Triplet Energy Transfers in Well-Defined Host-Guest Porphyrin-Carboxylate/Cluster Assemblies

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)], dppm = (Ph2P)2CH2) via ionic CO2(-)···Pd3(2+) interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd3(2+)] (∼8190 cm(-1)) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680-700 nm arising from a T1-T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1-T1 energy transfer ((3)dye*···[Pd3(2+)] → dye···(3)[Pd3(2+)]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 10(5) s(-1) on the basis of transient absorption measurements at 298 K.