Acid/base sensitive platinum terpyridyl complex: Switching between metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and intraligand charge transfer (ILCT) states

A platinum(II) 2,2′:6′,2″-terpyridyl complex (2) with a hydroxylphenyl substituent on the terpyridyl ligand and a dimethylamino substituent on the phenylacetylide ligand was synthesized and characterized. Complex 2 exhibits a metal-to-ligand charge transfer (1MLCT) absorption band at ca. 410 nm and a ligand-to-ligand charge transfer (1LLCT) band at ca. 536 nm. It exhibits dual emission at ca. 450 nm and ca. 560 nm at room temperature when excited at 334 nm, which originates from the 1π,π* state and the 3MLCT/3π,π* state, respectively. Dramatic color change was observed for 2 with addition of acid and base. Its emission at 560 nm was enhanced in acidic solution and quenched in basic solution. The changes in absorption and emission could be attributed to the variation of the nature of the lowest excited state from LLCT to MLCT in acidic solution and to LLCT/ILCT at basic solution. The drastic color and emission intensity changes in acidic and basic solutions suggest that 2 could potentially be a colorimetric and luminescent acid/base sensor.