Search

Metal-free chromophores that efficiently generate triplet excited states represent promising alternatives with respect to transition metal-containing photosensitizers, such as those featuring metal-to-ligand charge transfer excited states. However, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy metal-free triplet photosensitizers. We demonstrate that thionation of the lone C═O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O2 quantum yields at 1270 nm (ΦΔ = 0.78-1.0). Electronic structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in the proximity of a slightly higher energy S2 (π → π*) excited state. The distinct character in each of the two lowest-lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S1 → T1 intersystem crossing was thereby facilitated in this manner through spin-orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C═O moiety residing within its structure, valuable for the design of metal-free triplet photosensitizers.

A cyclometalated Ir(III) diimine complex [Ir(NBI)2(phen)]PF6, NBI = 1,8-naphthalenebenzimidizole and phen = 1,10-phenanthroline, exhibits excellent photostability as a sensitizer in a three-compone...

We present a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal-organic chromophores of the generic molecular formula [Re(phen)(CO)

In this investigation, the synthesis and photophysical properties of a series of new chromophores featuring Pt(II) σ-bonded to perylenediimide (PDI) cores are reported. A Pt(PPh3)2X (X = Cl, Br) moiety was attached to PDI in either the ortho or the bay position (2- or 1-positions respectively) or a Pt(PPh3)2 subunit was used to bridge two bay positions (1- and 12-positions) forming a Pt(II) cyclometalate. Through a combination of steady-state and transient absorption and photoluminescence spectroscopy, the excited-state dynamics of these molecules were revealed, indicating that the Pt atom location on the PDI has a substantial impact on observed intersystem crossing (ISC) rates. The ISC time constants for the bay-substituted and cyclometalated PDIs are between 2.67 and 1.29 ns, respectively, determined by the singlet fluorescence decays from the initially populated singlet excited states. In the case of the ortho-substituted PDI, ISC to the triplet state occurs on the ultrafast time scale with a time cons...

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Förster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.

As 2016 draws to a close, C&EN is taking stock of the year’s biggest moments in chemistry. In the pages that follow, we reflect on some of the year’s biggest chemistry headlines—the impact of a lab explosion at the University of Hawaii, the expected fallout from the U.S.’s national election, recent consolidation in the global agricultural chemicals industry, and many more. We also single out some of the year’s most notable research developments—including the discovery of a bacterium capable of biodegrading the ubiquitous plastic polyethylene terephthalate, the identification of a way to derivatize methylene C–H bonds catalytically and enantioselectively, and the proliferation of wearable personal sensors. Finally, because it often takes years for a research advance to reveal its true impact, we take a look back even further, to what’s become of some of our year-end picks from a decade ago.

IN THE DRUG INDUSTRY, as in fashion, occasionally what’s old becomes new again: The latest retro rage in the pharmaceutical world is deuterium substitution, a drug-design strategy that has faded in and out of vogue over the years. Now, firms large and small are banking on its potential to improve existing drugs. The strategy aims to take advantage of the edge deuterium has on hydrogen in terms of atomic mass. Carrying a neutron that hydrogen lacks, deuterium forms bonds with carbon that vibrate at a lower frequency and are thus stronger than C–H bonds. If a particular C–H bond in a drug molecule is known to be readily broken during metabolism, swapping that hydrogen for a heavier deuterium can in some instances slow the process down. So, the theory goes, heavy-hydrogen versions of drugs may sometimes last longer in the body and outshine the all-hydrogen originals. Biochemists have long exploited this phenomenon—dubbed the kinetic isotope ...

ONE YEAR AFTER HURRICANES Katrina and then Rita pummeled the Gulf Coast, the storms' devastation is still painfully apparent. In New Orleans, thousands of piles of trash dot the city, levee repair is incomplete, and few people have returned to their homes, particularly those in the Lower 9th Ward and in St. Bernard Parish. Outside the city, temporary trailer parks and still-empty houses with blue tarps in place of roofs remain a common sight up and down the Gulf Coast. Last November, C&EN described the plight of students, professors, and chemical company employees affected by the storms (C&EN, Nov. 21,2005, page 13). Scattered across the country in the aftermath of the hurricanes, they recounted tales of devastation, loss, and fear. Today, nearly all of them have returned to the area. But most are still coping with the havoc the storms wreaked on their personal and professional lives. "The flooding that resulted from the levee breaks ...

IT MIGHT BE THE TEST OF A TRUE scientist: Is your idea of the perfect summer trip a week relaxing on a sun-kissed beach or a week holed up in an otherwise deserted prep school campus in New England, debatingyour latest results with 100 of your colleagues and competitors? Despite their reputation for cafeteria food and dorm-style accommodations, the Gordon Research Conferences (GRC) remain the highlight of many a scientist's summer. Now in their 75th year, these much-loved conferences continue to shape both the lives of scientists and the frontiers of scientific research. The conferences have a long and storied history at the leading edge of science. Enrico Fermi described the slow-neutron process of atomic disintegration to a Gordon conference audience in 1936, a decade before the atomic bomb was unleashed on the world. The implications of recombinant DNA technology first came to the fore at the 1973 Gordon conference on nucleic acids. More recently, ...

BEFORE YOU READ ANY FURTHER, THINK OF A FEW organohalogen compounds commonly found in the environment. If you're like most people, your first thought will be man-made pollutants such as chlorofluorocarbons and dioxins. But in fact, nature's inventory of organohalogens is dizzyingly complex: Some 4,500 halogenated natural products, many with promising medicinal properties, have been identified to date. "And that list grows by more than 100 new natural organohalogens per year," says chemist and organohalogen aficionado Gordon W. Gribble of Dartmouth College. The ways in which bacteria, fungi, marine organisms, animals, and even humans produce these halogenated molecules have long intrigued chemists. A variety of chemical halogenation methods are available to the synthetic chemist, but none are so exquisitely specific as the enzymes nature uses to install halogen atoms, notes Christopher T. Walsh, a professor of biological chemistry and molecular pharmacology at Harvard Medical School. He and other scientist...

EARLIER THIS MONTH, PRESIDENT GEORGE W. BUSH traveled to India with an ambitious agenda, including expanding U.S.-Indian scientific cooperation. Bush and Indian Prime Minister Manmohan Singh announced the establishment of a new $30 million science and technology commission that will fund joint research in promising areas such as biotechnology. This bilateral push to promote ties between U.S. and Indian scientists comes at a time when India is vying to vault itself onto the world stage. Fueled by a highly trained workforce of synthetic chemists, India is seriously challenging the U.S. and Europe for supremacy in producing active pharmaceutical ingredients, advanced intermediates, and other fine chemicals. It now hopes to do the same in innovation-intensive sectors, including drug discovery, biotechnology, and materials science. The repatriation of Indian chemists from abroad is accelerating the new emphasis on innovation, as are increased links between academic and industrial scientists in the country. But...

RIKEN, JAPAN'S PREMIER SCIENtific research institute, is celebrating its 88th birthday this year. That milestone may not strike Westerners as noteworthy, but in Japan, "one's 88th birthday is a particularly happy and joyous occasion," says Ryoji Noyori, RIKEN's president. To explain, he draws the characters for 88 next to the one that signifies rice—a venerated staple that symbolizes happiness and longevity in Japan—and points to their resemblance. "RIKEN is among the most important research institutes not only in Japan but also all over the world," says Noyori, who was recruited to lead the institute in 2003 after a long career at Nagoya University that culminated in his receipt of the 2001 Nobel Prize in Chemistry. But he admits that RIKEN faces significant challenges, including how to reverse its historical isolation from the rest of the world. The institute was created in 1917 as the Institute of Physical & Chemical Research, the Japanese name of which ...

THE SIGHT OF THE SPANKING NEW single-crystal X-ray diffractometer sitting next to a microscope for mounting crystals makes you think you could be at a top university in the U.S. But walk outside, where students congregate in the shade of an ackee tree to escape the midday heat, and you can see Jamaica's Blue Mountains in the distance, and you'll know you are most certainly not in the U.S. This is in fact the oldest campus of the University of the West Indies (UWI), an unusual regional research institution serving a collection of small, English-speaking states in the Caribbean, including Anguilla, Antigua & Barbuda, the Bahamas, Barbados, Belize, the British Virgin Islands, Cayman Islands, Dominica, Grenada, Jamaica, Montserrat, St. Christopher & Nevis, St. Lucia, St. Vincent & the Grenadines, Trinidad & Tobago, and the Turks & Caicos. UWI began in Mona, Jamaica, in 1948 as an arm of the University of ...

BEFORE THE 1930S, THE GEMS OF CHOICE FOR ENGAGEment rings included opals, rubies, and sapphires. But in the 1940s, De Beers—the South African mining firm that controls the majority of the world's d...

CONSTRUCTION ON THE MIDDLE East's first synchrotron began this summer at Al-Balqa' Applied University in Jordan. The facility, slated to open in 2007 or early 2008, is known as SESAME, for Synchrotron light for Experimental Science and Applications in the Middle East. It will produce intense light beams for cutting-edge environmental, biomedical, and materials research. "SESAME will provide an environment for stimulating scientific activity and cooperation" in the Middle East, according to Zehra Sayers, chair of SESAMES scientific committee and a professor of engineering and natural sciences at Sabanci University, in Turkey But more important, Sayers tells C&EN, SESAME "will help to alleviate political tension among scientists and promote peace in the region." Synchrotrons send electrons hurtling through a circular tunnel at speeds close to that of light. The accelerated electrons give off light of varying energies—including infrared, ultraviolet, and X-rays—that scientists can harness to probe the molecu...

LAST DECEMBER, FORMER PRESIDENT JIMMY CARTER visited the Donald Danforth Plant Science Center in St. Louis. "I don't know of any other research center that I have ever visited that has a greater potential contribution to the well-being of the world," he remarked. "Within not too many years, the work that you do here will touch every human being who lives on Earth in a beneficial way" Research at the not-for-profit Danforth Center focuses on providing food for the world's rapidly expanding population in an environmentally friendly and sustainable way. Its mission extends beyond the food, nutritional, and health needs of the developed world: The center has dedicated 10% of its resources and facilities to research specifically related to the needs of agriculture in developing countries. This mission has drawn a diverse group of biochemists, chemists, geneticists, and plant pathologists to the center. Although they share a common mission, these investigators work on projects that range ...

HARVARD UNIVERSITY organic chemist William von E. Doering is a chemical legend. He is credited with, among other things, paving a route to quinine, pioneering carbene chemistry, and nailing down th...

IN THE WAKE OF LAST YEAR'S GULF COAST hurricanes, scientists from numerous governmental agencies and universities rushed to New Orleans to assess how the storms had affected the quality of the city...

IT'S NOT EVERY DAY THAT ARCHAEOLogists can claim provenance of a valuable drug discovery tool. But it was archaeologists' quest for more accurate dating of old bones that led to the development of ...

ROBERT ALONZO WELCH IS A case in point that you don't need to be a chemist to appreciate the importance of chemistry A self-made businessman who didn't even finish elementary school, Welch so valued the impact chemistry had made on his life that he left most of his millions to support chemical research after his death. His legacy, the Welch Foundation, now 50 years old, has transformed chemical research in Texas, the state Welch adopted as his home. "I have long been impressed with the great possibilities for the betterment of mankind that lay in the field of research in the domain of chemistry," Welch wrote in his will. His education in the subject was self-driven and on the job: To better understand the technical challenges of the businesses he was in—oil and sulfur extraction— Welch struck up friendships with chemists, geologists, and other scientists. When it came to deciding what to do with his ...

THE UBIQUITOUS CLASS OF CHANnel proteins known as aquaporins provides a path for water to cross cell membranes while blocking the passage of protons. When the first structural pictures of aquaporin...

THE OXIDIZED DNA BASE 8-OXOGUANINE is widely considered public enemy number one among the kinds of DNA damage caused by exposure to radiation or reactive oxygen species. Remarkably, this modified guanine—which can give rise to dangerous mistakes in the genetic code—differs from its undamaged parent by only one atom. The ease with which this lesion is formed and detected has made 8-oxoguanine the subject of intense research. That, coupled with a number of other factors, has made it one of the best characterized DNA lesions, notes Sheila S. David, an associate chemistry professor at the University of Utah. But recent work suggests that other DNA modifications caused by oxidation may also endanger cells. Cells hold dear the genetic information contained in their DNA and work diligently to maintain its sequence. The threat of DNA damage comes in many flavors. Among the troublemakers are breaks in the DNA helix and modification of DNA bases by anticancer ...

In some quarters of the chemical universe these days, it’s become commonplace to complain about how Chemistry Nobel Prizes go to discoveries that seem more biology than chemistry. Is the central science overlooked? Not this year, that’s for sure. Chemistry got the Nobel nod twice last week. Three biochemists—Tomas Lindahl, Paul Modrich, and Aziz Sancar—took home the Chemistry Nobel for revealing the inner workings of the repair systems that guard the integrity of our genomic DNA (see page 6). Each one of them spent years detailing the enzymatic mechanisms that protect our DNA from mistakes, chemical instability, and environmental insults. Meanwhile, the prize for medicine or physiology also went squarely to chemistry: Youyou Tu was recognized for leading the charge to isolate a game-changing malaria drug from a traditional Chinese herbal medicine during the height of China’s Cultural Revolution. She shared the prize with William Campbell and Satoshi Ōmura, who ...

A WEB OF COMPLICATED CROSS talk goes on in a cell: Information is relayed by protein-protein interactions, protease cascades, and highly choreographed stages of phosphorylation and dephosphorylation. Untangling these signaling pathways has mostly depended on the tools of genetics—deleting or altering a gene in yeast or mice and then observing the effects on signaling. But the relatively young field of chemical genetics is making its mark on signal transduction. "Chemists are developing a rich variety of chemical approaches that enable us to understand and manipulate signal transduction pathways," says Kevan M. Shokat, professor of cellular and molecular pharmacology at the University of California, San Francisco, professor of chemistry at UC Berkeley, and winner of this year's Eli Lilly Award for Fundamental Research in Biological Chemistry Shokat organized a symposium sponsored by the Division of Biological Chemistry at the American Chemical Society national meeting in Boston last month that showcased th...

The following is a guest editorial by Amanda Yarnell, C&EN’s managing editor, editorial. Like many chemists of a certain age, Derek Lowe remembers the exact moment when he learned about the chemical-structure-drawing program we now call ChemDraw. It was 1986, when Lowe, now a chemist at Vertex and the author of the influential In the Pipeline blog, was still in grad school at Duke University. “I remember the room, and where I was standing in it,” he said at a June 18 event in Cambridge, Mass., marking the program’s 30th anniversary. “The phrase ‘killer app’ applied.” Before ChemDraw, Lowe and his fellow chemists had to draw chemical structures by hand. Even with the help of the classic plastic template to create ring structures and rub-on letters to insert atoms, drawing structures pre-ChemDraw was painstaking work. Copying them, resizing them, or linking them within a reaction scheme was downright painful. Make ...

CHEMISTRY For developing superresolution microscopy methods, which have allowed scientists to collect nanometer-scale images of living cells Eric Betzig Howard Hughes Medical Institute’s Janelia Fa...

For a food industry searching to brand its products as “natural,” the color blue has long been a problem. But promising leads for natural blue food colorants have recently been plucked from the hot...