Synthesis of soluble bis-terpyridine ligands bearing ethynylene-phenylene spacers

Soluble and rigid terpyridine-based ditopic ligands bearing one to five phe nylene/ethynylene modules have been synthesized by way of a stepwise proced ure. Each module is attached to the terpyridine unit via an ethynylene frag ment and functionalized at the 4-position with an additional ethynylene con nector and in the a,5-positions with two flexible dodecyloxy chains. The sy nthetic protocol is based on sequential Pd(0)-catalyzed cross-coupling reac tions between a terpyridine subunit grafted with the necessary diethynyl/ph enyl or ethynylphenyl/bromide appendage. For ditopic ligands displaying an even number of phenyl/ethynylene modules, the final step involves a single cross-coupling reaction between 4'-ethynylene-2,2':6',6"-terpyridine and th e appropriate bromo derivative. In the case of the ligands having an odd nu mber of phenylene/ethynylene fragments, a double cross-coupling reaction be tween an extended dibromopolyphenylene intermediate and 4'-ethynylene-2,2': 6',6"-terpyridine or 1-(4'-ethynylene-2,2':6',2"-terpyridine)-4-ethynylene- 2,5-didodecyloxy- benzene is required. For ligands I-V, optimal preparative conditions were found with [Pd-0(PPh3)(4)] (6 mol %) in n-propylamine at 7 0 degrees C. Qxidative dimerization of the 1-(4'-ethynylene-2,2':6',2"-terp yridine)-4-ethynylene-2,5-didodecyloxybenzene derivative in the presence of cupric salts and oxygen gives the corresponding homoditopic ligand II2 bea ring a central diphenyldiacetylene spacer. Spectroscopic data for the new o ligomers are discussed in terms of the extent of pi-electron conjugation. U pon increasing the number of phenylene/ethynylene modules, there is a progr essive lowering in energy of absorption and fluorescence transitions.