Counteranion mediated controllable self-assembly and photo-physical properties of Au(I), Pt(II) metal complexes and metal-free organic compounds

Abstract

Organometallic complexes containing d8 and d10 metal ions have attracted significant attention due to their rich luminescent properties and ability to form functional supramolecular materials, which have a wide range of potential applications. This thesis concentrates on the exploration of supramolecular assemblies and excited states of d8 and d10 metal ions, including Au(I) and Pt(II). Additionally, the self-assembly of metal-free organic molecules has been investigated, revealing counteranion-modulated photo-physical properties and exciton dynamics.

Introduction of multiple kinetic aggregation states (Aggs) into the self-assembly pathway can bring complexity and flexibility to the self-assemblies, which is difficult to realize due to the delicate equilibria established among different Aggs bonded by weak non-covalent interactions. A series of chiral and achiral d10 Au(I) bis(N-heterocyclic carbene, NHC) complexes with different counteranions were synthesized, and the achiral complex could undergo counteranion-mediated self-assembly with multiple kinetic Aggs. Generation of multiple kinetic Aggs was realized by applying chiral or achiral seeds exhibiting large differences in elongation temperatures for their respective cooperative self-assembly processes. The chiral self-assemblies having non-centrosymmetric packing forms exhibit nonlinear optical response of second harmonic generation (SHG), while the SHG signal is absent in the achiral analogue. The crystalline achiral self-assemblies can function as optical waveguides with strong emission polarization.

Creating hierarchical molecular block heterostructures, with the control over size, shape, optical, and electronic properties of each nanostructured building block can help develop functional applications, such as information storage, nanowire spectrometry, and photonic computing. However, achieving precise control over the position of molecular assemblies, and the dynamics of excitons in each block, remains a challenge. The fabrication of molecular heterostructures with the control of exciton dynamics in each block was first demonstrated using metal-free organic imidazolium compounds with different counteranions. Additionally, these heterostructures are printable and can be precisely positioned using 3D printing technique, resulting in programmable patterns. Singlet excitons with emission lifetimes on nanosecond or microsecond timescales and triplet excitons with emission lifetimes on millisecond timescales appear simultaneously in different building blocks, with an efficient energy transfer process in the heterojunction. These organic materials also exhibit stimuli-responsive emission by changing the power or wavelength of the excitation laser. These organic heterostructures could have potential applications in integrated photonics, where the versatility of fluorescence, phosphorescence, efficient energy transfer, printability, and stimulus sensitivity co-exist in a single nanowire.

A stepwise counteranion exchange reaction of a Pt(II)-Au(I) double salt complex is demonstrated, associated with a dynamic self-assembly process and a change in intermolecular packing form. The metal-ligand coordination reaction and self-assembly process of the Pt(II)-Au(I) complexes can be further manipulated by a photochemical approach and monitored in real-time by using fluorescent microscopy.