Q.1. (c) Discuss how certain compounds produce fluorescence or phosphorescence when exposed to electromagnetic radiation. How this property is exhibited only by organic compounds with certain structural specifity? Explain
Ans.1.(c) When electromagnetic radiation passes through matter, a variety of phenomena may occur. One of them is; in some cases, the molecules after absorbing radiation become excited but they do not lose energy very quickly but with some delay. In such cases, the energy is re-emitted as radiation, usually of longer wavelength: than was originally absorbed. This phenomenon is termed as fluorescence. If in some cases, there is a detectable time delay in re-emission, the phenomenon is termed as phosphorescence.
This property is exhibited only by organic compounds with certain structural specifity. This can be explained as follows: Molecules have various states referred to as energy levels. Fluorescence spectroscopy is primarily concerned with electronic and vibrational states. Generally, the species being examined has a ground electronic state (a low energy state) of interest, and an excited electronic state of higher energy. Within each of these electronic states are various vibrational states.
In fluorescence spectroscopy, the species is first excited, by absorbing a photon, from its ground electronic state to one of the various vibrational states in the excited electronic state. Collisions with other molecules cause the excited molecule to lose vibrational energy until it reaches the lowest vibrational state of the excited electronic state.
The molecule then drops down to one of the various vibrational levels of the ground electronic state again, emitting a photon in the process. As molecules may drop down into any of several vibrational levels in the ground state, the emitted photons will have different energies, and thus frequencies. Therefore, by analyzing the different frequencies of light emitted in fluorescent spectroscopy, along with their relative intensities, the structure of the different vibrational levels can be determined.
.png)
