On Friday, the 16th of August 2024, Wai-Yeung Wong, Raymond, the Chair Professor of Chemical Technology and Dean of the Faculty of Science at the Hong Kong Polytechnic University, gave a private seminar at Imperial College London intended for CDT-ACM students entitled ‘Functional Metal-Based Nanomaterials from Metallopolymers’.

Sharing some of his work on metal-based nanomaterials with photofunctional properties and energy functions that can be derived from a variety of metal centers, from main group metals such as Sn and Pb to transition metals such as Fe, Ir, and Pt.

Phosphorescent transition metal complexes are a widely studied field for their photophysical properties, which have potential applications in areas such as organic light-emitting diodes (OLEDs), photocatalysis, and sensing with specific applications in biological labeling and probing of cells. My final year project as an undergraduate involved carrying out density functional theory and Hartree-fock calculations on a ruthenium complex, which falls under the category of phosphorescent transition metal complexes. Tris(bipyridine)ruthenium(II) chloride [Ru(bpy)₃]Cl₂ Emits strongly around 620 nm due to a metal-to-ligand charge transfer absorption at 452 nm. The absorption of light at this wavelength results in a long lifetime of 650 ns in water. The excited state takes a long time to relax back to ground state due to the forbidden triplet to singlet transition.

Wai-Yeung Wong, Raymond is no stranger to these metal complexes. He carried out a postdoc in Cambridge on metal-containing polymers, variations of metal centers, spacer groups, and auxiliary ligands. Focusing specifically on platinum, gold, and mercury complexes—fast forward 20 years, and Raymond is still working in the area—with a wide range of applications such as energy conversion with nonlinear optical properties.

In their Chemical Society Reviews paper “Organometallic acetylides of PtII, AuI and HgII as new generation optical power limiting materials” Gui-Jiang Zhou and Wai-Yeung Wong show off some benchmarking they had performed of polymetallaynes with optical limiting properties.

We also got some insight on the use of organometallic photovoltaics. Using the polymetallaynes W. Y. Wong and his team had developed for harvesting solar energy. An inside account of his paper on Organometallic Photovoltaics: A New and Versatile Approach for Harvesting Solar Energy Using Conjugated Polymetallaynes published in Accounts of Chemical Research in 2010. The use of a low-bandgap soluble platinum-acetylide polymer in PCSs was successfully used for efficient solar power generation. The team devised a hole-collection electrode consisting of indium tin oxide (ITO) with a spin-coating of poly(3,4-ethylene-dioxythiophene)/poly(styrene sulfonate) known as PEDOT/PSS. The metallopolyne was placed between this spin-coated layer and the aluminum, which acted as an electron-collecting electrode, creating a route for most of the photogenerated excitons to be split into free charge carriers. Despite its success, Reymond was quick to mention the infancy of this area of research at the time and pointed out that although platinum was the current research interest, this may be extended to other metals in the future and that more work needed to be done on increasing efficiency and to explore the many aspects of matellopolyyne as solar cells.

All of this work stems from an earlier paper by Wai-Yeung Wong published in Nature Materials back in May of 2007 titled “Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells“. This paper has received over 500 citations thanks to the synthesization of a soluble, intensely coloured platinum metallopolyyne with a bandgap of 1.85 eV. This is impressive given the range of lower bandgap polymers developed that don't match the efficiency of Poly(3-hexylthiophene) based cells.

Wai-Yeung Wong then proudly showed off the theory behind and the results of his 2012 Advanced Material paper on ‘A Polyferroplatinyne Precursor for the Rapid Fabrication of L10-FePt-type Bit Patterned Media by Nanoimprint Lithography’. Reymond was particularly proud of this advancement in patterning a new type of magnetic material with ultrahigh density with data recording capabilities. He initially showed some SEM images of nanoimprinted line array patterns, which show off the high conformity of a final pattern with the imprint mold using this novel technique with a feature size of 350 nm, before moving on to a comparison AFM image and MFM image side by side showing the adjustability of the FePt nanoparticles with respect to magnetization directions. In the paper, these adjustments are described as pointing “up” and “down”.

n his talk, Reymond went into much more detail about the practical applications of this effect for use in data storage, with the “up” and “down” being equated to binary ones and zeros.

Wai-Yeung Wong also described this fabrication method as being much better for commercial fabrication as opposed to previous fabrication methods. This is due to the development of the new polymeric material, which, by combining the merits of FePt nanoparticles and nanoimprinting lithography, can allow the fabrication process of BPM to be done in a single step with a relatively low cost.

Reymond went into detail on his research regarding CoPt nanoparticles and the characterization, which was published in 2017 in Nanoscale. The nanoparticles were prepared from a one-step pyrolysis method, and characteristics such as chemical ordering, morphology, and magnetic properties were reported using powder x-ray diffraction, transmission electron microscopy, and the physical property measurement system (PPMS), respectively.

Reymond remarked how nanoimprinting was a top-down approach, whereas self-assembly is a bottom-up approach, and ultimately they would like to combine the two techniques together. To date, there are only 4 papers in the literature regarding metallated graphene and their derivatives, and most use a top-down approach. The reason Raymond uses a bottom-up approach is because he can easily control properties and morphology by changing the base molecules. When you change the ligands, you can change the photophysical properties.

Reymond finished off his talk by going over some proposed topological structures of new metallated graphynes and graphdiynes; however, in our talk he was only able to go over type 1 and type 2 as research was still underway on the other types.

Type 1:

The most interesting part of the talk for me was type 1, which had been proven to exhibit optical limiting capabilities. Mercurated graphynes could be used for nonlinear optical applications where a standard Nd:YAG laser could be passed through a beam splitter with one laser beam passing through the graphene sample and the other beam going to a reference detector where it is shown that the graphene behaves as a reverse saturable absorber for lower power lasers and a saturable absorber for passively Q-switched lasers or mode-locking lasers with nanosecond or femtosecond output. More information can be found in his paper [10.1002/anie.202014835]

Reymond went on to show off some results related to a PQS Nd:YAG solid state laser and the pulse properties for different materials and how they compare to the graphynes Reymond and his team research. This research is still ongoing, and I will not be sharing the unpublished results.

A sincere thank you to professor Wai-Yeung Wong for taking the time to give us this talk and for sharing his exciting research with ambitious PhD students who will take inspiration from his scientific accomplishments.