Temperature dependence of the crystal structure and luminescence properties of [Pt{4′-(o-ClC6H4)trpy}Cl]SbF6 (trpy = 2,2′:6′,2″-terpyridine)

Abstract

The synthesis and characterisation of 4′-(2‴-chlorophenyl)-2,2′:6′,2″-terpyridine [4′-(o-ClC₆H₄)trpy] and [Pt{4′-(o-ClC₆H₄)trpy}Cl]SbF₆ are described. An X-ray crystal structure determination of the salt reveals columns of cations and anions with the cations stacked parallel and head-to-tail. At 295 K the Pt⋯Pt distances alternate between 3.374(1) and 3.513(1) Å while the perpendicular separation between the mean planes comprising the platinum and three bonded nitrogen atoms remains constant at 3.35 Å. As evidenced by crystal structure determinations at 260, 240 and 180 K, the shorter Pt⋯Pt distances become systematically shorter and the longer Pt⋯Pt distances become systematically longer as the temperature of the crystal is lowered. Structural evidence is presented that shows that this trend is a consequence of the more closely separated platinum atoms slipping into line with respect to a perpendicular to the above planes; at the same time the more widely separated platinum atoms fall out-of-line as the temperature is lowered. Emission spectra measured on a polycrystalline sample of the salt at 40 K intervals from 280 to 80 K comprise a single, asymmetric and featureless band [λ_max^em = 609 nm at 280 K] that systematically increases in intensity as the temperature is lowered. There is a slight blue shift in the wavelength of the emission maximum as the temperature is lowered to 240 K but, below this temperature, the emission maximum gradually and systematically shifts to the red [λ_max^em = 642 nm at 80 K]. This behaviour is rationalised in terms of ³MMLCT emission, the energy of which being determined by subtle changes in the extent of d_z²−d_z² orbital overlap between adjacent platinum atoms separated by the shorter distance on one hand, and between platinum atoms separated by the longer distance on the other.