The writing/erasing process and cycle.
The writing/erasing process and cycle.

A light-emitting hybrid combining the best of bioimaging organic molecules and luminescent solid-state materials into which data can be written and erased could pave the way to new anti-counterfeiting, data recording, and optoelectronic technologies, according to researchers from the Institute of Chemical Sciences in Rennes [Khlifi et al., Materials Today (2020),].

“By integrating light-emitting compounds in a polymer sheet, we created a material able to store data that are invisible in daylight but readable under ultraviolet (UV) light,” explains Yann Molard, who led the work.

The researchers started out trying to answer a series of fundamental questions, he says, from ‘is it possible to integrate two emitting species in a polymer?’ to ‘what would the interactions between the two emitters and the polymer be?’. By combining an organic blue-green emitting compound (3-oxindole) with an inorganic red phosphorescent metal cluster in polymethylmethacrylate (PMMA or plexiglass®), the team was hoping to achieve white emission. But what the researchers found was even more interesting.

“We checked [the material] under UV light and observed white emission,” says Molard. “We were quite excited and [left] the sample under the UV lamp. Looking back at the sample, it was not [emitting] white anymore but red. The next day it was emitting white again. Serendipity!”

Irradiation of the hybrid material changes the emission color from white to red as a result of energy transfer from the organic dye to the metal cluster.

“This is the first example of a material where these families of emitters are mixed and interacting,” says Molard. “Each come from different worlds: the blue emitter is an organic dye used in bioimaging, [while] the red emitter comes from the world of solid state chemistry and is a ceramic-like powder.”

The researchers developed a copolymerization synthesis that incorporates the two emitters in a homogeneous and stable manner. While both are chemically and photochemically stable, the emitters interact with each other and with oxygen in the air, giving rise to the change in emission color.

Data can be printed onto the material in two ways: either by irradiating with UV light through a mask or direct writing using a UV laser. The lifetime of the data can be tuned from a few minutes to many days, completely erased, and rewritten.

“These data, invisible in daylight, can be read by simple irradiation with a UV lamp like the ones used to check banknotes,” says Molard. “In addition to the reversible nature of the inscription, the process has the advantage of being simple and inexpensive.”

This dual emissive material is unprecedented, believe the researchers, and could provide a straightforward and cost-effective encryption-decryption system. Thin films of the material could be useful as oxygen sensors in food packaging or even as a solar concentrator on windows.