Switching of excited states in cyclometalated platinum complexes incorporating pyridyl-acetylide ligands (Pt-C C-py): a combined experimental and theoretical study

This article presents the design of cyclometalated platinum(II) complexes incorporating pyridyl-appended acetylide ligands of the form Pt-C C-py, acting either as sites for protonation or methylation reactions or as a host receptor for binding metal cations. The complexes studied are Pt(t-Bu(2)phbpy)(-C C-py), 2, which can undergo protonation at the pyridyl N; its cationic N-methylated derivative [Pt(t-Bu(2)phbpy)(-C C-pyMe)](+), 4, which serves as a model of the N-protonated species; and a derivative in which the pyridyl ring is incorporated into a macrocyclic diamide-crown ether ligand (3). The co-ligand t-Bu(2)phbpy is a cyclometalated, (NNC)-N-boolean AND-C-boolean AND-coordinated phenylbipyridine ligand carrying tert-butyl groups at the 4-positions of the pyridyl rings. The photophysical properties of the neutral compounds 2 and 3 have been compared to those of the pyridinium, methyl-pyridinium or metal-complexed species (namely 2-H+, 4 and 3-Pb2+). Detailed TD-DFT calculations provide a theoretical basis to account for the experimentally-observed changes upon protonation/methylation/complexation. The joint TD-DFT and experimental studies provide evidence for an unprecedented molecular switch in the nature of the excited state (from mixed L'LCT/MLCT to ML'CT) in which the acceptor ligand in the CT process switches from being the (NNC)-N-boolean AND-C-boolean AND ligand to the pyridyl acetylide.