Conjugated systems linear

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Hiickel s calculations on planar conjugated systems were extensively exploited, and I refer you once again to Streitwieser s classic book. Molecular Orbital Theory for Organic Chemists. What few calculations that had been done at that time on the (T framework had used the method of linear combination of bond orbitals. 

However, on deprotonation even by weak bases the linearly conjugated system is transformed by tautomerization into the cyclically conjugated aromatic [22]pentaphyrin(1.1.1.0.0). 

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

For the ir-conjugated systems as well as the tr-type diradicals, the triplet branched isomers are more stable than the linear ones (e.g., 1 vs 3 5 vs 4 21 vs 22). 

In conclusion, a variety of linear or cyclic oligo(phospholes)s and their derivatives are accessible via a set of efficient synthetic strategies. The potential of these compounds as advanced 71-conjugated systems is broadened by the presence of reactive trivalent P-centres, which allow a range of additional chemical modifications to be achieved. However, elucidation of structure-property relationships for these derivatives is still needed. 

This chapter summarizes our current knowledge and understanding of linear ji-conjugated systems for NIR dyes cyanines and cyanine-like molecules for one-and multiphoton applications, with a focus on the intramolecular spectroscopic properties and dynamics and the experimental methodologies used to characterize these particular organic systems and others. 

Webster S, Odom SA, Padilha LA, Przhonska OV, Peceli D, Hu H, Nootz G, Kachkovski AD, Matichak J, Barlow S, Anderson HL, Marder SR, Hagan DJ, Van Stryland EW (2009) Linear and nonlinear spectroscopy of a porphyrin-squaraine-porphyrin conjugated system. J Phys Chem B 113 14854-14867... 

Thus the ir-electron delocalization length Lj and the linear and nonlinear optical coefficients in ID conjugated systems reach their... 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

The photochemistry of linearly conjugated 2,4-cyclohexadiene-l-ones (e.g. 4) has been studied extensively7. These linearly conjugated systems generally photorearrange to a (Z)-dienylketene 5 (equation 1) this process is usually reversible, so that in the absence of a nucleophile little change is observed. In the presence of amines or alcohols, however, amides and esters are typically isolated. In the presence of weaker nucleophiles a slow formation of phenol derived products is usually observed. 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

Abstract Described are the synthetic routes to precisely defined molecular wires which are of discrete length and constitution. They are fully conjugated systems and are expected to have nearly linear current-voltage response curves. Their ends are functionalized with molecular alligator clips, based on chalconides and isonitriles, for adhesion between proximal probes. Both solution and solid-phase approaches have been used to prepare these molecular wires that are based on oligofthiophene ethynylene)s and oligofphe-nylene ethynylene)s. Molecular device syntheses are also described that would be expected to have nonlinear current-voltage responses. 

J. Roncali, Oligothienylenevinylenes as a new class of multinanometer linear ir-conjugated systems for micro- and nanoelectronics, Acc. Chem. Res., 33 147-156, 2000. 

Valence bond theory was initially the most widely accepted approach, probably because it depended on familiar concepts of mesomeric effects in conjugated systems. The theory assumed that the true wave function for the mesomeric state of a molecule is a linear sum of those of the contributing canonical forms. The technique was never successful for quantitative calculation of the absorption spectra of dyes, however, because of the difficulties encountered when introducing the numerous canonical structures necessary for computational precision. 

The non-linear optical (NLO) active aromatic chalcone l-(2-furyl)-3-(4-benzamidophenyl)-2-propene-l-one (FAPPO), shown in Fig.l, was found to give a rather big SHG activity of 1-2 order of magnitude higher than crystalline urea [6]. Due to their conjugated system all discussed NLO active molecules are approximately fiat. 

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