Spectroscopic Properties of Luminescent Platinum(II) Complexes Containing 4,4‘,4‘ ‘-Tri-tert-butyl-2,2‘:6‘,2‘ ‘-terpyridine (tBu₃tpy). Crystal Structures of [Pt(tBu₃tpy)Cl]ClO₄ and [Pt(tBu₃tpy){CH₂C(O)Me}]ClO₄

The 77 K emission spectrum of [Pt( t Bu 3 tpy)Cl]ClO 4, 1 (ClO 4 ) ( t Bu 3 tpy = 4,4‘,4‘ ‘-tri- tert -butyl-2,2‘:6‘,2‘ ‘-terpyridine), in a 10 - 3 mol dm - 3 MeOH/EtOH glassy solution exhibits a 625 nm band attributed to t Bu 3 tpy π−π interactions, whereas a dilute solution (10 - 6 mol dm - 3 ) shows a vibronic intraligand emission at λ max 467 nm. By comparing the UV−vis spectra of 1 with the 1 IL-dominated spectrum of [Au( t Bu 3 tpy)Cl] 2+ ( 4 ) in CH 3 CN, the moderately intense absorption band at λ max 373−386 nm for the former is ascribed to a 1 MLCT [(5d)Pt → π*( t Bu 3 tpy)] transition. The absorption spectrum of 1 contains a weak shoulder at 465 nm (ε = 57 dm 3 mol - 1 cm - 1 ) which obeys Beer's law in the concentration range 5 × 10 - 4 to 1 × 10 - 2 mol dm - 3, and a 3 MLCT assignment is proposed. Ketonyl platinum(II) complexes [Pt( t Bu 3 tpy){CH 2 C(O)R}]ClO 4 (R = Me, 2 and R = Ph, 3 ) have been prepared from 1 (ClO 4 ) in the presence of aqueous NaOH and the corresponding methyl ketone. The molecular structures of 1 (ClO 4 ) and 2 (ClO 4 )·C 2 H 5 OH reveal π-stacking orientations for the t Bu 3 tpy ligands with interplanar separations of ca. 3.8 and 3.6 Å, respectively. Complexes 2 and 3 are luminescent in solution at room temperature, and 3 MLCT excited states are assigned, while 1 is nonemissive. The superior photophysical properties of 2 and 3 are ascribed to the stronger σ-donating ability of the ketonyl ligands. Like 1, complexes 2 and 3 also display low-energy emission at ca. 620 nm in 10 - 3 mol dm - 3 MeOH/EtOH glass at 77 K. Complex 1 (ClO 4 ) is emissive in crystalline form at 77 K only (λ max 560 nm). Solid-state emissions of 2 (ClO 4 ) and 3 (ClO 4 ) at room temperature (λ max 607 and 615 nm, respectively) are derived from weak stacking interactions between t Bu 3 tpy groups, while their emission maxima are red-shifted at 77 K.