Broadband Nonlinear Absorbing Iridium(III) Complexes: Optimizing the Linear and Nonlinear Absorption via Rational Design

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This project is jointly funded by the Electronic and Photonic Materials Program (EPM) in the Division of Materials Research and by the Chemical Structure, Dynamics and Mechanisms B Program (CSDM B) in the Division of Chemistry (CHE).Non-technical Description: Nonlinear optical materials with strong and broadband nonlinear absorption (the absorptivity of the materials changes with the incident light intensity) are needed for numerous applications in information and image processing technology, such as optical switching, optical rectification, up-conversion lasing, three-dimensional optical data storage, and photodynamic therapy. This project focuses on studying the structure-property correlations in heavy transition-metal complexes with the aim to better understand how the chemical structural modifications influence their linear and nonlinear optical properties. With this understanding, synthesis of transition-metal complexes with broadband spectral (400-900 nm) and temporal (from picoseconds to continuous-wave) coverage can be a reality. The success of this project could provide a comprehensive picture of the relationship between nonlinear absorption and molecular structure, which enables a systematic design procedure for nonlinear absorbing materials. The structure-property correlations explored in this project could also be readily applied to other transition-metal complexes. This research is highly interdisciplinary. It provides students and postdoctoral researchers opportunities to gain knowledge and skills in organic synthesis, coordination chemistry, spectroscopic characterization, nonlinear optical measurements, and theoretical simulations. The interdisciplinary training motivates students' interest in research, and strengthens the workforce for the 21st Century. In addition, as part of the project effort, educational outreach activities involve the Native American students and high school students and inspire them to pursue future careers in materials research. Technical Description: Mononuclear and dinuclear heteroleptic cationic iridium(III) complexes with substituted 2-(2-quinolinyl)quinoxaline (quqo) ligand and various cyclometalating ligands are selected with the aim of understanding the structure-property correlations and achieving broadband nonlinear absorption in the Ir(III) complexes. These complexes are chosen because the strong spin-orbit coupling induced by the Ir(III) ion could lead to high triplet excited-state quantum yield, not only through rapid intersystem crossing but also through direct transition from the singlet ground state (S0) to the lowest triplet excited state (T1). The spin-forbidden S0-to-T1 absorption gives rise to broad and weak ground-state absorption in the 500-800 nm regime when appropriate diimine ligands are used, which is desirable for broadband reverse saturable absorbers. In addition, many of the Ir(III) complexes possess broadband excited-state absorption in the visible to the near-IR regions; and the structures of the Ir(III) complexes could be readily modified. In this project, mononuclear and dinuclear Ir(III) complexes with different pi-conjugations, substituents, and varied substitution positions on the quqo and/or cyclometalating ligands are synthesized. Their ground-state and excited-state properties, and nonlinear absorption, as well as the effect of inter- and intramolecular interactions on the photophysics and nonlinear absorption of these complexes in high concentration solutions and solid state are investigated systematically via a combination of spectroscopic characterizations and density functional theory (DFT) calculations. The combined experimental-theoretical approach allows the team to establish the correlations between nonlinear absorption and molecular structure of Ir(III) complexes and enables a rational design of materials with well-controlled broadband nonlinear absorption.