Piezochromism and hydrochromism through electron transfer: new stories for viologen materials

Abstract

A pyridinium-carboxylate compound undergoes reversible color change under pressure owing to the formation of radicals via electron transfer; dehydration and hydration can also trigger electron transfer.

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

“…In turn, such a change results in the alteration of the electrostatic interactions between those two counterparts and finally of the CT energy. (A very similar situation was described for the zwitterionic crystalline viologen by Sui et al [44] vide supra). Later on, Monk et al studied in depth the energetics and kinetics of the formation of such complexes [87] and reported useful information on this important type of CTC and very similar spectral results with More recently, various other research groups studied the effect of the viologen N-substituents on the CTC chromic performance (see various results on crystalline CTC of this family in Table 2).…”

“…The corresponding CTCs (59a,59b, Figure 25) are intensely thermochromic in the solid state being deep red-purple at room temperature and light red at temperatures as low as 80 K. The phenomenon exhibits reversibility, and it is attributed to the thermally sensitive nature of the crystal structure in which the distance between the viologen dications was found to intensely depend on the temperature. As mentioned above (zwitterionic crystalline viologen by Sui et al [44] vide supra), crystal changes can influence the energy of CT and thus the color of the complexes and this leads to chromic CTCs. The study by Kinuta et al is a very important example of materials with potential applications in solid-state supramolecular thermosensors.…”

“…The latter effect is not often observed and it resembles a very interesting case of supramolecular solvatochromism [45]. Another example of a viologen with "direct" (solvato)chromic behavior was designed and synthesized by Sui et al (Figure 9) [44]. Molecule 20 is π-conjugated and zwitterionic, comprising the bipyridinium dication and two para-benzoate units as functional groups of the two N-atoms.…”

“…(Top) The zwitterionic viologen bySui et al and (bottom) the interactions observed in the crystal state (red dashed line, π-π stacking interactions; blue dashed lines, donor-acceptor interactions)[44].…”

“…Sui et al[44] vide supra), crystal changes can influence the energy of CT and thus the color of the complexes and this leads to chromic CTCs. The study by Kinuta et al is a very important example of materials with potential applications in solid-state supramolecular thermosensors.…”