Guillermo C. Bazan - US grants

This grant will provide partial support for the acquisition of a modern differential scanning calorimetry package (DSC), including a theromogravimetric analyzer (TGA) and accessories for studying phase transitions at elevated pressure and at low temperature. This system will have a major impact on several materials oriented projects at the University of California, Santa Barbara (UCSB). Of major importance is the use of DSC data for understanding the formation or presence of microheterogeneous phases in multicomponent materials. The instrument will enhance projects concerned with the design of amorphous organic glasses suitable for use in emerging optoelectronic technologies, the synthesis of multi-component organic crystals, the characterization of biopolymers with liquid crystalline properties, the study of tough block copolymers and the solid state characterization of multicomponent networks for device applications. These studies require DSC analysis to understand long range order in sold samples and the spatial relationship of different components in complex mixtures.

The success of the research and teaching missions of UCSB's Materials program depends on the availability of modern instrumentation. A new Ph.D. program in Chemistry, which emphasizes research in Materials Chemistry, was established this year. In this program a broad education is encouraged and courses in fields such as Physics, Chemical Engineering and Materials Science are part of the curriculum. Modern instrumentation, such as the DSC system that is shared between groups of different departments will enhance collaborative interactions between students and faculty engaged in Materials research.
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This equipment will significantly enhance the research capabilities, and the educational experiences of students.

9985632
Bazan

Organic semiconducting materials are making a widespread impact in a range of emerging technologies. Recent efforts in materials synthesis, in conjunction with device engineering, has culminated in the fabrication of organic-based light emitting diodes, thin film transistors, solid state lasers, solar cells and photodetectors. There is even speculation about the creation of all-organic circuitry for computing purposes. Underlying all these functions is the spatial relationship between organic chromophores in the solid and the manner by which this parameter controls the electronic coupling between different subunits. Additionally, the level of order in the solid determines the ability of small molecule materials to form stable amorphous films and their potential inclusion into device structures.

The goals of this proposal over the next year are twofold. The first is to develop the necessary methodology to prepare well-defined molecules with geometrical attributes that yield amorphous materials. Materials of this type can be purified to a greater extent than polymer counterparts and offer advantages in devices where a small concentration of contaminants disproportionately affects charge migration. The second aim is to fabricate devices with the above compounds and to use the resulting performance profiles to probe the effect of molecular shape on the charge transport ability of the bulk.
The specific objectives during the grant period are:

(1) The synthesis of structurally-precise conjugated organic molecules with tetrahedral geometries. Of interest will be to examine how topology translates into bulk morphology. Spectroscopic studies will be executed to understand better the types and variability of environments present in organic glasses with a high chromophore content. This will be achieved by incorporating solvatochromic dyes into organic amorphous films and by measuring the resulting optical properties. The ultimate goal here is to develop guidelines that allow for tuning the cooperativity of chromophores in a disordered ensemble and to develop a better understanding of the local order that may exist in amorphous organic solids.

(2) To fabricate light emitting diodes using the above materials. Analysis of device performance will provide useful insight into how molecular shape affects important bulk properties such as conductivity and electroluminescence quantum yield.

This research program has the goals of understanding how bringing conjugated organic chromophores into close proximity influences the optical and electronic properties of the ensemble and how molecular topology can be used to control the organization of molecules in the solid state. To achieve these goals the PI will synthesize model compounds and study their optical, electronic and morphological properties. Specific objectives during the grant period are: (1) The synthesis of structurally defined bichromophoric and multichromophoric molecules that are held together by the paracyclophane framework. The target molecules will have a rigid structure that precisely determines the distance, orientation and number of interacting units. (2) To combine spectroscopic studies with quantum mechanical analysis to build a cohesive view of photoexcitation in multichromophore paracyclophane structures. (3) The synthesis of a homologous series of aggregated chromophores with the intent of identifying the limit at which the electronic communication between individual units ceases to be important. These are truly organic nanomaterials in that their electronic description intermediate between those of individual chromophores and of bulk materials (i.e. crystals, polymer films). (4) The synthesis of organic molecules with geometries that discourage crystallization. Of interest is to examine how topology translates into bulk morphology. Materials of this type can be purified to a greater extent than polymer counterparts and therefore may offer advantages in situations where a small concentration of contaminants disproportionately affects charge migration in the solid.
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The fundamental knowledge obtained through these studies will enable the engineering of organic materials for use in emerging optoelectronic technologies. This area of technology is likely to find widespread use in society. Specific examples include organic-based light emitting diodes for display applications, thin film transistors, solid state lasers, more efficient organic solar cells for energy generation and photodetectors. The issue of interchromophore delocalization is significant beyond the confines of materials design because it makes a profound impact in other areas of science, such the mechanism of photosynthesis and oxidative charge migration in double-stranded DNA.

The Advanced Materials Program in the Chemistry Division supports this award to University of California Santa Barbara. The focus of the research is the synthesis and characterization of conjugated polymer-lanthanide blends and to determine mechanisms for electronic couplings and charge transfer reactions in these systems. Under this award, ultra fast femto-second and time-resolved spectroscopic studies will be carried out by Vojislav Srdanov and Guillermo Bazan to determine the optical and electronic coupling between the conjugated polymers and lanthanides such as terbium neodymium and erbium in the prepared blends. Specifically, photoluminescence and electroluminescence properties of rare earth centered conjugated polymers will be studied with this award. Orbital energy levels of the polymer, the singlet and triplet levels of the ligand, and those of the lanthanides will be matched for the efficient combination of the polymer and the lanthanide. An understanding of the mechanism for the energy transfer reactions in conjugated polymers doped with different lanthanides may provide impetus for the development of monochromatic light-emitting diodes, flat panel displays and other related electro-optic devices.

An understanding of the mechanism for the charge transfer reactions in conjugated polymers with lanthanide rare-earth metals may provide impetus for the development of monochromatic light-emitting diodes, flat-panel display and other related electo-optic devices. The educational plan will integrate interdisciplinary research and education in the area of macromolecular chemistry and photochemistry.

This project explores the synthesis, design and application of conjugated polymers in biological assays that are relevant for bioterrorism threat assessment and defense. Specifically, the focus is on cationic conjugated polymers which behave as light harvesting macromolecules and which substantially improve, or "boost", the sensitivity of standard fluorometric detection and identification of DNA, RNA and protein structures. By improving optical sensitivity one will be able to examine samples, whether from the environment or from potentially infected methods to minimize non-specific interactions, which lead to sensor sensitivity losses, by molecular fine-tuning of the conjugated polymer. Polymers will be synthesized with specific molecular structures that probe the association between the optically active portion of the polymer and single stranded- or double stranded-DNA. Fluorinating substantial portions of the polymer chains will diminish hydrophobic interactions between the conjugated polymer and biological substrates. Potentially wasteful fluorescence quenching by photoinduced charge transfer mechanisms between the conjugated polymer and the electron rich sites of biological substrates will be examined by raising and lowering the electron affinity of the conjugated segments.
The proposal's intellectual merit stems from its inherent multidisciplinary approach. It
brings together distinct scientific fields, such as polymer synthesis and photophysics, mechanisms of biomolecular interactions, aqueous chemistry and analytical methodology. From the information generated by the proposed work we will be able to obtain insight not only on how to improve biosensors for bioterrorism assessment, but also on the weak intermolecular forces that exist between DNA and organic semiconducting materials. Such knowledge is woefully lacking and could provide the basis for a rational interfacing of electrical and optically active organic materials with biological matter. This award is supported jointly by the NSF and Directorate for Mathematics and Physical Sciences supports new concepts in basic research and workforce development with the potential to contribute to national security.
The broader impact of the proposal rests on the potential use of biosensors in disease diagnostics and forensic applications, which provide society with obvious benefits. It should be noted that in developing countries it is the cost of diagnostics, not treatment, which prevents medical treatment. With the materials proposed in this study, it may be possible to reach sensitivity levels that allow DNA examination with naturally occurring quantities. Such an improvement would circumvent the need to do polymerase chain reaction amplification and would substantially lower overall diagnostic costs. Students involved in the project are exposed to an interdisciplinary range of scientific techniques and approaches and will be well trained to capitalize on the opportunities offered at the interface of multiple scientific fields.
This award is supported jointly by the NSF and the Intelligence Community. The Approaches to Combat Terrorism Program in the Directorate for Mathematics and Physical Sciences supports new concepts in basic research and workforce development with the potential to contribute to national security.

TECHNICAL SUMMARY

Conjugated polyelectrolytes are described by a polymer backbone with a polarizable pi-delocalized electronic structure and pendant groups bearing ionic functionalities. These materials thus incorporate the useful optical and electronic attributes of semiconducting and light harvesting polymers with the properties of polyelectrolytes, which are modulated by complex electrostatic interactions. Such a combination of properties opens unique opportunities for designing materials that can be integrated into optoelectronic devices. One attractive target involves the creation of stable p-n junctions, which constitute a cornerstone in the fabrication of inorganic solid state devices. Such a fundamental module in organic counterparts has been difficult to attain due to difficulties in depositing thin films with independent p-doped and n-doped layers. The approach will take advantage of ion migration across polymer bilayers and concomitant formation of covalent bonds for permanently fixing the junction. Specific polymers will be designed and synthesized that have anion receptor groups and solubility properties for multilayer device fabrication. Another opportunity lies in the design, synthesis and applications of conjugated polyzwitterions. These materials are generated by attaching Lewis acids to lone pairs of electrons present along the polymer chain. When this approach is used on a conjugated polymer with a donor/acceptor motif, it is possible to attain large red shifts in the absorption profiles due to the stabilization of charge transfer excited states. Such polymers are essentially unexplored and offer new opportunities in the design of new materials for solar cell fabrication.


NON-TECHNICAL SUMMARY

Emerging organic optoelectronic technologies require materials with suitable electronic properties that can enable mass fabrication by solution methods, for example similar to those used in the production of photographic film. Conjugated polyelectrolytes are a new class of semiconducting polymers that open opportunities to create modules similar to those widely used with silicon-based technologies. One of the principal goals of this proposal is to integrate these conjugated polyelectrolytes and related materials into diodes and plastic solar cells. The work will include molecular design, materials preparation, examination of physical properties and device fabrication and testing. Students will benefit from a broad scientific perspective, which will serve them well in their future careers. Additionally, international links are in place for raising their awareness to global collaborations. Programs are also in place for educational programs with undergraduate and minority-serving institutions.

TECHNICAL SUMMARY:

One of the most important scientific challenges is how to efficiently harvest, convert, store and utilize solar energy. In recent years, there has been a growing interest of developing organic materials for solar cell applications. Organic solar cells offer a low-cost, large-area, flexible, light-weight, clean, and quiet alternative energy source for both indoor and outdoor applications. However, their power conversion efficiencies and operational lifetimes must be improved to enable large-scale commercialization and implementation and deep societal impact. Thus, there is an urgent need to understand fundamental processes in these devices. Currently, the synthesis and optimization of new materials is time consuming and labor intensive, and relies on trial and error approaches with poor success rates. There is therefore a great need to rationally anticipate materials performance from their chemical composition and bulk morphology to accelerate technology development and depart from empirical optimization. The goal of this interdisciplinary program is to mesh complementary expertise in chemistry, materials, physics, and mathematics, to make breakthroughs in the science and technology of organic solar cells. The team will address: 1) the development of new methods to simulate phase separation in BHJ solar cells; 2) the development of first-principles methods to predict carrier mobilities in organic semiconductors; 3) the nanoscale characterization of donor-acceptor interpenetrating networks; 4) the understanding of charge generation and transport process; 5) the synthesis of new conjugated polymers guided by the theoretical predictions; 6) evaluation of materials performance. The simulation methods will be validated extensively on well-studied materials and will then be capitalized for the design of more efficient new materials. As well-orchestrated theoretical and experimental efforts, the project strives to achieve transformative breakthroughs for the development of high-efficiency and low-cost organic solar cells.

NON-TECHNICAL SUMMARY:

The world demand for energy is expected to double by 2050. As of now, there is no viable technology to address this challenge without emission of carbon dioxide to the environment. In view of this, increasing the power conversion efficiency and operational lifetime of plastic solar cells is provides the opportunity to create a clean and potentially economically viable energy source with a wide range of applications. The goal of the proposed research is to assemble the team of scientists with complementary expertise in chemistry, materials, physics, and mathematics, to establish theoretical guidelines for a rational development of materials and solar cell device structures. Successful completion of the program is expected to relieve the current need to optimize device performance via trial and error procedures and will pave the way for an acceleration of technical innovation. This project integrates interdisciplinary research and education by involving the participation of undergraduate and graduate students and postdoctoral researchers, with special emphasis in the recruitment of underrepresented students. New courses on organic semiconductors and their applications in energy conversion will be offered at UCSB. The courses will be taught in an interdisciplinary environment, with which the investigators hope to address the urgent need to train researchers in the area of renewable energies. Furthermore, Workshops and demonstrations on solar energy for K-12 students, teachers, and parents in local schools will be developed to create awareness about renewable and sustainable energy sources. In addition, by exposing students to these ideas at such an early stage, the investigators hope to awaken in them the desire to pursue a career in sciences.

NON-TECHNICAL SUMMARY:

Recent studies have revealed that certain plastics can display electrical conductivity much higher than originally anticipated. Indeed, when introduced into transistor devices the conductivity of these plastic materials can be higher than certain forms of the commonly used silicon. Such a discovery opens new options for thinking about how plastic electronics can be utilized in a wide range of applications, including flexible solar cells, bright impact-resistant cellphone displays, and more energy-efficient white light sources. However, to reach high levels of electrical conductivity the polymer molecules that comprise the plastic material need to be very well organized across two important length scales. First, the molecular units that form the polymer chain have to be linked in a way that eliminates variations of structure. Second, the polymer chains themselves need to come together so that they pack into nanoscale fibers, which then coalesce to make up a conductive film. The latter can be achieved by a simple procedure that allows solutions of the polymer to dry on a substrate under controlled conditions. While these advances have been significant, the maximum possible conductivity of organized polymers remains unknown. The goals of this NSF-funded program are therefore to examine the properties of new polymer structures designed to increase electrical conductivity along the chain and to promote efficient interchain packing of molecules. Special attention will be paid to examine how these long molecules relate to each other as the solutions dry up, since this poorly understood process determines interchain relationships. Successful completion of the program will provide the scientific and engineering communities with new guidelines on how to design and process a new generation of highly conductive plastics for application in a range of emerging technologies.


TECHNICAL SUMMARY:

This program is centered on understanding unprecedented high charge-carrier mobilities in organic semiconductors based on conjugated polymers introduced into transistor devices. These materials comprise novel regioregular backbones with electron rich and electron poor heterocycles arranged along the backbone vector in a strict alternating sequence. Highly ordered registry between polymer chains in films is also a requirement, and this organization can be achieved via control of evaporation processes. While these findings have the potential of transforming our perspective of how to take advantage of plastic electronics, there are large gaps on how such high mobilities can be attained and the physical limits of this transport. One important question to address is how molecular weight determines carrier mobility, particularly because the carrier velocity appears to be dominated by motion along the polymer chain. Preparation and fractionation of specific average molecular weight systems will be carried out and subjected to characterization. Well-defined model compounds of intermediate dimensions will also be designed, synthesized and measured to understand the possible role of structural defects and to gain insight into the geometry of the interchain contacts. These materials will be incorporated into field-effect transistor devices to extract quantitative measures of charge mobility. Another important aspect of the work involves efforts to detail the self-assembly and evolution of the supramolecular structures with highly co-linear polymer chain crystals. Polymer chains with chiral side groups will also be prepared. Concentrated conditions or low temperatures lead these to form aggregates that exhibit strong circular-dichroism signals revealing the presence of chiral secondary (e.g., helical) structures. This simple spectroscopic tool will be used to understand the aspects of the molecular structure and the influence of substrate and solvent on the transition from isolated polymer chains to the highly ordered solid state.