Delocalized pi electrons

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

Which substance below is resonance stabilized by delocalized pi electrons ... 

BECAUSE benzene has delocalized pi electrons that stabilize its structure. 

If we look at a popular organic semiconductor, say pentacene, the hydrogen atoms which surround the carbon backbone are less electronegative than the carbon backbone itself and lend some electron density to the delocalized pi-electron cloud. The electron rich conjugated molecule has difficulty accepting another electron, but is able to lose an electron with relative ease. As a consequence, positive charge carriers dominate transport in pentacene thin films. 

Fig. 3.1. Some of the more common repeating functional units in conjugated organic materials. Most organic semiconductors and conductors are made from fused or linked elements like these, which are rich in sTj hybridized carbon atoms and delocalized pi electrons. It should be noted that chemical synthesis of organic semiconductors and conductors is often not performed using these materials as starting ingredients.

Two symbols are used to represent benzene. One is the Kekule structure, and the other is a hexagon with an inscribed circle, to represent the idea of a delocalized pi electron cloud. 

The benzyl group provides a delocalized pi-electron system that can carry substituents designed to enhance the NLO response. Alternatively, the parent polymer, poly(L-glutamic acid), may be derivatized through its pendent carboxyl group to bear an NLO active chromophore. 

Asymmetric molecules with delocalized pi-electron systems tend to large values of the first hyperpolarizability term, which is responsible for second harmonic generation. By incorporating electron donors and/or acceptors into the pi system, charge transfer interactions are enhanced, yielding increased values of higher order susceptibilities. 

Another species in which delocalized pi orbitals play an important role is benzene, QHg. There are 30 valence electrons in the molecule, 24 of which are required to form the sigma H H bond framework ... 

X values clearly reflect the poorer transmission of pi delocalization effects from the meta than para position. For the select meta sets of Table XI, for example, X" is typically about. 4, whereas for corresponding reactions, X is around unity. Evidence is meager with respect to the ortho position, but it appears that in accord with the classical ideas of pi electron transmission in the benzene ring, generally X > X > X"" for corresponding reactions (cf. subsequent discussion). 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

The pi-electrons cire delocalized over the entire ring structure, not localized between two carbons. This contributes to the observed stability of benzene. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

The ring appears to have alternating double and single bonds, but this is not technically correct. Benzene exhibits resonance, which means the double bonds are actually delocalized around the entire six carbon atoms in the ring. It s not that the double bonds are moving between different pairs of carbon atoms it s that the double bonds were never localized between any two carbon atoms in the first place. You will sometimes see the double bonds replaced by a circle, which emphasizes the delocalization of the pi electrons. These concepts are illustrated in Figure 11.16. 

L. Sobczyk et al., Interrelation between H-Bond and Pi-electron delocalization. Chem. Rev. 105, 3513-3560 (2005)... 

The Hiickel molecular orbital (HMO) model of pi electrons goes back to the early days of quantum mechanics [7], and is a standard tool of the organic chemist for predicting orbital symmetries and degeneracies, chemical reactivity, and rough energetics. It represents the ultimate uncorrelated picture of electrons in that electron-electron repulsion is not explicitly included at all, not even in an average way as in the Hartree Fock self consistent field method. As a result, each electron moves independently in a fully delocalized molecular orbital, subject only to the Pauli Exclusion Principle limitation to one electron of each spin in each molecular orbital. 

In situations like this one, the overlap of the carbon p orbital with one of the oxygen p orbitals cannot be ignored as Figure 3.15c attempts to do. Instead, all three p orbitals must be used to form MOs that involve the carbon and both oxygens. The three AOs interact to form three delocalized pi MOs. Two of these three delocalized MOs contain the four electrons the pi electrons and an unshared pair of electrons from the localized picture. Part d of Figure 3.15 shows how the orbitals overlap in the delocalized picture. 

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