Triplet Energy Transfers in Electrostatic Host-Guest Assemblies of Unsaturated Organometallic Cluster Cations and Carboxylate-Containing Porphyrin Pigments

The unsaturated Cyclic [M-3(dppm)(3)(CO)](2+) Clusters (M = Pt, Pd; dppm = Ph2PCH2PPh2; such as PF6- salt) exhibit a cavity formed by the six dppm-phenyl groups placed like a picket fence above the unsaturated triangular M3 dicationic center. Electrostatic interactions of the M3+ units inside this cavity with the carboxylate anion RCO2- [R = tetraphenylporphyrinatozinc(II), ZnTPP; p-phenyltritolylporphyrinatozinc(II), ZnTTPP; p-phenyltritolylporphyrinato-palladium(II), PdTTPP] form dyads for through-space triplet energy transfers. The binding constants are on the order of 20 000 M-1 in all six cases (298 K). The energy diagram built upon absorption and emission spectra at 298 and 77 K places the [Pt-3(dppm)(3)(CO)](2+) and [Pd-3(dppm)(3)(CO)](2+) as triplet energy donors, respectively, with respect to the ZnTPPCO2-, ZnTTPPCO2-, and PdTTPPCO2- pigments, which act as acceptors. Evidence for energy transfer is provided by the transient absorption spectra at 298 K, where triplet-triplet absorption bands of the metalloporphyrin chromophores are depicted at all time (at 298 K) with total absence of the charge-separated state in the nanosecond to microsecond time scale. Rates for energy transfer (ranging in the 10(4) s(-1) time scale) are extracted from the emission lifetimes of the [Pt-3(dppm)(3)(CO)](2 divided by) donor in the free chromophore and the host-guest assemblies. The emission intensity of [Pd-3(dppm)(3)(Co)](2+) is too weak to measure its spectrum and emission lifetime in the presence of the strongly luminescent metalloporphyrin-containing materials. For the [Pd-3(dppm)(3)(CO)](2+) ... metalloporphyrin dyads, evidence for fluorescence and phosphorescence lifetime quenching of the porphyrin chromophore at 298 K is provided. These quenchings, exhibiting rates of 10(4) (triplet) and 10(8) s(-1) (singlet), are attributed to a photoincluced electron transfer from the metalloporphyrin to the cluster due to the low reduction potential.