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

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Abstract
Several 4′-aryl-substituted 2,2′:6′,2″-terpyridines (tpy-C6H4R) have been prepared by palladium-catalysed cross-coupling of 4′-bromoterpyridine or 4′-triflate-terpyridine (triflate = trifluoromethylsulfonyloxy) with aryl boronic acids or esters, RC6H4B(OR′)2 (R = H, m-NH2, p-CHO, -NO2, -CN, -NMe2, -NPh2). The new ligand 4′-mesitylterpyridine (mesityl = 2,4,6-trimethylphenyl) was prepared in the same way. Similarly, 4′-bromophenylterpyridine (tpy-ϕ-Br) has been cross-coupled with aryl halides to generate several new biaryl-substituted terpyridines (tpy-ϕ-C6H4R where R = H, p-CN, NMe2, NPh2), together with two related compounds with pendent 3- or 4-pyridyl groups (tpy-ϕ-C6H4-py). For selected compounds, the alternative coupling strategy of reaction of a terpyridine-4-boronate or terpyridine-4-phenylboronate with the appropriate aryl halide has also been investigated (e.g. to prepare tpy-ϕ-C6H4NO2), but was generally found to be less satisfactory. All of the compounds are fluorescent in the UV region of the spectrum, the biaryl-substituted compounds being only slightly red-shifted compared to the monoaryl systems, but with the further red-shift that accompanies protonation being more significant for the former. Fluorescence lifetimes in solution are in the range 1–5 ns. The emission spectra of the aminobiphenyl-substituted compounds (tpy-ϕ-C6H4NR″2, where R″ = Me or Ph) display a large red-shift with increasing solvent polarity, suggesting the involvement of an intramolecular charge transfer state, as found previously for the two analogues omitting the phenyl ring (tpy-C6H4NR″2). In contrast to the latter, however, protonation or binding of a Lewis acidic metal ion to the aminobiphenyl compounds serves to quench almost completely their emission.

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