Preparation, Photophysical Properties and Conformational Behavior of Conjugated Metallopolymers Bearing Zinc(II)- and Platinum(II)-Schiff Base Moieties

Abstract

Considerable interest has been focused on the development of coilable polymers due to their structural diversity as well as the appealing range of applications, including molecular recognition and biomimetic materials. More recently, metal-organic moieties have been incorporated into polymeric structures to achieve good tunability, well-defined coordination geometries and luminescent properties. This thesis describes the design, synthesis, characterization, photophysical and supramolecular properties, as well as applications of conjugated metallopolymers bearing metal-Schiff base moieties.

Chapter 1 gives a general introduction to conjugated polymers and coilable metallopolymers, as well as a brief review of the recent progress in the development of chirality-responsive polymers. Chapter 2 describes general experimental details.

In Chapter 3, metallopolymers containing salphen-Zn(II) moieties with coilable and linear structures have been synthesized using the Sonogashira coupling reactions to polymerize terminal phenylene-diethynylenes (PEs) and salphen-Zn (aromatic Schiff base) units, and their properties and applications are described. The Zn-based polymers with m-PEs generally display CD signals that are stronger than those with p-PEs, and deprotected polymers tend to show greater intrachain interactions, resulting in stronger Cotton effects and red-shifted emission bands. Because of the existence of Zn⸱⸱⸱O(salphen) interactions, stronger Cotton effects are observed in non-coordinating solvents, but weaker Cotton effects are evident in coordinating solvents, which can interact more strongly with the folded polymer chains and disrupt the folded conformation, giving a more extended structure. Studies into concentration effects on emission and circular dichroism (CD) spectra have been undertaken, and the observed red-shifted emission maxima (> 1 × 10^‒4 M) and red-shifted CD bands (> 2 × 10^‒4 M) signify dominant interchain interactions at high concentrations. The reversible and dynamic nature of the Zn-based polymers are confirmed reversible CD changes in different solvents and at various temperatures. By taking advantage of biocompatible substituents and the ordered (helical) conformation of the polymer chains, the potential bioactivity of the deprotected polymers have been investigated.

Attention has been focused on incorporating Pt-Schiff base moieties into polymeric structures. In Chapter 4, the synthesis of Pt-based metallopolymers, as well as the study of their photophysical properties and conformational behavior have been undertaken. The Cotton effects observed for polymers with m-PEs are generally stronger than those with p-PEs, and this suggests more tightly coiled structures for the former. Removal of acetyl groups from the polymers have been achieved, and the resultant deprotected polymers display stronger and red-shifted Cotton effects compared with their protected analogues, indicating that the polymer chains are more tightly coiled, possibly due to stronger intrachain interactions at low concentrations (< 1 × 10^‒4 M). Organized interchain aggregation for P7 and P9 have been observed at higher concentrations (> 2 × 10^‒4 M). The CD spectra display reversible changes in different solvents, and variable temperature CD experiments show weakening Cotton effects at higher temperatures. These results demonstrate the flexibility, reversibility and dynamic nature of the folded conformation in these Pt-based polymers.

The consequences of different side-arm substituents, and employing m-diethynylpyridines (m-EPs) as building blocks, are examined in Chapter 5. For the Zn-based polymers, chirality transfer from the substituents on salphen moieties is more efficient than from the PE units, as indicated by CD spectroscopy. The Zn-based polymers with m-EPs show significantly stronger Cotton effects than the Pt-based analogues. Studies into aggregation behavior reveal that interchain interactions become dominant at high concentrations, which are consistent with the corresponding red-shifted emission spectra.       Discussions concerning general comparisons of the polymer systems described in Chapters 3-5 are presented in Chapter 6.

Date of Award	3 Jan 2019
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael Chi Wang CHAN (Supervisor)