Current Research Interests of Prof. Raymond W.Y. Wong
(a) Luminescent Organometallic Poly(aryleneethynylene)s and their Oligomers
Our research contribution in this area has been focused on synthetic metal-based multifunctional polymers which are rapidly emerging as interesting and useful materials given the immense structural diversity, range of properties and intermolecular interactions made possible by the metallic elements. These materials combine the processing advantages of polymers with the functionality provided by the presence of metal centers. Organometallic poly(aryleneethynylene) derivatives (or metallopolyynes) based on some heavy metal salts are recognized for their exciting functional properties, structural variability and important industrial applications and they can be used to sample triplet emission from soluble and processable materials. We have shown that the easily processed metallopolyynes can be used for optical limiting and transparency type applications. These materials also have ample applications as sensor protectors, as converters for light/electricity signals and as patternable precursors to magnetic metal alloy nanoparticles (Fig. 1). Fueled by these advances in synthesis and materials properties, new applications of metallopolymers would continue to emerge.
Important applications of metallopolyynes in various domains of materials science.
(i) Structure-Property-Function Relationships of Di-, Oligo- and Polymetallaynes
The growing interest in conjugated polymers stems from the fact that this new class of materials finds applications in polymer-based light-emitting diodes, solar cells and transistors. An identified problem in organic-based poly(aryleneethynylene)s is the lack of efficient routes to harvest the abundant triplet excitons over the singlet excitons. To address this issue, conjugated polymers containing transition metal atoms such as platinum have been widely studied by us as excellent model systems to explain aspects of the photophysics of excited states in this family of metallopolymers (Fig. 2). We have circumvented the problem of the triplet state being nonemissive by using platinum-containing poly(aryleneethynylene)s for which the spin-forbidden long-lived phosphorescence can be directly probed using optical methods since the incorporation of heavy atoms into the polymer backbone promotes efficient intersystem crossing that enables radiative decay from the triplet state. We have made unique contributions in elucidating the structure-property-function relationships of the triplet excited states at the molecular level and fine-tuning the triplet level over a wide energy range through optimization of the M, L and R groups. This allows a deep understanding of the relationship between triplet energy and the rate of nonradiative decay for this class of metallated polymers and the evolution of singlet and triplet excited states with chemical structure can be established. We have addressed the trade-off problem between the phosphorescence parameters and optical bandgaps. (see review articles in Coord. Chem. Rev.
, 2627; Dalton Trans.
, 4495; J. Inorg. Organomet. Polym. Mater.
General skeleton of transition metal polyyne polymer.
(ii) Mercury Alkynyls as Templates for New Organometallic Materials and Polymers
(iii) Optical Power Limiting (OPL) of Highly Transparent Metallopolyynes
With the rapid development of laser technology in the photonic era, the damage of human eyes, optical sensors, and sensitive optical components caused by exposure to powerful wavelength-agile laser pulses has driven many researchers to search for effective optical power limiters with high solubility, fast response speed, good linear transparency and high linear transmission. We have strong interest in developing solution-processable colourless homo- and heterometallic polyynes which reveal superior OPL/transparency trade-offs and outperform that of current state-of-the-art visible light-absorbing competitors such as C60
, metalloporphyrins and metallophthalocyanines (Fig. 3). (see Chem. Mater.
, 5209; Angew. Chem. Int. Ed.
, 6189; Adv. Funct. Mater.
, 963, Adv. Funct. Mater.
Metallopolyynes as highly-transparent OPL materials relative to some benchmark dyes.
(iv) Metallopolyynes as New Functional Materials for Organometallic Photovoltaics
Harvesting energy directly from sunlight using photovoltaic technology is increasingly recognized around the world as part of the solution to the growing energy challenge. Inorganic semiconductor materials typically exhibit higher solar power conversion efficiencies (PCEs) than their organic counterparts, but the use of inorganic solar cells remains limited due to the high costs of fabrication procedures. Bulk heterojunction solar cells based on conjugated organic polymers have been extensively studied due to their great potential for cost-effective photovoltaic devices. Although organometallic donor materials are commonly used in small-molecule solar cells, soluble conjugated organometallic polyynes have rarely been used in high-performance polymer solar cells. We recently invented a soluble, strongly-absorbing Pt(II) metallopolyyne which shows a low band gap of 1.85 eV. The bulk-heterojunction cells consisting of this metallated polymer and PCBM (1:4 blend ratio, PCBM = [6,6]-phenyl-C61
-butyric acid methyl ester) exhibited substantial photovoltaic responses with a high average PCE of 4.1 ¡Ó 0.9% (see Nature Materials 2007
, 521; Macromol. Chem. Phys. 2008
, 14.). The work represents an important conceptual breakthrough with new insight in this area. We have also described the tuning of polymer solar cell efficiency, as well as optical and charge transport properties, using different number of oligothienyl rings (see J. Am. Chem. Soc.
, 14372, Adv. Funct. Mater.
, 2824). Their photovoltaic behaviour depends to a large extent on the number of thienyl rings along the main chain. The methodology can be extended to efficient near-infrared photocurrent spectral responses (up to 900 nm) for extremely low-bandgap metallopolyynes (see Dalton Trans.
(v) Functional Metallopolymers as Novel Precursors to Magnetic Metal Alloy Nanoparticles
We are very interested in extending our research profile to the areas of nanotechnology and nanoscience based on metallopolymers. One of the latest applications of metal-containing polymers is as precursors for the synthesis of metal nanoparticles (NPs) via thermal or radiation treatment. As metallopolymers can be readily shaped and patterned using various lithographic techniques, they offer the prospect of access to patterned arrays of metal NPs with control of composition and density per unit area, which are crucial factors for device and catalytic applications. To date, reports of patterned FePt NP thin films are extremely rare. Among the various lithographic techniques, electron-beam lithography (EBL) and UV photolithography (UVL) are promising candidates for the next-generation lithography. We have recently developed an elegant one-step synthesis of ferromagnetic FePt NPs from a novel air- and moisture- stable, film-forming bimetallized polyferroplatinyne polymer precursor, which can be utilized directly as a negative-tone resist to fabricate FePt NP array patterns by both EBL (Fig. 4) and UVL (see Angew. Chem. Int. Ed.
, 1255). Future work will focus on the generalization of this approach to other magnetic metal alloy NPs and the creation of patterned magnetic films for the fabrication of spintronic switching devices (e.g. magnetoresistive random access memory (MRAM)) and devices for high-density magnetic data storage in which the convenient and rapid patterning of magnetic NPs is highly desirable.
Arrays of ferromagnetic fct FePt NP micropatterns can be fabricated from thin films of a novel air- and moisture-stable bimetallic polyferroplatinyyne precursor.
(b) Multifunctional Organometallic Electrophosphors for High-Performance Color-Tunable and White Light OLEDs
Organic light-emitting diodes (OLEDs) show great promise of revolutionizing display and lighting technologies in the scientific community. One successful approach for improved device efficiency has been to maximize the electron-hole recombination using metallophosphors that emit from the triplet excited state. Traditional room-temperature phosphorescent dyes are monofunctional materials working only as light-emitting centers but other key issues including charge generation and transport remain to be addressed in the electroluminescence. We aim at developing new synthetic strategies for multifunctional organometallic phosphors, which integrate both luminescent and charge carrier injection/transport functions into the same molecules so that they perform most, if not all, of the necessary functional roles (viz. photoexcitation, charge injection and transport as well as recombination) for achieving high-efficiency devices. Considerable focus is placed on the design concepts towards the tuning capability of charge-transport characteristics and phosphorescence emission colour of this prominent class of metallophosphors (Fig. 5). In particular, the latest endeavor in accomplishing novel triplet emitters with enhanced charge injection/charge transport of both hole and electron carriers can provide good implications regarding their possible routes for future research development in the field. The work can offer an attractive avenue to developing metal phosphors with optimized efficiency/color purity trade-offs for pure red and white light emissions (see review articles in J. Mater. Chem.
, 4457; Coord. Chem. Rev.
While lighting occupies a significant part of the world¡¦s energy consumption, with a large share still used up by inefficient incandescent lamps, the need for more efficient and environmentally friendly solutions to the impending world energy shortage has stimulated extensive research interest for new generation ambient lighting sources. White light OLEDs (WOLEDs) show promise as key role-players because of their low operating voltage and high brightness obtained. We are particularly interested in developing metallophosphors for simple two-color WOLEDs and single-layer polymer-based WOLEDs.
WOLED and phosphorescent dyes emitting different colours.