Singlet ground-state geometry optimization of the monomer, four dimers, and the trimer of [Pt(bph)(CO)(2)] where bph = biphenyl dianion, was performed at the B3LYP level of density functional theory (DFT) with a mixed basis set (6-311 G** on C, O, and H atoms; the Stuttgart/Dresden (SDD) effective core potential (ECP) on the Pt core; [6s5p3d] on the Pt valence shell). The aggregation was based on Pt-Pt binding as well as on pi-pi and electrostatic interactions. The lowest-lying triplet-state geometries of the monomer, one dimer, and the trimer of the complex were also optimized using the above theory. Significant shortening of the Pt-Pt bond was recorded in the triplet state compared to the singlet one. A number of low-energy singlet and triplet allowed excited states were calculated using time-dependent density functional theory (TDDFT) and analyzed with respect to absorption, excitation, and emission spectra collected under various conditions. Simulated spectra of the monomer and dimer based on the singlet excited states were correlated with the absorption spectrum. The emission in concentrated solution was due to the triplet dimer, and the emitting states were (MLCT)-M-3 and Pt-centered states.