The Molecular Construction And Properties Of Polydiacetylene Materials With Reversible Thermochromism

Polydiacetylenes, one kind of conjugated polymers, have attracted much attention in both theoretical and practical research fields. Polydiacetylenes have eminent optical and electrical properties, and are thus promising in nonlinear optical, photovoltaic and biomedical applications. Besides, polydiacetylenes have a variety of chromism; they change their color when exposed to the external stimuli such as heat, pH, ions, mechanical stress and magnetic or electrical field, which make polydiacetylenes promising for chemical sensors or biosensors. Among the chromism, thermochromism is most widely studied. The thermochromism of polydiacetylenes in their original form is irreversible and the irreversibility prevents them from being repeatedly used. For preparing polydiacetylenes with reversible thermochromism, the structures and interaction of the diacetylenes have to be deliberately controlled. The reversibility requires that the interaction among the side chains of polydiacetylenes is sufficiently enhanced (which was realized in literature through chemically modifying the diacetylenes), or the side chains of polydiacetylenes are fixed on a solid matrix including silica, carbon nanotube or polymers. It also requires that all the chemical and physical modifications should not prevent the backbone of diacetylenes from packing properly, otherwise they cannot be topochemically polymerized. These usually make the preparation of reversible polydiacetylenes difficult. In this thesis, we fabricated two kinds of polydiacetylenes nanocomposites with completely reversible thermochromism. We investigated the mechanism and the factors that affect the reversibility and also proposed a new model to explain this reversible process. The thesis includes the following two sections:1.Polydiacetylenes-melamine nanocrystals showing reversible thermochromismHerein we report an approach for preparing nanocrystals of a commercially available diacetylene (10,12-pentacosadiynoic acid, PCDA) and melamine (MA) with the cocrystal structure, whose polymerization product (PDA-MA) is completely reversible in thermochromism. Since the interaction between PCDA and MA is weak, and the latter molecule has a strong tendency to crystalize alone, existing methods cannot lead to PCDA-MA cocrystals. A Non-Covalent Connected Micelles (NCCM) plus annealing method for preparing nanocrystals is used to solve this problem. After polymerization, PCDA-MA crystals become PDA-MA crystals, in which each side chain of PDA interacts with MA molecules to make the PDA, completely reversible. At the same time, MA is easy to form two dimensional hydrogen boding layer and has a high melt point, which is a matrix or scaffold for diacetylene to tether on it. We found the proper annealing temperature and the interaction between PCDA and MA are the key points to the reversibility of PDA-MA.2.Polydiacetylenes-lanthanide ions nanocrystals with completely reversible thermochromism and modulating fluorescence resonance energy transfer process.Herein, we report a convenient and robust approach to the PDA-Ln (Ln represents the lanthanide ions) with the completely reversible thermochromism. PCDA-Ln nanocrystals were prepared simply by insertion the lanthanide ions into the bilayered PCDA crystal structure and then followed by annealing at the temperature slightly higher than the melting point of pure PCDA crystals. After the topochemical polymerization, PCDA-Ln nanocrystals became PDA-Ln nanocomposites, which the carboxylic groups of PDA are non-covalently cross-linked by the multivalent lanthanide ions. The cross-linking provides the restoring force for the PDA backbones to restore to their original conformation upon cooling back to room temperature, making the PDA-Ln nanocomposites completely reversible in the thermochromism. At the same time, due to the spectral overlap of the donor (Ln3+ions) emission spectrum and the acceptor (PDA) absorption spectrum, we found different lanthanide ions can enhance the intensity PDA’s fluorescence through fluorescence resonance energy transfer (FRET) process at different degrees.