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

该项目由材料研究部的电子与光子材料计划(EPM)和化学部的化学结构、动力学和机制B计划(CSDM B)联合资助。非技术描述：具有强的宽带非线性吸收(材料的吸收率随入射光强度变化)的非线性光学材料在信息和图像处理技术中的许多应用，如光开关、光整流、上转换激光、三维光学数据存储和光动力学治疗。本项目致力于研究重金属过渡金属配合物的结构-性质相关性，以期更好地了解化学结构修饰对其线性和非线性光学性质的影响。有了这种理解，合成具有宽带光谱(400-900 nm)和时间(从皮秒到连续波)覆盖的过渡金属配合物是可能的。该项目的成功将为非线性吸收与分子结构之间的关系提供一个全面的图景，从而使非线性吸收材料的系统设计成为可能。本项目中探讨的结构-性质相关性也可以很容易地应用于其他过渡金属络合物。这项研究是高度跨学科的。它为学生和博士后研究人员提供了在有机合成、配位化学、光谱表征、非线性光学测量和理论模拟方面获得知识和技能的机会。跨学科的培训激发了学生对研究的兴趣，并加强了面向21世纪的劳动力。此外，作为该项目努力的一部分，教育推广活动吸引了美洲原住民学生和高中生，并激励他们在材料研究领域追求未来的职业生涯。技术描述：为了了解Ir(III)配合物的结构-性质相关性并实现宽带非线性吸收，我们选择了单核和双核异构型阳离子Ir(III)与取代的2-(2-喹啉)喹恶啉(QuQO)配体和各种环金属化配体的配合物。选择这些配合物是因为Ir(III)离子诱导的强自旋-轨道耦合不仅可以通过快速的系间交叉，而且可以通过从单态基态(S0)到最低三态激发态(T1)的直接跃迁来产生高的三重态激发态量子产额。当使用合适的二亚胺配体时，自旋禁止的S0到T1吸收在500-800 nm范围内产生了宽而弱的基态吸收，这对于宽带反饱和吸收是可取的。此外，许多Ir(III)配合物在可见光到近红外区具有宽带激发态吸收，并且Ir(III)配合物的结构易于修饰。在这个项目中，合成了具有不同的pi-共轭、取代基和不同取代位置的单核和双核Ir(III)配合物。通过光谱表征和密度泛函理论(DFT)计算相结合的方法，系统地研究了它们的基态和激发态性质、非线性吸收以及分子间和分子内相互作用对它们在高浓度溶液和固体中的光物理和非线性吸收的影响。实验和理论相结合的方法使研究小组能够建立Ir(III)配合物的非线性吸收和分子结构之间的关联，并使合理设计具有良好控制的宽带非线性吸收的材料。

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.