View/Open - ARAN - National University of Ireland, Galway

Introduction
Figure 1.6: Perrin-Jablonski diagram showing the electronic processes occurring in a fluorescent
molecule after excitation by light. Here the singlet ground, first and
second excited states are depicted by S0, S1, S2 respectively. Each electronic
energy level has a number of vibrational energy levels, depicted by 0,1,2 etc.
Rotational energy levels are omitted from the diagram for clarity. The transitions
between states are depicted as vertical lines to indicate the instantaneous
nature of light absorption.
absorption (Stage 1) return to the ground state (S0) by fluorescence emission, instead
returning to the ground state by non-radiative transitions. Internal conversion from
S1 to S0 is also possible but is less efficient than conversion from S2 to S1, because
of the larger energy gap between S1 and S0. Under certain conditions such as heavy
atom spin-orbit coupling, molecules in the S1 state can undergo a spin conversion
to the first triplet state T1 via the process of intersystem crossing. Emission from
this state is termed phosphorescence and is typically of a much longer timescale than
fluorescence due to T1 −→S0 transitions being spin forbidden (or highly improbable)
by the spin selection rule for electronic transitions (∆S = 0).
Stage 3: Fluorescence Emission
Fluorescence emission usually occurs from the lowest vibrational level of the S1
state and can return to different vibrational levels of the ground state. The emission
of a photon is as fast as the absorption (≈ 10 −15 s), but excited molecules stay in the
Intersystem crossing is in principal forbidden by quantum mechanics but coupling of the magnetic
field generated by orbital motion and the electronic spin magnetic moment can lead to a conversion
or flip of the spin state. This condition is enhanced by the presence of heavy atoms with high atomic
numbers such as Br or Pb.
14