Cyclophane Ring Research Articles

Photoinduced electron transfer in molecular organizates at the gas-water interface

The influence of the molecular geometry is of fundamental importance for a better understanding of the photoinduced electron transfer mechanism. Because of their typical molecular structures, cyclophane rings have proved to be suitable for this purpose as electron acceptor molecules adsorbed under an amphiphile monolayer. We used a pyrene-labelled phospholipid derivative both as molecular anchor for the cyclophane ring and electron donor molecule. The co-spreading technique was used to prepare the complex monolayers. Surface pressure and surface potential measurements have indicated similar monolayer behaviour as with dimyristoylphosphatidic acid as anchor molecule, leading to the same parallel orientation of the molecular plane of the cyclophane ring with respect to the water surface. Surface reflection has revealed a change in orientation of the pyrene moiety during compression, but no relevant dimer formation was observed. Fluorescence spectra were almost of excimer emission nature only. Steady state fluorescence quenching of the pyrene depends on the pyrene/cyclophane co-spreading ratio.

Reduction of the 1,2,5-thiadiazole ring of 3,6:12,15-di-1,4-benzo[6.6](3,4)-1,2,5-thiadiazolo- and 3,5:11,13-di-1,3-benzo-[6.6](3,4)-1,2,5-thiadiazolocyclophanes. Selective preparation ofcis- andtrans-[23]cyclophane-1,2-diacetoamide

The effect of the [2.2.2]cyclophane ring structure on the reduction of 1,2,5-thiadiazole ring incorporated in cyclophanes 1a-c and 2a-c was investigated. When reduced by sodium metal in ethanol followed by acetylation, para[2 3 ]cyclophane 1 gave a mixture of the expected cis- and trans-diamides, 3 and 4, in which 4 was the major product. On the other hand, reduction of 1 with lithium aluminum hydride proceeded in a cis-selective manner and gave 3 as a major product after a treatment of the reduced products with acetic anhydride. The reduction of metacyclophane 2, which is less strained than 1, proceeded exclusively in cis-fashion and a subsequent treatment of the reduction product with acetic anhydride gave only cis-diamide 6

Tautomerism (hydroxyazo versus ketohydrazone form) of [2.2.2] cyclophane-1,2-dione monoarylhydrazones: an 1H-NMR study

Hydroxyazo-ketohydrazone tautomerism of arylhydrazones of para-para-para-, para-meta-para-, and para-ortho-para-[2.2.2]cyclophane-1,2-dione 1a, 1b and 1c was studied by means of 1H-NMR spectroscopy. Except for the phenylhydrazones 4ba and 4ca of 1b and 1c, which exist as the ketohydrazone 4-H, the azo tautomer 4-A is predominant in solution, and is more favored in chloroform and benzene than in acetonitrile. A good correlation was found between the strain of the cyclophane ring and the δ-value of OH proton in 4-H.

New strained and chiral aromatic molecules

Abstract

Investigations of the influence of molecular geometry on the spectroscopic and photochemical properties of .alpha.-oxo[1.n]paracyclophanes (cyclophanobenzophenones)

The spectroscopic and photochemical properties of a family of (Y-OXO( l.n)paracyclophanes (8-12) for n = 8-12, respectively) have been investigated. Compared to a model structure possessing a nominal T,(na*) state (4,4'-dimethyl- benzophenone, DM-BZ), photochemical and photophysical evidence is presented that the na* character of TI decreases as the cyclophane ring size decreases. For example, the rate constants for hydrogen atom abstraction from 1,4-cyclohexadiene in acetonitrile are 1.6 X lo8, 0.93 X lo8, 0.30 X lo8, 0.06 X lo8, and 0.04 X lo8 M-' s-) for triplet 12-8, respectively, a result consistent with decreasing nr* character and decreasing reactivity toward hydrogen atom abstraction with decreasing value of n. The spectroscopic properties (vibrational structure of the phosphorescence spectrum, phosphorescence lifetime, phos- phorescence excitation spectra, triplet-triplet absorption spectra) also vary in a manner consistent with decreasing na* character as the cyclophane ring size decreases. Since the early 1960s it has been known that aryl-alkyl ketones and diary1 ketones may possess nearly isoenergetic 3na* and %a* The relative energies of the two states, each with its own distinctive photophysical and photochemical behavior, can and have been manipulated by chemical substitution and solvent polarity. Substantial efforts have been devoted to understanding these effects. Some ketones, such as acetophenone, have their two triplets so close in energy that they may be inverted simply by adjusting the polarity of the solvent. When the states are very close, interesting consequences may arise due to the proximity effect, state mixing, or dynamic equilibrium between the states.' In spite of extensive research on the role of conformation on photochemical and photophysical beha~ior,~ there has been rel- atively little work reported on the influence of molecular geometry on the photochemical and photophysical properties of a family of structurally related compounds.

NMR of the methylene group: Artifacts in the proton–proton decoupled heteronuclear two‐dimensional chemical shift correlation experiment

AbstractWhen the proton–proton decoupled heteronuclear two‐dimensional chemical shift correlation experiment is applied to a molecule containing methylene groups, the spectra show not only the expected signals at the two proton chemical shifts, but also a strong artifact. This artifact appears at the average of the two proton chemical shifts, and arises from coherence transfer between the two protons caused by the BIRD pulse. It is not caused by strong coupling between the protons, and so may appear in any spectrum. The dependence of this artifact on experimental conditions was analysed, and is illustrated with results obtained on the methylene groups from the cyclophane ring in N6′,N9‐octamethylenepurinecyclophane.