Helicenes, description

The staircase model is based on the existence of planes through 6 (or 4) atoms, what points to a well regulated geometry. Accepting a regular arrangement of these planes a small number of independent parameters, related to the mutual positions of the planes should be sufficient for a full description of a helicene structure. 

To investigate whether computational methods could be used for an accurate description of the conformation of helicenes a tt-SCF-force field method was applied to hexahelicene, its 1-methyl- and 1,16-dimethyl derivatives and heptahelicene ll2). To avoid predestinate outcome of the calculations minimization of the energy was started from a nearly planar model, and the minima obtained were compared with those of a procedure starting with the geometry found by X-ray analyses. 

The logical development of the linear fusion of odd rings is described in [73], That report presents both the IUPAC nomenclature for molecules formed by the fusion of tricyclo through heptacyclo pentane modules and a binary code that simplifies the description of the complete set of fused tricyclo through hexacyclo pentane aggregations. The latter examples in Table 3 of Chapter 2, shall focus on cyclopentane as the module of interest. In a similar manner, the extrapolation of linear chains of cyclopentane modules to form the counterpart of helicenes, henceforth referred to as "helicanes", is described in [73]. 

M-), (P-) Stereochemical descriptors (M = minus, P = plus) introduced to describe the chirality of helical molecules. Extension of the CIP system to planar chirality gave an alternative description aR/aS for helical molecules such as helicenes, aR invariably = (M) and aS = (P), but for compounds showing planar chirality the reverse, with pR = (P) and pS = (M). Best avoided. See Section 7.3.2. 

The reader is referred to the Number of Benzenoids and Poly hex Hydrocarbons subsection for the definition and classification of benzenoids and polyhex hydrocarbons, as well as for additional references for this class of compounds, which is only partially reviewed because of space limitations. Let us recall that the direct counting approach has difficulties with molecules that cannot be represented by tree-like structures, such as pericondensed polyhexes. Furthermore, the counting approach caimot separate nonplanar polyhexes (helicenes) from planar benzenoids. Consequently, for benzenoids and polyhexes, enumeration is not only a valuable tool that provides a concise description of the structures being enumerated, but also enumeration computes isomer numbers that cannot be derived otherwise. 

A new approach to second-order nonlinear materials was reported by Verbiest et al. (56) in which chirality plays the key role. These authors investigated Langmuir-Blodgett films of chiral helicenes which lack features commonly associated with a high SHG response. The molecules adopt a helical structure on a solid support and this chiral supramolecular arrangement enhances the second-order NLO susceptibility by a factor of 30 when compared to the corresponding racemic mixture. An adequate description of the SHG response in a chiral system requires additional tensor elements. Experimental evidence was provided that those tensor elements which are only allowed in a chiral environment dominate the SHG response of the helicene system. 

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