Multi-responsive Tetradentate Phosphorescent Metal Complexes as Highly Sensitive and Robust Luminescent Oxygen Sensors: Pd(II) versus Pt(II) and 1,2,3-Triazolyl versus 1,2,4-Triazolyl.

Two Pd(II) complexes based on tetradentate chelate ligands with either 1,2,4-triazolyl (Pd1) or 1,2,3-triarolyl (Pd2) unit were synthesized and their structure-property relationships were studied. Both Pd1 and Pd2 are rare bright deep blue Pd(II) phosphors with contrasting properties. Pd1 displays stimuli-responsive luminescence in response to UV irradiation, concentration or temperature change, which is ascribed to the facile switching of monomer to excimer emission. In contrast, similar stimuli-responsive luminescence was not observed for Pd2. Crystal structures and TD-DFT computational studies established that the excimer formation of Pd1 is caused by electronically favored intermolecular π-π interactions and less steric protection of the Pd core due to the position of its alky chains, compared to Pd2. In solution, the excimer emission of Pd1 shows a much greater sensitivity toward oxygen than the monomer emission with a very large Stern-Volmer constant (Ksv) that is more than twice of that of the monomer emission. Both Pd(II) complexes are found to be outstanding oxygen sensors in ethyl cellulous (EC) films with superior sensitivity (K_sv^app = 0.228-0.346 Torr-1) over their Pt(II) equivalents (K_sv^app = 0.00674 - 0.0110 Torr-1), owing to their long phosphorescence decay lifetimes. Furthermore, Pd1 shows an excellent photostability, comparable to the Pt(II) analogue, making it one of the best and highly robust oxygen sensors based on cyclometalated metal complexes.