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Patterns in the cyanide stretching frequencies have been examined in several series of monometal-and CN -bridged transition metal complexes. Metal-to-cyanide back-bonding can be identified as a major factor contributing to red shifts of ν CN in monometal complexes. This effect is complicated in cyanide-bridged complexes in two ways: (a) when both metals can back-bond to cyanide, the net interaction is repulsive and results in a blue shift of ν CN ; and (b) when a donor and acceptor are bridged, ν CN undergoes a substantial red shift (sometimes more than 60 cm -1 lower in energy than the parent monometal complex). These effects can be described by simple perturbational models for the electronic interactions. Monometal cyanide complexes and CN --bridged backbonding metals can be treated in terms of their perturbations of the CNπ and π* orbitals by using a simple, Hu ¨ckel-like, three-center perturbational treatment of electronic interactions. However, bridged donor-acceptor pairs are best described by a vibronic model in which it is assumed that the extent of electronic delocalization is in equilibrium with variations of some nuclear coordinates. Consistent with this approach, it is found that (a) the oscillator strength of the donor-acceptor charge transfer (DACT) absorption is roughly proportional to the red shift of ν CN and (b) there are strong symmetry constraints on the coupling. The latter point is demonstrated by a 10-fold larger red shift of the symmetrical than of the antisymmetrical combination of CNstretching frequencies in the centrosymmetric trans- ([14]aneN 4 )Cr(CNRu(NH 3 ) 5 ) 2 5+ complex ([14]aneN 4 ) 1,4,7,11-tetraazacyclotetradecane). The coupling of the metal dπ orbitals to CNπ and π* orbitals can be formulated in terms of ligandto-metal (LMCT) and metal-to-ligand (MCLT) charge transfer perturbations. The associated charge delocalizations provide a basis for the synergistic weakening of the C-N bond and D/A coupling.

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Photoresponsive asymmetrically organized systems based upon small unilamellar dihexadecyl phosphate (DHP) vesicles were constructed by entrapping high concentrations of potassium ion within the vesicular aqueous core and incorporating either an amphiphilic trans-azobenzene-containing phosphate monoester or an amphiphilic trans-stilbene-containing carboxylic acid into its membrane structure. Spectroscopic measurements indicated that the azobenzene derivative was molecularly dispersed in the hydrocarbon phase of the vesicle and that the extent of aggregation of the membrane-localized stilbene derivative was minor. Thermal K+ leak rates from the doped vesicles were very low, with calculated permeability coefficients (P) of ∼4 × 10-12 cm/s at 40 °C for DHP vesicles containing 5.5 mol % of the trans-azobenzene derivative and ∼1.5 × 10-11 cm/s at 38 °C for vesicles containing 5.5 mol % of the trans-stilbene derivative; for comparison, P ≃ 2 × 10-12 cm/s for undoped vesicles at 40 °C. Photoexcitation of the azobenzene-doped vesicles at 360 nm caused >90% trans → cis photoisomerization over the measured temperature range (25−40 °C), with complete reversion to the trans isomer upon photoexcitation at 450 nm. Photoexcitation of deoxygenated suspensions of the stilbene-doped vesicles at 315 nm gave ∼80% conversion to the cis isomer in the photostationary state, which was not reversible. At 25 °C, K+ leak rates for the isomeric azobenzene-doped vesicles were nearly identical; at 40 °C, K+ leakage for the DHP vesicles containing the cis-azobenzene isomer corresponded to P ≃ 2 × 10-11 cm/s, 5-fold greater than that of the trans isomer. In trans → cis → trans photocycling experiments, K+ leak rates alternately increased and decreased, indicating that the vesicles remained intact. At 40 °C, K+ leakage from the vesicles containing predominantly cis-stilbene was ∼2-fold greater than that from vesicles with the corresponding trans isomer. In electrochemical experiments, viologen-mediated reduction of the DHP-bound trans-azobenzene occurred at E < −0.44 V (NHE), with hydrazobenzene reoxidation at E ≃ −0.16 V; addition of viologen radicals to aqueous suspensions of the trans-azobenzene-doped DHP vesicles caused immediate decolorization of the dye.