Single-bond twisting

In the family of stilbazolium dyes DASPMI, DPS etc., a similar three-state mechanism is active leading in this case to single-bond twisted TICT species with a high intrinsic nonradiative rate [108,109] probably connected with the ionic nature of the dyes (charge shift, see Sect. 4.5). This results in similar lifetime maxima as a function of temperature as for DCS but sizeably shifted to lower temperatures with respect to DCS [96]. 

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

Because there are so many ways that these molecules can rotate and twist, they do not remain rodlike. The molecules have only single bonds, which allow rotation about the bonds, and so each molecule can adopt many configurations. 

It is easy to see how simple MO arguments lead one to expect that two non-bonding orbitals will result if a single bond is stretched sufficiently (Fig. 6), as a double bond is twisted by 90 degrees (Fig. 5), or as atoms are rearranged to make a carbene. Return through such biradicaloid... 

In neutral polyenes a clear distinction can be made between essential single bonds (which can be twisted fairly easily) and double bonds (which offer much more resistance to rotation). It has become customary to distinguish stable isomers (classified E/Z with regard to the configuration around the double bonds) and easily interconvertible conform-ers (classified as s-cis, s-trnns or gauche with regard to the substituents on the essential single bonds) in polyenes. 

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