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  1. With the aim to construct a new type of photoswitchable photochromic material modulated by specific radiation in the microwave region, the spin dynamics of radical pairs (RPs) from ion-pair complexes between viologen and tetraarylborate compounds have been investigated in the presence of microwave (μw) radiation, using steady-state electron paramagnetic resonance (SSEPR) to follow the radical pair (RP) dynamics. This strategy is realized by excitation of the charge transfer (CT) absorption band of the ion-pair complex in the solid phase (powders and dispersed in polymer matrices) at 410 nm, which leads to electron transfer from borate to viologen, producing RPs. In the singlet excited state or Partially Separated Charge (PSC) state, an electron transfer process occurs between the ions, and the subsequent (purple) viologen radical is observed as a Fully Charge Separated (FCS) state. In solid state SSEPR experiments, μw radiation deactivates the FSC state by inducing back electron transfer, which subsequently increases the population of a Partially Separated Charge (PSC) state, recovering the initial color of the ion-pair complex. State-of-the-art photophysical and photochemical studies show that deactivation of the FSC state can take place using μw radiation on the RPs in a switchable, reversible fashion. The results have potential impactmore »for a number of applications including photo-writing and photo-erasing processes and spintronics. Examples of laser writing using a polymer matrix to lock the relative positions of the radicals, and then erasing the color using microwaves, are presented and discussed.« less
    Free, publicly-accessible full text available May 11, 2023
  2. Free, publicly-accessible full text available March 8, 2023
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    The title compound, [Cu 2 (C 19 H 23 N 7 O)(C 2 H 3 O 2 ) 4 ] n , was obtained via reaction of copper(II) acetate with the coordinating ligand, 6-ethoxy- N 2 , N 4 -bis[2-(pyridin-2-yl)ethyl]-1,3,5-triazine-2,4-diamine. The crystallized product adopts the monoclinic P 2 1 / c space group. The metal core exhibits a paddle-wheel structure typical for dicopper tetraacetate units, with triazine and pyridyl nitrogen atoms from different ligands coordinating to the two axial positions of the paddle wheel in an asymmetric manner. This forms a coordination polymer with the segments of the polymer created by the c -glide of the P 2 1 / c setting of the space group. The resulting chains running along the c -axis direction are held together by intramolecular N—H...O hydrogen bonding. These chains are further packed by dispersion forces, producing an extended three-dimensional structure.
  4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been used to study the proton coupled electron transfer (PCET) reaction between a ruthenium complex (Ru(bpz)(bpy) 2 ) and several substituted hydroquinones (HQ). After excitation at 355 nm, the HQ moiety forms a strong hydrogen bond to the exposed N atoms in the bpz heterocycle. At some point afterwards, a PCET reaction takes place in which an electron from the O atom of the hydrogen bond transfers to the metal center, and the proton forming the hydrogen bond remains on the bpz ligand N atom. The result is a semiquinone radical (HQ˙), whose TREPR spectrum is strongly polarized by the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Closer examination of the CIDEP pattern reveals, in some cases, a small amount of radical pair mechanism (RPM) polarization. We hypothesize that when the HQ moiety has electron donating groups (EDGs) substituted on the ring, S–T − RPM polarization is observed in HQ˙. These anomalous intensities are accounted for by spectral simulation using polarization from S–T − mixing. The generation of S–T − RPM is attributed to slow radical separation after PCET due to stabilization of the positive charge on the ringmore »by EDGs. Results from a temperature dependence support the hypothesis.« less
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