Scientists from the University of St Andrews have created the first samples of organic materials capable of simultaneously absorbing two photons and emitting circularly polarized light. This rare combination of properties promises a revolution in deep scanning of biological tissues.
Conventional dyes for fluorescence microscopy scatter rapidly in the thickness of tissue, and the signal fades already at a depth of a few hundred micrometers. The new chiral molecules from the class of multiresonant thermally activated delayed fluorescence materials (MR-TADF) behave differently: they absorb two photons of infrared light at once, and then emit visible light with a precisely defined polarization.
Imagine a flashlight that shines not with one but with two beams at once and at the same time "remembers" which way the light is twisted. That is exactly how these compounds work. The chirality of the molecules — their mirror asymmetry — forces the emitted light to spin predominantly in one direction, which makes it possible to filter out noise and obtain a clearer image.
Two-photon absorption is already used in medicine in its own right: infrared light penetrates deeper than visible light and damages tissues less. The addition of circularly polarized luminescence makes the signal even more informative — researchers will be able to distinguish structures that previously merged into a single blurred mass.
The work, published in Nature Communications, was carried out by Professor Eli Zysman-Colman's group in collaboration with colleagues from Durham University. For now these are laboratory samples, but it is already clear: such materials could become the basis for non-invasive methods of visualizing tumors, blood vessels and nerve fibers at a depth of several millimeters.
In the future, doctors will be able to obtain three-dimensional images of internal organs without X-rays and strong magnetic fields, while biologists will be able to observe living processes in real time inside a whole organism. A single material solves two problems at once: deep penetration and a high-contrast polarized signal.
The researchers have already begun testing the new compounds in biological media.
