Structure-Property Relationships and 1O2 Photosensitisation in Sterically Encumbered Diimine PtII Acetylide Complexes

A series of sterically encumbered [Pt(L)(sigma-acetylide)(2)] complexes were prepared in which L, a dendritic polyaromatic diimine ligand, was held constant (L=1-(2,2-bipyrid-6-yl)-2,3,4,5-tetrakis(4-tert-butylphenyl)benzene) and the cis ethynyl co-ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of conjugation and steric bulk of the ethynyl ligands. Replacing electron-withdrawing phenyl-CF3 substituents (4) with electron-donating pyrenes (5) resulted in a red shift of both the lowest-energy absorption (E=3300cm(-1), 61nm) and emission bands (E=1930cm(-1), 64nm). The emission, assigned in each case as phosphorescence on the basis of the excited-state lifetimes, switched from being (MMLLCT)-M-3-derived (mixed metal-ligand-to-ligand charge transfer) when phenyl/polyphenylene substituents (3, 4, 6) were present, to ligand-centred (3)* when the substituents were more conjugated aromatic platforms [pyrene (5) or hexa-peri-hexabenzocoronene (7)]. The novel Pt-II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited-state lifetimes suggested they would be good candidates for use as singlet-oxygen photosensitisers. Determined by in situ photooxidation of 1,5-dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl-5 as sensitiser (k(obs)=39.3x10(-3)min(-1)) was over half that of the known O-1(2) sensitiser tetraphenylporphyrin (k(obs)=78.6x10(-3)min(-1)) under the same conditions. Measured O-1(2) quantum yields of complexes 5 and 7 were half and one-third, respectively, of that of TPP, and thus reveal an efficient triplet-triplet energy-transfer process in both cases.