Terthiophene appended with terpyridine units as sensors of protons, and Zn2+ ions: photoinduced energy and electron transfer processes

Two ligands consisting of 2,2' : 6',2 ''-terpyridine (tpy) and terthiophene units (3T) connected by a methyleneoxy (TTT) or an alkynyl bridge (TAT) show different photophysical properties: in the former there is no evidence of a ground state interaction between the two constituent chromophores and the expected 3T emission is observed. On the other hand, the latter exhibits strong p-conjugation between the two chromophores mediated by the alkynyl bridge: this results in a substantially redshifted absorption and an emission displaced toward the red with a decreased quantum yield. Both of these ligands can be diprotonated: protonation occurs at the tpy unit in two successive steps, leading finally to TTT center dot 2H(+) and TAT center dot 2H(+). In both cases, strong changes in the absorption spectra and very efficient quenching of the luminescence takes place by a photoinduced electron transfer from 3T to the protonated tpy unit. Both TTT and TAT can also perform as Zn2+ ligands, giving rise to complexes with 1:2 metal/ligand stoichiometry in acetonitrile solution, as demonstrated by changes in the photophysical properties. Also in this case, the resulting complex is not luminescent because of photoinduced electron transfer from 3T to the [Zn(tpy)(2)](2+) moieties.