Radiochromic Hydrogen‐Bonded Organic Frameworks for X‐ray Detection

Abstract

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through self-assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-26), as a new class of "allorganic" radiochromic smart material, opening a gate for HOFs in this field. PFC-26 is active upon both X-ray and UV irradiation, while PFC-25 is only active upon X-ray irradiation. The same building block yet different radiochromic behaviors of PFC-25 and PFC-26 allow us to gain a deep mechanistic understanding of the factors that control the detection specificity. Theoretical and experimental studies reveal that the degree of π-conjugation of viologen ligand is highly related to the threshold energy of triggering a charge transfer, therefore being a vital factor for the particularity of radiochromic materials. Thanks to its convenient processibility, nanoparticle size, and UV silence, PFC-25 can be further fabricated into a portable naked-eye sensor for X-ray detection, which shows obvious color change with the merits of high transmittance contrast, good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent stability. The work exhibits the promising practical potentials of HOF materials in photochromic technology.

A Three-Component Donor–Acceptor Hybrid Framework with Low-Power X-ray-Induced Photochromism

“…In addition, there are plenty of supramolecular interactions (Figures S7 and S8), such as anion−π interactions, O–H···N H-bonds, C–H···N H-bonds, O–H···O H-bonds, among [Co­(CN) 6 ] 3– anions, free or/and coordinated H 2 O molecules, and bcbp ligands in the D–A hybrid architecture, which consolidate the D–A hybrid structure. Moreover, the anion−π interactions between Co­(CN) 6 3– anions and bcbp ligands in the D–A hybrid architecture may provide electron-transfer pathways to realize the reversible dual photochromism. ,,,− …”

“…All of them indicated that the photochromic behavior involves only electron transfer because no composition loss or skeleton change was observed in irradiated samples. According to refs and , the appropriate distance between the acceptor and donor is quite necessary to give rise to electron transfer upon X-ray and UV irradiation (<3.8 Å for UV photochromism and ∼4 Å for X-ray photochromism). As shown in Figures S7 and S8, there are strong anion−π interactions between electron donor Co­(CN) 6 3– anions and electron acceptor bipyridyl units.…”

“…Photochromic materials usually exhibit reversible and visible color changes upon exposure to external light sources and have become an active research area in the past several years. Among them, considerable effort has been devoted to exploring ultraviolet (UV) or visible light-induced photophysical properties such as photomagnetism, photoconductance, and photoluminescence as well as applications in the field of erasable printing and smart windows. , However, the development of X-ray-induced photochromic materials is still in infancy. − At present, the scintillation counters, ionization gauges, and other devices have been developed for X-ray detection. − However, X-ray-sensitive materials used in these devices often require electronic attachments or tedious operations to process the signals. − Compared to commercial X-ray-sensitive materials, donor–acceptor (D–A) hybrid materials with organic and inorganic photoactive moieties fabricated at the molecular level make up an emerging class of photochromic materials that can be directly “visualized” by X-ray irradiation. − Although a few D–A hybrid materials with X-ray-induced photochromism (XIP) have been reported, most of them are produced in high-power environments, − and it is still rather rare for those with low-power XIP. , Accordingly, developing new radiosensitive D–A hybrid materials that can directly show a color change upon low-power irradiation with convenient operation is meaningful.…”

“…18−22 Although a few D−A hybrid materials with X-ray-induced photochromism (XIP) have been reported, most of them are produced in high-power environments, 23−27 and it is still rather rare for those with lowpower XIP. 22,28 Accordingly, developing new radiosensitive D−A hybrid materials that can directly show a color change upon low-power irradiation with convenient operation is meaningful.…”

Design Rules of Hydrogen-Bonded Organic Frameworks with High Chemical and Thermal Stabilities

“…The successful preparation of a stable HOF, tecton-1 ( L2 ), in 1997 by Wuest led to a rapid expansion of experimental and theoretical investigations in this field . Since then, numerous HOFs with diverse structures and unique properties have been reported at an astonishing pace, and this has led to the successful implementation of these crystalline porous materials in applications that include gas storage and separation, − chiral separation, chemical sensing, proton conduction, radiation detection, water remediation, antibacterial activity, enzyme encapsulation, − and catalysis. ,− …”