Multiresponsive 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 a 1,2,4-triazolyl
(Pd1) or 1,2,3-triazolyl (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, a similar stimuli-responsive
luminescence was not observed for Pd2. Crystal structures
and time-dependent density functional theory 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 because of the position
of its alkyl 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
that of the monomer emission. Both Pd­(II) complexes are found to be
outstanding oxygen sensors in ethyl cellulose films with superior
sensitivity (Ksvapp = 0.228–0.346 Torr–1) over their Pt­(II) equivalents (Ksvapp = 0.00674–0.0110 Torr–1), owing to their long phosphorescence decay lifetimes.
Furthermore, Pd1 shows an excellent photostability, compared
to the Pt­(II) analogue, making it one of the best and highly robust
oxygen sensors based on cyclometalated metal complexes.