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The compounds do not exhibit fluorescence in solution. These unprecedented scaffolds, featuring a helical PAH backbone and a reactive P atom, present promising redox and optical properties that can be further used in optoelectronic applications, including chiroptical applications, provided that enantiomers of 31 — 33 can be separated.

In all of these examples, the phosphorus atom retained its reactivity toward organic reagents or metal ions. So far, only a few examples have been described and the main reasons are clearly the synthetic difficulties associated with the preparation of such products.

We thank Prof. Duffy for fruitful discussions. Volume 18 , Issue The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.


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If the address matches an existing account you will receive an email with instructions to retrieve your username. ChemPhysChem Volume 18, Issue Minireview Free Access. Gellert ter 4, H Budapest, Hungary Search for more papers by this author. Muriel Hissler Corresponding Author E-mail address: muriel. Tools Request permission Export citation Add to favorites Track citation.

Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract Polycyclic aromatic hydrocarbons PAHs are highly appealing functional materials in the field of molecular electronics. Figure 2 Open in figure viewer PowerPoint.

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NICS 1 local aromaticities in ppm of compounds 15 , 9 and Figure 9 Open in figure viewer PowerPoint. Figure 10 Open in figure viewer PowerPoint. Figure 11 Open in figure viewer PowerPoint. Figure 12 Open in figure viewer PowerPoint. Figure 13 Open in figure viewer PowerPoint. Figure 14 Open in figure viewer PowerPoint.

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Figure 15 Open in figure viewer PowerPoint. Figure 16 Open in figure viewer PowerPoint. Figure 17 Open in figure viewer PowerPoint. Figure 18 Open in figure viewer PowerPoint. Figure 19 Open in figure viewer PowerPoint. Conflict of interest The authors declare no conflict of interest. References 1 K. Novoselov , A. Geim , S. Morozov , D. Jiang , Y. Zhang , S. Dubonos , I. Grigorieva , A. Firsov , Science , , — Wiley Online Library Google Scholar. Google Scholar. Citing Literature. Figures References Related Information.

Close Figure Viewer. Browse All Figures Return to Figure. Previous Figure Next Figure. Email or Customer ID. Forgot password? Old Password. The diene-like chemical behavior may be illustrated in such a way. However, the question is how the aromaticity of these compounds is to be explained?

Cyclopentadiene does not display an aromatic behavior. So, what is really the difference between cyclopentadiene and the aromatic five-membered heterocycles? In contrast to cyclopentadiene's sp 3 carbon, the nitrogen, oxygen or sulfur of the aromatic heterocycles possess at least one lone electron pair that occupies a p orbital perpendicular to the ring plane. Furthermore, they are planar. According to the rules for aromaticity, they are aromatic compounds which may additionally be substantiated through the application of the Frost circle.

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The second lone electron pair of oxygen and sulfur in furan and thiophene, respectively, is located in an sp 2 orbital that is parallel to the ring plane. In comparison to furan and thiophene, the dipole moment of pyrrole unexpectedly has the opposite direction. This is surprising, as the positive pole of pyrrole's dipole is located at the nitrogen, though nitrogen is more electronegative than carbon.

Why is the highest electron density of pyrrole not located at the nitrogen? The illustration of pyrrole's resonance structures makes it clear that negative charges appear at the four carbons, though not at the nitrogen, while a positive charge only appears at the nitrogen. That is, the effect of resonance on the electron distribution obviously surmounts the effect that nitrogen's higher electronegativity has on electron distribution.

Pyrrole's chemical behavior corresponds to the electron distribution indicated by the respective resonance structures. Through a relatively weak acid, for instance, it is not the nitrogen but almost only the carbons that are protonated.

Phosphorus Heterocycles II

The dipole moment of pyrrolidine, which is the corresponding saturated compound, is not influenced by resonance. Thus, in this case, the negative pole is located at the nitrogen.

Rings with seven or more members

In furan and thiophene, the dipole moment is influenced by resonance in the same way as in pyrrole. However, the effect of resonance is obviously not sufficient enough to exceed the effect of the heteroatom's electronegativity. Thus, the negative pole is located at the heteroatom. However, due to the resonance, the dipol moments of furan and thiophene are smaller than that of the corresponding saturated heterocycles, tetrahydrofuran and tetrahydrothiophene. In the case of pyrrole, the influence of resonance on the dipole moment is so strong, that the dipole moment is inverted in comparison to the corresponding saturated heterocycle, pyrrolidine.

As is expected of an aromatic compound, pyrrole can react in electrophilic substitutions. Due to the higher electron density at the carbons, substitution takes place at a carbon instead at nitrogen. However, there are two different carbon positions.