Abstract

One of the most compelling reasons for undertaking the study of indole and its derivatives under jet-cooled conditions is to elucidate the unusual excited state properties which make them useful as fluorescent probes in biological systems. The remarkable sensitivity of the fluorescence quantum yield and emission spectra of these compounds to the environment of the indole nucleus has been extensively used as a probe of conformational fluctuations in polypeptides and proteins. Efforts to understand this sensitivity are based on a description involving two electronic states which underlie the lowest energy absorption band systems in condensed phases. The highly structured absorption near the red edge has been attributed to transitions to the ¹L_b state, while a broad continuum peaking to higher energies has been assigned to the ¹L_a state, which appears to lie above the ¹L_b state for most indole derivatives studied. It is generally agreed that the ¹L_a state has a much larger excited state dipole moment than the ¹L_b state. It is therefore subject to a larger stabilization in polar solvents, which can lead to an inversion in the ordering of the excited states, thus providing an explanation for the extreme sensitivity to solvent polarity.

© 1990 Optical Society of America