Question

Explain the following observations: (Please type the answers):

1. Fluorescence is generally not observed from saturated hydrocarbons.

2. Fluorescence is generally in red shifted from the absorbance.

Answer 1

Saturated hydrocarbons have only sigma bonds, these sigma bonded electrons are not easily exited so the electromagnetic radiations used can not excite the sigma electron so the absorbance phenomenon will not take place i.e.Fluorescence will not be observed.

By absorbance of radiation, the molecule reaches to the exited level, and before its deexcitation, it stays in the excited state for some time. In exited state during this time interval molecule may come from high vibrational level to some lower vibrational level, and now it comes to the ground state again starting from that lower vibrational level so the higher wavelength or lower energy radiation in comparison to absorbed wavelength will get emitted.