Triplet energy transfers in electrostatic host-guest ...
|Title||Triplet energy transfers in electrostatic host-guest assemblies of unsaturated organometallic cluster cations and carboxylate-containing porphyrin pigments.|
|Author(s)||Shawkat M. Aly, Charfedinne Ayed, Christine Stern, Roger Guilard, Alaa S. Abd-El-Aziz, Pierre D. Harvey|
|Abstract||The unsaturated cyclic [M3(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 M(3+) units inside this cavity with the carboxylate anion RCO2(-) [R = tetraphenylporphyrinatozinc(II), ZnTPP; p-phenyltritolylporphyrinatozinc(II), ZnTTPP; p-phenyltritolylporphyrinatopalladium(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 [Pt3(dppm)3(CO)](2+) and [Pd3(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 [Pt3(dppm)3(CO)](2+) donor in the free chromophore and the host-guest assemblies. The emission intensity of [Pd3(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 [Pd3(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 photoinduced electron transfer from the metalloporphyrin to the cluster due to the low reduction potential.|
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