北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

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.