Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions

Abstract

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions

“…41 In compound 2, such phenomena would support the EPR measurements of a J AB value of À734 cm À1 . 22 In contrast, the J AB variations along the longitudinal slippage exhibit a double minimum curve (see Fig. 4b).…”

“…À734 cm À1 ). 22 Unfortunately, it was not possible to measure a clear EPR signal for compounds 1 and 3. Quite interestingly, the crystal structure of 2 was obtained for the tetra-cation [CYC] 4+ , 23 a regime where doubly-charged organic subunits are held together.…”

“…Density functional theory (DFT) calculations were then performed on isolated cyclophanes in a model of water, stressing a marked propensity towards antiferromagnetism. 22 More recently it has been suggested that large dispersion forces dominate the bonding regime in viologen cation-radical p-dimer (so-called pimer). 24 The analysis was carried out from DFT calculations including dispersion corrections and concluded on the competition between covalent and dispersion attractions with Coulomb repulsions.…”

Photo/redox-responsive 2D-Supramolecular assembly involving Cucurbit[8]uril and a star-shaped porphyrin tecton

“…Our group has been focusing over the past few years on the development of tailor-made redoxcontrollable molecular or supramolecular systems involving electrogenerated viologen-based -cation radicals R1VR2 +• in Figure 1B) as key responsive and/or assembling elements, using their ability to self-assemble into sandwhich-like, dimeric entities called -dimers ([(R 1 VR 2 ) 2 ] 2+ in Figure 1B) [10][11][12][13][14][15][16][17][18][19][20]. These studies, and many others [16,[21][22][23][24][25][26][27][28][29], have served to establish that the effective formation of such multi-center bonded dimers in solution actually requires either very low temperatures and/or high concentrations in radicals. As an alternative, viologen-based -dimers can also be observed in standard temperature and concentration ranges, when using barrel-shaped cavitan ds known as cucurbit [8]urils (CB [8]), made of 8 glycoluril moieties connected by methylene bridges, whose inner cavity is ideally suited to the inclusion of two viologen-based radicals [30][31][32][33][34][35].…”

Electron‐Rich, Lewis Acidic Diborane Meets N‐Heterocyclic Aromatics: Formation and Electron Transfer in Cyclophane Boranes

“…[4][5][6][7] Figure 1a shows the Lewis representations of some literature-known examples of viologen cyclophanes with aromatic or flexiblea lkyl linkers, cyclobis(paraquat-p-phenylene) (CBPQT 4+ + ), cyclobis(paraquat-p-biphenylene) (CBPQTBP 4+ + ), and ExBox 4+ + ,a box-like cyclophane comprising two 4,4'-phenylenelinked "extended" bipyridinium units. [8][9][10][11] Very recently,f irst cyclotris(paraquat-p-phenylenes) were synthesized. [12] Cyclophanes with 1,2-bis(4-pyridyl)ethylene units, that could be photoisomerized, were also reported.…”