Answer to Question #115499 in Physical Chemistry for DEEPAK SINGH

Question

discuss fluorescence with suitable examples. And explain phosphorescence with example.

Answer 1

Fluorescence is a case of photoluminescence, i.e. irradiation of a photon due to electron transition from an excited state to the ground state. Energy difference between these states corresponds to energy of emitted photon. The excitation photon commonly has higher energy than the luminescent photon, because part of excitation energy is lost in the form of atomic vibrations. Fluorescence is the case when the electronic system has same spin state (usually, singlet) in the excited and ground states, so that the transition between these states is allowed and, hetherefore, proceeds rapidly (in 10^-11 to 10^-6 s). The examples of fluorescent substances that are excited by UV photons and fluoresce Visible light photons are polyaromatic compounds, like quinine, rhodamine B, pyranine; f-elements compounds, like Eu2(SO4)3, UO2F2 etc. In these subtances where are available excitation energy levels (lowest unoccupied molecular orbitals in organic molecules of f-electrons energy levels in f-element cation). These excited levels have energy about 2-3 eV higher than the ground states (highest occupied molecular orbitals), so that fluorescence is with visible light.

Phosphorescence is a kind of photoluminescence when electron transition occurs between an excited and ground states having different spin states (e.g. triplet excited state - to - singlet ground state). Such transition is prohibited by exception rules, that is why it takes longer time (10^-4 to 10^-2 s). An example is a polyaromatic molecule of erythrosine. Upon UV light excitation electron jumps from a singlet ground state to a singlet excited state, i.e. spin state does not change. But next, due to the presence of iodine in the molecule, it can relax from from to a triplet excited state, i.e. spin changes. Electron becomes pinned at the triplet excited state and its transition to the singlet ground state enables phosphorescence.