Direct triplet state excitation of indole derivatives: novel applications of phosphorescence

Abstract

Photophysics is by definition the study of the interaction of light and matter. When this interaction takes places, molecules can absorb light and release a photon of certain energy. This is called photoluminescence. Photoluminescence can be separated into two emission processes: fluorescence (which happens on the time scale of nanoseconds) from the singlet state and phosphorescence (which happens in the scale of microseconds – seconds) from the triplet state. Phosphorescence emission is several orders of magnitude weaker than fluorescence emission because there needs to be a so-called “forbidden” process- singlet to triplet intersystem crossing- to achieve phosphorescence emission. Indole is an aromatic organic compound which has photoluminescence properties. It also is the side chain in the essential amino acid tryptophan. There have been efforts to use the spectroscopic properties of tryptophan to study proteins and protein interactions since the 1950’s. However, most of the work has been done using fluorescence because phosphorescence is much weaker. In this study we propose a new method of achieving phosphorescence emission: direct triplet excitation. Instead of populating the singlet state first and then having intersystem crossing to the triplet state, if one uses longer wavelength (redder) excitation, only the triplet state is populated. This allows us to use the long timescale (microseconds-seconds) and high anisotropy of phosphorescence to study protein dynamics of much larger biomolecular systems. I investigated tryptophan’s phosphorescence as well as the indole and indole derivatives, 2-Phenylindole and 5-Bromoindole in poly (vinyl alcohol) film. The indole derivatives were chosen to enhance the probability of exciting tryptophan directly to the triplet state.