Synthesis and Characterization of Multinanometer-Sized Expanded Dendralenes with an iso-Poly(triacetylene) Backbone

Abstract

A series of [n]dendralenes (n 3, 4, 8, 3b ± d (Fig. 1)) expanded with buta-1,3-diynediyl moieties between the CC bonds were prepared by repetitive acetylenic scaffolding of 3-(cyclohexylidene)penta-1,4-diyne building blocks (Scheme). These remarkably unstable iso-poly(triacetylene) (iso-PTA) oligomers were characterized by 1 H-and 13 CÀNMR ( Fig. 3 and Table 1), IR, and UV/VIS (Figs. 4 and 6 and Table 2) spectroscopy, as well as mass spectrometry (Fig. 2). The expanded [8]dendralene contains 40 C(sp)-and C(sp 2 )-atoms in the backbone and represents the longest iso-PTA oligomer prepared to date. HOMO-LUMO Gap energies were determined as a function of oligomeric length ( Fig. 5 and Table 3), providing insight into the degree of p-electron delocalization in these cross-conjugated chromophores. A continous drop in the HOMO-LUMO gap with increasing number of monomeric repeating units provides evidence that cross-conjugation along the oligomeric backbone is effective to some extent. The limiting HOMO-LUMO gap energy for an infinitely long, buta-1,3-diynediyl-expanded dendralene was extrapolated to about 3.3 ± 3.5 eV. The conformational preferences of the expanded dendralenes were analyzed in semi-empirical calculations, revealing energetic preferences for planar or slightly twisted s-cis and −U-shaped× geometries.1. Introduction. ± Dendralenes are polyene hydrocarbons in which CC bonds are aligned in a cross-conjugated arrangement [1a]. This requires the presence of at least three CC bonds, and the simplest dendralene, therefore, is 3-(methylidene)penta-1,4-diene (a [3]dendralene). Novel synthetic approaches to dendralenes are currently being developed [1d,e], and these chromophores are increasingly investigated for their structural, electronic, and advanced materials properties [1]. Upon insertion of one or more acetylene units between the CC bonds, series of expanded dendralenes are obtained (Fig. 1). Diederich and co-workers [2] reported in 1995 the synthesis of the iso-poly(triacetylene)s 1a ± d by acetylenic scaffolding starting from appropriate tetraethynylethene (TEE, 3,hex-3-ene-1,5-diyne) precursors. Compounds 1c and 1d in this series are the first examples of expanded dendralenes, with buta-1,3-diynediyl bridges inserted between the CC bonds. More recently, Tykwinski and coworkers [3] introduced a new class of expanded dendralenes 2c ± i with the isopoly(diacetylene) backbone, featuring ethynediyl spacers between the CC bonds (the names iso-poly(diacetylene) (iso-PDA) and iso-poly(triacetylene) (iso-PTA) were introduced by Tykwinski and co-workers [3b]). In addition to compounds 2a ± i, with peripheral isopropylidene moieties, derivatives with peripheral cyclohexylidene and adamantylidene fragments were also reported [3d].

Oligomeric and Polymeric Systems with a Cross-conjugated π-Framework

“…Unlike the peralkynylated derivatives 75 − 76 , the longer oligomers for both series 77 and 78 showed much reduced kinetic stability, decomposing over a period of days, even when stored under nitrogen at low temperature. The same kinetic instability was found for iso -PTAs based on the cyclohexylidene subunit, 82 , reported by Diederich and co-workers …”

“…While little evidence for a lowering of the HOMO-LUMO gap was found for either the TEE (75) 70 or isopropylidene (77 and 78) 72 series, analysis of the cyclohexylidene (82) series by three different methods demonstrated a consistent drop in HOMO-LUMO energy as length was increased, 74 suggesting a contribution from cross conjugation to the overall electronic makeup of the molecules. The electronic delocalization of iso-PTAs in comparison to linearly-conjugated PTAs has also been studied theoretically by Lüthi and coworkers.…”

Conjugated Oligoenynes Based on the Diethynylethene Unit

“…Thus, the longest wavelength absorption of 63 is at λ max = 347 nm (Table ), while it is at λ max = 262 nm for ( Z )- 1 . Table lists the absorption maxima for a variety of simple bis-DEEs. 11h,− Comparing linearly conjugated ( Z )- and ( E )-bis-DEEs 82 and 83 with the same substitution pattern, we notice that the E -configuration ( 83 ) leads to a slightly lower HOMO−LUMO gap. Cross-conjugated dimers 84 − 86 exhibit higher HOMO−LUMO gap energies.…”

“…(Figure ) . A large selection of expanded dendralenes containing the gem -DEE or gem -TEE units has been prepared, such as 131a − i , 19a, 132a − d , 133a − c , 134 , and 135a − c , as well as hybrid systems, such as 136a − d (Chart ). ,

7 Definition of dendralenes and expanded dendralenes, iso -PAs, iso -PDAs, iso -PTAs, etc.

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Advanced Optoelectronics Materials by Fullerene and Acetylene Scaffolding

“…200°C and can be stored in the air at ambient temperature for prolonged periods of time, the corresponding cross-conjugated expanded dendralenes 2a-d [39] and tend to decompose rapidly, an observation that still awaits good explanation (for other work on expanded dendralenes, see [40,41]). According to the UV/vis spectra, cross-conjugation seems effective in transmitting π-electron delocalization along the oligomeric backbone to some extent, although it is much weaker and less efficient than linear conjugation.…”