Twisted aromatic compounds

Typical for aromatic hydrocarbons and their derivatives are the Lb, Lj, Bb, and Bj, bands. Chirally twisted aromatic compounds, for example, hexa-helicene (2), are inherently dissymmetric n chromophores. The rotatory strength of the various transitions can be calculated by means of the common r-electron methods. 

Figure 1.7. Axially chiral molecules (a)-(d) and a molecule with a chiral plane (e). (a) Dichlorallen, (b) twisted biphenyl, (c) helicene, (d) highly twisted aromatic compound [25], and (e) paracyclophane.

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

The value of 0 in a particular radical can be estimated by comparison of the experimental half-wave potential and HFSC with the results obtained from a series of HMO calculations using different assumed values of 0. A large number of phenyl-substituted aromatic compounds [62] and ethylenes [63] have been treated in this fashion. Similar evidence for the twisting of the nitro group in nitroaromatic anion radicals is summarized in Reference 1. Restricted rotation of alkyl substituents is also discussed in Reference 1, but this torsion does not significantly affect the electrochemical behavior. 

The effects of a number of substituent groups on the fluorescence of aromatic compounds are listed in Table 2.3. There are exceptions to this table since a number of other factors must be considered. For example, molecules which are able to rotate, bend or twist have a tendency to lose energy from the excited state through molecular collision and other vibrational processes. It is not possible to compile a complete set of rules for determining whether a molecule will fluoresce, as there are many anomalies. 

A similar phenomenon was noticed in the case of aromatic compounds. Thus, Bellamy [24] has pointed out that in polynitro compounds when one nitro group remains coplanar whilst another is twisted out of the plane of the ring under the influence of steric hindrance, the degree of aromatic conjugation is reduced and a new higher frequency band appears. 

In general, alkanes have small rotations and aromatic compounds have large rotations, so it is reasonable to expect that it is the interaction of plane-polarized light (electromagnetic radiation) with the electrons in the twisted pi system (which can also be considered as having wave properties) that causes this enormous rotation. 

The realization that polycyclic aromatic compounds are not necessarily planar is not a new concept. A book published in 1964 on benzenoid hydrocarbons (24) contains a chapter on nonplanar, overcrowded, aromatic hydrocarbons this chapter primarily describes high degrees of nonplanarity that are due to steric interactions of hydrogen atoms in benzenoid compounds that contain substructures related to benzo[c]phenanthrene (5) (twist angle = 31°) or to dibenzo[c,d]phenanthrene (6) (twist angle = ca. 50°) (26-28). 

Structures determined by modern X-ray techniques show that significant nonplanarities exist in benzenoid compounds of this type. This finding infers that nonplanar polycyclic aromatic compounds are quite common. Structures 7-9, which have twist angles ranging from 30° to 42°, are examples... 

In contrast, none of the hydrocarbon analogs (e.g., 95) of compounds 87-92 include, whereas 96 does. " Addition of the two further phenyl rings now has switched the latter into the twisted aromatic hosts family (Section 4.1). 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

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