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The principle of operation of the developed compounds. Credit: Taken from Inorganic Chemistry, St. Petersburg State University
Chemists from St. Petersburg University has developed a new family of luminescent iridium complexes that, for the first time, realize a unique mechanism of photoactivated proton transfer. In the future, this discovery will potentially allow for the creation of a fundamentally new class of "smart" antitumor drugs that can be activated directly inside tumor cells and tracked in real time by the change in the color of their glow.
How proton-hopping molecules work
The ability of molecules to change their properties under the influence of light is a phenomenon widely studied and used in chemistry. One of the key mechanisms responsible for this process is Excited-State Intramolecular Proton Transfer (ESIPT). Such a molecule contains two functional groups: a donor, capable of donating a proton, and an acceptor, which accepts it.
Upon absorption of a light quantum, the electron density redistributes, causing the proton to quickly "jump" from the donor to the acceptor. This process underlies many biological phenomena, including the bioluminescence of some living organisms, and is actively used in industry.
Limits of traditional organic systems
For a long time, scientists managed to create such "switchable" luminescent systems primarily based on organic molecules. However, they often do not glow brightly enough and are frequently unstable, prompting researchers to seek new ways to create more effective substances with desired properties.
Attempts to integrate a metal atom into such systems, which could stabilize the molecule and impart new useful properties, did not yield the desired results. This is because, typically, the metal ion, upon coordinating with the organic molecule, would simply displace the labile proton, thereby completely suppressing the ESIPT switching mechanism.
Therefore, the creation of organometallic complexes where the metal atom does not block but rather facilitates proton transfer remained an important challenge.
Iridium complex that changes color
Scientists at St. Petersburg University have managed to construct an iridium complex with a special organic ligand where the central metal atom becomes a key player. It actively intervenes in the distribution of electron density, which, upon light irradiation, leads to ultrafast proton transfer within the excited molecule and a change in its emission color—from blue-green to orange-red.
The research is published in the journal Inorganic Chemistry.
This is the first example of an organometallic luminescent molecule in which the metal atom directly governs the proton transfer process, and where the donor and acceptor centers are spatially separated.
Potential for smart cancer therapies
"Such compounds could potentially be used in the future to create therapeutic drugs or theranostic agents sensitive to the microenvironment. For example, a design is fundamentally possible where the change in emission color would be triggered only under the specific conditions of a tumor cell," explained Professor Mikhail Kinzhalov from the Department of Physical Organic Chemistry at St. Petersburg University.
"This would allow not only localizing the therapeutic effect but also tracking the state of the molecule in real time at the cellular level. However, we are currently at the fundamental research stage: it was important for us to prove that a metal can not suppress ESIPT, but rather facilitate it. And we have shown for the first time that such a mechanism is achievable."
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Thus, the university's chemists have created an iridium complex with an organic "framework"—an acyclic diaminocarbene ligand—into which they incorporated a pyrazine fragment with two nitrogen atoms. This structure acts as a "trap" for the proton.
When the molecule absorbs light, the iridium acts as a molecular pump: it shifts electron density onto the pyrazine, sharply increasing its ability to attract a proton. As a result, the hydrogen atom "jumps," forming a new form of the molecule that now emits not blue-green, but orange-red light—a wavelength shift of about 100 nanometers.
To confirm the efficiency of the transfer, the scientists conducted a series of observations. First, they found that the luminescence depends on the environment: in some solvents, the complex glows orange, while in alcohol it reverts to green, as alcohol blocks the transfer. Second, computer calculations confirmed that proton transfer in the excited molecule is energetically favorable and should indeed lead to such a color shift.
Finally, the effectiveness of the development was confirmed by an isotope experiment: when the ordinary hydrogen in the molecule was replaced with its heavy isotope, deuterium, the orange glow disappeared. This convincingly proves that the color shift is caused specifically by proton transfer, and not by any other intramolecular process.
This work opens up prospects for the creation of "smart" drugs and sensors. For example, one could design a molecule that activates and starts glowing with a specific color only inside a tumor cell, allowing not only for precise targeting of the tumor but also for its visualization in real time.
Besides therapy, such switchable luminescent molecules will find applications in medical diagnostics and the development of new materials for electronics.
Publication details Polina O. Skripnyak et al, Photoinduced Tautomerisation of ESIPT-Capable Iridium(III) Complexes with Rationally Designed Acyclic Diaminocarbene Ligands, Inorganic Chemistry (2026). DOI: 10.1021/acs.inorgchem.5c04206 Journal information: Inorganic Chemistry
— Source: Phys.org (https://phys.org/news/2026-03-chemists-iridium-compounds-synthesis-smart.html)
