DMREF: SusChEM: Simulation-Based Predictive Design of All-Organic Phosphorescent Light-Emitting Molecular Materials

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

DMREF: SUSCHEM: SIMULATION-BASED PREDICTIVE DESIGN OF ALL-ORGANIC PHOSPHORESCENT LIGHT-EMITTING MOLECULAR MATERIALS Non-technical Description: Organic light emitting diodes (OLED) exhibit remarkable energy efficiency in applications ranging from urban lighting to large-screen display panels. Current technologies are based on phosphorescent materials that contain organo-metallic compounds, which involve heavy-metal ions. These are expensive to procure, present limitations with regard to device longevity, and in some cases are considered environmentally unsafe or even toxic. The goal of this research is to eliminate the need for heavy-metal ions by developing a fundamentally new class of all-organic phosphorescent molecules. The principal task is to design molecules in which the juxtaposition of electronic orbitals promotes the processes underlying phosphorescence while at the same time the chemical bonding patterns provide the structural rigidity needed to minimize the non-radiative decay of electronic excitations. To this end an integrative computational-experimental approach is employed, in which molecular simulations, chemical synthesis, and materials characterization are combined in a synergistic and iterative sequence. The expected outcomes of this project are novel environmentally benign phosphorescent materials that are based on sustainable chemistries and that are immediately deployable for lighting applications. The new insights into the functional response of molecular materials gained while perfecting metal-free OLED benefits organic electronics in general, and advance technologies such as photovoltaics, sensors, and displays. Finally, software toolkits, data management utilities, and workflows for simulation-based predictive materials design are established as a new paradigm for materials development. Technical Description: The efficiency of phosphorescent materials is based on the ability to emit not only from singlet but also triplet excited states, which are populated as a result of spin-orbit coupling. The strength of this coupling is attributed to the presence of heavy-metal ions in organo-metallic compounds. However, organo-metallics are accompanied by significant challenges: besides the high cost of precious metals, dislocated metal ions in the emitting layer may trap charge, which jeopardizes device longevity. By serendipity, the co-PI demonstrated metal-free organic phosphors with unprecedented high solid-state phosphorescent quantum yield of up to 68% at ambient conditions. The current research aims to further develop this fundamentally new, environmentally benign, and chemically sustainable class of all-organic phosphorescent molecules with improved performance characteristics by employing an integrated computational-experimental approach. Specific objectives are to (i) eliminate the heavy metal ions form the emitting molecules with the aim to lower materials cost and obtainability, improve ease of fabrication, and prolong device lifetime and dependability; (ii) deconvolute the dual roles of halogen bonding, i.e., to promote spin-orbit coupling and suppress vibrational energy dissipation, and supplant the intermolecular secondary bonding-induced phosphorescence enhancement mechanism with intramolecular analogs; (iii) optimize the molecular architectures of both the emitting and host species so as to minimize vibration-mediated non-radiative decay of excited states through stiffening of intramolecular bonding patterns, stabilization of emitters by host molecules designed to suppress detrimental vibrations within effectively packed geometries, and crystallization of emitters within nano-confinement. To this end, concept emitter and host molecules are constructed and their structure and electronic properties, e.g., excited state energies, singlet-triplet transition rates, charge mobilities, etc., predicted using first-principles calculations. Structural models are generated using shape packing algorithms and molecular simulations, and possible crystal structures are predicted. Best candidate molecules are synthesized, characterized, and their emissive and vibrational properties measured.