Excited-state behavior of tryptamine and related indoles. Remarkably efficient intramolecular proton-induced quenching

Abstract

The excited state behavior of tryptamine and 1,2,3,4-tetrahydrocarbazoles possessing alkylamino side chains in the absence and presence of 18-crown-6 in MeOH-H/sub 2/O (9:1) mixtures has been studied by means of nanosecond single-photon counting, fluorimetry, and photochemical H-D isotope exchange. The fluorescence intensity of these indoles increases significantly with increasing concentration of 18-crown-6. The relatively short lifetime of the tryptamine ammonium ion 1 is not ascribed to external quenching but rather to internal quenching. The rate constant k/sub q/ for internal quenching can be estimated from the equation k/sub q/ = tau/sub 0//sup -1/ - tau/sub max//sup -1/, where tau/sub 0/ and tau/sub max/ represent the fluorescence lifetimes for free 1 and the 1:1 1-crown ether complex, respectively. Internal quenching originates from electrophilic proton attack by the -N/sup +/H/sub 3/ (or -N/sup +/D/sub 3/) group of 1 at the C-4 position of the excited indole ring. For 3 (the tetrahydrocarbazole derivative R(CH/sub 2/)/sub 3/N/sup +/H/sub 3/) the k/sub q/ value comprises the electrophilic proton attack at the C-8 position plus other quenching (probably charge-transfer quenching) between the excited indole moiety (R*) and the -N/sup +/H/sub 3/ (or -N/sup +/D/sub 3/) group. The stabilization constant K/sub g/ for the corresponding ammonium ionmore » and 18-crown-6 can be determined by fluorimetry. The kinetic and thermodynamic parameters for the internal quenching and the complex formation, respectively, have been described.« less