I have synthesized a compound which is pure as well as no other possible diastereomers are present in it, as evident from ¹H NMR. On inspection of its photophysical properties an anomaly was noted as described below:
- when the compound was excited in the region (~400 - 450 nm) of its highest absorbance a low intensity emission peak was observed at about 500-550 nm with indication of excimer like entity formation (600 - 650 nm) on varying the concentrations.
- On the other hand , excitation of the molecule at a peak of lower absorbance, near to the λmax around 350 nm a very high intensity and sharp single emission peak was observed around 380 nm with no excimer like entity at higher concentrations.
What could be the plausible reasons behind these observations?
I would be grateful for valued suggestions, if any.
If your fluorescence spectrometer allows, I would suggest doing a full EEM (excitation emission matrix) measurement. Or at least measure excitation spectra for your different emission bands. If excitation spectra are the same for two bands, there must be an excited state process leading to those two emissions. If they are differing, there must be two ground state compounds present with different absorbance spectra. A highly fluorescent impurity might get unnoticed from measuring absorbance spectrum or NMR, but still exist in small amounts.
Generally, the shape of fluorescence spectra from a pure compound, with no excited state transformations going on, should be independent of excitation wavelength (look for "Kasha's rule")
Dr. @Friedrich Menges, thank you so much I will try to carry out EEM if our machine Fluoromax-4 permits and also check the excitation spectra. However, I would like to add that this anomaly has shown in other related derivatives of the same compound as well.
Be careful that you are not being fooled by Raman scatter from the solvent, which appears as a sharp peak a certain wavenumber (not wavelength) separated from the excitation wavelength. If you shift the excitation wavelength by a small amount (e.g. 5 nm), the emission peak wavelength due to Raman scatter will also shift, whereas this will not happen for fluorescence.
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