Organic materials are important to development of organic light emitting diodes (OLEDs) used in display technologies, organic photovoltaics (OPVs) used in the manufacturing of plastics that convert light to energy, and organic semiconductor technologies used for information processing and computing. Research in the Frank group utilizes theory and techniques in photochemistry /photophysics, charge-transfer/charge transport, and spin-state dynamics/spin-spin exchange, and electronic structure theory to design complex functional materials with unique applications for quantum computing, biosensing, and low energy demand device architectures. Using the principles of organic synthesis, coordination chemistry, and polymer synthesis, we design and prepare conjugated organic molecules, coordination complexes, and polymers that combine optical, magnetic and electrical properties. We then investigate the effect of stimuli ( optical, magnetic, electrical) on the electronic structure and secondary functional properties (optical, magnetic, electrical) to determine the effects of structural parameters on electronic coupling, reorganization energies, electrochemical behavior, and efficiencies, for device fabrication. The effect of such perturbations on the electronic structure or targets is studied by solution-state and spectroscopy, physical measurements in the solid state, and computation. The goal of our research is to gain insight into (i) the effect of spin state on excited state processes and ground state charge transport processes, (ii) the effect of structure on long-range and short range order in organic materials on surfaces, and (iii) the effect of gated topological changes on electronic coupling relevant to electron and energy transfer processes. Such insights ultimately inform design principles for materials development in quantum information processing, spintronics, energy harvesting and storage, and chemosensing/biosensing applications.
(I) Organic Spintronics: Spintronics or magnetoelectronics is an emergent technology in which both the quantum spin and charge of the electron is used to carry and store information in charge-based devices. In traditional electronics, equal populations of ms ±1/2 charge carriers exist under thermal equilibrium conditions. In contrast, charge carriers can be induced to undergo a spin preference (i.e. through spin-spin interactions) resulting in spin-polarized current. Electronic devices based on spintronics have the advantages of nonvolatile memory storage, decreased electrical power consumption, increased speed of data processing, and the exciting possibility to both store and process data on a single chip. Our group is investigating the relationship between spin-spin exchange interactions and charge transport parameters in organic semiconducting materials through preparation of n-type and p-type pi-conjugated molecules and polymers that exhibit correlations between semiconducting behavior and spin-spin exchange interactions. Computational modeling is used to provide insight into spectroscopic and physical property measurements carried out on these systems to determine the dominant structural parameters that dictate spin-charge correlations in organic semiconductors. The goal of this project is to understand the mutual effects of spin state and magnetic exchange on charge transport phenomena toward design of ferromagnetic semiconductors with high Tc's for spintronics applications.
(II) Organic Electronics: Organic electronic materials are of interest for development of organic photovoltaic cells (OPVs), organic field effect transistors (OFETs), and electronic inks. Challenges exist in developing new organic materials with high mobilities and conductivities for electronics applications.We have developed synthetic methodology for a novel class of delocalized stable radicals that exhibit low ionization energies/electron affinities and on-site coulomb energies due to spin delocalization. These radicals can be functionalized for incorporation into a wide variety of polymeric and oligomeric structures and exhibit strong p-type or n-type behaviour. By preparing structural series of small molecule/polymer conjugates and studying the effect of structure on charge transport pathways, accompanied by computational modelling, we can determine how open shell ground states and mid-gap states effect the mobilities and electrical properties of redox-active organic semiconducting materials.
(III) Optically-Gated Functional Materials: By utilizing light as an external stimuli for advanced functional materials, the advantages of temporal and spatial resolution can be incorporated into materials for data processing and storage, biosensors, and energy/charge storage devices. We have developed a novel strategy for optical gating of charge and energy transfer processes in metal-organic complexes through the use of photochromic ligands. The photochemical control of electronic structure observed in photochromic spirooxazines is coupled to ligand-field sensitivity in transition metal complexes and pi-conjugated dyads/triads to give optical gating of redox state, spin state, excited state energies, and relaxation kinetics. The systems provide a useful strategy for optical gating of sensing based on charge state, spin state, emissive state, or the presence of biological targets.
(IV) Noncovalent-interactions at organic interfaces. The performance of multilayer electronic devices incorporating solution-caste organic materials is subject to the degree of order/disorder and coexistence of localized/delocalized electronic states inherent in the material. The coexistence of multiple length scales of electronic and structural order decreases charge mobilities, exciton migration, and lifetimes of excited states. Theoretical modelling is also hindered by the inability to model a uniform electronic system. We are currently probing the effects of structure, noncovalent boding, and weak orbital mixing between organic layers, and adsorbates/organic surfaces to determine the dominant structural factors that govern such interactions in organic solid state materials.
(V) Magnetoreception. Hundereds of organisms utilize subtle variations in the earth's magnetic field to navigate over long distances, or to reach their native spawning grounds, such as sea turtles, birds, butterflies, trout, and salmon. In avian magnetoreception, a light dependent mechanism is under investigation, in which a putative chromophore undergoes excitation to form singlet and triplet excited states, the relative yields of which are dependent on magnetic field. In other organisms, a light-independent mechanism is operative, governed by the behavior of biomineralized magnetic Fe2O3 nanoparticles that are tethered to, we believe, mechanosensitive ion channels. The mechanism of how nanoparticle magnetization affects ion channel gating for signal transduction in higher organisms is currently under investigation through probing the relationship between magnetism and ionic conductivity in model systems for higher biological systems.