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 CC 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 1H- and 13CNMR (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(sp2)-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 π-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.