SOLUBLE SYNTHETIC MULTIPORPHYRIN ARRAYS .3. STATIC SPECTROSCOPIC AND ELECTROCHEMICAL PROBES OF ELECTRONIC COMMUNICATION

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 a series of dimeric and trimeric porphyrin-based arrays. All the arrays consis t of tetraarylporphyrins linked via ethyne groups at the p-positions o f the aryl rings. The complexes investigated include zinc-free base an d bis-zinc dimers which contain varying degrees of torsional constrain t between the porphyrin rings and the aryl group of the linker, and li near and right-angle trimers in which two zinc porphyrins are bridged by either a zinc or free base porphyrin. The spectroscopic studies wer e performed on singly and multiply oxidized complexes as well as the n eutral species. The electrochemical and spectral properties of the arr ays 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 media ted by the diarylethyne linker. In the case of the torsionally unconst rained dimers, unusually large RR intensity enhancements are observed for aryl-ring and ethyne-bridge stretching modes. The RR intensity enh ancements are attributed to an excited-state conformational change tha t enhances the conjugation between the pi-electron systems of the porp hyrin 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 e nergy-transfer rates observed for these arrays (unhindered, similar to (24 ps)(-1); monohindered, similar to(46 ps)(-1); bis-hindered similar to(88 ps)(-1)) and indicates that the excited-state electronic commun ication can be tuned via structural modification of the diarylethyne l inker. In contrast, the optical and RR signatures of the linear and ri ght-angle trimeric arrays are essentially identical indicating that th e geometrical arrangement of the porphyrins does not significantly inf luence the excited-state communication. The half-wave potentials for o xidation of the zinc porphyrins in the dimers and trimers are essentia lly identical. The EPR spectra of the oxidized arrays exhibit complex temperature-dependent signatures that reflect hole/electron hopping an d/or spin exchange interactions in the ground electronic state. Hole/e lectron 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 geometri cal arrangement of the constituent porphyrins (trimers) has any affect on the EPR signatures of the monocations indicating that this structu ral element does not affect ground-state electronic communication as r eflected 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 enhan ced upon solvent freezing. Unlike the hole/electron hopping, the excha nge interactions in the dimers are influenced by the degree of torsion al constraint. In contrast, the geometrical arrangement of the constit uent porphyrins in the trimers has no measurable effect on this proper ty. Collectively, the static spectroscopic and electrochemical studies provide new insights into the electronic communication pathways in th e diarylethyne-linked multiporphyrinic arrays.