Excimer-Monomer Photoluminescence Mechanochromism and Vapochromism of Pentiptycene-Containing Cyclometalated Platinum(II) Complexes.

The ability of the bulky H-shaped pentiptycene scaffold in promoting the mechanochromic and vapochromic luminescence properties for organometallic materials has been demonstrated with the N^C^N cyclometalated platinum(II) complexes [X-NCNPtY], where X = Br or Pa, the substituent on the terdentate dipyridylbenzene N^C^N ligand, and Y = Cl or Pa, the ancillary ligand, in which Pa = pentiptycene acetylene. Intermolecular interactions between the planar NCNPt cores are described by π-π and d-π interactions with negligible PtII ···PtII bonding, corresponding to ligand-centered excimer rather than metal-metal-to-ligand charge-transfer emission, for these platinum(II) complexes in aggregates and in the solid state. Interplay of the relative excimer-to-monomer emission intensity in response to external force and/or vapor stimuli accounts for the luminescence mechanochromism and vapochromism of the pentiptycene-incorporated platinum(II) complexes [Pa-NCNPtCl], [Br-NCNPtPa], and [Pa-NCNPtPa], whereas the pentiptycene-free counterpart [Br-NCNPtCl] undergoes little or no emission color response. In particular, the complex [Pa-NCNPtCl] displays a distinct response to aromatic versus nonaromatic organic vapors: namely, aromatic vapors such as benzene convert the excimer emission to monomer emission, but the opposite is true with nonaromatic vapors. A two-stage emission color change from red to orange and then to yellow could thus be achieved by grinding and by subsequent benzene-vapor fuming. Another feature associated with [Pa-NCNPtCl] is an aggregation-induced green → magenta luminescence color change in water/tetrahydrofuran mixed solutions. The structure-luminescence property relationship is discussed in relation to intermolecular interactions and packing modes that depend on the number and positions of pentiptycene groups.