INVESTIGATION OF ELECTRONIC COMMUNICATION IN MULTI-PORPHYRIN LIGHT-HARVESTING ARRAYS

A comprehensive electrochemical (cyclic and square-wave voltammetry, c oulometry) and static spectroscopic (absorption, resonance Raman (RR), electron paramagnetic resonance (EPR)) study is reported for several pentameric and dimeric porphyrin-based arrays and their monomeric buil ding blocks. The pentameric arrays consist of a central tetraarylporph yrin linked to four other tetraarylporphyrins via ethyne groups at the p-positions of the aryl rings. The complexes investigated include Zn- 5 pentameric and Zn-2 dimeric porphyrin arrays, a pentameric array in which the four peripheral porphyrins are zinc and the central porphyri n is a free base (Zn(4)Fb(1)), and a mixed zinc-free base porphyrin di mer(Zn(1)Fb(1)). The center-to-center inter-porphyrin distances in the arrays are similar to 20 Angstrom. The dimensions of the dimeric and pentameric arrays are similar to 40 and similar to 60 Angstrom, respec tively. The spectroscopic studies were performed on singly and multipl y oxidized complexes as well as the neutral species. The electrochemic al and spectral properties of the arrays indicate that the electronic communication between the macrocycles is relatively weak in the ground and excited electronic states. The absorption characteristics of the arrays can be explained in terms of long-range, through-space excitoni c interactions. The RR, electrochemical, and EPR data indicate that th rough-bond electronic communication pathways are also open in the arra ys. Extremely large RR intensity enhancements are observed for aryl-ri ng and ethyne-bridge stretching modes. The RR intensity enhancements a re attributed to an excited-state conformational change that enhances the conjugation between the pi-electron systems of the porphyrin ring and bridging diarylethyne group. The half-wave potentials for oxidatio n of the zinc units in Zn-2, Zn(4)Fb(1), and Zns are slightly differen t. Up to four electron equivalents can be removed from Zn-2 and Zn(1)F b(1) without compromising the sample integrity; up to five electron eq uivalents can be removed from Zn(4)Fb(1) and Zn-5. The EPR spectra of the oxidized assemblies exhibit complex temperature dependent signatur es that reflect hole/electron hopping and/or spin exchange interaction s. Hole/electron hopping is rapid (10(7) s(-1) or faster) on the EPR t ime scale in liquid solution and slow in frozen solution. The state of the solvent rather than the temperature mediates the hopping process. Exchange interactions are significant (probably 1000 MHz or greater) in both liquid and frozen solutions and, in certain cases, are enhance d upon solvent freezing. Collectively, the studies reported provide ne w insights into the electronic communication pathways in the porphyrin ic arrays. The studies also indicate the following: (1) The center-to- center distance of similar to 20 Angstrom provided by the diarylethyne linker affords weak electronic interactions. These weak interactions facilitate rapid energy transfer but not deleterious electron-transfer quenching reactions. (2) The ability to remove five electrons from th e pentameric arrays indicates their possible application as controlled -potential electron reservoirs. (3) The rapid mobility of the hole in the oxidized pentamers indicates that arrays of similar design, if fas hioned in a linear architecture, could function as molecular wires or as components of electron-transport chains.