(Invited) Emissive Charge-Transfer Excited-State between Organic Conjugated Molecule and Two-Dimensional Inorganic Nanosheet
Authors: Umeyama Tomokazu 
Published: December 2024

The unique properties of two-dimensional (2D) materials have boosted intensive interests in combining distinct 2D materials into organic-inorganic heteronanostructure interfaces for device and sensor applications. The hetero-interfaces, integrating atomically thin inorganic materials with an unlimited variety of organic molecules, provide an ideal platform for broader, superior, and on-demand functional applications by incorporating customized organic molecules that particularly exhibit excellent optical and optoelectronic properties. Especially, unique electronic and optical properties discovered for mono- or few-layered transition metal dichalcogenides, represented by MoS2, have rendered them as suitable platforms to host organic π-conjugated compounds. As such, there have been investigations on photodynamic processes at interfaces between MoS2 and various π-conjugated compounds.1 However, previously reported hybrid interfaces were constructed by amorphous-deposition or covalent linkage using long, flexible bridges and thus, the inhomogeneous interfacial structures make it difficult to elucidate the relationship between the interface structure and photodynamics.

In this study, we designed and synthesized a hetero-nanostructure interface between an organic π-conjugated molecule (pyrene, Py) and 2D inorganic nanosheet (mono- or few-layered MoS2) with a well-defined bridge at a molecular level (Figure 1).2 As the bridge between Py and MoS2 basal plane, we employed N-benzylsuccineimide structure to form Py-Bn-MoS2.

Spectroscopic measurements revealed that the locally-excited (LE) state of pyrene in Py-Bn-MoS2 efficiently generates a long-lived charge-transfer (CT) excited state that straddles the interface of pyrene–MoS2 nanosheet. The CT excited-state is highly emissive (0.68) and relaxes to the ground state without forming the complete charge-separated (CS) state. To the best of our knowledge, this is the first observation of unusually intense emission from the CT excited state at well-defined hetero-nanostructure interface of organic π-conjugated molecules and 2D inorganic nanosheets. Theoretical studies elucidated the interaction of MoS2 vacant orbitals with the pyrene LE state to form the CT excited-state showing distinct solvent dependence of the emission energy. These findings provide important implications for manipulating the LE, CT, and CS states at the interface of photoactive organic molecules and inorganic semiconducting nanosheets.

References

Baek, J.; Umeyama, T.; Choi, W.; Tsutsui, Y.; Yamada, H.; Seki, S.; Imahori, H. Formation and Photodynamic Behavior of Transition Metal Dichalcogenide Nanosheet-Fullerene Inorganic/Organic Nanohybrids on Semiconducting Electrodes. Chem. Eur. J.
2018, 24, 1561−1572.

Umeyama, T.; Mizutani, D.; Ikeda, Y.; Osterloh, W. R.; Yamamoto, F.; Kato, K.; Yamakata, A.; Higashi, M.; Urakami, T.; Sato, H.; Imahori, H. Emissive Charge-Transfer Excited-State at Well-Defined Hetero-Nanostructure Interface of Organic Conjugated Molecule and Two-Dimensional Inorganic Nanosheet. Chem. Sci.
2023, 14, 11914−11923.

Figure 1