Dear Maixian Liu,

which molecule have fluorescence properties only that molecules are contribute in excitation spectra but in absorption spectra molecules have not fluorescence properties but they also contribute in absorption spectra...

So Reason of both spectra are not overlap are varies system to system (which compound you are using)...

like as if your record spectra highly aggregated dye both spectra are different because aggregate molecule are non fluorescent....

The answer is already provided by Raman.

"which molecule have fluorescence properties only that molecules are contribute in excitation spectra but in absorption spectra molecules have not fluorescence properties but they also contribute in absorption spectra"

So you can say that the sample is mixture of chromophore and the major absorbing species is not the emissive species. Remember that due to extreme sensitivity associated with fluorescence (with respect to absorption) even if the sample is analytically pure (NMR, Mass, XRD) you may see emission from species which is present in very minute quantity (which avoided detection in NMR). Please have a look at the attached (Alert: Self Promotion)

Article The Prospect of Salophen in Fluorescence Lifetime Sensing of Al3+

Could you elaborate more on the issue of non-overlapping of absorbance and excitation spectra? It would be best if you can show the spectra. Are you concerned about the different shapes of the two or different intensities? For example, the absorbance maximum appears at 250 nm, but excitation maximum is at 350 nm ? This could be an instrumental artifact as well along with chemical effects mentioned above. Without knowing anything else, it is difficult to make a conclusion.

Dear Maixian Liu,

As suggested by Raman Khurana, presence of impurity can indeed lead to such differences between the excitation and absorption spectra. The other reason could be the use of the solutions that are not optically very dilute. Record the excitation spectrum using optically very dilute solution. If still the excitation spectrum do not match with the corresponding absorption spectrum, there is certainly an impurity issue for the studied systems.

Good Luck!!!

Haridas Pal

Among the reasons listed here, for the difference between absorption and fluorescence spectra, the main physical principal is not named, the origin of which is due to the inhomogeneous broadening (IB) of the vibronic spectra. There are two types of inhomogeneous broadening: dynamic and static.

The origin of IB is associated with fluctuations of intermolecular interactions, which lead to the appearance of statistical distributions over the frequencies of purely electronic 0-0 transitions . The main contribution to the fluctuation is made by orientational dipole-dipole interactions. If the solvent is low-viscosity, these fluctuations are dynamic in nature, i.e. molecules of the polar dye (solute), absorbing a photon of light at one 0-0 frequency emit light with the same statistical distribution of dye molecules over frequencies of purely electronic 0-0 transitions. If a glass-forming polar solvent is used (a good example is isobutyl alcohol), then, as the temperature is lowered, the excitation spectrum deviates from the absorption spectrum. Thus, when the glass solution of the dye is frozen, a change in the broadening of the vibronic spectra from the dynamic spectrum to the static spectrum takes place. This phenomenon is most clearly observed when the emission at the blue edge of the fluorescence spectrum is recorded. This phenomenon and its theory are described in detail in the references cited below.

Article Dependence of the spectra of excitation of dipole molecule s...

Article Correlation between excitation and absorption spectra of sol...

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