Congested Cyclometalated Platinum(II) Ditopic Frameworks and Their Phosphorescent Responses to S-Containing Amino Acids

New phosphorescent platinum(II) molecular hosts featuring a tridentate N,C,N-coordinating ligand, a conformationally rigid organic linker, and a binding group have been prepared. The complexes have been fully characterized by various spectroscopic techniques, and the X-ray crystal structure of one derivative has been determined. Their photophysical properties have been studied, and intense green metal-perturbed (IL)-I-3 emission is observed in solution at room temperature. The luminescent responses of these Pt(II) hosts to amino acids have been investigated: emission quenching and UV-vis absorption changes in polar/aqueous media are detected for terminal thiols only, and unusual preferential binding is apparent for cysteine over homocysteine. The nature of the host guest interactions has been examined by quantitative and comparative binding studies, mass spectrometry, and DFT calculations, which indicate that these observations may be ascribed to the presence of rigidly positioned ditopic binding sites.