Enhancing Solid-State Phosphorescence in Pi Electronic Molecules

Anion binding and ion-pairing of organoplatinum(II) complexes with countercations increases solid-state phosphorescence 75 times

Enhancing Solid-State Phosphorescence in Pi Electronic Molecules

Pi Electronic molecules are luminescent materials with applications in photonics. However, they lose their luminosity in the solid state due to self-association. To address this issue, researchers from Ritsumeikan University, Japan introduced chloride ions and cations to dipyrrolyldiketone PtII complexes, creating a charge-by-charge arrangement. This innovative approach prevents self-association of Pi-electronic molecules, maintaining luminescent properties in the solid state. The study opens avenues for new emissive materials with potential applications in organic electronics and flexible displays.

Photoluminescent molecules, capable of absorbing and re-emitting light, play an important role in the development of technologies such as light-emitting diodes, sensors, and displays. Among them, ordered arrangements of Pi-electronic molecules such as crystals of organoplatinum(II) complexes, where a platinum(II) ion is coordinated by organic ligands in a square-planar arrangement, stand out for their applications in energy-efficient flexible displays.

However, their luminescence in the solid state is short-lived due to the interaction between excitons (bound electron-hole pairs) of neighboring molecules. To address this issue, bulky foreign molecules are introduced into the molecular structure to prevent or minimize the electronic interactions between molecules.

Using this strategy, a research team led by Professor Hiromitsu Maeda from Ritsumeikan University, Japan, recently enhanced the solid-state phosphorescence in multiple organoplatinum(II) complexes, increasing the phosphorescence by upto 75 times. “Spatially and electronically isolated ordered arrangement of emissive ?-electronic molecules is a principal point for the preparation of emissive solid-state materials. This concept can be used in materials for organic electronics, particularly organic light-emitting diodes for flexible displays,” explains Prof. Maeda.

In their study published in Chemical Science on December 5, 2023, the research team synthesized dipyrrolyldiketone PtII complexes consisting of four different C^N ligands. These molecules display strong phosphorescence in solution but show extremely weak phosphorescence in the solid state due to self-association. To enhance their luminosity in the solid state, the team introduced ion pairs consisting of a chloride anion and tetraalkylammonium countercations: TPA+ (tetrapropylammonium), TBA+ (tetrabutylammonium), and TPeA+ (tetrapentylammonium).

This resulted in ion-pairing assemblies consisting of chloride ion-binding PtII complexes and countercations. The chloride ions bind to the PtII complex via hydrogen bonds, while the cations form layers between the ?-electronic molecules. X-ray analysis confirmed the complex’s rigid structure, where PtII complexes are separated by cations in charge-by-charge arrangements.

By isolating the electronic molecules from each other, the researchers enhanced the luminescent properties of the organoplatinum(II) complexes in the solid state. Compared to the original anion-free states where the complex is not bonded to the chloride ion, the relative intensity of phosphorescence in Cl-binding PtII complexes with cations showed improvements ranging from 1% to 7.5%, a 75-fold increase over the original molecule.

The luminescence also lasts significantly longer, with certain ion-pairing assemblies achieving an emission lifetime nearly 200 times longer than the monomeric PtII complex. Theoretical studies using DFT calculations revealed that the charge-by-charge packing structure prevents the delocalization of the electron wavefunction over PtII complexes.

“To the best of our knowledge, such a room-temperature phosphorescence enhancement by anion binding and ion-pairing assembly has not been demonstrated thus far,” remarks Prof. Maeda.

Such a strategy can be used to design emissive materials and improve the phosphorescence of solid-state materials for novel applications. “The chemistry of ion-pairing assembly of charged electronic molecules is a new topic in a research area of supramolecular chemistry.

Understanding the interactions between charged species and the formation of assembled structures through research will affect in a further design and fabrication of functional ion-pairing assemblies such as efficient electric conductive materials, ferroelectric materials, and chiral transfer in ion pair and the ion-pairing assemblies exhibiting fascinating optical properties,” concludes Prof.  Maeda.

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