Conventional fluorescence spectra in liquid solutions are usually broadbanded and contain little spectral information. Two approaches are being used to improve the spectral resolution: Shpol’skii spectroscopy and fluorescence linenarrowing (FLN) spectroscopy. The resulting spectra can be used for fingerprint identification, for studying fundamental photophysical processes, or to obtain detailed information on the local environment of the fluorophore. The setup consists of a XeCl excimer/dye laser tunable excitation source, a 5 K closedcycle refrigerator, and a triple monochromator with an intensified CCD camera for sensitive, time-resolved detection.
In Shpol’skii spectroscopy, polycrystalline matrices such as frozen n-alkanes are used. The technique was used to study ultrafast intramolecular proton tunneling rates in 3-hydroxyflavone and its derivatives. Subtle broadening effects in the excitation and emission spectra and comparison with the spectra after deuteration allowed us to determine the femtosecond tunneling rates.
In FLN spectroscopy a variety of amorphous frozen matrices can be used and laser excitation into the first excited state is carried out to select an iso-energetic sub-population of fluorophores. Current emphasis is on the interactions of fluorophores with their nano-environment. FLN spectra were used to determine the orientation of the pseudo-estrogenic hydroxybenzo[a]pyrenes within the estrogen receptor (collaboration with the Dept of Molecular Toxicology) (see figure).
Normalised FLN spectra of 3-hydroxybenzo[a]pyrene in the estrogen
receptor ER, top) and in various solvents (or solvent combinations). Similarity
to the spectrum in methylcyclohexane plus triethylamine (+TEA) indicates a
largely non-polar receptor pocket and H-bonding to histidine.