We report a combined theoretical and experimental study of both the structural and optical properties of phosphorescent cyclometalated square-planar (phenylbipyridyl)platinum(II) acetylide complexes, namely (Pt(tBu2-ĈN̂N)(C≡C-Ph)] and (Pt(hex2-ĈN̂N)(C≡C-thienyl)] that exhibit, at high concentrations, an additional emission band at longer wavelength. The geometry optimizations of both the ground and the lowest triplet excited states of the considered monomers and different possible dimers have been performed in solution using several density functional theory (DFT) functionals corrected for dispersion effects. For the dimers, which are shown to exhibit a head-to-tail configuration, a significant shortening of the Pt···Pt distance, compared to that in the ground state, is observed in the first triplet state. Moreover, we show that trimeric species are highly improbable in solution. The UV-visible absorption spectra of the complexes are well rationalized using a vertical time-dependent DFT (TD-DFT) protocol relying on a global hybrid exchange-correlation functional. Finally, the new emission band at high concentration of the complexes can be assigned to a metal-metal to ligand charge transfer excited state ((3)MMLCT).