Metal-Based Molecular Functional Materials - From Discrete Metal Complexes to Supramolecular Assembly, Nanostructures and Functions

The continuous search for smart and advanced materials with enhanced photofunctional features has attracted a world-wide interest and resulted in great efforts towards the investigations and developments of molecular-based functional materials. It has been known that the chemical and physical properties of various organic moieties can be influenced by and correlated to their molecular structures. As a result of this, the coordination chemistry of transition metal complexes, through the utilization of various transition metal centers and their incorporation into the organic moieties with diverse combinations, has been found to give rise to superior advantages of greater versatility of molecular design and thus leading to more in-depth understanding for the exploration of photofunctional materials with tunable photophysical and excited state properties. In the past few decades, the photophysics and photochemistry of transition metal complexes have drawn inter-disciplinary attention and resulted in significant impact towards materials and energy research, especially with the detailed investigations on the class of ruthenium(II) polypyridine complexes with the characteristic metal-to-ligand charge transfer (MLCT) excited state, arising from their rich photoredox and photoluminescence properties and good stability. These have provided insights into the exploration of molecular-based functional materials with concepts of supramolecular photochemistry. In addition to the ruthenium(II) polypyridine system, researchers also investigated the photophysics and photochemistry of iridium(III) system because of their synthetic versatility, high photoand thermal stabilities with tunable emission color and thus can be applied as triplet emitters and phosphorescent dopants in organic light emitting devices (OLEDs). In addition, lots of efforts have also been devoted into the exploration of new classes of luminescent transition metalligand chromophores and their application in materials chemistry and science by understanding and rationalization of their excited state properties. By the utilization of ligands with versatile structural and electronic properties, the excited state properties of transition metal complexes can be fine-tuned and controlled. For example, the incorporation of strong s-donating ligands has been found to cause drastic luminescence enhancement of some classes of transition metal complexes because of the destabilization of non-emissive d–d ligand field excited states. Compared to the d ruthenium(II) and iridium(III) complexes with octahedral geometry, the square-planar d transition metal complexes with coordinationunsaturated nature has been found to exhibit intriguing spectroscopic and luminescence properties due to their unique tendency to form non-covalent metal···metal interactions. Platinum(II) complexes have been extensively explored in previous years due to their intriguing chromphoric and aggregation properties. The introduction of supramolecular assembly components involving non-covalent interactions could lead to other dimensions of unlimited possibilities and opportunities. In this review, the utilization of this class of complexes in supramolecular assembly and various functions will be discussed. Examples include applications in ion-binding, solvent-induced aggregation, nucleic acid-induced aggregation responsive materials and light harvesting molecular devices. The unique features for this class of complexes could also be attributed to their versatile emissive excited states that are strongly affected by subtle changes in the local environment. Metal-Based Molecular Functional Materials From Discrete Metal Complexes to Supramolecular Assembly, Nanostructures and Functions Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong Vivian Wing-Wah Yam* Received March 27, 2017; E-mail: wwyam@hku.hk