Abstract
Binding of alkanediammonium ions +H3N(CH2)nNH3+, n = 2–6, to (E)-bis(18-crown-6)stilbene initiates the [2 + 2] photocycloaddition reaction leading mainly to the rctt isomer of 1,2,3,4-tetrakis(benzo-18-crown-6)cyclobutane. The high efficiency and stereoselectivity of this photoreaction in dilute solution are due to the formation of bis-pseudosandwich complexes consisting of two stilbene molecules and two alkanediammonium ions. With a decrease in the length of the polymethylene spacers in the 2:2 complex, the quantum yield of supramolecular [2 + 2] photocycloaddition increases from 0.02 to 0.27, while the quantum yield of the competing E–Z photoisomerization decreases from 0.12 to 0.06. Spectrophotometric observations and density functional theory calculations revealed that (Z)-bis(18-crown-6)stilbene, unlike the E isomer, is able to form pseudocyclic 1:1 complexes with +H3N(CH2)nNH3+, n = 2–6, owing to the proximity of two crown ether moieties. The peculiar spectral properties of these complexes are associated with large torsion angles around the ethylene–benzocrown single bonds. X-ray diffraction analysis showed that co-crystallization of (E)-bis(18-crown-6)stilbene with 1,6-hexanediammonium diperchlorate affords a linear polymer in which the alternating stilbene molecules and diammonium ions are connected by hydrogen bonds.
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