New Stories for Viologen Materials - Dr. Lin Wang

JANUARY. 23, 2017

Viologen materials have been well known for electrochromism and photochromism from electro and photo stimulus. New work from a team led by Prof. Enqing Gao of East China Normal University and Dr. Lin Wang from HPSTAR, found piezochromis in viologen compounds. This is not only a new chromic phenomena for viologen compounds but also represents the first example of organic mechanochromism and hydrochromism associated with radical formation via electron transfer, proposed in the paper published in the journal Chemical Science (DOI: 10.1039/C6SC04579K).

Colour change or chromic phenomenon plays an important role in our daily lives. Chamaeleo changes colour in different environments via the active tuning of a lattice of nanocrystals contained in some cells. There are many ways in which colour can be caused to arise both by chemical and physical forces, all of which are used or have the potential to be used in technological applications. These colour change phenomena are classified and named after the stimulus that causes the change. Such as photochromism, electrochromism, mechanochromism and solvatochromism.

Piezochromism refers to the reversible color change of a solid in response to external pressure. We know that pressure will lead to structural changes, it may also lead to color change through modulation of electronic energy gaps or irreversible inter-/intramolecular chemical transformations such as polymerization and isomerization, concomitant with irreversible (undesirable) color change. Stress/pressure could also lead to free colored radicals - often referred to free radicals which are atomic or molecular species with unpaired electrons on an otherwise open shell configuration- to develop colour change.

Viologen derivatives have long been known for electrochromism and photochromism through reversible redox acitivty- electron/charge transfer, to form colored cation radicals. In this work, researchers demonstrated that a viologen-carboxylate zwitterionic molecule - with both positive and negative charges (equal positive and negative charge) in the crystalline state exhibits piezochromic phenomenon.

The yellow viologen comound will turn to red under several gigapascals, to green after pressure release, and finally return to yellow at ambient conditions.

“External pressure will force the molecules in the lattice to move closer, shortening donor-acceptor distances - be favorable to electron transfer from the electron-rich carboxylate group to the electron-deficient viologen unit,” explained Prof. Gao.

“Pressure could be a more powerful than light for induction of intermolecular electron transfer (and chromic phenomena) in viologen crystals. Light can be absorbed by individual molecules and have no direct effect on intermolecular distances, so it would not induce electron transfer if the donor and the acceptor were not placed at a favorable distance. While pressure works directly on intermolecular distances, so it can proactively reduce the distance between the donor and the acceptor in favor of electron transfer,” Dr. Lin Wang added.

In addition, they found that hydration-dehydration will also change the color of the viologen compound just like what pressure does.

“The compound behaves like a “chromic sponge”: the yellow sample in moist atmosphere will turn to green upon heating to be dried, then return to yellow when put back in the air.” Said Prof. Gao. “We could assume that the loss of lattice water will also lead to closer crystal packing of the zwitterionic molecules so that the distance between electron donors and acceptors is reduced in favor of electron transfer.

“So despite the remarkably different nature, the physical stimuli (compression/decompression) and the chemical stimuli (dehydration/rehydration) have one thing in common: the ability to modulate intermolecular donor-acceptor contacts in favor of electron transfer,” Prof. Gao explained.

Caption: Pressure/water induced colour change in viologen.

变色现象是指物质在外界环境的影响下，而产生的一种对光的反应的改变。这种现象普遍存在于自然界中，比如变色龙，它的体色会随着周围环境的变化而改变。人们感兴趣的是一类具有可逆变色现象的材料，即可利用一定的外界条件将它们的颜色进行改变并且在另一种条件下将其还原。目前发现的变色现象主要有4 类: 电致变色、光致变色、热致变色和压致变色。紫罗精是一种最具有代表性的有机电致变色材料-具有良好的变色性能，选择合适的取代基，通过改变分子轨道能级和分子间电荷的迁移能可以方便地调节其电色效应。人们发现紫精同时具有电致变色，及光致变色的性能。华东师范大学及北京高压科学研究中心带领的研究团队发现紫精化合物也表现出压致变色，及吸水脱水变色的行为。压力及水虽然一个是通过物理过程，一个是化学的方法使紫罗精变色，但具有一个共同的机理-改变分子间距，转移电荷，产生阳离子自由基。本工作不仅发现了紫罗精化合物新的变色现象，也是首次在有机物中同时发现由于电荷转移形成的自由基而导致压致变色及水致变色的现象。