Mapping nanomagnetic fields using a radical pair reaction

Single-Molecule Magnetoluminescence from a Spatially Confined Persistent Diradical Emitter

Luminescent radicals are an emerging class of materials that exhibit unique photofunctions not found in closed-shell molecules due to their open-shell electronic structure. Particularly promising are photofunctions in which radical's spin and luminescence are correlated; for example, when a magnetic field can affect luminescence (i.e., magnetoluminescence, ML). These photofunctions could be useful in the new science of spin photonics. However, previous observations of ML in radicals have been limited to systems in which radicals are randomly doped in host crystals or polymerized through metal complexation. This study shows that a covalently linked luminescent radical dimer (diradical) can exhibit ML as a single-molecular property. This facilitates detailed elucidation of the requirements for and mechanisms of ML in radicals and can aid the rational design of ML-active radicals based on synthetic chemistry.

Peptoid-Conjugated Magnetic Field-Sensitive Exciplex System at High and Low Solvent Polarities

The magnetic field effect (MFE) in exciplex emission (ExE) has been studied for decades, but it has been observed to occur only in solvents with a limited range of polarity. This limitation is mainly due to the reversible interconversion collapse between two quenching products of the photoinduced electron transfer, the exciplex and magnetic field-sensitive radical ion pair (RIP) beyond that polarity range. In a nonpolar solvent, the formation of RIPs is suppressed, whereas in a polar solvent, the probability of their re-encounter forming the exciplexes decreases. In this study, we developed new exciplex-forming (phenyl-phenanthrene)-(phenyl-N,N-dimethylaniline)-peptoid conjugates (PhD-PCs) to overcome this limitation. The well-defined peptoid structure allows precise control of the distance and the relative orientation between two conjugated moieties. Steady-state and time-resolved spectroscopic data indicate that the PhD-PCs can maintain the reversibility, which allows MFEs in ExE regardless of the solvent polarity. Subtle differences between the ExEs of the PhD-PCs were observed and explained by their exciplex geometries obtained through time-dependent density functional theory (TD-DFT) calculations.

Structural Dynamics of Lipid Bilayer Membranes Explored by Magnetic Field Effect Based Fluorescence Microscopy

Lipid bilayer membranes are known to exist as heterogeneous and dynamic structures where the molecules are always moving and fluctuating under physiological conditions. Magnetic field effects (MFEs) studied herein are phenomena in which the exciplex emission from an electron donor-acceptor dyad increases or decreases by applying an external magnetic field. The characteristic dependence of MFEs on the viscosity and polarity of the surrounding medium has been applied to investigate the local environments around the probe molecule. In this study, a novel MFE-based fluorescence microscopy technique was developed to explore the structural dynamics of lipid bilayer membranes. The vesicle formation during the membrane deformation was selectively visualized through the MFEs, thus allowing the extraction of information on the cellular dynamics at high temporal and spatial resolutions. This highly versatile and powerful technique is applicable to a wide range of areas, such as biology and material science.

Quantitative imaging of magnetic field distribution using a pyrene-based magnetosensing exciplex fluorophore

Quantitative imaging of magnetic field distribution was carried out using a pyrene-based magnetosensing exciplex fluorophore, pyrene-(CH₂)₁₂-O-(CH₂)₂-N,N-dimethylaniline (Py-12-O-2-DMA), on a conventional fluorescence microscope with an off-the-shelf LED lamp. No continuous sample supply was required for the process. The solvent system (anisole : DMF, 50 : 50 (v/v)) was carefully selected for monitoring the extent of modulation caused by the external magnetic field. The emission from Py-12-O-2-DMA increased by ca. 1.5 times under an external magnetic field of 50 mT. The pyrene-based reporter was ca. 24.7 times brighter than a previously reported phenanthrene-based complex when excited by using the widely available 355 nm excitation. Moreover, the maximum wavelength up to which Py-12-O-2-DMA could be excited (up to 380 nm) was longer than the wavelength up to which Phen-12-O-2-DMA could be excited. The combined advantages allowed the capture of magnetic field images with a high S/N ratio under milder conditions such as low illumination power, reduced sample concentration, and simpler optical setup. The system was also found to be feasible for 3D magnetic field distribution imaging by two-photon fluorescence microscopy.

Can a hybrid chemical-ferromagnetic model of the avian compass explain its outstanding sensitivity to magnetic noise?

While many properties of the magnetic compass of migratory birds are satisfactory explained within the chemical model of magnetoreception, its extreme sensitivity to radio-frequency magnetic fields remains a mystery. Apparently, this difficulty could be overcome if the magnetoreceptor model were augmented with a magnetite nanoparticle, which would amplify the magnetic field at the position of the magneto-sensitive cryptochrome molecule. However, comparison of the radio-frequency power used in the experiment with intrinsic magnetization noise of such a particle, estimated from the theory of fluctuations, shows that the required sensitivity cannot be reached with realistic parameters of iron-oxide nanocrystals.

Comment on “Magnetic-field-enabled resolution enhancement in super-resolution imaging” by M. Zhang et al., Phys. Chem. Chem. Phys., 2015, 17, 6722–6727

Spectral fluorimetry demonstrates that common organic fluorophores such as Alexa 647 exhibit no magnetic field enhanced fluorescence in the absence/presence of a strong magnet.

Magneto-optical micromechanical systems for magnetic field mapping

A new method for magnetic field mapping based on the optical response of organized dense arrays of flexible magnetic cantilevers is explored. When subjected to the stray field of a magnetized material, the mobile parts of the cantilevers deviate from their initial positions, which locally changes the light reflectivity on the magneto-optical surface, thus allowing to visualize the field lines. While the final goal is to be able to map and quantify non-uniform fields, calibrating and testing the device can be done with uniform fields. Under a uniform field, the device can be assimilated to a magnetic-field-sensitive diffraction grating, and therefore, can be analyzed by coherent light diffraction. A theoretical model for the diffraction patterns, which accounts for both magnetic and mechanical interactions within each cantilever, is proposed and confronted to the experimental data.

Two-photon imaging of a magneto-fluorescent indicator for 3D optical magnetometry

We developed an optical method to visualize the three-dimensional distribution of magnetic field strength around magnetic microstructures. We show that the two-photon-excited fluorescence of a chained donor-bridge-acceptor compound, phenanthrene-(CH2)12-O-(CH2)2-N,N-dimethylaniline, is sensitive to ambient magnetic field strength. A test structure is immersed in a solution of the magneto-fluorescent indicator and a custom two-photon microscope maps the fluorescence of this compound. The decay kinetics of the electronic excited state provide a measure of magnetic field that is insensitive to photobleaching, indicator concentration, or local variations in optical excitation or collection efficiency.

Optical Absorption and Magnetic Field Effect Based Imaging of Transient Radicals

Short-lived radicals generated in the photoexcitation of flavin adenine dinucleotide (FAD) in aqueous solution at low pH are detected with high sensitivity and spatial resolution using a newly developed transient optical absorption detection (TOAD) imaging microscope. Radicals can be studied under both flash photolysis and continuous irradiation conditions, providing a means of directly probing potential biological magnetoreception within sub-cellular structures. Direct spatial imaging of magnetic field effects (MFEs) by magnetic intensity modulation (MIM) imaging is demonstrated along with transfer and inversion of the magnetic field sensitivity of the flavin semiquinone radical concentration to that of the ground state of the flavin under strongly pumped reaction cycling conditions. A low field effect (LFE) on the flavin semiquinone-adenine radical pair is resolved for the first time, with important implications for biological magnetoreception through the radical pair mechanism.

Magnetic-field-enabled resolution enhancement in super-resolution imaging

A novel strategy for modulating the photophysics of organic dyes in super-resolution fluorescence imaging using an external magnetic field was reported. The magnetic field induced increase in fluorescence intensity, localization number of probe molecules, and the number of photons emitted per molecule as compared to those acquired without a magnetic field were experimentally confirmed. Improved dSTORM localization precision and imaging resolution were consequently achieved.

Enhanced self-assembly of metal oxides and metal-organic frameworks from precursors with magnetohydrodynamically induced long-lived collective spin states

Magneto-hydrodynamic generation of long-lived collective spin states and their impact on crystal morphology is demonstrated for three different, technologically relevant materials: COK-16 metal organic framework, manganese oxide nanotubes, and vanadium oxide nano-scrolls.

Modulation of Visible Room Temperature Phosphorescence by Weak Magnetic Fields

Magnetic control over excited states of molecules presents interest for many applications. Here we show for the first time that visible room temperature phosphorescence in multichromophoric donor-acceptor systems can be modulated by weak magnetic fields (<1 T) via magnetic field effects (MFE) on the spin dynamics in photogenerated radical pairs (RPs). The studied compounds comprise Pt porphyrin (PtP)-Rosamine B (RosB) dyads, which possess strong visible absorption bands and phosphoresce at room temperature. The observed MFE is unique in that it occurs upon direct excitation of the PtP in the dyads, whereby ultrafast quantitative formation of the local PtP triplet state precedes the occurrence of radical intermediates. A model explaining the effect is proposed, which is based on reversible electron transfer between the local triplet state and a long-lived RP. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet RP recombination channel and thereby influencing phosphorescence.