The 77 K emission spectrum of [Pt((t)Bu(3)tpy)Cl]ClO4, 1(ClO4) ((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 pi-pi interactions, whereas a dilute solution (10(-6) mol dm(-3)) shows a vibronic intraligand emission at lambda(max) 467 nm. By comparing the UV-vis spectra of 1 with the (IL)-I-1-dominated spectrum of [Au((t)Bu(3)tpy)Cl](2+) (4) in CH3CN, the moderately intense absorption band at lambda(max) 373-386 nm for the former is ascribed to a (MLCT)-M-1 [(5d)Pt --> pi*((t)Bu(3)tpy)] transition. The absorption spectrum of 1 contains a weak shoulder at 465 nm (epsilon = 57 dm(3) mol(-1) cm(-1)) which obeys Beer's law in the concentration range 5 x 10(-4) to 1 x 10(-2) mol dm(-3), and a (MLCT)-M-3 assignment is proposed. Ketonyl platinum(II) complexes [Pt((t)Bu(3)tpy){CH2C(O)R}]ClO4 (R = Me, 2 and R = Ph, 3) have been prepared from 1(ClO4) in the presence of aqueous NaOH and the corresponding methyl ketone. The molecular structures of 1(ClO4) and 2(ClO4). C2H5OH reveal pi-stacking orientations for the (t)Bu(3)tpy ligands with interplanar separations of ca. 3.8 and 3.6 Angstrom, respectively. Complexes 2 and 3 are luminescent in solution at room temperature, and 3MLCT excited states are assigned, while 1 is nonemissive. The superior photophysical properties of 2 and 3 are ascribed to the stronger a-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(ClO4) is emissive in crystalline form at 77 K only (lambda(max) 560 nm). Solid-state emissions of 2(ClO4) and 3(ClO4) at room temperature (lambda(max) 607 and 615 nm, respectively) are derived from weak stacking interactions between (t)Bu(3)tpy groups, while their emission maxims are red-shifted at 77 K.