Nanometer Length-Dependent Triplet−Triplet Energy Transfers in Zinc(II)Porphyrin/trans-Bis(ethynylbenzene)Platinum(II) Oligomers

The synthesis and characterization of organometallic oligomers of the type [(p-C₆H₄)CCPt(P(n-Bu)₃)₂CC(p-C₆H₄)Zn(P)]_n with the corresponding models [(C₆H₅CC)Pt(P(n-Bu)₃)₂CC(p-C₆H₄)Zn(P)(p-C₆H₄)CCPt(P(n-Bu)₃)₂(CCC₆H₅)], where Zn(P) is zinc(II)-10,20-di(mesityl)- (n = 4, 9) or zinc(II)-10,20-di-n-pentyl-porphyrin (n = 3, 6, 9), are reported. The electronic spectra (absorption, excitation, emission, and transient absorption) and the photophysical properties (emission lifetimes and quantum yields) of these species in 2-methyltetrahyrofuran at 298 and 77 K are presented. Rates for triplet (T₁) energy transfer, k_ET, from the [(p-C₆H₄CC)Pt(P(n-Bu)₃)₂(CC-p-C₆H₄)] spacer to Zn(P) vary from 2.4 × 10⁴ to 1.3 × 10⁶ s⁻¹. For the n-pentyl case, a rate dependence of oligomer size is noted as k_ET increases with the number of units, n. This phenomenon is interpreted by the presence of an excitonic process (i.e., delocalization of the energy along the Zn(P) array).