Unravelling the quenching mechanisms of a luminescent Ru(II) probe for Cu(II)

We have investigated the photophysical and photochemical features of a luminescent heteroleptic Ru(II)-polypyridyl probe and of its corresponding Ru(II)-Cu(II) dinuclear complex formed upon the analyte binding through extensive density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The molecular probe contains the tailored imidazo[4,5-f]-1,10-phenanthroline (IIP) ligand for simultaneously binding the Ru(II) core and the target metal ion in aqueous solution. We have rationalized the static photoluminescence quenching observed upon the Cu(II) coordination, on the grounds of distinct excited state deactivation mechanisms which are absent in the free Ru(II) complex probe. Additionally, the emission quenching found upon increasing the solution pH has also been investigated. When coordinated IIP deprotonates, the nature of the lowest excited state of its complex changes from (3)MLCT to (3)LLCT/(3)IL. The strong base-induced emission quenching can be understood in terms of both the energy-gap law, since the (3)LLCT/(3)IL states lie at a significantly lower energy than the (3)MLCT state increasing the contribution of non-radiative mechanisms, and the expected slower radiative rates from such (3)LLCT/(3)IL states. After Cu(II) binding, the lowest triplet excited state is similar to the analyte-free probe in both energy and electronic nature. However, Cu-centered non-radiative excited states, populated after photoinduced electron transfer and intersystem crossing processes, are responsible for the population drainage of the emissive state.