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.