Copyright 2008 by the American Chemical Society
VOLUME 112, NUMBER 21, MAY 29, 2008
Kathy Sauber, University of Washington Photography
Biography of Larry Dalton Ten years ago it was not clear whether organic molecules could become the basis for viable commercial solid-state optoelectronic and electronic technologies. Today that question can be answered in the affirmative. Even five years ago, there were many who argued that success was unlikely. However, at a Conference on Electrical, Optical, and Magnetic Properties of Organic and Hybrid Materials held on the University of Washington campus in June, 2003, [see J. Phys. Chem. B 2004, 108 (25)] the mood of the participants seemed to become more upbeat with each paper presented. Last summer (2007), a second such conference was held at The University of Washington’s Friday Harbor Laboratories in the San Juan Islands. Here the conversation was not about “whether” but about “when”, and about which applications might be the most substantive commercial forerunners. This issue of The Journal of Physical Chemistry grew out of the 2007 Conference, which was dedicated to Professor Larry Dalton. Professor Dalton has played a pivotal role in changing the perspective regarding the potential of organic molecules for nonlinear optical applications. The impact of this shift promises to be transformational in both the range of scientific research
topics as well as commercial initiatives that will be pursued in the coming years. Indeed, it can be argued that we now stand on the threshold of new era in the use of organic molecules for a wide range of photonics applications including telecommunications, computing, energy efficient solid state lighting, renewable energy through new photovoltaic technology, optical signal processing, and new sensor technologies based on the integration of organics with silicon photonics. It is hoped that these emerging photonic technologies will achieve the same kind of dominance in the years to come that has already been achieved by the closely related applications of organic molecules to the established field of liquid crystal displays. It is to highlight these paradigm-changing achievements that this issue of The Journal of Physical Chemistry is dedicated to Professor Dalton. Born on April 25, 1943 on a farm near Belpre, Ohio, Larry pursued undergraduate studies in the Honors College of Michigan State University. He pursued a dual degree (BS/MS) program in chemistry and soon became involved in undergraduate research (that included work performed at Argonne National Laboratory) on pulsed radiolysis of the solvated electron with E. J. Hart and Professor James Dye. He received his BS degree
10.1021/jp801441z CCC: $40.75 2008 American Chemical Society Published on Web 05/22/2008
7774 J. Phys. Chem. C, Vol. 112, No. 21, 2008 with highest honors in 1965, and an MS degree in 1966 under Professor James Dye’s supervision. The following year he enrolled at Harvard University for doctoral studies. At Harvard he decided to cast his lot with a young faculty member, Alvin Kwiram, to pioneer the field of electron-nuclear double resonance (ENDOR) as applied to the study of free radicals in irradiated organic crystals. His productivity was remarkable, and within the next several years, after fulfilling his course requirements, he completed three major projects, any one of which would have qualified for a fine dissertation. The three topics covered in his 610 page dissertation included (i) the detailed “ENDOR Detected NMR” study of the deuteron quadrupole tensors in several cyclic organic molecules, (ii) the development of a powder ENDOR methodology for characterizing hyperfine tensors in systems which could not be obtained easily in single crystal form, and (iii) the spin-lattice relaxation behavior of a variety of organic free radicals in the solid state over the temperature range from roughly 2 to 400 K (much of this latter work remains unpublished despite the extensive data collected). He was awarded the Ph.D. by Harvard in 1971 (in addition to an AM degree he picked up along the way). As already suggested, Larry’s intensity and prodigious capacity for work were evident very early on. He seemed to require little food and even less sleep. He could be found in the laboratory at all hours of the day and night including weekends and holidays. Indeed, he was the only student for whom I stipulated a mandatory vacation in which he was not to engage in any scientific endeavor. That intensity does not seem to have diminished over the years. The quality and breadth of his research was sufficient to convince Vanderbilt University that this was someone who was on a steep upward trajectory, and the Vanderbilt Chemistry Department appointed him as an Assistant Professor in 1971 as part of an initiative supported by an NSF Centers of Excellence award. There he soon linked up with Bruce Robinson, who had been working with Professor Lawrence Schaad, and their subsequent collaboration, first with Bruce as a student, then postdoc, and eventually as a colleague at the University of Washington, has remained highly productive to the present day. It was at Vanderbilt that the work on electron-electron double resonance (ELDOR) was launched and eventually resulted in a exhaustive theoretical and experimental description of saturation transfer spectroscopy that remains in use to this day. More specifically, through a coordinated theoretical and experimental effort, Dalton, working with Bruce Robinson and other collaborators, developed the concept of “saturation transfer magnetic resonance spectroscopy” whereby molecular dynamics and inter/intramolecular distances could be measured by precisely measuring the transfer of saturation between different portions of magnetic resonance spectra. Dalton and Robinson demonstrated how the “perturbation theory” approaches of Redfield and Abragam could be replaced by a general theory of magnetic resonance that could describe dynamical events of any frequency (i.e., frequency domain spectra and time domain relaxation could be described continuously from the “fast motion” to the “rigid lattice” limits). Dalton and co-workers introduced the density supermatrix theory of magnetic resonance as well as the eigenfunction expansion method for describing the rotational diffusion of proteins and DNA/RNA and a new mathematical method for computing the remainders of slowly converging mathematical series. Professor Dalton played a major role in introducing and advancing the full range of magnetic resonance spectroscopies from cw modulation techniques, to
multiple (double and triple) magnetic resonance techniques, to pulsed time domain techniques. Indeed, his research helped define the magnetic resonance instrumentation being sold by Varian Associates and Bruker Physik (he served as consultant for both companies) in the 1970s, 1980s, and 1990s. In 1974, he designed and published the first fast (nanosecond response) pulse programmer that formed the basis of commercial programmers permitting complex (including multidimensional) pulse sequences. This was one of the first emitter coupled logic (ECL)-based electronic devices to be commercialized. He also designed and patented many of the magnetic resonance probes that were subsequently marketed by Varian, Bruker, and IBM Instruments. Professor Dalton and his students also pioneered the application of new spectroscopic techniques to critical biomedical problems, including DNA dynamics and mutagenesis, the supermolecular interaction of red blood cell proteins, and the supermolecular interaction and dynamics of muscle proteins using, in many cases nitroxide spin label methods. This research was recognized by NIH awards and service on critical advisory committees (the Parent Committee for Oversight of the National Sickle Cell Program). It was this work that also led to his appointment as Research Professor of Biochemistry and Physiology at Vanderbilt’s College of Medicine in 1975, an appointment that lasted until 1987, well past the time when he had left Vanderbilt. Dalton moved to SUNY Stony Brook in 1976 as Associate Professor of Chemistry where he remained until 1982 when he moved to the University of Southern California. He spent the major portion of his career (the middle years) at USC, and there he made the transition from magnetic resonance to the field of nonlinear optics. This process began when Professor Dalton brought advances in theoretical and experimental techniques to the task of characterizing the wave functions and dynamics of solitons and polarons in π-electron polymers. He was the first to apply ENDOR, pulsed relaxation (including electron spin-echo), and magnetic resonance imaging techniques to the characterization of solitons and polarons in systems such as polyacetylene and the heteroaromatic ladder polymers. It is generally agreed that his research, together with that of others using the techniques that he introduced, provide the best characterization of these novel species. Theoretically, he was the first to quantitatively demonstrate the importance of electron Coulomb interactions in defining the wave functions of solitons and polarons. Professor Dalton and his students also led the effort to develop methods to solubilize and process electroactive polymers, which to that time had been considered to be largely intractable. They would later apply such physical and chemical techniques to the solubilization, processing, and study of carbon nanotubes. It was his work on electro-active polymers that introduced him to the potential of these and related materials for molecular electronics and photonics. This field had seen sporadic efforts to exploit the properties of organic molecules as conductors and nonlinear optical materials. Indeed, several major corporations had made a concerted effort to develop electro-optic materials but failed to achieve the results they had hoped for, and they eventually abandoned the field. In the late 80s, Professor Dalton, undaunted by their failures, took up this challenge and launched a program to develop new nonlinear optical materials and device concepts. In parallel with the materials development (about which more will be said later), he pioneered wavelength-agile, femtosecond-time-resolution, phase-sensitive-detection techniques for the characterization of various orders of nonlinear optical susceptibilities. He designed an optical amplifier leading
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7775 to record signal-to-noise ratio measurements in pulsed experiments. He also introduced the “Signal Matrix” technique for simultaneously measuring the real and imaginary components of nonlinear susceptibilities in femtosecond pulsed experiments. His laboratory became the reference laboratory for the characterization of instantaneous two-photon absorption coefficients for the Department of Defense sensor protection efforts and his research was marked by extensive collaborations with the Naval Research Laboratory (Washington, DC) and the Air Force Research Laboratory (Wright Patterson AFB, Dayton, OH). He is generally credited with the definitive characterization of the fullerenes (C60 and C70) by two-photon absorption spectroscopy, for example, as well as a number of other nonlinear optical materials. Dalton, and his students, also developed quantitative theoretical (Density Matrix) methods for the characterization of virtually any nonlinear optical experiment including those where phase relaxation times and pulse widths are comparable. Although his group had begun to make some progress on improving the characteristics of the materials for nonlinear optical applications, his pioneering work in the field was catapulted to a new level when he was enticed to move to the University of Washington in 1998 as Professor of Chemistry (with a subsequent appointment in the Department of Electrical Engineering as well). Here he teamed up again at close range with his long-time collaborator, Bruce Robinson, in pursuing his singular efforts to demonstrate the potential of organic materials for nonlinear optics. A few years later, Professor Alex Jen arrived at the University of Washington in the Department of Materials Science and Engineering. The collaboration of Dalton, Robinson, and Jen in exploring and characterizing the theoretical and experimental properties of complex chromophores led to dramatic advances in understanding these systems, and a new chapter in the work on nonlinear optical materials was written. Whereas the critical parameter, r33, was hovering around 10-25 pm/V in the mid 90s (comparable to the industry standard nonlinear optical material, LiNbO3, which has an r33 of about 30 pm/V), this number began to move up with the design and synthesis of ever more complex donorbridge-acceptor chromophores. By about 2000, r33 values for the organic chromophores had moved ahead of lithium niobate. In the five year period from about 2002 to 2007, the value for r33 shot off the charts first reaching 50 pm/V, then the “benchmark” 150 pm/V (five times that of lithium niobate), then the daunting goal set by DARPA of 300 pm/V by the middle of 2006, and the dramatic doubling again to 600 pm/V at the end of 2007. To reiterate, these materials have led to record performance milestones as electro-optic materials (exhibiting orders of magnitude greater (purely electronic) electro-optic activity than previous materials such as crystalline lithium niobate) and have led to new device technologies (e.g., ultrahigh bandwidth, low drive voltage devices; high density integrated device technologies; and lightweight and conformal devices). It should be noted that much of the foundation for these advances was provided first by the Lumera Corporation and second by the National Science Foundation (NSF). The Lumera Corporation was a start-up company based entirely on Professor Dalton’s research vision. Initially created and financed by Microvision Corporation (an earlier UW spin-out) in 2000, Lumera’s goal was to exploit the new materials for building electro-optic modulators for the telecommunications industry. The timing was not optimum since by 2001 it was clear that the telecommunications industry had greatly overestimated the demand and had massive excess capacity. As a result, that sector
of the industry suffered even more than the market in general in the 2001 stock market fall. Nevertheless, Lumera funded substantial research at the UW to advance the material development and became a public company in 2004. In 2002, the NSF awarded a Science and Technology Center (STC) on Materials and Devices for Information Technology Research (MDITR) to the UW with Professor Dalton as the Principal Investigator leading a team of researchers from around the country (some half-dozen institutions were part of the original NSF-STC proposal). The five-year benchmark for the electro-optic material portion of the Center proposal was an r33 of 150 pm/V, a goal which was surpassed within the first couple of years. It was this dramatic progress that finally caught the attention of a DARPA program manager, the late Bill Schneider, who began discussions with Professor Dalton about possible funding. Professor Dalton’s research had been supported by various programs within the Department of Defense for a number of years through a Multidisciplinary Research Initiative (MURI) Center on Polymeric Smart Skin Materials, through the Office of Air Force Research and other programs. However, the DARPA program was highly targeted and sought to capitalize on the large and ongoing foundational investments that had been made by others. Unfortunately, the DARPA goal posts kept moving and the already daunting phase I goal of 300 pm/V was “enhanced” with demands for simultaneous values of low optical loss (less than 2 db/cm) and a thermal stability goal of 200 °C for Tg, the glass transition temperature, all in the same material. Remarkably, those goals were achieved within an 18 month window. It should be noted that the synthetic work on these materials was accompanied by an ever expanding understanding at a theoretical level of what the key issues are in developing materials with the desired characteristics. An early insight showed that an oblate spheroid was the preferred configuration for the embedded guest chromophore (to reduce the dipole-dipole interaction strength that tends to create centrosymmetric structures and reduce the electro-optic activity of the material). These design concepts are also relevant to the design and realization of new classes of ordered supra/supermolecular materials including noncentrosymmetrically organized discotic liquid crystals and binary organic electro-optic glasses including the design of a new generation of organic photovoltaic, photorefractive, light-emitting, and field-effect transistor devices. Further theoretical work by Professor Dalton and co-workers (including particularly Bruce Robinson) involved the development of new theoretical methods, including Pseudo-Atomistic (United Atom Approximation) Monte Carlo/Molecular Dynamical Statistical Mechanical calculations, to treat the full range of critical intra- and intermolecular electrostatic interactions necessary to predict supra/supermolecular organization of the bulk material and the electronic/photonic properties derivative from specific structures. In particular, Dalton and his students were the first to quantitatively treat long-range and spatially anisotropic intermolecular dipole-dipole interactions characteristic of molecules with extended π-electron conjugation. All of these efforts provided an increasingly refined understanding of the factors that yield optimum values for r33 and guided experimental design of the basic chromophores as well as the hosts in which these chromophores are embedded (including dendrimeric systems in which the chromophore is an integral part of the dendrimer). One example of the quantitative success of these theoretical advances is the prediction and experimental verification that multichromophore-containing dendrimer materials exhibit elec-
7776 J. Phys. Chem. C, Vol. 112, No. 21, 2008 tro-optic activity consistent with the nanoenvironment of chromophores acting to shield the chromophores from each other such that the chromophores behave as independent particles. Thus, no attenuation of electro-optic activity is observed due to “centrosymmetric pairing” phenomena even at high chromophore concentrations. Another specific example of the pioneering research of Professor Dalton is his theoretical demonstration of the effect of host lattice symmetry on noncentrosymmetric order of both dopant chromophores and host-embedded chromophores in electrically poled “binary chromophore” organic glasses comprised of chromophore-containing dopants and chromophorecontaining hosts. The acentric order parameter is theoretically predicted and experimentally observed to increase dramatically progressing from Langevin (3-D) to Bessel (2-D) to Ising (1D) lattice symmetries. While doping of chromophores into crystalline lattices was previously known, the theory of Dalton and co-workers permits quantitative simulation of photonic properties for doping chromophores into hosts of varying degrees of order. Moreover, the effect of electric field poling on both dopant and host chromophores and their impact on each other (through strong intermolecular dipole-dipole interactions) is quantitatively described. Dalton and Jen have designed a new class of materials, binary organic chromophore glasses, which permit very high chromophore number densities and improved material homogeneity leading to dramatic electro-optic activity (e.g., 600 pm/V at telecommunication wavelengths) with reduced optical loss associated with the absence of line broadening and solvatochromic shifts with changes in material composition. Stated thermodynamically, binary chromophore glasses yield dramatically improved enthalpies and entropies of mixing compared to conventional chromophore/polymer composite materials. The theoretical work has provided a meaningful framework on which to build future developments. The research of Dalton and co-workers has led to new processing methodologies including electric and optical/electric field poling. For example, a polarized optical field may produce Bessel- or Ising-like symmetry in a host chromophore lattice (containing a chromophore that can undergo photoinduced trans-cis-trans isomerization leading to effective molecular reorientation), which in turn can influence the order of a second (dopant) chromophore [Olbricht et al. J. Phys. Chem. C 2008, 112, 7983–7988]. The net result of this aspect of Professor Dalton’s research has been the systematic realization of electrooptic materials characterized by both high chromophore number density and by a significant acentric order parameter. The task of increasing the order parameter remains one of the key challenges and provides an enticing path for improving electrooptic activity even further. Theoretically inspired design (which permits systematic control of the relative positioning of donor and acceptor ends of charge transfer chromophores) also permits control of properties such as electron and hole transport (electronic conductivity) critical to organic electronics, photovoltaic activity, photorefractive activity, and light emitting properties as well as to electric field poling and bias voltage drift characteristics of organic electro-optic materials. Professor Dalton also enlisted colleagues in electrical engineering, such as his long-time collaborator Professor William Steier, to incorporate these organic materials into device structures, as well as colleagues in physics to work on the propagation properties of light through these highly nonlinear materials. This work is also advancing the frontiers for the optical properties of these novel materials.
Dalton and his co-workers have provided not only the theoretical guidance for the development of new materials but have also led the development of synthetic and processing methodologies that have facilitated the development of costeffective materials and have provided the intellectual property that has inspired new industrial activity. Dalton and Jen have been “early adopters” of dendritic and sequential synthesis techniques, as well as microwave-assisted synthesis methodologies, and have pioneered the application of these synthetic techniques to the development of materials with extended π-electron conjugation. They have also developed new lattice hardening chemistries based on novel cycloaddition reactions that permit the glass transition temperatures of materials to be tuned and elevated to values that satisfy Telcordia standards. They have demonstrated novel processing methodologies uniquely suited to their new materials including the application of soft and nanoimprint device fabrication technologies permitting the “printing” of complex photonic and opto-electronic circuitry and the cost-effective manufacture of high density circuitry. Dalton and his co-workers have also pioneered many of the analytical techniques now used to characterize organic electroactive (and particularly, electro-optic) materials. Among the array of new applications greatly facilitated by the materials and devices advances of this research are new sensor applications and terahertz spectroscopy and imaging methods. During the early stages of the NSF STC program, Professor Dalton also initiated collaborations with engineering colleagues, Professor Axel Scherer at Caltech and Professor Michal Lipson at Cornell, who were pioneering the new field of silicon photonics. He convinced them that there was a future in integrating organic materials with silicon, and that entirely new devices and technologies could, in principle, emerge from such a marriage. He received significant criticism initially for “spreading the Center program” too thinly in nonproductive directions, but he stuck by his convictions. That decision was rewarded not long after by the discovery of slotted silicon waveguides and their unique properties. If these 70-100 nm slots are filled with a nonlinear optical organic material, an entirely new range of devices is made possible. New concepts in “resonant” photonic devices have been theoretically and experimentally defined including the low optical loss transition of light from high index of refraction passive silicon waveguide to low index of refraction organic electro-optic waveguide for active control of light. With these hybrid device structures, light has been focused into waveguides of lateral dimensions as small as 70 nanometers leading to first time observations of optical rectification and all-optical modulation with diode laser input power levels in the milli- to microwatt power range. Moreover, modulation has been demonstrated to 5 THz. Electro-optic modulation and switching with tens of gigahertz bandwidth have been demonstrated using millivolt drive voltages. Dalton and co-workers have also demonstrated new concepts for and the successful integration of new semiconductor photonic circuitry with very large scale integration (VLSI) electronics. Professor Dalton’s vision and conviction once again has broken open a new field of endeavor in organic-silicon integration that will resonate for many years to come with exciting new technologies for a wide range of applications. Although the work of the NSF Center is much broader than Professor Dalton’s own research program, the potential applications emerging from the Center touch virtually every field of technology that relies on light or signal processing. These include areas such as solid state lighting, solar energy conver-
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7777 sion, telecommunications, transportation (collision avoidance radar), defense (including phased array radar), computing (optical interconnects), ubiquitous sensors, biomedical (photonics) applications in both diagnosis and therapeutics, and so on. The work in Professor Dalton’s laboratory, as well as that of the teams he has led in various NSF and DOD initiatives, has demonstrated that organic materials are realistic candidates for any number of such applications. Indeed, it can be argued that the work accomplished under the NSF Center award represents a watershed event that has transformed an intriguing curiosity-driven research effort in nonlinear optics using organics into an endeavor of commercial viability. It would appear that the nearly two decade transition from initially parrying with the innumerable skeptics to ultimately advising government agencies and corporate executives on the most promising applications of this revolutionary technology has now been largely accomplished. Arguably, a large part of the credit for this changed perspective has to be attributed to Professor Dalton’s vision, his unrelenting drive, and his impressive grasp of disparate disciplines from electrical engineering, to physics, to materials science, to chemistry and to the commercial and defense opportunities that will ultimately benefit from these advances. It is appropriate to note that the research on electro-optic materials, just as the earlier work on saturation transfer spectroscopy, represents a veritable tour de force, ranging endto-end from the most fundamental theoretical and experimental advancement of scientific concepts and knowledge at the molecular level to the development of new materials and new device concepts which are already beginning to transform the commercial landscape. Another area of Professor Dalton’s research merits comment. With Professor Martin Gouterman and Professor James Callis of the University of Washington, he extended the Gouterman/ Callis “sensor paint” technology, and this work formed the basis of a very successful Multidisciplinary University Research Initiative (MURI) Center on Polymeric Smart Skin Materials. Gouterman, Callis, and their students pioneered commercially viable multisensor paints, beads (for the study of fluid dynamics), and “smart worms”. “Ideal” pressure-sensitive paints have been developed and commercialized and an impressive array of physical and chemical phenomena is now monitored routinely with a variety of device structures including those adapted for embedded network sensing. Professor Dalton’s contributions expanded the range of phenomena directly sensed and imaged which now include pressure, temperature, shear, stress, strain, pH, corrosion of ferrous metals, corrosion of aluminum, fluid flow, singlet oxygen concentration, various ion concentrations, and sensing of specific chemicals including for IED detection. Professor Dalton’s research achievements have strongly influenced the science and engineering communities as is evidenced by his leadership of numerous multi-investigator Federal centers and groups some of which have already been mentioned. It is worth noting that in many of these large Center grants Professor Dalton has been remarkably generous in his support and mentoring of young faculty, in many cases ensuring that they received funding even at the expense of his own interests. In some cases his own research program received no
funding from grants on which he was the principal investigator because he wanted to be sure that the young faculty had support and were integrated into these larger programs. His contributions to the research enterprise have also resulted in a long and distinguished career of Federal Advisory Service including his most recent appointments: Advisory Committee, Mathematical and Physical Science Directorate, National Science Foundation (2005-2008); Advisory Group on Electron Devices (AGED), Office of the Undersecretary of Defense (2006-present)/Defense Science Board Standing Subcommittee on Electronics Technology (2006-present); NSF ACGPA (National Science Foundation Advisory Committee for the Government Performance and Results Act) (2006, 2007); Committee of Visitors, Division of Materials Research, National Science Foundation (2005); and Committee of Visitors, Chemistry Division, National Science Foundation (2007); Nanotechnology Technical Advisory Group of the President’s Council of Advisors on Science and Technology (PCAST). His research has also been recognized by professional society awards: American Chemical Society 2003 Chemistry of Materials Award and the 1996 Richard C. Tolman Medal; and the Institute of Electrical and Electronics Engineers the 2006 IEEE/ LEOS William Streifer Scientific Achievement Award. His research was the subject of a lecture in the National Science Foundation Distinguished Lecture Series. He is a Fellow of the AAAS, SPIE, and the Optical Society of America. Professor Dalton’s career has also been characterized by innovative contributions to education and to the advancement of under-represented minorities. He has been a popular teacher even at the freshman chemistry level and has given generously of his time to guide both high school as well as undergraduate students in undergraduate research projects. He has served on advisory boards of a number of minority-serving institutions. These efforts in the advancement of under-represented minorities have been recognized by the QEM (Quality Education for Minorities)/MSE (Mathematics, Science, and Engineering) Network 2005 Giants in Science Award. However, probably one of his signature contributions in this arena has been his role in the creation of the first Ph.D. program in science and engineering at Norfolk State University. This was a long-term effort starting with his contributions to their proposals to NSF and to NASA which resulted in significant funding for NSU, followed by his work with the faculty and administration to develop the vision of a Ph.D. program, to his advocacy for the approval of the Ph.D. program before the Legislature of the Commonwealth of Virginia, to his rallying of his Center team to work with NSU in developing a series of introductory lectures for a semester long course at NSU in the optical properties of organic materials, and to his ongoing encouragement of faculty, students, and administrators to press toward the goal. The entering class of Ph.D. candidates (about a dozen in all) started their program in the fall of 2007. This initiative would probably not have happened or happened as soon as it has but for the diligent efforts made by Professor Dalton. This will be a legacy that will endure for generations.
Alvin L. Kwiram UniVersity of Washington
