On the basis of an extended series of monodisperse oligomers of the dialkoxy-substituted phenyleneethenylenes la-i the Eqs.(3) and (4) were conceived in order to determine the limiting values of the energies Ei and the wavelengths di of the W / v i s absorption. The convergence of the Ei and l i values with a growing number n of repeating units pennits a precise prediction of the Ei,,,, and ,Ii,,,, values of the corresponding polymer 1 j as well as a statement about the overall effect of conjugation mi and the effective conjugation length ECL. A great variety of different conjugated oligomers 2-14 can be evaluated by the same algorithm. Acta Polymer., 48,379-384 Q VCH Verlagsgesellschaft mbH, D-69451 Weinheim 1997 0323-7648/97/0808-0379$17.50+.50/0 Acta Polymer., 48,379-384 (1997) Effective conjugation length and UV/vis spectra of oligomers 383

Four series of oligo(1,4-phenyleneethynylene)s (OPEs), 1−4 (a−d), each having a terminal dialkylamino group as their electron donor, were prepared by applying Sonogashira−Hagihara reactions and a protecting group strategy. To study the influence that the push−pull effect has on the long-wavelength absorption, three of the four series of OPEs contain terminal acceptor groups (CN, CHO, NO 2 ). Extending the conjugation (increasing the number of repeat units, n) lowers the energy E(n) of the electron transition in the purely donor-substituted series 1a−4a (bathochromic shift). This effect is superimposed in the push−pull series 1−4 (b−d) by the effect of the intramolecular charge transfer (ICT), which decreases with increasing the distance between

Four OPV series 1-4 (a-d) with a terminal dialkylamino group as electron donor were prepared by Wittig-Horner reactions. To study the influence of the push-pull effect on the long-wavelength absorption, three of the four series contained terminal acceptor groups (CN, CHO, NO(2)). The length of the chromophores strongly affects the intramolecular charge transfer (ICT)-an effect which superimposes upon the extension of the conjugation. Increasing numbers n of repeat units cause an overall bathochromic shift for the purely donor-substituted series 1 a-4 a and the series 1 b-4 b with CN as weak acceptor. The two effects annihilate each other in the series 1 c-4 c with terminal CHO groups, so that the absorption maxima are almost independent of the length of the chromophore. A hypsochromic shift is observed for the series 1 d-4 d, which contains the strong acceptor group NO(2). This anomaly disappears on protonation of the dialkylamino group because the push-pull effect disappears in the ammonium salts. The results can be explained by semiempirical quantum mechanics (AM1, INDO/S). The HOMO-LUMO transition, which is mainly responsible for the ICT, becomes less important in the electron transitions S(0)-->S(1) when the distance between donor and acceptor is increased. The commonly used VB model, which contains an electroneutral and a zwitterionic resonance structure, is contrasted with a MO model with dipole segments at both ends of the OPV chains. The latter model turned out to be more appropriate-at least for donor-acceptor-substituted OPVs with n >/= 2.

Individual but connected synthetic routes for the preparation of the all-E-configured 2,5-dipropoxy-substituted oligo(l,4-phenyleneethenylene)~ 1 a-g were developed An increasing number of conjugated stilbene units from n = 1 to n = 11 led to a convergent bathochromic shift (Ah = h, -h, = 127 nm) in the UV/Vis absorption An exponential function for the convergence of the absorption energies (wavelengths) in conlugated systems is proposed By a simple extrapolation of this function the effective chain length may be determined Among the conjugated polymers poly( 1,4-phenyleneetheny1ene)s [polyb-phenylenevinylene)s, PPV] are receiving increasing attention as conductive materials (upon doping), photoconductors, materials for light-emitting diodes and nonlinear optics ['].A crucial question for all these applications is: How does the length of the conjugated chain influence the properties? Polymers exist as polydisperse mixtures and therefore contain components with suitable or less suitable properties.Another disadvantage is the occurrence of structural defects. PPVs, mostly synthesized by the precursor method[';'], contain for example saturated carbon centers in their chain, which interrupt the conjugation. Incomplete elimination processes are responsible for these defects. A further defect in the PPV chain can originate from the formation of (aconfigured double bonds, which induce a high torsion of the chain and impair therefore the conjugation. All these drawbacks can be avoided by the preparation of specific oligoniers, but one should not forget in the discussion polymers versus oligomers, that polymer systems mostly exhibit better mechanical properties, especially the capability of generating films.Some time ago we elaborated a novel procedure for the preparation of 2-alkoxy-, 3-alkoxy-, and 2,5-dialkoxy-substituted oligo-and poly( 1,4-phenyleneethenylene)~[~"]. The alkoxy side chains enhance the solubility and lower the HOMO-LUMO gap and the band gap, respectively'']. On the other hand, conjugation is influenced by the torsion in the chain which reflects the contribution of different conformations based on the substitution pattern[? Concerning the photoconductivity the best results were obtained for 2,s-dipropoxy-substituted PPVS[~.~I. However, again the question of the optimum length of the main chain remained still unanswered. Therefore, we decided to synthesize and investigate the monodisperse 2,5-dipropoxy-substituted oligo( 1,4-phenyIeneethenylene)s 1 a-g. SynthesesThe final step in all synthetic sequences of la-g was the formation of one or two CC double bonds by applying the

Three new monoterpenoid indole alkaloids, 19(S),20(R)-dihydroperaksine (1), 19(S),20(R)-dihydroperaksine-17-al (2), and 10-hydroxy-19(S),20(R)-dihydroperaksine (3), along with 16 known alkaloids 4-19 were isolated from hairy root culture of Rauvolfia serpentina, and their structures were elucidated by 1D and 2D NMR analyses. Taking into account the stereochemistry of the new alkaloids and results of preliminary enzymatical studies, the putative biosynthetical relationships between the novel alkaloids are discussed.