Acid-sensitive Pt(II) 2,6-di(pyridin-2-yl)pyrimidin-4(1H)-one complexes

Three Pt(II) 2,6-di(pyridin-2-yl)pyrimidin-4(1H)-one complexes (2-4) with chloride or 4-ethynyltolyl ancillary ligands were synthesized and characterized. The photophysical properties of 2-4 were investigated in different solvents and at different acid concentrations. Their electronic absorption and emission responses at various acid concentrations were compared to those of 5. 2-4 all exhibit a broad charge-transfer band in their electronic absorption spectra from 380 to 500 nm and emit at about 560 nm in acetonitrile at room temperature, presumably ascribed to the triplet metal-to-ligand charge transfer ((3)MLCT) state. All of them exhibit broad and relatively strong triplet transient absorption in the visible to the near-IR region (450-820 nm). Upon addition of p-toluenesulfonic acid, the original charge-transfer band intensity decreases, accompanied by the increase of the absorption in the 350-400 nm region in their electronic absorption spectra. Meanwhile, the (3)MLCT emission is quenched, and the triplet transient absorption intensity decreases. The changes in electronic absorption, emission, and the transient absorption spectra are reversible upon the addition of base, that is, triethylamine. The reversible acid sensitivity is caused by the protonation and deprotonation of the carbonyl oxygen on the terdentate ligand. Therefore, these platinum complexes could be potential chromogenic and luminescent sensors for acids.