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Synthesis of substituted phenylene-ethynylene- based conjugated rods

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Abstract

A series of -substituted-4-(4-trimethylsilanylethynyl-phenylethynyl)benzenes (4a-e) have been prepared via Pd(^1’")/Cu(^1)-catalysed cross-coupling of 1-iodo- or bromo-p-substituted benzenes with l-(trimethylsilylethynyl)-4-ethynyl benzene (3). Deprotection by removal of the TMS from compounds (4a-e) was used to obtain p-ethynyl tolans with different functional groups (5a-e) at the other para position. A number of new symmetrical conjugated diynes (6a-e) were formed by homocoupling of the terminal alkynes (5a-e).A Harnett plot λ(_masx) (Abs) versus Hammet constants Cp for compounds (4a-e), (5a-e) and (6a-e) showed that both donors and acceptors red shift absorption. Also, diynes have been synthesised from esters containing a long chain aliphatic group to improve solubility. Thus, 4-bromobenzoic acid was reacted with octan-1-ої to obtain 4- bromobenzoic acid octyl ester, 7, which has a Cg chain and a Br group in the preposition for subsequent coupling reactions. Then compound 7 was used in two pathways. In path A, compound 7 was coupled with TMSA to obtain the protected alkyne, 8. Then the TMS protecting group was removed under mild condition to give the terminal alkyne, 9, which was oxidatively homocoupled to give diyne, 10.In path B, compound 7 was coupled with compound 3 to give 4-(4- trimethylsilanylethynylphenylethynyl) benzoic acid octyl ester, 11. The next step was deprotection of 11 to form the terminal alkyne 12. Lastly, 2 equivalents of compound 12 was oxidatively homqcoupled to give 4,4'-bis-(4"-carbooctyloxylphenylethynyl)diphenylbuta-1,3-diyne, 13.

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