Soluble Synthetic Multiporphyrin Arrays. 1. Modular Design and Synthesis

A set of porphyrin building blocks has been developed for the construction of light-harvesting model compounds and related molecular photonic devices. The porphyrins are facially encumbered to enhance solubility in organic solvents, are employed in a defined metalation state (free base (Fb) or zinc chelate), and bear peripheral functional groups such as iodo or ethyne for joining the porphyrins via covalent bonds. The coupling of an iodophenylporphyrin and an ethynylphenylporphyrin via mild Pd-mediated reactions (2−4 mM of each porphyrin in toluene/triethylamine (5:1) with Pd2(dba)3 and AsPh3 at 35 °C for 2 h) yields the corresponding diphenylethyne-linked multiporphyrin array in 70−80% yield. The arrays are easily purified by a sequence of flash silica chromatography, preparative size exclusion chromatography, and gravity elution silica chromatography. The diphenylethyne linkers give a center-to-center separation of the porphyrins of ∼20 Å. Model light-harvesting compounds are easily prepared using Zn and Fb porphyrin building blocks. In order to investigate the role of the linker in through-bond electronic communication, and the effect of through-bond electronic communication on the rates and yields of photoinduced energy transfer in the arrays, four ZnFb dimers have been prepared that have a systematic increase in steric hindrance in the diphenylethyne unit. The presence of steric hindrance inhibits rotation of the phenyl group toward coplanarity with the porphyrin, thereby modulating the electronic communication. A linear ZnFbZn trimer and a right-angle ZnFbZn trimer have been prepared to probe the effects of geometry on electronic communication pathways. A linear ZnZnFb trimer has been synthesized to investigate the photodynamics of energy migration among isoenergetic zinc porphyrins. These multiporphyrin arrays have sufficient solubility (∼5 mM) for routine handling in organic solvents such as toluene, CH2Cl2, or CHCl3, and can be examined spectroscopically (1−10 μM) in diverse solvents such as tetrahydrofuran, acetone, dimethyl sulfoxide, and castor oil. This building block approach should make diverse multiporphyrin arrays readily available.