Using electronic absorption and fluorescence spectroscopic techniques, as well as quantum chemical calculations, we have studied the electronic spectra of thia-bridged stilbenophane (TSP) with close cofacial contact of two trans-stilbene (t-SB) units. Compared to the t-SB monomer, the experimental consequences of the cofacial arrangement are (i) a splitting of the main absorption band with a weakly allowed emitting state, and (ii) a strongly red-shifted, unstructured emission spectrum with long fluorescence decay times. According to the theoretical investigations, the two t-SB units are strongly bent in the electronic ground state (So), because of repulsive π-π overlap. In the first excited state (S 1 ), the t-SB units become almost planar, because of attractive π*-π* overlap. As a consequence, the symmetry-forbidden S 0 ↔ S 1 transition couples strongly to interchromophore breathing modes of low frequency (v 1 = 67 cm - 1 , v 2 = 117 cm - 1 ), yielding structureless spectra with large Stokes shifts. The features of the calculated spectra are in good agreement with the experimental data. The results indicate that strong intermolecular vibronic coupling is also responsible for "excimer-like" emission in organic molecular crystals of cofacially arranged molecules. Furthermore, the different geometries in the S 0 and S 1 states of TSP give evidence for the mechanism of [2+2]photodimerization of t-SB in solutions.

Modeling of the Optical Properties of Cofacial Chromophore Pairs: Stilbenophane

Thin films of poly(chloro-p-xylylene) (PPXC) grown by the pyrolysis and evaporation of chloro-[2.2]paracyclophane in an evacuated chamber contain free-standing, slanted, parallel columns that are 50 μm long and are assemblies of 50- to 100-nm-diameter nanowires, which thus can have an unprecedented aspect ratio as high as 1000:1. The nanostructured thin films organize spatially with a chemical structure similar to that of planar PPXC thin films, but the former also possess nanostructured morphology. Nanostructured thin films of poly(p-xylylene) (PPX) and its derivatives shall be useful as functionalized interfaces for antifouling coatings and biomedical devices.

Spatially organized free-standing poly(p-xylylene) nanowires fabricated by vapor deposition

The title compound, [Ru2(O2CCF3)4] (1), has been obtained without any exogenous ligands and crystallized by deposition from the gas phase at 170 C. Its crystal structure has been determined for the first time to confirm an infinite chain motif built on axial Ru···O interactions of the diruthenium(II,II) units. The X-ray diffraction studies at variable temperatures showed no phase transitions in the range of 295-100 K but revealed a significant decrease in the volume per atom from 14.2 to 13.3 A3. This noticeable thermal compressibility effect is discussed in connection with the solid-state packing of the [Ru2(O2CCF3)4] chains. The highly electrophilic character of the diruthenium(II,II) units has been shown by the gas-phase deposition reaction of [Ru2(O2CCF3)4] with an aromatic donor substrate, namely [2.2]paracyclophane (C16H16). As a result of the above reaction, a new arene adduct [Ru2(O2CCF3)4·C16H16] (2) has been isolated in crystalline form. It has an extended one-dimensional (1D) chain structure comprised of alternating building units and based on the rare bridging mode of [2.2]paracyclophane, [Ru2(O2CCF3)4·(2-2:2-C16H16)]. The magnetic susceptibility of 1 and 2 has been measured and compared in the range of 1.8-300 K. In addition, in the course of synthesis of 1 by the carboxylate exchange reactions, a new mixed-carboxylate diruthenium(II,II) core complex [Ru2(O2CCF3)3(O2CC2H5)] (3), bearing no exogenous ligands, has also been isolated and structurally characterized. It exhibits an interesting polymeric structure in which the ruthenium(II) centers selectively form axial interdimer contacts with the O-atoms of the propionate groups only.The title compound, [Ru2(O2CCF3)4] (1), has been obtained without any exogenous ligands and crystallized by deposition from the gas phase at 170 C. Its crystal structure has been determined for the first time to confirm an infinite chain motif built on axial Ru···O interactions of the diruthenium(II,II) units. The X-ray diffraction studies at variable temperatures showed no phase transitions in the range of 295-100 K but revealed a significant decrease in the volume per atom from 14.2 to 13.3 A3. This noticeable thermal compressibility effect is discussed in connection with the solid-state packing of the [Ru2(O2CCF3)4] chains. The highly electrophilic character of the diruthenium(II,II) units has been shown by the gas-phase deposition reaction of [Ru2(O2CCF3)4] with an aromatic donor substrate, namely [2.2]paracyclophane (C16H16). As a result of the above reaction, a new arene adduct [Ru2(O2CCF3)4·C16H16] (2) has been isolated in crystalline form. It has an extended one-dimensional (1D) chain structure comprised of alternating building units and based on the rare bridging mode of [2.2]paracyclophane, [Ru2(O2CCF3)4·(2-2:2-C16H16)]. The magnetic susceptibility of 1 and 2 has been measured and compared in the range of 1.8-300 K. In addition, in the course of synthesis of 1 by the carboxylate exchange reactions, a new mixed-carboxylate diruthenium(II,II) core complex [Ru2(O2CCF3)3(O2CC2H5)] (3), bearing no exogenous ligands, has also been isolated and structurally characterized. It exhibits an interesting polymeric structure in which the ruthenium(II) centers selectively form axial interdimer contacts with the O-atoms of the propionate groups only.

Unligated diruthenium(II,II) tetra(trifluoroacetate): the first X-ray structural study, thermal compressibility, Lewis acidity, and magnetism.

The first generation of [1.1]paracyclophane (1a) and its bis(methoxycarbonyl) derivative (1b) from the corresponding bis(Dewar benzene) precursors, 3a and 3b, has been investigated. Irradiation of 3a in a glassy mixture of ether−isopentane−ethanol at 77 K leads to the formation of species exhibiting absorption extending to 450 nm, which readily undergoes secondary photolysis to give an isomer showing λmax at 244 nm. On the basis of these UV/vis spectral observations and the accompanying 1H NMR measurement, the structures of 1a and the corresponding transannular [4 + 4] adduct (21a) are assigned to the initial and the secondary products, respectively. Compound 3b undergoes similar successive phototransformation into 1b and 21b. [1.1]Paracyclophanes, 1a and 1b, and their photoisomers, 21a and 21b, are sufficiently stable to permit the measurement of 1H NMR spectra at low temperature, but are consumed fairly rapidly in solution at ambient temperature, defying their isolation. The results of geometrical optim...

[1.1]Paracyclophane. Photochemical Generation from the Corresponding Bis(Dewar benzene) Derivative and Theoretical Study of Its Structure and Strain Energy

The enthalpies of formation [Δ (g)] of tricyclo[8.2.2.24,7]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.24,7]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol-1, SE(3) = 42.0 kcal mol-1] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol-1 for the parent tricyclo[8.2.2.24,7]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol-1) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bendin...

Heats of formation of [2.2]paracyclophane-1-ene and [2.2]paracyclophane-1,9-diene - an experimental study.

IN the determination of metallic traces, insufficient attention has been paid either to the advantages of direct spectrography of chromatographic columns, or to the chromatographic behaviour of organic metal complexes, although in the latter sphere the use of oxine1 and of violuric acid2 has been reported.

A. P. Seyfang

Papers

Spectrographic Examination of Chromatographic Columns