Highly Functionalized Dimeric Tetraethynylethenes and Expanded Radialenes: Strong Evidence for Macrocyclic Cross-Conjugation

Abstract

A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.

Synthesis and Characterization of Expanded Radialenes, Bisradialenes, and Radiaannulenes

“…The same trend was found for the larger expanded radialenes 3 , as described by Diederich and co‐workers, where similar redox potentials were observed irrespective of the size of the macrocycle ( n =1–3) 21. The smaller radialenes 5 – 7 are considerably more difficult to reduce than any of the reported radialenes 3 17. 21 The difficult reduction of compound 5 may substantiate the prediction of Chauvin and co‐workers8 that a one‐electron reduction of a [3]radialene analogue to 5 (i.e., 2 n =0, R=H) should lead to an antiaromatic molecule, although this premise inspires the analysis of additional substitution patterns (which is currently under investigation).…”

“…Conversely, it may arise from an increase in macrocyclic cross‐conjugation, which has been invoked to account for the red shift in the λ max values observed for trimeric derivatives of the larger expanded radialenes 3 4. 17 The latter argument is supported through a comparison of the λ max values for 5 – 7 with those of the acyclic precursors 9 – 11 , which show λ max =374 nm when measured in THF 10. 18 Since the longest linearly conjugated segment is identical in 5 – 7 and 9 – 11 (shown in bold in Scheme ), the low‐energy absorption of 5 must arise from other factors 19…”

Two‐Dimensional Acetylenic Scaffolding: Extended Donor‐Substituted Perethynylated Dehydroannulenes

“…They are studied both experimentally and theoretically to enhance further the understanding of aromaticity/antiaromaticity and, in general, π conjugation in unsaturated macrocyclic systems 8. With their extended π chromophores, a number of representatives feature interesting optoelectronic properties such as high third‐order optical nonlinearities 2d. 9 Furthermore, some of these acetylenic macrocycles represent hypothetical constituents of two‐dimensional all‐carbon networks such as graphyne or graphdiyne, which are predicted to display fascinating electronic and mechanical properties 1b.…”

“…Over the past decade, we developed a large library of tetraethynylethene (TEE, 3,4‐diethynylhex‐3‐ene‐1,5‐diyne) building blocks1c, g and applied them to the construction of novel families of acetylenic macrocycles such as perethynylated dehydroannulenes,13 expanded radialenes,2d, 9a, 13b, 14 and radiaannulenes 13d. 15 With their numerous sp‐hybridized carbon atoms, the all‐carbon cores of these systems feature potent electron‐accepting properties.…”

“…Among the first derivatives prepared were the per(silylethynylated) octadehydro[12]annulene 1 a , with an antiaromatic macrocyclic perimeter as revealed by UV/Vis and 1 H NMR spectroscopy, and the larger, aromatic dodecadehydro[18]annulene 2 a . [13a, b] Later, the terminally N,N‐ dimethylanilino‐substituted derivatives 1 b and 2 b were synthesized,13c, d as well as a series of anilino‐substituted expanded radialenes2d, 9a and radiaannulenes 13d. 15 The combined investigation of all three classes of acetylenic macrocycles clearly demonstrated three beneficial effects obtained upon introduction of the peripheral π‐electron donor groups: 1) the solubility of the compounds is enhanced, 2) the electron‐deficient all‐carbon cores are stabilized against nucleophilic attack and cycloadditions, and 3) intense bathochromically shifted charge‐transfer (CT) bands result from strong intramolecular CT interactions between these groups and the electron‐accepting all‐carbon cores 13d…”