A joint structural, kinetic, and thermodynamic investigation of substituent effects on host-guest complexation of bicyclic azoalkanes by β-cyclodextrin

Derivatives of the azoalkane 2,3-diazabicyclo[2,2,2]oct-2-ene (1a) with bridgehead 1,4-dialkyl (1b), 1,4-dichloro (1c), 1-hydroxymethyl (1d), 1-aminomethyl (1e), and 1-ammoniummethyl (1f) substituents form host-guest inclusion complexes with beta-cyclodextrin. They were employed as probes to assess substituent effects on the kinetics and thermodynamics of this complexation by using time-resolved and steady-state fluorimetry, UV spectrophotometry, induced circular dichroism (ICD) measurements, and H-1 NMR spectroscopy. The kinetic analysis based on quenching of the long-lived fluorescence of the azoalkanes by addition of host provided excited-state association rate constants between 2.6 x 10(8) and 7.0 x 10(8) M-1 s(-1). The binding constants for 1a (1100 M-1), 1b (900 M-1), 1c (1900 M-1), 1d (180 M-1), 1e (250 M-1), and 1f (ca. 20 M-1) were obtained by UV, NMR, and ICD titrations. A positive ICD signal of the azo absorption around 370 nm was observed for the beta-cyclodextrin complexes of 1a, 1d, and 1f with the intensity order 1a much greater than 1d approximate to 1f, and a negative signal was measured for those of 1b, 1c, and 1e with the intensity order 1c < 1b approximate to 1e. The ICD was employed for the assignment of the solution structures of the complexes, in particular the relative orientation of the guest in the host (co-conformation).