Nanometer Length-Dependent Triplet−Triplet Energy Transfers in Zinc(II)Porphyrin/<i>trans</i>-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, kET, 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 kET 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).