Water modulation of photo- and piezochromism in viologen - Dr. Lin Wang

NOVEMBER 3, 2017

It is proposed that the lattice water does not act as an electron donor but serves to change the electro transfer energetic through its unique polarity and hydrogen bonding capability. New study led by Dr. Lin Wang of HPSTAR and Prof. Enqi Gao of East China Normal University found that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure in a series of different viologen pseudopolymorphic solids. The story is reported in the recent issue of JPCL.

Viologen will change colors through forming radicals by electron transfer (ET) under various stimuli, such as light/photo or pressure, which are called photochromism and piezochromism, respectively.

Although the two phenomena are similar in associating with electro transfer and consequential radical formation, the stimuli are quite different in the way they operate on molecular materials.

So the team aims to find correlations and difference between piezochromism and photochromism in viologen. Drs. Qi Sui of East China Normal University and co-workers give a comparative study on a group of viologen compounds with different water contents.

They found that the viologens show different sensitivities to light, less lattice water facilitating electro transfer and racicals forming and thus inducing piezochromism at lower pressure.

It’s interesting that photochromism can also be tuned by lattice water, less water stabilizing the radical-containing state and facilitating electro transfer.

“In this way, we can say that piezochromism and photochromism show the same reaction to water, whilst the underling mechanism is different”, said Dr. Lin Wang.

“Light can be directly absorbed by individual molecules, while has no direct intermolecular effect, it would not induce electro transfer if the donor and viologen were placed at unfavorable distances or in impedimental environments”, explained Prof. Enqi Gao, the other lead-author of the study. “However, pressure forces molecules to move more closer, and the energy is absorbed by intermolecular interactions”.

So for piezochromic viologen compounds, “pressure directly reduces the donor-acceptor distance to facilitate electro transfer, which in turn mitigates the intermolecular tension”, added Enqi Gao.

Caption: Impact of lattice water on solid-state electron transfer in viologen pseudopolymorphs.

紫罗精作为代表性的有机电致变色材料-具有良好的变色性能，通过选择合适的取代基，改变分子轨道能级和分子间电荷的迁移能来调节其电色效应。之前的研究提出晶格水不会作为紫罗晶中的电子供体而是通过改变电荷转移的动力学能。北京高压科学研究中心与华东师范大学合作，通过一系列的对比性实验发现紫罗晶晶格中的水含量越高，其对光及其压力的敏感性越低，因而导致紫罗晶越不容易变色，同时提出压力比光更容易实现电荷转移，因而更容易实现变色。