Volume 14 Issue 7 - July 2, 2010 PDF
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Synthesis, Structure and Photophysical Properties of Highly Substituted 8,8a-Dihydrocyclopenta[a]indenes
Yao-Ting Wu,[a],* Ming-Yu Kuo,[b] Yu-Ting Chang,[a] Chien-Chueh Shin,[a] Tsun-Cheng Wu,[a] Chia-Cheng Tai,[a] Tzu-Heng Cheng,[a] Wei-Szu Liu[b]
[a]Department of Chemistry, National Cheng-Kung University, No.1 Ta-Hsueh Rd., 70101 Tainan, Taiwan.
[b]Department of Applied Chemistry, National Chi Nan University, No. 1 University Rd., 54561 Puli, Nantou, Taiwan
 
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Alkynes contain one or more carboncarbon triple bonds and belong to the class of unsaturated hydrocarbons. They are important building blocks in common organic molecules, natural products and organic conducting materials. The π nature of acetylenes can be easily activated and allows for various reaction types, particularly for CC bond formation. For example, acetylene, the simplest member of the alkyne family, in thermolytic conditions or under the catalysis of transition metals, such as Ni, Pd and Rh, forms benzene. This formal [2+2+2] cyclotrimerization becomes an elegant method for the generation of benzene derivatives and polycyclic aromatics. In contrast, we recently found a new cycloadduct 3a (R1 = R2 = R3 = H) generated from diphenylacetylene 2a (Ar = Ph, Scheme 1). Herein, we describe the systematic optimization of this reaction and the versatility of this method for several alkynes.

Exhaustive studies of the reaction conditions for the synthesis of cycloadduct 3a from diphenylacetylene (2a) showed that the palladium catalyst, the organophosphine, water, benzoquinone and Cu(OAc)2•H2O all play key roles in this reaction. In the absence of any one of them, 3a was only obtained in very low yield. The structure of 3a was determined by the X-ray diffraction analysis. The reactivity of several other alkynes was examined under these optimized conditions, and they generate corresponding 3 in moderate to excellent yields (2885%). In general, byproducts 1 were also produced in small amounts under these reaction conditions, but they can be removed by either careful chromatography or crystallization.
Scheme 1. Metal-catalyzed cyclotrimerizations of alkynes 2.
A solution of 3a in dichloromethane or THF is practically nonfluorescent, but it becomes highly luminescent in the crystal (or solid). As the amount of water in the solution of THF/water was increased, a dramatic change in fluorescence intensity and relative quantum yield (Φ) can be observed (Figure 1). The Φ value of a solution of 3a in THF/water (10/90, W90) is ca. 70 times higher than in pure THF (W0). Moreover, the size of particles plays a key role in their luminescent properties. In a mixture of THF/water (ca. 1:2), 3a aggregates to form spherical nanoparticles with a mean diameter of ca. 10-30 nm. As with other compounds that exhibit aggregation-induced emission, 3a could potentially be used as a sensor for detecting organic solvents.
Figure 1. Photophysical and aggregation properties of 3a. Left: Aggregation-induced emission of 3a (ca. 10-5 M) in a THF/water mixture under illumination of UV light (365 nm). Right: Cycloadduct 3a in THF/water (1:2) forms nanoparticles.
In summary, we have developed a new and versatile synthetic method for the preparation of highly substituted 8,8a-dihydrocyclopenta[a]indenes 3 in one pot by the cyclotrimerization of diarylalkynes. Further studies of the photophysical and electroluminescence properties of functionalized cycloadducts 3 are currently underway, as these molecules may potentially find use as organic light emitting diodes (OLEDs).
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