Convenient and Efficient Synthesis of Functionalized Oligopyridine Ligands Bearing Accessory Pyrromethene-BF2Fluorophores

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Abstract
The synthesis of stable and highly luminescent pyridine-, bipyridine-, phenanthroline-, bipyrimidine-, and terpyridine-based ligands bearing one or two 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) modules has demonstrated the advantages of three different protocols which have been adapted in light of the chemical stability of the alkyne-grafted starting building blocks and the chemical reactivity of the bromo-substituted starting materials. A classical method of condensation of aldehydes or acid chlorides with Kryptopyrrole has been used for direct linkage of a bodipy to the oligopyridinic platform. For the phenylethynyl-linked molecules, direct coupling between the bodipy-phenyliodo and the stable 4‘-ethynyl-2,2‘:6‘,2‘ ‘-terpyridine, 6,6‘ ‘-diethynyl-2,2‘:6‘,2‘ ‘-terpyridine, 5-ethynyl-2,2‘-bipyridine, 5,5‘-diethynyl-2,2‘-bipyridine, 6,6‘-diethynyl-2,2‘-bipyridine, and 5,5‘-diethynyl-2,2‘-bipyrimidine substrates is feasible and is promoted by Pd catalysts and sonication. This procedure provides the advantages of efficiency, versatility, and rapidity. A second set of experimental conditions is required to produce the 4-substituted pyridine, 3,8-disubstituted-1,10-phenanthroline, and 5,5‘ ‘-disubstituted-2,2‘:6‘,2‘ ‘-terpyridine derivatives. Cross coupling of a bodipy-phenylethynyl molecule with the bromo-substituted partners takes place smoothly with the pyridine but with low yields in the other cases due to the efficient formation of the homocoupled diphenylbutadiyne bodipy compounds. A third convenient protocol enabled the preparation of these target molecules in a one-pot reaction where the deprotection of the alkyne was conducted in situ by a phase-transfer process with aqueous NaOH and with Et3BnN+Cl- as mediator and the cross-coupling reaction realized in the benzene phase with [Pd(PPh3)4] as catalyst and CuI as co-reagent. The nascent acid was quenched in the aqueous phase. This method is much more efficient when a trimethylsilyl protecting group is used instead of a propargylic alcohol. The pyridino-bodipy bases were alkylated smoothly in good yields in the presence of methyl iodide. The photophysical and electrochemical properties for these new molecules have been investigated and are discussed in terms of substitution patterns of the bipyridine and terpyridine platforms. In the pyridinium salt 18, the fluorescence from the first singlet excited state at λem = 547 nm is totally quenched at the expense of a low-lying charge-transfer emitting state located at λem = 660 nm.