Through-Bond versus Through-Space T1 Energy Transfers in Organometallic Compound−Metalloporphyrin Pigments

Abstract

The preparation and characterization of two d9−d9 M2-bonded Pt2(dppm)2(C≡CC6H4-M(P))2 complexes (where M = Zn or Pd, and P = diethylhexamethylporphyrin) were achieved. The central [Pt2(dppm)2(C≡CC6H4)2] organometallic unit appears to be an independent chromophore and is suspected to be luminescent at 77 K (in 2MeTHF) in the porphyrin-containing complexes, as this is the case for the unfunctionalized Pt2(dppm)2(C≡CPh)2 parent compound. However, when this spacer is connected (by a single C−C bond) to either M(P) (M = Zn, Pd), even in the absence of conjugation (as the computed dihedral angle between the C6H4 and porphyrin planes is ∼84.5°), total quenching of the luminescence of the [Pt2(dppm)2(C≡CC6H4)2] central unit is observed. Only T1−Tn absorption bands of the metalloporphyrins are observed in the nanosecond transient absorption spectra, indicating the absence of a charge-separated state in these systems, thus indicating the presence of T1 species only. Consequently, efficient T1 energy transfers from [Pt2(dppm)2(C≡CC6H4)2] donor to the M(P) acceptor occur. Using a lower limit of measurable value of Φe < 0.0001, the rate for T1 energy transfers is estimated to be >108 s−1. This rate indicates a fast process, which is 4 orders of magnitude larger than as recently reported in electrostatically held dyads between the unsaturated clusters M3(dppm)3(CO)2+ (M = Pd, Pt) and metalloporphyrins (M′(TPP-CO2−); M′ = Zn, Pd; TPP = tetraphenylporphyrin) as the donor and acceptor, respectively (Aly, S. M.; Ayed, C.; Stern, C.; Guilard, R.; Abd-El-Aziz, A. S.; Harvey, P. D. Inorg. Chem., 2008, 47, 9930−9940). In the latter cases, the through-space T1 energy transfer rates were on the order of 104 s−1.