Highly functionalized dimeric tetraethynylethenes and expanded radialenes:Strong evidence for macrocyclic cross-conjugation

A selection of dimeric tetra-ethynylethenes (TEEs) and perethynylated expan ded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structu re of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)ph enyl substituents, is presented. This macrocycle, the all-carbon core of wh ich is isomeric with fullerene C-60, adopts a non-planar, "chair-like" conf ormation. Also a TEE dimer, carrying NN-dimethylaniline donor substituents, has been subjected to an X-ray crystallo graphic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, provid ing fundamental insight into mechanisms of pi -electron delocalization in t he acyclic and macrocyclic chromophores. Donor or donor-acceptor-substitute d dimeric TEE derivatives show very strong absorptions extending over the e ntire UV/Vis region; their longest wavelength absorption bands have high ch arge-transfer character. Macrocyclic cross-conjugation in the expanded radi alenes becomes increasingly efficient with increasing donor-acceptor polari zation. A dual, strongly solvent-polarity-dependent fluorescence was observ ed for a tetrakis(NN-dimethylaniline)-substituted dimeric TEE; this interes ting emission behavior is explained by the twisted intramolecular charge-tr ansfer (TICT) state model. Donor-substituted expanded radialenes display hu ge resonance-enhanced third-order nonlinear optical coefficients.