Trans-polyenes

Trans-polyenes H-(-HC=CH-),, -H, trans-polyenynes H-(HC=CH-C=C) -H, cumulenes H2C=(C=C) =CH2 and polyynes H-(C=C) -H have been studied (M=N-1). For eentrosymmetrie molecules, the first order hyperpolarizability p is equal to zero so that non linear effects are of second order nature. Furthermore, (the x axis goes through the middle of the C-C bonds of the polyenes, or is the intemuclear axis in the case of linear molecules) is the most important component of the second order y hyperpolarizability tensor, the other components being negligible. Both y and the mean hyperpolarizability... 

Kofraneck and coworkers24 have used the geometries and harmonic force constants calculated for tram- and gauche-butadiene and for traws-hexatriene, using the ACPF (Average Coupled Pair Functional) method to include electron correlation, to compute scaled force fields and vibrational frequencies for trans-polyenes up to 18 carbon atoms and for the infinite chain. 

TABLE 1. Observed absorption maxima (Amax) and molar absorption coefficients (g) of unsubstituted all-trans-polyenes... 

Due to steric hindrance by cis substituents, coplanarity and conjugation of cis polymers is hindered. As a result the absorption of trans polyenes occurs at higher wave lengths compared with the absorption of cis polyenes. The same effect is observed in cis trans stilbenes. 

To link the two half moieties of the molecule, a Julia-Kocienski olefmation was carried out between the C19 building block 59 (again prepared by syn-SN2 -substitu-tion of a propargylic oxirane with DIBAH) and the C20 building block 60, formed via oxidation of 58 with Mn02 (Scheme 18.19). Although this reaction initially led to the formation of the Z-isomer as the major product, the latter was readily isomerized at room temperature to the desired all-trans-polyene peridinin (6). 

The cyclic polyenes (in PPP approximation) have been examined in (Kapuy et ah, 1984) too. The correlation energy contributions obtained for the zW.-trans polyenes further were analyzed both in canonical and localized representation, respectively (Kapuy et ah, 1994). The results are in agreement with those found for the smaller cyclic polyenes in the series. 

It was mentioned above (see Sect. 2) that for centrosymmetic molecules, such as all-trans polyenes and a,c<>-disubstituted frans-polyenes, there is a complementarity between the selection rules for IP- and 2P-allowed transitions. Transitions from the groimd state (which is of gerade type) to a B state are visible in the IPA spectriun, while transitions from the ground state to an Ag state appear in the 2PA spectrum. For this reason, 2P spectroscopy has... 

Linear systems such as trans-polyenes in Fig. 2 are particularly simple, since inversion amounts to reading a legal diagram backwards and thus relates pairs of diagrams k > and cr k >= k >. 

Table 3. One and two-photon thresholds (in eV) of trans-polyenes, CnHn+2, in alkane matrices[58] and in the Pariser-Parr-Pople model[21]. The solid-state shift of 0.40eV of the ionic 1 1B state yields gas-phase values up to n = 12 covalent 2M+ shifts of 0.05eV are neglected.

F1F and DFT methods have been used by Smith et al.205 to calculate the dipole polarizabilities, a, of the all trans polyenes up to CigH2o, and polyacenes to CigH12 and their molecular ions. The static values and the dynamic response at 800 nm have been calculated. For the smaller molecules the results have been compared with those of correlated methods. It is found that the uncorrelated results are about 20% higher than those of the correlated methods for the neutral molecules but are very similar for the molecular ions. General conclusions about the scaling and frequency dependence of a values obtained by simpler methods are drawn. 

Figure 25. Possible tunnel transitions between the two different configurations of a trans-polyene chain.

We have also compared PDA results with earlier PPP calculations on trans polyenes with t -2.26 and t2 = -2.56 ev for single and double bonds of 1.45 and 1.35A, respectively. The larger PDA alternation due to the triple bond (t3 -3.04 ey, r 1.20A) Increases as expected the excitation energy to 1 6 for the ir system, but this Is more than compensated by ir-ir coupling. The optical gap of II In Table II and of 4.543 ev for I are both lower than all-trans hexatrlene gap of 5.04 ev. The larger PDA alternation Is clearly... 

As previously noted, each LHC-II monomer contains two lutein molecules [see Figs. 4 (A) and Fig. 5 (B)]. These two dl -trans polyene chains, each 30 A long, are in the highly hydrophobic environment of helices A and B, and are inclined at an angle of 50° relative to the membrane normal. The shortest dis-... 

All-trans- polyenal Solvent Band max (nm ) Extinction coeff. at hand max (M" cm" X 10" ) Triplet state decay constant ps y K... 

A stereoselective synthesis of all-trans-stereomer of ethyl retinoate (111) was reported by Babler and Schlidt [56] starting from the easily available -ionone 112 (Scheme 30). The key step of the synthesis was a base-catalyzed isomerization of the vinylphosphonate 113, which was obtained in the Horner-Wittig reaction of the aldehyde 114 and tetraethyl bisphosphonate, to give the allylic phosphonate 115 as the sole product. The Horner-Wittig reaction of the latter with ethyl trans-3-methyl-4-oxo-2-butenoate concluded a facile synthesis of the all-trans-polyenic retinoate 111. 

In all calculations described in this section idealized geometries for all considered r-systems are used. In these geometries all -C-C- bond lengths are equal to 1.4 A. Geometries of all zig-zag tmns carbon chains of polyene systems are planar. All -C-C-C- bond angles for trans polyenes are equal to 120°. All cyclic systems and polycyclic hydrocarbons are equilateral (regular) polygons. 

---