The synthesis of 4′-aryl substituted terpyridines by Suzuki cross-coupling reactions: substituent effects on ligand fluorescence

摘要

Several 4u0001-aryl-substituted 2,2u0001:6u0001,2u0002-terpyridines (tpy-C6H4R) have been prepared by palladium-catalysed cross-ncoupling of 4u0001-bromoterpyridine or 4u0001-triflate-terpyridine (triflate = trifluoromethylsulfonyloxy) with aryl boronicnacids or esters, RC6H4B(ORu0001)2 (R = H, m-NH2, p-CHO, -NO2, -CN, -NMe2, -NPh2). The new ligand 4u0001-mesityl-nterpyridine (mesityl = 2,4,6-trimethylphenyl) was prepared in the same way. Similarly, 4u0001-bromophenylterpyridinen(tpy-u0001-Br) has been cross-coupled with aryl halides to generate several new biaryl-substituted terpyridines (tpy-u0001-nC6H4R where R = H, p-CN, NMe2, NPh2), together with two related compounds with pendent 3- or 4-pyridyl groupsn(tpy-u0001-C6H4-py). For selected compounds, the alternative coupling strategy of reaction of a terpyridine-4-boronatenor terpyridine-4-phenylboronate with the appropriate aryl halide has also been investigated (e.g. to prepare tpy-u0001-nC6H4NO2), but was generally found to be less satisfactory. All of the compounds are fluorescent in the UV region ofnthe spectrum, the biaryl-substituted compounds being only slightly red-shifted compared to the monoaryl systems,nbut with the further red-shift that accompanies protonation being more significant for the former. Fluorescencenlifetimes in solution are in the range 1–5 ns. The emission spectra of the aminobiphenyl-substituted compoundsn(tpy-u0001-C6H4NRu00022, where Ru0002 = Me or Ph) display a large red-shift with increasing solvent polarity, suggesting theninvolvement of an intramolecular charge transfer state, as found previously for the two analogues omitting the phenylnring (tpy-C6H4NRu00022). In contrast to the latter, however, protonation or binding of a Lewis acidic metal ion to thenaminobiphenyl compounds serves to quench almost completely their emission.