Stable
stimulus-responsive materials are highly desirable due to
their widespread potential applications and growing demand in recent
decades. Despite the fact that viologen derivatives have long been
known as excellent photochromic and electrochromic materials, the
development of stable viologen-based multifunctional smart materials
with short coloration times remains an exciting topic. To obtain photochromic
and electrochromic dual responsive materials, embedding the viologen
ligand into a robust metal oxide cluster to increase its stability
and sensitivity is an effective strategy. Herein, a viologen-based
metal–organic polyhedron (MOP) {[Zr6L3(μ3-O)2(μ2-OH)6Cp6]·8Cl·CH3OH·DMF} [Zr-MOP-1; H2L·2Cl = 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium
dichloride, and Cp = η5-C5H5] was successfully prepared and characterized. It consists of trinuclear
Zr–oxygen secondary building units and exhibits reversible
photochromic and electrochromic dual responsive behaviors. As expected,
the designed robust viologen-based nanocage with a V2E3 (V = vertex, and E = edge) topology can maintain its stability
and rapid photo/electrochromic behaviors with an obvious reversible
change in color from purple (brown) to green, mainly due to the enclosed
cluster structure and the abundant free viologen radicals that originate
from the effective Cl → N and O → N electron transfers.
Spectroelectrochemistry and theoretical calculations of this Zr-MOP
were also performed to verify the chromic mechanism.
Through the complementary use of single-crystal X-ray diffraction and X-ray absorption spectroscopy, we present in this paper the first direct results on the site occupancy of thorium in the fluorapatite structure and the structural distortion created by its substitution. Structure refinements based on single-crystal X-ray diffraction data from synthetic Th-doped fluorapatite indicates that Th substitutes almost exclusively in the M2 site. A single-crystal X-ray study of natural fluorapatite from Mineville, New York, also indicated that substituting heavy scatterers (including Th) are concentrated in the apatite M2 site, but definitive site assignments of specific elements were not possible. Extended X-ray absorption fine-structure spectroscopy (EXAFS) was used to probe the local structure of Th in the synthetic fluorapatite (single-crystal form) with a Th concentration of roughly 20 000 ppm, as well as Th in the natural Mineville fluorapatite (powder form) with a Th concentration of ~2000 ppm. The EXAFS fitting results also indicate that Th partitions into the M2 site and yield a ~0.05-0.08 Å decrease of average M2-O bond distances associated with local structure distortions that are not obtainable from single-crystal X-ray diffraction studies.
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