The short Pt … Pt distance found in the solid state stimulated us to look for signs of the metalophilic Pt … Pt interaction in solution . Emission and absorption spectroscopy studies were hence undertaken at a concentration of 50 μm in methanol or in aqueous solutions . In methanol , both complexes were highly soluble , and spectroscopically speaking , comparatively highenergy absorption ( 451 nm or lower ) and emission maxima ( 604 for [ 1]OAc and 556 and 591 nm for [ 2]OAc , see Figure 2a ; Table S6 , Supporting Information ) were found , which suggested an absence of metalophilic interaction and hence monomeric species . The distinct absorption of both isomers in MeOH can be attributed to their different HOMO - LUMO gap ( Figure S6a , Supporting Information ) , which critically depends on the proximity between the PtC bond and the non - bonding electron pair on the NMe bridge . [ 22 ] While in aerated methanol ( 50 μm ) , the luminescence of both complexes was weak , as characterized by low quantum yields ( below 0.0015 , Table S6 , Supporting Information ) ; in degassed methanol , their luminescent quantum yield increased ≈2 - 4 times ( still remaining lower than 0.005 ) , which demonstrated the triplet nature of the excited states of these monomers , but also suggested that O 2 is not so good at quenching the monomer emission . This observation was confirmed by the singlet oxygen generation quantum yield , which was moderate in aerated methanol solution ( φΔ = 0.44 for [ 1]OAc , 0.11 for [ 2]OAc ) . The photodynamic properties of both complexes were hence limited as monomers in methanol . In frozen MeOH / EtOH mixtures ( 77 K , Figure S8b , Supporting Information ) , the phosphorescence quantum yield was further increased by a factor > 20 for [ 1]OAc and > 100 for [ 2]OAc ( see Table S6 , Supporting Information ) , suggesting possible intermolecular ( triplet - triplet annihilation ) or intramolecular ( vibrational ) quenching routes at room temperature . Limited quenching by O 2 was confirmed by luminescent lifetime measurements ( Table S6 and Figure S8 , Supporting Information ): for both isomers , biexponential decay was observed , but considering the very low phosphorescence intensity , we interpret the short component τ 1 as a consequence of scattered light from the laser pulse ; the longer lifetime τ 2 , which we interpret as that of the Pt complex , increased by a factor of 2 - 4 upon degassing ( e.g. , from 150 to 602 ns for   for [ 2]OAc , see Table S6 , Supporting Information ) . According to TDDFT calculation on the monomeric cations [ 1 ] + and [ 2 ] + and to data above , the weak emission in methanol can be assigned to triplet ligand - to - ligand charge - transfer ( 3 LLCT ) transitions with a small triplet metal - to - ligand charge - transfer ( 3 MLCT ) character .