Soluble Synthetic Multiporphyrin Arrays. 3. Static Spectroscopic and Electrochemical Probes of Electronic Communication

Abstract

A comprehensive electrochemical (cyclic and square-wave voltammetry, coulometry) and static spectroscopic (absorption, resonance Raman (RR), electron paramagnetic resonance (EPR)) study is reported for a series of dimeric and trimeric porphyrin-based arrays. All the arrays consist of tetraarylporphyrins linked via ethyne groups at the p- positions of the aryl rings. The complexes investigated include zinc-free base and bis-zinc dimers which contain varying degrees of torsional constraint between the porphyrin rings and the aryl group of the linker, and linear and right-angle trimers in which two zinc porphyrins are bridged by either a zinc or free base porphyrin. The spectroscopic studies were performed on singly and multiply oxidized complexes as well as the neutral species. The electrochemical and spectral properties of the arrays indicate that the electronic communication between the macrocycles is relatively weak in the ground and excited electronic states. This communication is through-bond, rather than through-space, and is mediated by the diarylethyne linker. In the case of the torsionally unconstrained dimers, unusually large RR intensity enhancements are observed for aryl-ring and ethyne-bridge stretching modes. The RR intensity enhancements are attributed to an excited-state conformational change that enhances the conjugation between the π-electron systems of the porphyrin ring and bridging diarylethyne group. The intensity of the aryl and ethyne-bridge vibrations monotonically decreases as the degree of torsional constraint increases. This trend parallels the decrease in energy-transfer rates observed for these arrays (unhindered, ∼(24 ps) - 1; monohindered, ∼(46 ps) - 1; bis-hindered ∼(88 ps) - 1 ) and indicates that the excited-state electronic communication can be tuned via structural modification of the diarylethyne linker. In contrast, the optical and RR signatures of the linear and right-angle trimeric arrays are essentially identical indicating that the geometrical arrangement of the porphyrins does not significantly influence the excited-state communication. The half-wave potentials for oxidation of the zinc porphyrins in the dimers and trimers are essentially identical. The EPR spectra of the oxidized arrays exhibit complex temperature-dependent signatures that reflect hole/electron hopping and/or spin exchange interactions in the ground electronic state. Hole/electron hopping in all the monocations is rapid (10 7 s - 1 or faster) on the EPR time scale in liquid solution and slow in frozen solution. Neither the degree of torsional constraint (dimers) nor the geometrical arrangement of the constituent porphyrins (trimers) has any affect on the EPR signatures of the monocations indicating that this structural element does not affect ground-state electronic communication as reflected in hole/electron hopping rates. Exchange interactions in the multiply oxidized arrays are significant (probably 1000 MHz or greater) in both liquid and frozen solutions and, in certain cases, are enhanced upon solvent freezing. Unlike the hole/electron hopping, the exchange interactions in the dimers are influenced by the degree of torsional constraint. In contrast, the geometrical arrangement of the constituent porphyrins in the trimers has no measurable effect on this property. Collectively, the static spectroscopic and electrochemical studies provide new insights into the electronic communication pathways in the diarylethyne-linked multiporphyrinic arrays.