SOLUBLE SYNTHETIC MULTIPORPHYRIN ARRAYS .1. MODULAR DESIGN AND SYNTHESIS

A set of porphyrin building blocks has been developed for the construc tion of light-harvesting model compounds and related molecular photoni c devices. The porphyrins are facially encumbered to enhance solubilit y in organic solvents, are employed in a defined metalation state (fre e base (FD) or zinc chelate), and bear peripheral functional groups su ch as iodo or ethyne for joining the porphyrins via covalent bonds, Th e coupling of an iodophenylporphyrin and an ethynylphenylporphyrin via mild Pd-mediated reactions (2-4 mM of each porphyrin in toluene/triet hylamine (5:1) with Pd-2(dba)(3) and AsPh(3) at 35 degrees 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 s ilica chromatography, preparative size exclusion chromatography, and g ravity elution silica chromatography. The diphenylethyne linkers give a center-to-center separation of the porphyrins of similar to 20 Angst rom. 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 th rough-bond electronic communication on the rates and yields of photoin duced energy transfer in the arrays, four ZnFb dimers have been prepar ed that have a systematic increase in steric hindrance in the diphenyl ethyne unit, The presence of steric hindrance inhibits rotation of the phenyl group toward coplanarity with the porphyrin, thereby modulatin g the electronic communication, A linear ZnFbZn trimer and a right-ang le ZnFbZn trimer have been prepared to probe the effects of geometry o n electronic communication pathways, A linear ZnZnFb trimer has been s ynthesized to investigate the photodynamics of energy migration among isoenergetic zinc porphyrins. These multiporphyrin arrays have suffici ent solubility (similar to 5 mM) for routine handling in organic solve nts such as toluene, CH2Cl2, or CHCl3, and can be examined spectroscop ically (1-10 mu M) in diverse solvents such as tetrahydrofuran, aceton e, dimethyl sulfoxide, and castor oil. This building block approach sh ould make diverse multiporphyrin arrays readily available.