Zwitterionic resonance structures

Molecules with alternating single and double bonds ( conjugated polyenes ) often exhibit unusual physical and chemical properties. Chemists have postulated the involvement of zwitterionic resonance structures to account for these properties. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

The chemistry of the so-called calicene 23 and its stable derivatives has recently been reviewed [14, 15], as has that of pentafulvalene 24 and its derivatives [14]. In view of the possible stabilization of at least the cationic moieties in the zwitterionic resonance structures 23 1 and 24 +, it would certainly be interesting to prepare the percydopropylated hydrocarbons 25 and 26. The easily accessible tetracydopropylcydopentadiene 14 [13] appears to be an appropriate precursor for these target molecules (Scheme 5). 

A -Methyl pyrroloisothiazolones 34 (R =C02Me, NO2 R = H, Me) show downfield H NMR chemical shifts with respect to N-methylated open-chain or cyclic sulfenamides (see Sections 4.05.6.3.4 and 4.05.9.1.1). This behavior can be attributed to a strong deshielding effect caused by a partial positive charge on the N-atom, as depicted in the zwitterionic resonance structure 34a (Equation 4) <2003HCA2471>. 

Actually, a number of resonance hybrid structures of NNO can also be written. In addition to the hypervalent configuration I there are three relevant zwitterionic resonance structures ... 

The interaction of NLO-active organic molecules with intensive light induces a dipole moment, which results in strong hyperpolarization (see Section 13.3.1). In the simplest case of neutral compounds with uncharged donor and acceptor moieties, the CT and the zwitterionic resonance structures are shown below (Scheme 13.1). 

Scheme 11.22. Conjugated oligomers with terminal donor-acceptor substitution charged cyanines (symmetrical polymethine dyes) 51 below the so-called cyanine limit DAOPVs 52/DAOPEs 53 having electroneutral and zwitterionic resonance structures.

Scheme 11.25. Push-pull substituted OPVs whose zwitterionic resonance structure is compatible with aromatic subunits in the conjugated chain.

Push-pull systems with n linkers of the OPV or OPE type represent in materials science a new and highly promising class of conjugated oligomers. In addition to D- r-A chains, which are characterized by electroneutral resonance structures with benzene rings and zwitterionic resonance structures with p-quinoid substructures (Scheme 11.22), series like 61 should be studied, in which the zwitterionic resonance structure is compatible with aromatic subunits (Scheme 11.25) [Iz, 216]. 

A large class of organic reactions that lead to the formation of five-membered heterocyclic molecules, have been designated as 1,3 dipolar cycloaddition reactions °. Huisgen has defined the 1,3 dipole to be a species which is represented by zwitterionic resonance structures (i.e. the standmd Lewis octet structures) and which undergoes 1,3 cycloadditions to a multiple bond system, the dipolarophile , as in structures (18) and (19). 

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