Homochiral [2.2]Paracyclophane Self-Assembly Promoted by Transannular Hydrogen Bonding.

[2.2]paracyclophane (pCp), unlike many π-building blocks, has been virtually unexplored in supramolecular constructs. Reported here is the synthesis and characterization of the first pCp derivatives capable of programmed self-assembly into extended cofacial π-stacks in solution and the solid state. The design employs transannular (intramolecular) hydrogen bonds (H-bonds), hitherto unstudied in pCps, between pseudo-ortho-positioned amides of a pCp-4,7,12,15-tetracarboxamide (pCpTA) to preorganize the molecules for intermolecular H-bonding with π-stacked neighbors. X-ray crystallography confirms the formation of homochiral, one-dimensional pCpTA stacks helically laced with two H-bond strands. The chiral sense is dictated by the planar chirality (Rp or Sp ) of the pCpTA monomers. A combination of NMR, IR, and UV/Vis studies confirms the formation of the first supramolecular pCp polymers in solution.

Effect of alkyl substituents on photorelease from butyrophenone derivatives.

Photolysis of 1a yields 4a in argon-saturated methanol, whereas 1b is photostable. Laser flash photolysis of 1a in acetonitrile shows formation of biradical 2a (lambda(max) = 340 nm, tau = approximately 60 ns), which undergoes intersystem crossing to form Z-3a (lambda(max) = 380 nm, tau = 270 ns) and E-3a (lambda(max) = 380 nm, tau = 300 ms). Z-3a regenerates the starting material, whereas E-3b undergoes intramolecular lactonization to release the alcohol moiety and form 4a. Similar laser flash photolysis of 1b shows formation of biradical 2b (lambda(max) = 340 nm, tau = 1.9 micros in acetonitrile), which is longer-lived than 2a is. However, 2b only undergoes intersystem crossing to form Z-3b (lambda(max) = 380 nm, tau = 4.3 micros). Calculations demonstrate that intramolecular pseudo hydrogen bonding between the OH moiety and the radical centered on the isopropyl carbon in 2b and the bulkiness of the isopropyl group prevent the necessary rotation to form E-3b. In comparison, 2a does not form an intramolecular pseudo hydrogen bond between the methylene radical center and the OH group, and as a consequence, it undergoes intersystem crossing to form both E- and Z-3a.