Question

Explain the mirror image rule for electronic absorbance and fluorescence spectra. Why does the role sometimes not apply? Thank you

Answer 1

For many fluorophores, the absorption spectrum is a mirror image of the emission spectrum. This is known as the mirror image rule.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 0^th ? 2^nd vibrational transition during S₀ ? S₁ transition is highest, the 2^nd ? 0^thtransition will be most probable in the reciprocal transition.This results in an emission spectrum that is a mirror image of the S₀ ? S₁ transition in terms of the shape.

There are several exceptions to the mirror image rule that arise largely due to the excited state reactions of the molecule.

Potential energy diagrams showing the Franck-Condon principle