[Pt((Bu-n)(3)P)(2)(ethynylbenzene)(2)] and [Pt((Bu-n)(3)P)(2)(1,4-diethynylbenzene)(2)], which possess D-2h micro-symmetry in their ground states, were studied at 77 K by time-resolved infrared (TRIR), FTIR, steady-state emission, and time-resolved photoluminescence spectroscopies. The primary luminescence peaks are at 22 573 cm(-1) and 20 408 cm(-1), respectively, with additional resolved structure from both phenyl ring and ethynyl vibrations. A quantum chemical modeling study showed that the HOMO is composed of conjugated pi-obitials, which include contribution from the d(xy) orbital on the platinum metal, while the LUMO consists of only the ligand pi* antibonding orbitals, thus the excitation is a mixture of pipi* and MLCT. From the results of the TRIR study and from group theoretical requirements, it has been concluded that the excitation in the lowest triplet manifold is confined to one ligand, resulting in a reduction of symmetry to C-2v. For the ethynylbenzene compound, the lowest energy grow-in in the TRIR spectrum is in the range of a stretching vibration for a carbon-carbon double bond. In the 1,4-diethynylbenzene compound, the low-energy grow-in in the TRIR spectrum is intermediate between a doubly and triply bound carbon stretch. An explanation of the localization is presented through a coupling of the lowest energy B-3(3u) electronic manifold to a B-3u anti-symmetric ethynyl stretch on the peripheral ligands.