Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

Two novel derivatives of 2,3-diazabicyclo[2.2.2] oct-2-ene were synthesized, carrying a carboxyl (4) and a methylcarboxyl (5) substituent at the bridgehead position. The photodecomposition quantum yields (51% for 4 and 2.9% for 5) and fluorescence lifetimes (29ns for 4 and 345 ns for 5) in water were determined. The higher photoreactivity and fluorescence quenching for 4 was attributed to its higher propensity to undergo photochemical elimination of nitrogen as a consequence of the presence of the radical-stabilizing carboxyl group in the alpha-position. The absolute photodecomposition rate constants of 4 became faster upon protonation (e. g., at pH 2), which contrasted anticipated substituent effects on the C-N bonds strengths. The complexation behavior of both acids, 4 and 5, with tetrakis(N,N'-dimethylammoniomethyl) tetrahydroxycalix[4] arene (17) and beta-cyclodextrin was studied by fluorescence and induced circular dichroism, respectively, in order to evaluate their suitability as potential sensor systems for anions and analytes with hydrogen- bonding abilities, respectively. The binding constants of 4 and 5 with the calixarene 17 were unexpectedly small (< 2000 M (-1)) which was attributed to reduced Coulomb attractions as a consequence of the 1,3-alternate conformation which this host adopts in water. The binding constants towards beta- cyclodextrin were also low at pH 7.0 (< 120 M-1), which was attributed to the low hydrophobicity of the anionic form of the guests; in line with this interpretation, the binding constants with beta-cyclodextrin increased at pH 2.0, by about one order of magnitude.