Monodisperse Dialkoxy‐Substituted Oligo(phenyleneethenylene)s

Abstract

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

Solid Film versus Solution‐Phase Charge‐Recombination Dynamics of exTTF–Bridge–C60 Dyads

“…[34] Dyad 2 (exTTF-TVP-C 60 ) was obtained in a multistep synthetic procedure as depicted in Scheme 1. Thus, a twofold Wittig-Horner olefination reaction of bisphosphonate 3 [37] with commercially available 4-bromo-2-formylthiophene Scheme 1. Synthesis of the C 60 -TVB-exTTF dyad (2).…”

Optical Bandgaps of π‐Conjugated Organic Materials at the Polymer Limit: Experiment and Theory

“…A linear relationship is typically observed between the experimental transition energies and 1/N for oligomer sizes ranging from 2 to 6 repeat units (n), while the transition energy of the monomer (n = 1)deviates from the linear behavior, as illustrated in Figure 5 for oligothiophenes. Because longer oligomers of well-defined chain length (n > 6) were not available until the mid 1990s, [29] the linear dependence prevailing for mediumsize oligomers stimulated a large number of research groups to extrapolate the oligomer values to the limit of an ideal infinite conjugated polymer chain by linear regression. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] However, the experimental optical bandgaps of oligomers with n > 6 [45][46][47][48] were always significantly larger than the extrapolated values.…”

Giant meso–meso‐Linked Porphyrin Arrays of Micrometer Molecular Length and Their Fabrication

“…[10] Meier et al synthesized monodisperse oligo(1,4-phenylene ethenylene)s up to 12-mer as a more planar p-conjugated oligomer. [11] Tour et al developed an approach to growing molecular systems by means of an iterative divergent/convergent method, which resulted in the synthesis of phenylethynyl 18-mer bearing protected thiol groups for connection onto gold electrodes. [12] This method, when combined with solid-phase synthesis, furnished a block-alternating 23-mer consisting of oligo(1,4-phenylene ethynylene)s and oligo(2,5-thiophene ethynylene)s, the molecular length of which reached about 160 .…”