Dark-Field Oxidative Addition-Based Chemosensing: New Bis-cyclometalated Pt(II) Complexes and Phosphorescent Detection of Cyanogen Halides

Heavy metal complexes that are phosphorescent at room temperature are becoming increasingly
important in materials chemistry, principally due to their use in phosphorescent organic light-emitting devices
(OLEDs). Their use in optical sensory schemes, however, has not been heavily explored. Homoleptic bis-cyclometalated Pt(II) complexes are known to undergo oxidative addition with appropriate electrophiles
(principally alkyl halides) by either thermal or photochemical activation. We have applied this general reaction
scheme to the development of a phosphorescence-based sensing system for cyanogen halides. To carry
out structure−property relationship studies, a series of previously unreported Pt(II) complexes was prepared.
Most of the complexes (excluding those that incorporated substituents on the ligands that forced steric
crowding in the square plane) were strongly orange-red phosphorescent (Φ = 0.2−0.3) in a room-temperature oxygen-free solution. These sterically demanding ligands also accelerated the addition of
cyanogen bromide to these complexes but slowed the addition of methyl iodide, indicating that the oxidative
addition mechanisms for these two electrophiles is different. The lack of solvent-polarity effect on the addition
of BrCN suggests a radical mechanism. Oxidative addition of BrCN to the metal complexes in solution or
dispersed in poly(methyl methacrylate) gave blue-shifted emissive Pt(IV) complexes. The blue-shifted
products give a dark-field sensing scheme that is in sharp contrast to energy transfer-based sensing
schemes, which have limited signal-to-noise because of the presence of lower-energy vibronic bands of
the energy donor that can overlap with the emission of the acceptor.