Transition Types in Fluorescence

Fluorescence rarely results from absorption of ultraviolet radiation of wavelength shorter than 250 nm because radiation at this wavelength has sufficient energy to deactivate the electron in the excited state by predissociation or dissociation. The bond of some organic molecules would rupture at 140 kcal/mol, which corresponds to 200-nm of radiation. For this reason, σ→σ∗ transition in fluorescence are rarely observed. Instead, emissions from the less energetic transition will occur which are either π∗→π or π∗→n transition.

Molecules that are excited electronically will return to the lowest excited state by rapid vibrational relaxation and internal conversion, which produces no radiation emission. Fluorescence arises from a transition from the lowest vibrational level of the first excited electronic state to one of the vibrational levels in the electronic ground state. In most fluorescent compounds, radiation is produced by a π∗→π or π∗→n transition depending on which requires the least energy for the transition to occur.

Quantum Efficiency and Transition Types

Fluorescence is most commonly found in compounds in which the lowest energy transition is π→π∗ (excited singlet state) than n→π∗ which suggest that the quantum efficiency is greater for π→π∗ transitions. The reason for this is that the molar absorptivity, which measures the probability that a transition will occur, of the π→π∗ transition is 100 to 1000 fold greater than n→π∗ process. The lifetime of π→π∗ (10^-7 to 10^-9 s) is shorter than the lifetime of n→π∗ (10^-5 to 10^-7).

Phosphorescent quantum efficiency is the opposite of fluorescence in that it occurs in the n→π∗ excited state which tends to be short lived and less suceptable to deactivation than the π→π∗ triplet state. Intersystem crossing is also more probable for π→π∗excited state than for the n→π∗state because the energy difference between the singlet and triplet state is large and spin-orbit coupling is less likely to occur.