SCIENCE/TECHNOLOGY
Advance In Optical limiting Achieved With Organic Dye-Based Device • Device absorbs laser pulse as never before, but further improvement in performance still is needed for practical applications A metal phthalocyanine dye has en/ % abled scientists to achieve the J L J L best results yet in optical limiting, a method used to protect optical sensors or the human eye from the damaging effects of ultrabright pulses of laser light. The ideal optical limiter would be transparent at ordinary light intensities. But it would become opaque within nanoseconds of being exposed to a high-intensity laser burst. And after the pulse was over, it would immediately become transparent again. In everyday life, a poky sort of optical limiting occurs in photochromic sunglasses: Indoors, the lenses are highly transparent, but in bright sunlight, they darken considerably. The change from one extreme to the other is gradual and can take a minute or two. Optical limiters intended to protect the human eye from laser flashes must respond much faster and absorb much more light than sunglasses. No one has yet found an optical-limiting material that provides the desired level of protection. But scientists are getting closer. The latest advance was reported in late November at the Materials Research Society meeting in Boston by physical chemist Joseph W. Perry of California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena. Perry described a rather simple device, based on a chloroindium phthalocyanine, that darkens from about 70% light transmission to 0.18% in 5 nanoseconds when exposed to a focused laser beam. The degree of light absorbance by the device is an order of magnitude bet24
JANUARY 1,1996 C&EN
ter than the previous record, set by the same group. One of Perry's coworkers on the project, Seth R. Marder, a chemist at JPL and Caltech's Beckman Institute, calls the achievement "a performance breakthrough/' The JPL team is one of several groups around the world that have been exploring the optical-limiting potential of different kinds of compounds, such as phthalocyanines, mixed-metal clusters, and fullerenes. By the late 1980s, scientists had recognized that phthalocyanines have useful optical properties. Indeed, chloroaluminum phthalocyanine was one of the first laser dyes. Perry says he and his coworkers showed in 1989 that the optical-limiting response of this dye was "quite impressive" compared with anything else they had seen. But it still left a lot of room for improvement. By introducing a heavymetal atom such as indium, tin, or lead into phthalocyanine molecules, the JPL researchers markedly improved optical limiting by these molecules for the first time. For maximum optical limiting to occur, Perry explains, a large proportion of the dye molecules—at least 90%— must be promoted from the ground state to the triplet excited state. That's because the excited-state molecules absorb photons more strongly than groundstate molecules. By adding heavy atoms to the molecules, the researchers were able to increase the efficiency of excitation to the triplet state. So as the incoming light becomes more intense, light absorption also increases, effectively limiting the number of photons getting through the material. But it's not that simple. Normally, as a laser beam traverses an absorber, the light intensity is sapped to the point that it can no longer excite molecules in its path. To overcome this difficulty, Perry A. Miles, a collaborator of Perry's at Logicon R&D Associates in Los Angeles, used mathematical modeling to design a simple device having a dye concentra-
tion gradient that maintains a suitably high level of optical excitation across the absorber. The device used to test the design concept basically is a stack of three absorbing disks and two spacers mounted in a cylinder. The absorbing disks contain the metallophthalocyanine dye dispersed throughout a host material such as poly (methyl methacrylate). The mathematical model prescribes the concentration of the dye in the disks and the thickness of the disks and the spacers separating them. The spacing of the disks approximates the model's requirement that the dye distribution in the absorber be nonuniform. Even though the device approximates the model in a rather crude way, Marder says, it still provides an orderof-magnitude improvement in optical limiting, compared with the best previous value. Moreover, this new record in optical limiting—0.18%—is 63 times better than the figure quoted for C60, which has served as a benchmark of sorts since its optical-limiting behavior was reported in 1992, and, some say, subsequently "overhyped." Nevertheless, 0.18% is still not good enough. Goggles that allow that much
Perry: applications go beyond military
Chloroindium phthalocyanine used in optical-limiting device
light through would not protect the wearer from retinal damage. What is needed is a further 10-fold reduction in light transmission—to 0.02%—before the system could be considered for use in eye protection. Perry and Marder are confident that this level of optical limiting is achievable with phthalocyanines, perhaps by better approximating the ideal dye
concentration gradient and by "tweaking the molecules a bit," in Marder's words. Likewise, Larry R. Dalton, a chemistry professor at the University of Southern California who heard Perry's talk in Boston, is "cautiously optimistic" that a further 10-fold improvement in optical limiting will be achieved. The JPL group's demonstration that a nonuniform distribution of dye leads to significantly enhanced optical limiting opens "a new and unanticipated direction" in this field, he tells C&EN. "It's a nice accomplishment." The JPL research—funded by the Advanced Research Projects Agency, the Office of Naval Research, the Air Force Wright Laboratory, and the Air Force Office of Scientific Research—is of particular interest to the military. That is because it promises to protect eyes from damage due to inadvertent exposure to lasers (that can occur while using a laser range-finder) as well as from intentional
Aldol catalysts produced by reactive immunization Researchers at Scripps Research Institute, La Jolla, Calif., have devised a technique called reactive immunization. The technique generates catalytic antibodies that bond covalently to antigens, making the antibodies capable of catalyzing a class of reactions previously inaccessible to antibody catalysis. Catalytic antibodies are immune-system proteins that stabilize the transition state of chemical reactions and thus increase their reaction rates. The Scripps team has demonstrated the practical potential of the technique by producing an antibody that catalyzes the aldol reaction, a fundamental carboncarbon bond-forming reaction for which versatile catalysts have been lacking. The reactive immunization technique was conceived by Scripps President Richard A. Lerner and chemistry professors Kim D. Janda and Carlos F. Barbas III. Lerner and Janda, with Scripps coworkers Peter Wirsching, Jon A. Ashley, and Chih-Hung L. Lo, used the concept to produce covalent antibody-antigen intermediates [Science, 270, 1775 (1995)]. The method was then used by Lerner, Barbas, and coworker Jiirgen Wagner to produce a catalytic antibody for the aldol reaction [Science, 270,1797 (1995)]. In reactive immunization, says Lerner, 'Instead of immunizing with an in-
ert compound, you immunize with a reaction. In essence, one immunizes with a chemical reaction rather than with a chemical. That's a big change. It's the first attempt to immunize with reactive compounds." Antibodies normally bind noncovalently with their substrates. But reactive immunization results in the generation of antibodies that bind covalently, making it possible for them to catalyze reactions that require covalent activation. "What we're trying to do is cause a chemical reaction to take place in the binding pocket of an antibody, because there are elements of that chemical reaction that will be later used in a catalytic event," Lerner explains. "The antibodies go forward and their progeny will make that covalent bond with a similar substrate. That covalent bond then becomes part of the catalytic event." The technique mimics nature, he says, because "a full one-third of natural enzymes work by covalent mechanisms." The researchers have demonstrated the technique by using it to generate catalytic antibodies that catalyze the aldol reaction. "The aldol reaction is arguably the most important carbon-carbon bondforming reaction in biology and chemistry," says Lerner, "but the natural catalysts are too highly restricted in the sub-
exposure (such as from laser weapons designed to blind soldiers). Beyond the military applications, Perry also holds out the possibility of adapting this organic-based optical-limiting system for use in protective goggles or visors for labs and other settings where lasers are used. The most sophisticated goggles in use today rely on heavy interference filters to block a specific wavelength—but they also interfere with normal vision. Accidents can occur when the wearer momentarily lifts the goggles to get a good look at what he or she is doing and gets a reflected laser beam in the eyes. A system with an optical limiter might be more practical because it would protect the eyes without encumbering normal vision. Even if such applications come to pass, don't expect to see phthalocyanines in commercial sunglasses anytime soon. Sunlight, the researchers point out, just doesn't have the intensity to trigger the optical-limiting effect. Ron Dagani strates they use to be of general use to chemists." The goal was to generate antibodies that use the reaction mechanism that gives aldolases their efficiency but that are capable of catalyzing reactions of a greater range of substrates. The mechanism of the catalytic antibodies they developed mimics that of natural class I aldolase enzymes, which catalyze biological aldol reactions. In such reactions, a nitrogen atom on a lysine residue in the aldolase active site can react with a substrate to form a Schiff base (iminium ion). The Schiff base tautomerizes to form an activated enamine intermediate. The intermediate then reacts with an aldehyde to form a new carboncarbon bond. After hydrolysis of the Schiff base, the enzyme and product separate. In the immunization used by Lerner and coworkers, lysine residues in antibody binding sites formed Schiff-base linkages with a hapten (an antigen used to induce antibody formation). Some antibodies induced by the immunization therefore form a similar Schiff-base linkage with substrates that resemble the hapten structurally. The antibodies are then able to catalyze the aldol reaction, using a mechanism similar to that of the aldolases. The researchers were able to use one of these antibodies to successfully catalyze the reaction of aldehydes with acetone and other ketones to form aldol adJANUARY 1,1996 C&EN
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