Cross-coupling reactions in the development of new luminescent iridium bis-terpyridyl complexes
Luminescent complexes of 2(^nd) and 3(^rd) row transition metal ions with polypyridyl ligands are of growing interest as molecular luminescent sensors. The long luminescence lifetime of iridium (III) bis-terpyridine, [Ir(tpy)(_2)](^3+), makes it and its derivatives of particular interest, but the further development of such compounds requires improved synthetic routes as well as a greater understanding of their photophysical properties. The palladium-catalysed Suzuki cross-coupling methodology has been investigated as a means for the preparation of 4'-aryl and biaryl substituted 2, 2': 6', 2"-terpyridines under mild conditions. Optimal yields were obtained through the coupling of 4'-bromoterpyridine or 4'-bromophenylterpyridine with aryl boronic acids or esters, to yield aryl or biaryl-substituted terpyridines respectively. A photophysical study has revealed that most of the terpyridines prepared in this way emit in the near-UV region, from a ligand-centred (πtpy-πtpy*) state, with lifetimes in the 1-5 ns range. However, those bearing a para-aminq group in the pendent aryl or biaryl substituent display solvatochromic emission assigned to an intramolecular charge transfer (ICT) state (πph-πpy*). The ICT state is stabilised upon binding of zinc ions to the terpyridyl moiety, leading to a large red-shift in emission, a response which is almost unique to this metal ion. The utility of the cross-coupling strategy for the in situ elaboration of bromo-functionalised iridium bis-terpyridyl complexes has also been demonstrated. The complex [Ir(tpy)(tpy-ɸbr)](^3+) reacts with aryl boronates to yield biaryl-substituted complexes directly. These complexes display yellow-orange emission in degassed solution, with luminescence lifetimes in excess of 100 μs, assigned to an excited state of primarily (^3)π-π* character. Homoleptic iridium (III) complexes of para-amino-substituted 4'-aryl terpyridines have been prepared directly by complexation of the metal to the ligands. These complexes display weak, short-lived emission in the near infra-red region of the spectrum, assigned to a 'ILCT state which is stabilised relative to the free ligand by the binding of the iridium (III) ion to the terpyridine unit.