Steady-state and time-resolved fluorescence anisotropy of 7-azaindole and its derivatives

Abstract

The fluorescence excitation and excitation anisotropy spectra at -60 OC in a propylene glycol glass are reported for 7-azaindole and three related derivatives: 7-azatryptophan, N1-methyl-7-azaindole (1 M'IAI), and 7-methyl- 7H-pyrrolo[2,3-b]pyridine (7M7AI). At the reddest excitation wavelengths, steady-state anisotropy values are observed in the range 0.17 to 0.23, which is significantly less than the theoretical limiting anisotropy of 0.4. The anisotropy spectra indicate the presence of closely-spaced 'Laa nd ~LLba, nds as in indole. The low temperature anisotropies are compared with results from time-dependent measurements. An alternative method of collecting fluorescence depolarization data in time-correlated single-photon counting experiments is also presented. It is reliable, provides long-term stability, which is essential for weakly fluorescent samples, and obviates the need for "tail-matching" scaling procedures. This method is tested with the well-characterized fluorescein derivative, rose bengal, and is employed to compare the rotational diffusion times of the normal and the tautomer species of 7-azaindole in methanol and butanol with that of the fluorescent, "blocked," species of 7-azaindole in water.