Silylenes electronic states

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

While carbenes are found with both singlet and triplet ground electronic states, only singlet silylenes and germylenes are well documented. 

The absorption spectra of silylene polymers and telomers have been extensively reported and it has been shown that the position of the absorption maximum shifts to the red with increasing degree of polymerization (6,15-19). We and others have reported the existence of narrow, line like fluorescence with no observed vibrational structure for a number of medium and high molecular weight polymers (2,9,16,20,22). The narrow, line-like fluorescence and the chain length dependence of the absorption spectra both indicate substantial delocalization of the electronic states involved in the transitions. 

The fluorescence of the phenyl polymer is similar in shape to the fluorescence from the alkyl polymers and the similar shape of the phosphorescence spectrum, as well, suggests that the origins of the electronic spectrum are also much the same. The apparent increased quantum yield for phosphorescence in poly(phenyl methyl silylene) probably reflects a mixing of the ring electronic levels with the levels of the chain. Both the fluorescence and phosphorescence of the naphthyl derivative are substantially altered relative to the phenyl polymer. Fluorescence resembles that of poly(B Vinyl naphthalene) (17,29) which is attributed to excimer emission. Phosphorescence is similar to naphthalene itself. These observations suggest that the replacement of an alkyl with phenyl moiety does not change the basic nature of the electronic state but may incorporate some ir character. Upon a naphthyl substitution both the fluorescence and phosphorescence become primarily tt-tt like. 

Phosphorus-, tin-, boron- and indium-doped silicon samples give different photo-EMF signals as well as modified chemical reactivity in the direct synthesis. The actual concentrations of n- and p-dopands control the reactivity of the silylene intermediates. Tests on these silicon samples were demonstrated that the electronic state of the educt silicon significantly affects the selectivity. Thus the amounts of disilanes in the crude silane mixture decreased if the n-type semiconductor behavior of the educt silicon was intensified by doping. 

The various computational results agree well with these data. For silylene the situation is opposite to that for methylene in fact, a recent experimental nuclear recoil study seems to establish for SiH2 the existence of a singlet ground state. However, in this case, the energy separation with the Bj state is not known, since, in the various spectroscopic studies reported for SiH2, no transitions involving triplet electronic states have been identified. 

The spin conservation rule requires that the silylene thus formed be in the singlet electronic state. 

Studies of SiFj formed in the nuclear recoil systems are also consistent with the singlet being the ground state. In fact, essentially all the kinetic studies of various silylenes are consistent with singlet ground electronic states. 

TABLE 3. Bond Angles and Bond Lengths of Silylenes in Their Ground Electronic States... 

The polysilylenes continue to be of interest because of the novel electronic and optical properties of these macromolecules. These properties stem from the presence of a o-delocalized -Si-Si- polymer backbone. The interplay between conformational states, side-chain order/disorder transitions, and electronic states creates interesting temperature and pressure dependent optical phenomena. These novel properties have stimulated interest in improved synthetic methods. Thus, the first three papers in this symposium focus on understanding and improving catalytic methods for generating silylene chains. These efforts are followed by Worsfold s research on improving the traditional Wurtz coupling method of synthesis. 

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