A curious interplay in the films of N-heterocyclic carbene Pt(II) complexes upon deposition of alkali metals.

Anna A Makarova,Denis V Vyalikh,Anastasia I Solomatina,Elena V Grachova,Alexander V Fedorov,Vera S Neudachina,Clemens Laubschat,Sergey P Tunik,Dorota Niedzialek,Oleg Yu Vilkov,Simon Sonntag

doi:10.1038/srep25548

Abstract

The recently synthesized series of PtII complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned PtII complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination PtII centre to Pt0 and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system “Pt complex-alkali metal ion”; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the PtII complexes for a variety of biomedical, sensing, chemical, and electronic applications.

Highlights

Within the past decades, a lot of attention has been paid to the investigation of molecular organic semiconductors (OSCs) to be used in various optical and electronic devices, including organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), organic photovoltaic cells (OPVCs), and organic spintronic devices[1,2,3,4]
This is because the system has two types of nitrogen atoms (N1 and N2) in the N-heterocyclic carbene (NHC) and N^C ligands trans-located in the PtII coordination sphere
The N1 feature with a binding energy (BE) of 400.9 eV corresponds to nitrogen atoms in the NHC ligand[32], while the component N2 at 400.2 eV can be assigned to the electron emission from the nitrogen atoms in the N^C ligand

Summary

Results and Discussion

Growth of the PtII complex film in UHV. First, we applied the well-known PVD approach for growing high quality molecular layers on solid state substrates to obtain the [Pt(N^C)(NHC)Br] complex film on Au(111) under UHV conditions. Considerable increase in the electron density localized at the nitrogen and platinum sites clearly points to charge transfer from the alkali metal atoms to the complex molecule resulting in formation of an ion pair: radical anion and positive counterion M+ (where M = Li, Na, K, Cs). Shifts of the Br 3d peak towards higher BEs. Presumably, during the deposition, the gradually increasing number of alkali metal atoms surrounding the [Pt(N^C)(NHC)Br] molecules leads to the formation of a supramolecular structure within the molecular film, which is held together by a network of electrostatic interactions between the alkali cations and bromide ligands in the platinum complex. Under exposure of the doped system to oxygen the “Pt complex-alkali metal ion” structure is able to partly recover to give the initial PtII complex

Conclusion

Author Contributions

Additional Information