Metal–Organic Frameworks with Extended Viologen Units: Metal-Dependent Photochromism, Photomodulable Fluorescence, and Sensing Properties

Abstract

The photoresponsive viologen unit has been widely used to endow metal–organic materials with photochromic and other photomodulable properties. Herein we report the first examples of the metal–organic materials functionalized by extended viologens (ExVs), of general formula [ML]·2H2O (M = Zn for 1, M = Mn for 2, M = Co for 3, and L is a tetracarboxylate ligand with the p-phenylene-extended viologen spacer). Of the three isomorphic metal–organic frameworks, only 1 is photochromic owing to formation of extended viologen radicals through photoinduced electron transfer (PET). The incapability of 2 and 3 to undergo photochromism can be ascribed to longer intermolecular donor–acceptor contacts, emphasizing the sensitivity of solid-state PET to structural changes. 1 also shows strong fluorescence owing to interligand charge transfer, and the fluorescence can be reversibly modulated and switched on/off in the photochromic process. Furthermore, 1 shows excellent hydrolytic stability and can be used as a sensitive, selective, and recyclable fluorescence sensor for detecting Fe3+ in water. The results demonstrated the great potential of extended viologens as functional units for the design of novel responsive metal–organic materials.

Mono- and Di-Quaternized 4,4′-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

“…In recent years, the derivatives of 4,4 -bipyridine have been employed in numerous high-tech applications including molecular motors [2], machines [3,4], and switches [5,6]. 4,4 -Bipyridine is highly abundant in supramolecular chemistry as well, where it mainly serves as an electro-and photo-sensitive building block as well as a bidentate coordinating ligand, very often appearing in covalent organic and metalorganic frameworks (COFs and MOFs, respectively) [7][8][9]. A key characteristic of the derivatives of this important building block is their multifunctionality [10,11].…”

“…The most prominent class of 4,4 -bipyridine is that of viologens, i.e., the N,N -disubstituted 4,4 -bipyridines, also known as paraquats, due to the presence of two quaternary nitrogen atoms of the two pyridine rings lying at para-positions. Viologen-based materials often exhibit very interesting electronic, optical, electrochemical, and magnetic properties [1, 9,10], which can be combined in novel multifunctional systems suitable for a wide range of applications. There is a constant increase of published research works involving viologens (see Figure 1) and this perhaps is associated with the need for new materials corresponding to a variety of hi-tech applications.…”

“…There is a constant increase of published research works involving viologens (see Figure 1) and this perhaps is associated with the need for new materials corresponding to a variety of hi-tech applications. The importance of viologens is reflected through the numerous review papers (many of them very recent) underlining the high versatility of this class of compounds [9,10,[12][13][14][15]. Perhaps, the most reviewed subject regarding the viologens is their electrochromism [13,14,16].…”

“…Two important classes of 4,4 -bipyridine-based products with solvatochromic and/or environment-responsive character are reviewed: viologens (i.e., N,N -disubstituted derivatives) and monoquats (i.e., monosubstituted 4,4 -bipyridine derivatives). The multifunctional nature of these derivatives is analyzed and structure-property relations are discussed in connection to the role of these derivatives in various novel applications.Molecules 2020, 25, 1 2 of 30 recent) underlining the high versatility of this class of compounds [9,10,[12][13][14][15]. Perhaps, the most reviewed subject regarding the viologens is their electrochromism [13,14,16].…”

“…Molecules 2020, 25, 1 2 of 30 recent) underlining the high versatility of this class of compounds [9,10,[12][13][14][15]. Perhaps, the most reviewed subject regarding the viologens is their electrochromism [13,14,16].…”