MULTIPLE-STATE EMISSION FROM PLATINUM(II) DIIMINE DITHIOLATE COMPLEXES - SOLVENT AND TEMPERATURE EFFECTS

The solvent and temperature dependence of the emission of Pt(diimine)(dithiolate) complexes is reported where diimine = 4,4'-dimethyl-2,2'-bipyridine (dmbpy) or 4,7-diphenyl-1,10-phenanthroline (dpphen) and dithiolate = 1-(ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolate (ecda) or 1-(tert-butoxycarbonyl)-1-cyanoethylene-2,2-dithiolate (tbcda). In fluid solution at room temperature, a single emission band is observed at approximately 16 670 cm-1 (600 nm) for the complexes Pt(dmbpy)(ecda) (1), Pt(dpphen)(ecda) (2), and Pt(dpphen)(tbcda) (3), which are studied in detail. The radiative quantum yields and lifetimes of the emission decrease with increasing solvent polarity for the three complexes. In DMF/CH2Cl2/MeOH the emission intensity for the complexes increases between 298 and 175 K with no change in the emission energy. Between 175 and 165 K the emission undergoes a red shift to approximately 15 625 cm-1 (640 nm), corresponding to a rigidochromic effect, and below 140 K two new bands appear at higher energies (17 000-18 500 and 18 600-19 900 cm-1). The latter bands have a relative separation of 1360 cm-1 and are assigned to a pi-pi* diimine transition on the basis of their observed lifetime and comparison with PtCl2(diimine) analogues. Between 80 and 6 K, the lifetime of the 15 625-cm-1 (640-nm) emission band increases, while, below 50 K, its emission intensity decreases with decreasing temperature, suggesting the presence of closely spaced excited states. From a kinetic analysis of the radiative quantum yield and lifetime data, the low-energy emission is assigned to a charge-transfer manifold of two emitting states separated by 50-60 cm-1.