I excited a fused aromatic conjugated naphthalene-based system from 240-360 nm and the emission bank was continuously red shifted from 500-800 nm. The absorption maximum is around 360 nm.
All what has said above is right. In principle, if your change the excitation wavelength but keep exciting the same electronic level, the emission spectrum should not change if your compound is pure and your solvent impurity-free; you should simply see intensity changes reflecting the absorption spectrum and the lamp emission intensity.
A good way of trying to disentangle what is happening is to record
(i) blank spectra of the solvent for each excitation wavelength that you use; use preferably spectroscopy quality solvents.
(ii) excitation spectra for several wavelengths within the emission band; this may (a) help finding out if you have impurities generating spurious bands and (b) show you what is the best wavelength for exciting your sample. Note that excitation spectrum is the product of the absorption spectrum by the instrumental function, so that the most efficient wavelength for excitation may not correspond to the maximum in the absorption spectrum. If you correct the excitation spectrum for the instrumental function (mainly the emission spectrum of the lamp,the transmittance of the monochromator and the response of the detector), then both absorption and corrected excitation spectra should be identical.
Have fun!
You do not expect the emission wavelength to change with excitation wavelength. Though red-edge excitation shift is an exception. Does the quantum yield also changes? Please read about Kasha -Vavilov's rule. Before that, have you recorded excitation spectra? 500-800 nm emission looks like too extreme for a naphthalene based system. Why don't you show the spectra.
All what has said above is right. In principle, if your change the excitation wavelength but keep exciting the same electronic level, the emission spectrum should not change if your compound is pure and your solvent impurity-free; you should simply see intensity changes reflecting the absorption spectrum and the lamp emission intensity.
A good way of trying to disentangle what is happening is to record
(i) blank spectra of the solvent for each excitation wavelength that you use; use preferably spectroscopy quality solvents.
(ii) excitation spectra for several wavelengths within the emission band; this may (a) help finding out if you have impurities generating spurious bands and (b) show you what is the best wavelength for exciting your sample. Note that excitation spectrum is the product of the absorption spectrum by the instrumental function, so that the most efficient wavelength for excitation may not correspond to the maximum in the absorption spectrum. If you correct the excitation spectrum for the instrumental function (mainly the emission spectrum of the lamp,the transmittance of the monochromator and the response of the detector), then both absorption and corrected excitation spectra should be identical.
Have fun!
Thanks Jean-Claude G Bünzli and Tuhin Khan for your detailed answers. I understand that no matter the excitation wavelength, the same range of excited level will be reached before emitting back a photon, after having lost energy.
Then i wonder for the high wavelength excitation case (for exemple 680nm for AF647), we still get the blue spectrum, and thus can potentially have emission at a lower wavelength, meaning the molecule gained power somewhere in the process ?
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