A novel hybrid charge transfer crystal with reversible color-changing property

Transfer of charge is a process in which electrons move within a molecule or between two molecules. It is a crucial chemical process that can be applied to a wide range of technologies. Intramolecular charge transfer (ICT) occurs when electrons are exchanged between donor and acceptor groups within a molecule via a series of overlapping electron orbits. This exchange creates a shift in light wavelength towards the red end of the light spectrum (redshift). This observable color shift due to ICT has applications in dye manufacturing and organic LEDs (OLEDs).

Meanwhile, intermolecular charge transfer (CT), when electrons are exchanged between different molecules, can be realized using ‘π-conjugated organic molecules’ through which electron movement occurs from donors to acceptors. CT plays a crucial role in photovoltaic devices, semiconductors, and other applications. Combining CT with ICT in a single hybrid system could lead to the development of novel materials. However, achieving this has been challenging because it requires precise control over the molecular design and intermolecular interactions. Additionally, the hybrid system must be composed of material that remains stable under such rapid transfer conditions.

Pyrazinacenes, a class of aromatic (ring-like) molecules, could be a promising candidate for such a task. Pyrazinacenes can serve as a bridge between a donor and acceptor molecule, facilitating CT. Being deficient in electrons, pyrazinacene enables electrons to move easily within its ring-like structure, facilitating ICT. This may lead to the formation of a CT-ICT hybrid system, though its efficacy remains untested.

Now, in a new study, scientists from the Graduate School of Engineering and Science, Shibaura Institute of Technology (SIT), Japan—Professor Akiko Hori, Mr. Kazushi Nakada, and Dr. Gary James Richards—describe a new CT-ICT system (compound 1) that utilizes the novel pyrazinacene derivative, 6,7-bis{4-(diphenylamino)-phenyl}-pyrazino[2,3-b]pyrazine-2,3-dicarbonitrile. This pyrazinacene core links triphenylamine groups, strong electron donors, with cyano groups, electron acceptors. The study was made online on January 24, 2025, and published in Volume 31, Issue 18 of Chemistry – A European Journal on March 25, 2025.

Compound one co-crystallized in a 1:1 ratio with naphthalene. The resulting crystals exhibited a dramatic color change, shifting from greenish-blue to red-violet. This interaction was specific to naphthalene. Experiments with naphthalene derivatives, such as octafluoronaphthalene, did not result in co-crystallization; instead, they led to electronic repulsion. Thermogravimetric analysis and powder X-ray diffraction confirmed compound 1’s specificity for naphthalene and demonstrated successful co-crystallization. Density functional theory (DFT) calculations revealed that the distinct color shift arises from an intermolecular CT event facilitated by the novel pyrazinacene. The CT event disrupts ICT, resulting in a blue shift.

“Our molecule’s design achieves competition between intramolecular and intermolecular charge transfer,” says Mr. Nakada, a graduate student at SIT and the first author of this paper. Elaborating further, he says, “This allows our molecule to act as a sensor that can, through a simple color change, identify even trace amounts of naphthalene—an environmentally regulated substance—in freshwater and seawater.”

Upon analyzing the crystal structure, the researchers found that the molecular recognition process, responsible for the color shift in these crystals, is facilitated by π-hole···π interactions. The hydrogen atoms of naphthalene are extended toward the nitrogen atoms of the pyrazinacene (compound 1); however, the atoms are not close enough to form strong hydrogen bonds. Instead, the crystal structure is stabilized by weaker Van der Waals forces. These weaker bonds can be easily broken and reformed, making the color shift a reversible process. For instance, heating the violet crystals to 180 °C caused naphthalene to separate, restoring the crystals to their original greenish-blue color.

“Our study establishes a foundation for synthesizing nonporous adaptive crystals with reversible color-changing properties. This breakthrough opens new avenues for the development of sensor technologies and materials for selective molecular recognition,” concludes Prof. Hori.

***

Reference

Title of original paper: Colorimetric Detection of Naphthalene Enabled by Intra-to Intermolecular Charge Transfer Interplay Induced by π-hole···π Interactions of a TPA-Attached Pyrazinacene

Journal: Chemistry – A European Journal

DOI: 10.1002/chem.202404487

About Shibaura Institute of Technology (SIT), Japan

Shibaura Institute of Technology (SIT) is a private university with campuses in Tokyo and Saitama. Since the establishment of its predecessor, Tokyo Higher School of Industry and Commerce, in 1927, it has maintained “learning through practice” as its philosophy in the education of engineers. SIT was the only private science and engineering university selected for the Top Global University Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology and had received support from the ministry for 10 years starting from the 2014 academic year. Its motto, “Nurturing engineers who learn from society and contribute to society,” reflects its mission of fostering scientists and engineers who can contribute to the sustainable growth of the world by exposing their over 9,500 students to culturally diverse environments, where they learn to cope, collaborate, and relate with fellow students from around the world.

Website: https://www.shibaura-it.ac.jp/en/

About Professor Akiko Hori from SIT, Japan

Dr. Akiko Hori is a Professor at the Department of Applied Chemistry, SIT, Japan. She heads the Laboratory of Molecular Assemblies. Her research is focused on crystal engineering and supramolecular chemistry based on the combination of inorganic and organic materials. Her lab also explores related research topics, including design and environmental applications of coordination compounds, molecular recognition, and gas adsorption in molecular crystalline states, as well as the photo-functionality and dynamic transformation of crystals in response to external stimuli.

Funding Information

This work was supported by Grants-in-Aids for Scientific Research C, 24K08401 (G.J.R) and Grant-in-Aids for Scientific Research B, 23K21122 (A.H) of JSPS KAKENHI.