Fluorescent Hybrid Changes Color with Polarization

A fluorescent hybrid material that changes color based on light polarization could enhance a number of applications in nanotechnologies.

Creating this material required a host inorganic material of crystalline structure with parallel nanochannels and pore size of less than 1 nm, as well as two dyes of similar size and shape with complementary optical properties that would respond differently when stimulated by light.

Researchers from the Molecular Spectroscopy Group at the UPV/EHU University of the Basque Country, in collaboration with the Institute of Catalysis and Petroleum Chemistry of the Spanish National Research Council, used an aluminophosphate (AIPO-11) host material and the dyes pyronin, featuring green fluorescence, and acridine, with blue fluorescence.

At left, a chromaticity diagram characterizes the colors. Green emission obtained using linearly polarized light along the channels, shown at top right. Blue emission obtained using light linearly perpendicular to the channels, shown at bottom right. The arrows indicate the direction in the polarization of the light used. Courtesy of UPV/EHU Molecular Spectroscopy Group.

“The dyes enter in order, they align themselves along the nanochannels, and their fluorescent properties are improved in them,” said researcher Virginia Martinez. She said this improvement was prompted by the dyes’ restricted molecular flexibility and separate inclusion into the channel.

In the experiment, pyronin was inserted first, creating a highly luminescent material. The researchers then incorporated that with the acridine into a synthesis process, which produced rectangular crystals of 30 µm×20 µm. This allowed the colors to change based on the light’s polarization — along the channel it appeared green, while perpendicularly it displayed blue — and indicated that energy was transferring between the dyes.

“The tuning of color is an instantaneous, efficient process that can be fully reversed and reproduced with high resistance to fatigue,” said Iñigo López-Arbeloa, a researcher with the group.

The synthesis process was fundamental to the work, the researchers said. Unlike traditional photoactive hybrid materials, in which the organic part is inserted into the inorganic part from the gas or liquid phase by diffusion, the new method included insertion of the dyes into a gel with which the inorganic material is synthesized. As the crystal grew, the organic chromophore was gradually incorporated.

The research is published in ACS Photonics (doi: 10.1021/ph4000604).