Matrix-isolated I-silacyclopropenylidene (7) has been generated by pulsed flash pyrolysis of 2-ethynyll,l,l-trimethyldisilane (4). By subsequent photolysis 7 can be isomerized into ethynylsilanediyl (S), vinylidenesilanediyl (S), and silacyclopropyne (9). The identification of the C2HzSi isomers and their I3C and D isotopomers is based on the comparison of their experimental and calculated (MP2/6-31G**) infrared spectra. In case of 9, the first example of a "formal" cyclopropyne, the definitive assignment of the structure was only possible by studying I3C isotopomers. Detailed ab initio calculations have been carried out in order to understand the unique bonding situation in 9. It is found that the Lewis structure 9B, where the dashed line indicates an electron pair which occupies a CC nonbonding orbital, is the best possible compromise to describe the electronic structure of 9 correctly. The value of 6.2 kcal mol-' for the barrier of 9 to the planar transition structure 13 represents the lowest inversion barrier of tetra-coordinated silicon described so far.

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The association of the discotic mesogens is crucial for the formation of columnar liquid crystals, which we have proved for 2 h.k by X-ray small-angle diffraction, differential scanning calorimetry (DSC). a n d polarization microscopy. The DSC measurements for 2k are shown in Figure 2. The cooling and heating curvcs indicate an extremely wide range for the LC phase. Irrndiution ( j . 2 300 nm) at 230 'C leads to a quantitative t Endo ----t ' 1.7 Jg-' 266 "C 0 50 100 150 200 250 V C -!,I<. 2. I>S(' mi'iis~ii-eiiieiit of 2 k (tipper cooling curve. lower heating citrvc. 10 K 111111 ' iii each c a w ) .formation of the belt cyclophane 3 k . The LC properties are irreversibly lost in the process. At lower temperatures the quantum yield drops to zero. At room temperature the LC phase is photostable. Compound 2 h shows analogous behavior. The large temperature effect on the photoreactivity cannot be explained by a thermal activation barrier in the S, state, since then the temperature dependence would also appear in the solution photolysis. We explain this effect by the temperature-dependent mobility. The photocyclodiiiierization requires not only a parallel orientation of the large discoid mesogens. but also a small separation o f the olefinic centers. Only with the increased molecular mobility at higher temperatures and the thereby increased rotation about the column axis can a photodimerization take place in the LC phase. Under the described conditions this is even more efficient than in solution.

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