PROFILE Profile of Vivian W.-W. Yam

Farooq Ahmed Science Writer

Primary school children do not typically Che worked on metal-mediated and metal- handle droplets of the toxic element mercury catalyzed oxidation chemistry with the ele- with bare hands. However, for a young ments ruthenium and osmium. These metals Vivian Yam, an early experience with a bro- can exist in a high valence or high oxida- ken thermometer sparked a career synthe- tion state, reflecting the number of free sizing metal complexes for use in light- electrons in the elements that are capable of harvesting and light-emitting materials. Yam, forming bonds. a recently elected foreign associate of the In the 1980s, a decade before Hong Kong National Academy of Sciences and a chemis- formed its Research Grants Council to fund try professor at the University of Hong Kong, science, the University of Hong Kong lacked says, “I was amazed by mercury’s ability to resources for purposes unrelated to teaching. flow and coalesce—unlike most metals we’re Instead, the institution relied on the in- exposed to.” Over the past three decades, genuity of professors such as Che who had through a combination of molecular design been trained by prominent chemists from and physical inorganic chemistry, Yam has around the world; Che studied with Caltech helped create new classes of metal-containing chemist Harry Gray. Yam recalls that “many chromophores and luminophores—molecules of the chemicals we ordered were not allowed that absorb or radiate light and some of to be transported by plane, and so they ar- which may help mitigate the growing global rived months later by ship from the United demand for energy. Yam’s Inaugural Article States or Europe.” As a result, the Chinese (1) explores how a platinum(II)-containing chemists had to carefully schedule experi- Vivian W.-W. Yam. Image courtesy of the compound assembles and forms varied 3D ments. “Occasionally, you’d order a chemical University of Hong Kong. structures with individual spectroscopic and then by the time it arrived half a year properties. later you wouldn’t remember what it was as suitable models because they belong to the for,” Yam adds with a laugh. Chemist from Hong Kong same group as iron and have similar chemical Born in Hong Kong when the region was Oxidation and Life properties. Being less reactive than iron, under British rule, Yam attended an Anglican Despite the challenges, Yam remained at the however, ruthenium and osmium are more grammar school. Her mother was a home- University of Hong Kong for her doctorate, amenable to experimentation and studies of maker, and her father, a civil engineer. “I studying with Che. She met her husband reaction mechanisms. really admired him and still remember him Patrick Shing-Tat in Che’s laboratory. To- In Che’s laboratory, Yam crystallized a ru- using the slide rule back in the days when gether, the couple worked on high-valent thenium precursor and demonstrated that you didn’t have a calculator,” she says. How- metal and oxidation chemistry, the study of the compound can be used in the synthesis ever, Yam’s parents did not urge her to enter which, Yam notes, is inspired by a funda- of a ruthenium(V)–oxygen multiple-bond professional schools or study medicine. She mental biological reaction. complex (2) (the number in parentheses says her curiosity with the natural world and “Interest in this type of chemistry origi- refers to the valence of the metal). “We were an analytical disposition prompted her to nates from our understanding of the enzyme the first to obtain this species back in the late pursue science. In her secondary school cytochrome P450—aformofitexistsin 1980s,” she says. “It had important implica- biology teacher, Yam found another in- nearly every species and even in some vi- tions in proton-coupled electron transfer, spiration. “She embodied the Chinese ruses.” Cytochromes use a metal ion, often which is at the heart of reduction and oxi- commitment to one’s field—that whatever iron, to help metabolize chemical bonds and dation reactions, like water oxidation for you do, you should respect your work, enjoy harness their energy. “The enzyme’s ability to example.” it, and be very much devoted to it. Even oxidize unactivated carbon–hydrogen bonds when she was pregnant, she taught until the in particular is a very difficult task, but cy- Department Building last moment!” tochromes do so under the typically mild Unlike many academic scientists, Yam did Yam majored in chemistry at one of Hong conditions in the body. Life may not exist not begin a postdoctoral fellowship after re- Kong’s two universities of the time, the as we know it without these enzymes and ceiving her graduate degree. Instead, in 1988, University of Hong Kong. She also competed their reactions,” says Yam. she accepted a junior faculty position at the on the university’s badminton team, playing Oxidation chemistry is also of crucial then-new Department of Applied Science at several times a week. Yam conducted re- importance to the conversion of sunlight into

search with inorganic chemist Chi-Ming Che solar fuels or for splitting water, a process This is a Profile of a recently elected member of the National while at the University of Hong Kong, and he similar to a key reaction in photosynthesis. Academy of Sciences to accompany the member’s Inaugural remains influential in her career. The metals ruthenium and osmium emerged Article on page 7986.

7964–7966 | PNAS | May 14, 2013 | vol. 110 | no. 20 www.pnas.org/cgi/doi/10.1073/pnas.1307201110 Downloaded by guest on September 30, 2021 PROFILE the City Polytechnic of Hong Kong (now the women interested in science has been: “Pur- The work of Yam and others on organo- City University of Hong Kong). The de- sue your dreams, and do not be burdened by metallic luminophores has contributed to the partment, an ambitious attempt to bridge social or familial prejudices.” development of organic light emitting diodes several physical and life sciences, had little (OLEDs). Similar to standard light emitting precedent in an institution that lacked chem- Illuminating Chemistry diodes, such as those found in car headlamps istry, biology, and physics departments. Like her mentor Che, Yam traveled widely to and traffic signals, OLEDs convert energy Yam recalls, “A colleague and I literally amassskillstofurtherherresearch.She into light efficiently and require little power started from scratch. In the library, there paid a short visit to Caltech in the late 1980s to achieve the same level of brightness as wasn’t a single book on chemistry. We split to perform spectroscopic measurements in ’ incandescent lamps. OLEDs can also be in- the chemical catalog and ordered material for Harry Gray s laboratory and spent the sum- corporated into flexible sheets of glass and both teaching and research purposes: The mer of 1990 studying photochemical prop- plastic; thin, lightweight layers produce suf- very first beaker, the first kilograms of so- erties in microenvironments like spherical ficient luminosity for smartphones, portable dium chloride, the first gas chromatograph, micelles with David Whitten, then at the tablet computers, and even large flat-screen and on and on. We built the infrastructure University of Rochester in New York. Yam televisions. Yam has helped develop gold(III) and facilities by hand.” Although it was a spent the summers of 1991 and 1992 in phosphorescent materials as well as solution- daunting process, she says that the effort is England, learning from another distinguished processable gold(III) phosphors for OLED among her fondest memories as a young chemist, Nobel laureate Geoffrey Wilkinson “ applications (6). Solution processing allows scientist. of Imperial College, London. Iwantedto materials to be easily deposited onto glass, For the next two years at the City Poly- study organometallic synthesis because not plastic, and other surfaces. technic of Hong Kong, Yam taught under- a lot of people were working with excited ” The novelty of these compounds, Yam graduates, and in her limited free time, states in these compounds, Yam explains. fi emphasizes, pales in comparison with their endured long commutes to conduct research Notoriously dif cult to create because they potential applications. “Energy consumption using the facilities in her former advisor involve air- and moisture-sensitive chemicals, is perhaps the most pressing issue we face Che’s laboratory. Her work also took a new organometallics, such as the now-banned today. Nineteen percent of the total power direction as she began studying the lumi- gasoline additive tetraethyllead, consist of used on the earth annually, almost 3 tera- nescence of complexes with metal–metal metal centers bound to carbon-containing watts, is due to lighting. These organometal- interactions. ligands; their study bridges organic and in- lics have the potential to reduce that sig- “This research originated from Che’sas- organic chemistry. nificantly and help meet our ever-increasing sociation with Harry Gray at Caltech,” Yam The visits with Wilkinson helped launch ” ’ demands. She explains that the internal notes. Che and Gray had been investigating Yam s career in organometallic luminophores quantum efficiency of these metal-containing a system called platinum pop, a chemical and metal alkynyl chemistry. Alkynyls are “ ’ – materials can reach 100% in theory. That s complex with two closely interacting plat- complexes that contain a carbon carbon what aroused our and other’s interest,” inum(II) centers. The complex is held to- triple bond; the linear geometry of the bond fi she says. gether by covalent bonds and noncovalent, gives the complexes de ned structures and She adds: “[Researchers] have to start us- – metal metal bonding interactions, which allows them to dissolve in common solvents. ing cheaper, more abundant, and robust have strengths similar to those of hydrogen Organometallic materials that contain strong materials if we want this work to drive bonds. The interactions between the plati- electron-donating alkynyl groups often have change globally, reduce energy consumption, “ ” num centers create an excited or elevated characteristic spectroscopic properties. The and repair the environment. We’ve got to be energy state that produces a strong lumi- luminescence originates from the valence of innovative, and work across disciplines— nescence in the visible region. Yam pursued the metals as well as the interactions between chemistry, physics, biology, engineering, – gold(I), which also luminesces due to metal the metal centers and the carbon atoms. etc.—tosolvetheproblemsthatwearefacing metal interactions. Upon returning to Hong Kong, Yam and on our planet.” In 1990, she returned to the University of colleagues began developing new classes of Hong Kong to join the faculty and continue organometallic luminophores. Many of the Sensing Changes working on luminescence. She has since compounds showed long-lived luminescence Yam has also been studying another use of remained at the university and now holds and produced excited states with lifetimes in luminescent metal complexes—as biological both the chair professorship of chemistry and the microsecond range—orders of magnitude or environmental sensors. Her group at the the Philip Wong Wilson Wong professorship longer than that of green fluorescent protein University of Hong Kong developed a plati- of chemistry and energy. Along the way, in (GFP) and other fluorescent materials (3). num(II) alkynyl-containing compound with 2001, she became the youngest member “We found that, by varying the number and square-planar geometry. The organometallic elected to the Chinese Academy of Sciences, type of metal centers, gold(I) versus the iso- self-aggregated in solution, and its spectro- breaking the record held by her mentor Che. electronic copper(I) for example, we could scopic properties differed depending on the “Working in Hong Kong,” Yam says, “you affect not only the excited state reduction– way it assembled; it displayed colors from sometimes feel isolated from mainland China oxidation properties but could also tune lu- red to emerald green and blue (7). and the rest of the scientific community. So minescence colors and behavior,” she adds. “Water-soluble derivatives of this and the honor from the Chinese government In a gold(I)ÁÁÁgold(I) compound, the binding similar complexes could function like GFP, along with my membership in the National of potassium ions instigates conformational which is now widely used to detect mRNA Academy of Sciences and in 2011 the L’Oréal- changes that produce low-energy red and expression or protein levels,” she explains. UNESCO for Women in Science Award were near infrared emissions (4). By swapping the “Our compounds are much smaller, less recognitions I’d never dreamed of.” Yam atom that bridged the gold(I) centers, the susceptible to background interference, and cherishes her role as a mentor for female researchers further tweaked the color of have increased emission lifetimes.” Yam used scientists. Yam says her advice to young the luminescence (5). her platinum(II) alkynyl derivative as an

Ahmed PNAS | May 14, 2013 | vol. 110 | no. 20 | 7965 Downloaded by guest on September 30, 2021 indicator to bind short sequences of DNA (8) To these carbon compounds, her group tion of the compound’s self-assembly, her and also to detect the enzyme lysozyme as it attached metal-containing chromophores— group discovered, can lead to folding and bound to a specificnucleotidesequence(9). small clusters of molecules that absorb light. unfolding events, which emerge as spectro- “The simple act of binding produced a rear- The chromophore’s excited state sensitized scopic changes. rangement of the platinum compound in so- the resulting material. When the researchers Yam believes that understanding and lution and a different luminescence,” she shined different wavelengths of light, the manipulating 3D molecular structures holds adds. “So we can easily see the results.” metal—diarylethene complex shifted confor- a key to harnessing synthetic materials. mation, reversibly changing between struc- To support that notion, she takes a cue from Materials of the Future “ – turally different forms. Yam says that this the natural world: In proteins, whatever the In more recent work, Yam exploited metal kind of photochromism, as the phenomenon amino acid sequence, they would not func- metal interactions in the organometallic is called, “is ideal for any system that relies on tion without the correct secondary, ter- compounds to create luminescent materials binary on- and off-states like digital memory” tiary, and quaternary conformations. We in nontraditional forms. Her group made (13). In her Inaugural Article, she examines need to better control structure in the ra- metallogels (10, 11) and created spherical the noncovalent, metal–metal interactions tional design of new materials so that they vesicles and cylindrical rod nanostructures — that stabilize the 3D structures of a plati- endupwiththedesiredfunctionsand that could interconvert each with different num(II)-containing compound. Modula- performance.” colors and luminescent properties (12). “I’m a synthetic chemist, and what we like to do is to create new molecules,” she notes. “How- ever, it’s equally important to understand 1 Leung SY-L, Lam WH, Yam VW-W (2013) Dynamic scaffold 8 Yu C, Chan KHY, Wong KMC, Yam VWW (2006) Single-stranded of chiral binaphthol derivatives with the alkynylplatinum(II) nucleic acid-induced helical self-assembly of alkynylplatinum(II) how these molecules interact with each other terpyridine moiety. Proc Natl Acad Sci USA 110:7986–7991. terpyridyl complexes. Proc Natl Acad Sci USA 103(52): and understand the factors that govern 2 Che CM, Yam VWW, Mak TCW (1990) A novel monooxo- 19652–19657. ruthenium(V) complex containing a polydentate pyridyl amine ligand. 9 Yeung MCL, Wong KMC, Tsang YKT, Yam VWW (2010) Aptamer- their interactions. This way, we can provide III Syntheses, reactivities, and X-ray crystal structure of [Ru (N4O)(H2O)] induced self-assembly of a NIR-emissive platinum(II) terpyridyl guiding principles on how to control their (ClO4)2. J Am Chem Soc 112(6):2284–2291. complex for label- and immobilization-free detection of lysozyme and assembly.” 3 Yam VWW (2002) Molecular design of transition metal thrombin. Chem Commun 46(41):7709–7711. alkynyl complexes as building blocks for luminescent metal-based 10 Tam AYY, Wong KMC, Yam VWW (2009) Unusual luminescence Yam has also been designing molecular materials: Structural and photophysical aspects. Acc Chem Res enhancement of metallogels of alkynylplatinum(II) 2,6-bis(N- materials that react to visible light in atypical 35(7):555–563. alkylbenzimidazol-2′-yl)pyridine complexes upon a gel-to-sol phase ways. She began by examining classes of 4 Yam VWW, Li CK, Chan CL (1998) Proof of potassium ions transition at elevated temperatures. J Am Chem Soc 131(17): by luminescence signalling based on weak gold–gold interactions 6253–6260. photochromic compounds including diary- in dinuclear gold(I) complexes. Angew Chem Int Ed 37(20): 11 Tam AYY, Wong KMC, Wang GX, Yam VWW (2007) lethenes, which reversibly change from one 2857–2859. Luminescent metallogels of platinum(II) terpyridyl complexes: structural form to another. Unlike alkynyls, 5 Lee TKM, Zhu N, Yam VWW (2010) An unprecedented luminescent Interplay of metal···metal, π–π and hydrophobic–hydrophobic polynuclear gold(I) μ(3)-sulfido cluster with a thiacrown-like architecture. interactions on gel formation. Chem Commun 2007(20): these molecules contain a carbon–carbon JAmChemSoc132(50):17646–17648. 2028–2030. double bond that bridges two separate aro- 6 Tang MC, Tsang DPK, Chan MM, Wong KMC, Yam VWW (2013) 12 Po C, Tam AYY, Wong KMC, Yam VWW (2011) Supramolecular “ Dendritic luminescent gold(III) complexes for highly efficient solution- self-assembly of amphiphilic anionic platinum(II) complexes: A matic groups. Think of a bicycle where the processable organic light-emitting devices. Angew Chem Int Ed Engl correlation between spectroscopic and morphological properties. J wheels are rings of carbon atoms or occa- 52(1):446–449. Am Chem Soc 133(31):12136–12143. sionally nitrogen and sulfur and the frame in 7 Yam VWW, Wong KMC, Zhu N (2002) Solvent-induced 13 Yam VWW, Ko CC, Zhu N (2004) Photochromic and – — aggregation through metal–metal/π–π interactions: Large luminescence switching properties of a versatile diarylethene- between is a carbon carbon double bond solvatochromism of luminescent organoplatinum(II) containing 1,10-phenanthroline ligand and its rhenium(I) complex. J that’s similar to a diarylethene,” Yam explains. terpyridyl complexes. J Am Chem Soc 124(23):6506–6507. Am Chem Soc 126(40):12734–12735.

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