Photoluminescence

Introduction to Fluorescence and Phosphorescence
Credits:
This notes are a summary from “Modern Analytical Chemistry” by
David Harvey, “Molecular Fluorescence: Principles and
Applications” (Wiley 2001) by Bernard Valeur, and application
notes from Perkin Elmer Instruments.
Prepared by Jose Hodak for 2302303 Spectroscopy for Chemistry BSAC
program 2008.
Photoluminescence in theultraviolet-visible comprises two similar phenomena:
fluorescence and phosphorescence.
Molecules have energy levels determined by the molecular orbitals that hold the
molecule bound together. In the case of atoms it is the atomic orbitals what
determines the energy levels of the electrons. In this section we will concern
ourselves with molecular photoluminescence. We will leave the atomic emissionphenomena for the next section.

Absorption of an ultraviolet or visible photon promotes a valence electron
from its ground state to an excited state with conservation of the electron’s
spin. For example, a pair of electrons occupying the same electronic ground
state have opposite spins (Figure 1) and are said to be in a singlet spin state.
Absorbing a photon promotes one of the electrons to asinglet excited state
(Figure 1b). This phenomenon is called “excitation”
The ectited states are not stable and will not stay indefinitely. If we observe
a molecule in the excited state, at some random moment it will spontaneously
return to the ground state. This return process is called decay, deactivation or
relaxation. Under some special conditions, the energy absorbed during theexcitation process is released during the relaxation in the form of a photon.
This type of relaxation is called emission. Emission of a photon from a singlet
excited state to a singlet ground state, or between any two energy levels with
the same spin, is called fluorescence. The probability of a fluorescent
transition is very high, and the average lifetime of the electron in the excited
state is only10–5–10–8 s. Fluorescence, therefore, decays rapidly after the
excitation source is removed. In some cases an electron in a singlet excited
state is transformed to a triplet excited state (Figure 1c) in which its spin is
no longer paired with that of the ground state.
Emission between a triplet excited state and a singlet ground state, or between
any two energy levels that differ in theirrespective spin states, is called
phosphorescence. Because the average lifetime for phosphorescence ranges from
10–4 to 104 s, phosphorescence may continue for some time after removing the
excitation source.

1

Figure 1

History:
The use of molecular fluorescence for qualitative analysis and semiquantitative
analysis can be traced to the early to mid-1800s, with more accuratequantitative methods appearing in the 1920s. Instrumentation for fluorescence
spectroscopy using filters and monochromators for wavelength selection appeared
in, respectively, the 1930s and 1950s. Although the discovery of phosphorescence
preceded that of fluorescence by almost 200 years, qualitative and quantitative
applications of molecular phosphorescence did not receive much attention until
afterthe development of fluorescence instrumentation.
The use of molecular fluorescence for qualitative analysis and semiquantitative
analysis can be traced to the early to mid-1800s, with more accurate
quantitative methods appearing in the 1920s. Instrumentation for fluorescence
spectroscopy using filters and monochromators for wavelength selection appeared
in, respectively, the 1930s and 1950s.Although the discovery of phosphorescence
preceded that of fluorescence by almost 200 years, qualitative and quantitative
applications of molecular phosphorescence did not receive much attention until
after the development of fluorescence instrumentation.
Photophysical proceses:
A molecule in the excited state can decay in several ways. It might also use the
extra energy gathered during...