Characteristics of fluorescence:
Figure 6.2 shows absorption and fluorescence emission spectrum of a hypothetical fluorophore. The important characteristics of the fluorescence emission can be briefly summarized as follows:
Figure 6.2 Absorption and fluorescence emission spectrum of a hypothetical fluorophore |
Stokes shift: A fluorescence emission spectrum is always shifted towards longer wavelengths with respect to the absorption spectrum. This shift is known as Stokes shift and is expected as excited molecules lose energy through processes like internal conversion and vibrational relaxation. The emitted radiation is therefore expected to be of lower energy i.e. higher wavelength.
Kasha’s rule: As fluorescence emission is observed from S1 → S0 transtions (except a few exceptions), fluorescence absorption spectrum is independent of the excitation wavelength.
Franck-Condon principle: The Franck-Condon principle states that the positions of the nuclei do not change during electronic transitions. The transitions are said to be vertical. This implies that if the probability of 0th → 2nd vibrational transition during S0 → S1 transition is highest, the 2nd → 0th transition will be most probable in the reciprocal transition (Figure 6.3).