Photophysics of [Pt{4-(o-tolyl)isqbipy}Cl]SbF6, where 4-(o-Tolyl)isqbipy is the New 4-(o-Tolyl)-6-(3´´ -isoquinolyl)-2,2´-bipyridyl Ligand

Abstract

The synthesis and characterisation of the 4-(o-tolyl)-6-(3´´-isoquinolyl)-2,2´-bipyridyl [4-(otolyl) isqbipy] ligand is described. A single-crystal X-ray structure determination shows that it adopts a trans-trans conformation about the interannular bonds linking the central pyridine ring to the outer pyridine ring and the isoquinolyl moiety. The o-tolyl group is twisted by an angle of 51.7° out of the plane of the central pyridine ring. The synthesis and characterisation of the [Pt{4-(otolyl) isqbipy}Cl]SbF6 complex is described. The absorption spectrum of the complex measured in acetonitrile exhibits MLCT bands at 362 and 393 nm, as well as intraligand π-π* absorptions in the 200 - 350 nm range; the MLCT bands are shifted significantly to higher energy when compared to those recorded for the parent trpy complex, viz. [Pt{4´-(o-tolyl)trpy}Cl]SbF6, where trpy is 2,2´ : 6´,2´´-terpyridine. Similarly, the 3MLCT emission measured for the 4-(o-tolyl)isqbipy complex in a 1 : 1 CH2Cl2/CHCl3 solution is blue-shifted with respect to the emission spectrum recorded for the 4´-(o-tolyl)trpy complex. We attribute the higher energy MLCT absorption and emission for the 4-(o-tolyl)isqbipy complex to a significantly higher energy for its π*-LUMO than for that of the 4´-(o-tolyl)trpy complex. Emission spectra of the title compound have also been measured in a low-temperature 1 : 5 : 5 (v/v) DMF/methanol/ethanol (DME) glass as a function of concentration. These spectra show that aggregation of the complex occurs at rather low concentrations of 5 - 10 μM, probably to dimers. Variable-temperature emission spectra recorded on a solid sample of [Pt{4-(otolyl) isqbipy}Cl]SbF6 comprise relatively narrow asymmetric bands whose maxima are shifted to the red as the temperature is lowered: specifically λem(max) is 641 nm at r. t. (τ = 235 ns) and 676 nm at 77 K (τ = 1.36 μs). Temperature-dependent emission of this type is typical of a metal-metal-to-ligand charge transfer (MMLCT) excited state that has its origin in dz2 (Pt)-dz2 (Pt) orbital interactions in the crystal.