Spectroscopic methods:

You have learnt about two significant spectroscopic methods, namely fluorescence and phosphorescence.  These are totally based on the phenomenon of emission of radiation through electronically excited species and are collectively termed as photoluminescence methods. The origin of these luminescence spectra are explained in terms of Jablonski diagram that provides a representation of ground and variant excited electron states of a molecule and the processes related along with absorption and emission (radiative and nonradiative) of energy.

The absorption of a photon of appropriate energy causes the molecule to get excited from the ground state to one of the excited states. These excited or activated states get deactivated through a number of processes that are commonly grouped under nonradiative and radiative processes. The nonradiative relaxation is by three various processes namely, vibrational relaxation, internal conversion and intersystem crossing while the radiative deactivation is by fluorescence and phosphorescence.  A wavelength of fluorescence emission is commonly greater than the excitation wavelength and that of phosphorescence emission is at still longer wavelengths.

There are two kinds of fluorescence spectra. These are fluorescence emission and fluorescence excitation spectrum. Within the former the molecule is excited along with a given wavelength and the fluorescence emission at various wavelengths is measured while in the later the excitation wavelength is varied and the intensity of fluorescence emission at a predetermined wavelength is monitored. Commonly the excitation spectrum of a molecule is the similar as its UV-VIS absorption spectrum. The wavelength difference among the absorption and fluorescence maxima is known as the Stokes shift.

The fluorescent behaviour of a molecule is determined through the chemical structure of the molecule and is affected through the chemical environment in that it exists. The structural features whose presence in a molecule might make it fluorescent are the presence of benzene ring, fused ring system and rigidity within the molecule. External factors affecting the fluorescent behaviour of a molecule are temperature, pH, paramagnetic species such as dissolved oxygen and the solvent.