| Abstract
| - Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner.The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-likearray with a diameter of ∼35 Å. One array is comprised of five zinc (Zn) porphyrins and one freebase (Fb) porphyrin (cyclo-Zn5FbU) while the other is comprised of an alternating sequence oftwo Zn porphyrins and one Fb porphyrin (cyclo-Zn2FbZn2FbU). The prior synthesis employed aone-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Znporphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated couplingreactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, oriodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction ofa porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presenceof a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5+ 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing thestructure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successiveethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn2FbZn2FbUemployed a magnesium porphyrin, affording cyclo-Zn2FbZn2MgU from which magnesium wasselectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectraof the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showedthat the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrinsto the Fb porphyrin(s) was 60% in cyclo-Zn5FbU and 90% in cyclo-Zn2FbZn2FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesizedfor use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies.
|
