n-hexane solution, their photophysical behavior was studied and compared with a system of freely moving naphthalene and the free radical TEMPO. The linked molecules showed strong quenching of the singlet and triplet states of the naphthalene moiety, compared to when naphthalene and TEMPO were not linked. The quenching efficiency decreased with increasing the length of the spacer group. In addition, new electronic absorption and emission bands, along with the usual bands of the individual moieties, were also seen. These news bands have been attributed to the formation of electron donor-acceptor charge-transfer complexes in the ground state, arising from the interaction between the two moieties in close proximity. The photophysical dynamics of the linked molecules has been rationalized by assuming the existence of two types of population of the linked molecules: folded and extended. The ground state complex formation is proposed to occur only in the folded conformation of the linked molecules. To our knowledge, this is possibly the first example of a ground state charge-transfer complex formation involving a TEMPO free radical and naphthalene.

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.

Observation of splitting of EPR spectral lines without any concomitant splitting in energy levels.

Splitting of spectral lines is generally associated with corresponding splitting of appropriate energy levels. However, here we report our observation of the splittings of hyperfine lines in the time-resolved EPR spectrum of a stable free radical that participates in the quenching of an excited molecule. The observed splitting does not arise from any splitting of the energy levels of the radical, nor due to Torrey oscillations but is a culmination of a detailed interplay of photophysical and magnetic resonance dynamics of the quenching process. In particular, sequential quenching of an excited singlet and triplet states by the free radical, generation of opposite electron spin polarization, and time-dependent EPR line width evolving according to the Bloch equations contribute to the appearance of unusual lineshapes of the observed EPR spectrum. This effect is sufficiently general and should always be observable in the time-resolved EPR experiments on free radicals involved in photophysical quenching of excited molecules. We also point out that this effect cannot be seen in Fourier transform EPR spectroscopy.

Dominance of the triplet mechanism in the electron spin polarisation of slowly reacting triplets.

For the triplet mechanism (TM) of electron spin polarisation in photochemically generated free radicals to dominate, the chemical reaction has to be very fast and capable of competing with the electron spin-lattice relaxation process of the triplet. This is a well-established condition. Here we report a counter example of electron spin polarisation, where this condition appears to be violated, and a dominant spin polarisation characteristic of the TM is observed, even when the triplet reacts very slowly. The system studied is the photoreduction of 4,4'-dihydroxy-benzophenone (DHBP) in 2-propanol producing the DHBP ketyl radical and the (CH3)2C˙OH radical. At room temperature, electron spin polarisation of the time-resolved EPR (TREPR) spectra of this system is dominated by the radical pair mechanism involving ST0 mixing, as the rate of the hydrogen abstraction reaction cannot compete with the spin-lattice relaxation of triplet DHBP. However, with the lowering of temperature, TREPR spectra dominated by a net emissive polarisation are seen, even though the rate of the reaction remains slow. Existence of this single-phase, hyperfine-line independent emissive polarisation due to the TM is established by the simulation of their TREPR spectra at -48 °C. To explain the increased contribution of the TM with the lowering of temperature, we proposed that the presence of two hydroxyl groups of DHBP could form hydrogen bonds with the solvent and steadily increase the micro-viscosity experienced by DHBP appreciably as the temperature was lowered. This would progressively increase the rotational correlation time of triplet DHBP, enabling the TM route to contribute increasingly to the radical generation. TREPR experiments with 4,4'-dimethoxy-benzophenone in 2-propanol, where similar hydrogen bonding is not possible, showed the spectra to be dominated by the ST0-RPM with little contribution of a net emissive signal. This observation qualitatively confirmed our proposal of enhanced micro-viscosity experienced by DHBP in 2-propanol at low temperatures, resulting in the enhanced emissive spin polarisation due to the TM, even though the chemical reaction remains slow.