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43328-G7
Photochromic Conjugated Polymers and Dendrimers: Design, Synthesis, Characterizations, and Properties Investigation
Pi-conjugated polymers and oligomers represent a new class of semiconductive materials that have received considerable attention in the past 20 years. These materials have been demonstrated to exhibit interesting optoelectronic properties and they have found potential applications in areas such as field-effect transistors, organic light-emitting devices, displays, chemo and biosensors. Precisely defined oligomers with well-defined chemical compositions are useful for the structure-property study of conjugated polymers. They serve as excellent models for their corresponding polydisperse macromolecular counterparts and provide useful information into the structural and conformational properties. In recent years, the design of new molecular structure has expanded from linearly conjugated system to two- and three-dimensionally structured conjugated polymeric systems.
Dendrimer is a unique example of well defined polymer with perfect three-dimensional structural control. Dendrimers have attracted considerable attention because of their unique molecular architecture and novel properties and they have found uses as catalysts, drug delivery systems, and membranes. Among the different classes of dendrimers reported, conjugated dendrimers have been synthesized and studied as potential materials for applications in the area of sensors, organic light-emitting diodes, and organic photovoltaics. Well-known examples of conjugated dendrimers include phenylacetylene dendrimers as developed by Moore and coworker, phenylenevinylene dendrimers by Yu as well as more recently, Peng's unsymmetrical dendrimers based on the 1,3,4-substituted phenylacetylenes linkage.
Photochromic molecules and systems have been studied for many decades and photochromism refers to the reversible photo-isomerization of a chemical species between two forms having different absorption spectra. It can be envisaged that by integrating the concept of photochromic systems and g-conjugated polymers or dendrimers, it may be possible to furnish new organic conductive polymers exhibiting interesting optoelectronic properties that are potentially photoswitchable. Recently, we have described a conjugated polymer incorporating the spirobenzopyran unit in the main chain of poly(p-phenyleneethynylene). Spirobenzopyran was introduced to the system due to the reversible change between the closed-ring colorless spiro (SP) structure and the open-ring colored merocyanine (MC) form and the substantial difference in electronic structure between the two isomers. Upon irradiation, the system will become more conjugated upon ring opening of the spirobenzopyran moieties.
In this past year, our research has been focused on extending this design concept to the dendritic system as there will be multiple delocalization pathways available within the dendritic network that could result in higher on/off signal ratio between the two isomeric states. To this end, four new dendrimers, each bearing either two or six spirobenzopyran units, have been synthesized by palladium-catalyzed Sonogashira cross-coupling between an AB2-type diiodide and a terminal acetylene in each successive generation built-up. Two of these dendrimers have been substituted with long alkyl chains in order to enhance their solubility properties. For comparison, it was also found that analogous dendritic structures without the long chains are also soluble in common solvents such as dichloromethane, chloroform, THF or toluene, rendering their structural and optical characterization relatively simple. A preliminary investigation of the photochromic properties of these dendrimers was accomplished with UV/vis spectroscopic measurement and all of them show intense absorption band below 400 nm, ascribable to the extended pi-conjugated backbone. Upon irradiation with UV light (365 nm), there is a new absorption band appearing in the region between 500 and 700nm, a signature of the open-ring merocyanine state. This work has demonstrated the initial proof-of-concept that optical switching could be observed in architecturally interesting three-dimensional dendritic macromolecular structures as well.
During the second year of the funding period, this project was conducted by a postdoctoral fellow and a summer undergraduate student. These researchers have now developed not only modern organic materials synthetic techniques but also the tools needed to study other related applications such as photovoltaics and field-effect transistors. Both of them are planning to pursue a career in academic research by moving to an academic institution as a faculty and as a graduate students focusing on organic photovoltaic materials research in the coming year.
