In optoelectronic materials, including solar cells, photocatalysis and photon upconversion, extensive research has been conducted on organic-inorganic nanohybrids using semiconductor nanocrystals (NCs) coordinated with aromatic organic molecules. In these materials, it is generally assumed that the coordination bonds of the ligand molecules will remain stable during optical processes. However, this assumption is only sometimes true.

Researchers from Ritsumeikan University, Japan demonstrate that the coordination bonds between ligand molecules and NCs are quasi-reversibly displaced by carboxyl groups by light irradiation using zinc sulfide (ZnS) NCs coordinated with perylene bisimide (PBI) as a model system. This is the first time that researchers have demonstrated the quasi-reversible movement of ligands in organic-inorganic nanohybrids by exposing them to visible light.

As a result, they observed photoinducible displacement that could lead to the development of advanced photofunctional materials with greater functionality.

Researchers made a breakthrough by demonstrating the quasi-reversible movement of organic ligands on the surface of nanocrystals. The results suggest that the coordination bond between perylene bisimide with a carboxyl group (PBI) and inorganic zinc sulfide (ZnS) nanocrystals can be reversibly moved upon exposure to visible light.

The findings offer a new take on the common belief that the organic ligands are anchored to the surface of the nanocrystals.

Professor Yoichi Kobayashi from Ritsumeikan University, Japan, said: “We investigated the ligand properties of organic-inorganic nanohybrid systems by using perylenebisimide with a carboxyl group (PBI)-coordinated zinc sulfide (ZnS) NCs (PBI-ZnS) as a model system. Our findings provide the first example of photoinduced displacement of aromatic ligands with semiconductor nanocrystals.”

To understand the material’s peculiar photoinducible properties, the researchers conducted theoretical analysis and experimental research in their study. The researchers first performed calculations using density functional theory to understand the structure and orbitals of PBI-ZnS ([PBI-Zn25S31]-) both in the ground state and in the first excited state. They then used an ultrafast laser to excite the material using time-resolved impulsively stimulated Raman spectroscopy. This aided their analysis of the associated Raman spectrum, which showed the excited state characteristics of PBI-ZnS.

According to experimental findings and mathematical analysis, one electron is excited from the PBI molecule upon photoexcitation, and the corresponding “hole” (the vacancy created by the lack of the electron) quickly transfers from the aromatic ligand (PBI) to ZnS. This ejects a long-lasting, negatively charged PBI ion from the surface of the ZnS nanocrystal.

However, the displaced ligands eventually recombine with the surface defects of the ZnS nanocrystal, producing a quasi-reversible photoinduced displacement of coordinated PBI. In particular, the dynamic behavior of coordinated ligand molecules in this work differs from that of conventional photoinduced charge transfer processes, where the hole often remains on the donor molecule to allow rapid recombination with the electron.

prof. Kobayashi says: “The precise understanding of the interaction between ligand and nanocrystal is important for basic nanoscience and the development of advanced photofunctional materials using nanomaterials. These include photocatalysts for breaking down persistent chemicals using visible light and photoconductive microcircuit cartridges for wearable devices.

“Indeed, the results of this study offer a promising way to improve the tunability and functionality of inorganic materials with aromatic molecules. This in turn could have a significant impact in the fields of basic nanoscience and photochemistry in the near future.”

Magazine reference:

  1. Daisuke Yoshioka, Yusuke Yoneda, I-Ya Chang, Hikaru Kuramochi et al. Quasi-reversible photoinduced displacement of aromatic ligands of semiconductor nanocrystals. ACS Nano. DOI: 10.1021/acsnano.2c12578