The Principles of Fluorescence - Aquatic Organic Matter Fluorescence - page 14

+ + + -

σ σ *

nodal plane

Figure 1.2. Overlapping molecular orbitals interfering constructively (same signs) and destructively
(different signs). Constructive interference results in a ( σ ) molecular bonding orbital while destruc-
tive interference produces an antibonding sigma ( σ *) molecular bonding orbital.

1.3.1 Electronic Transitions

The interaction between a molecule and optical radiation manifests itself in various radia-
tive and nonradiative processes as shown in Figure 1.3 . Many of the colors that we expe-
rience in everyday life (flowers, green vegetation, synthetic dyes, etc.) are the result of the
transition of an electron from one electron orbital into another. Sigma electrons require
high energies found only at very short wavelengths in the vacuum UV (100 nm-200 nm)
if they are to be promoted to an available molecular orbital at a higher energy level. They
are of little relevance to most fluorescence techniques, which are usually concerned with
wavelengths between 200 nm and 1000 nm. Pi electrons are less tightly bound to the nuclei
than σ electrons. As a result, the energies required for ionization and electronic transi-
tion are lower than for σ electrons but still mainly in the vacuum UV or the middle UV
(200 nm-300 nm). However, for delocalized π electrons in conjugated systems the ener-
gies required for electronic transitions are much lower and easily obtained in the near
UV (300 nm-400 nm). For extensively delocalized systems or super-delocalized systems
electronic transitions can occur at energies ranging from the near UV to the near infrared
(200 nm-1500 nm). The preference of a particular process over another is dictated by the
energy of the “exciting” field, that is, the wavelength of the light, the configuration of the
molecule(s), and its environment.

1.3.1.1 Spin Multiplicity
The electronic state of a molecule determines both the distribution of negative charge and
its overall geometry. All molecules exhibit different electronic states (illustrated as S 0 , S 1 ,
and S 2 in Figure 1.3 ), depending on the total electron energy and the symmetry of vari-
ous electron spin states. Each electronic state contains a number of vibrational and rota-
tional energy levels associated with the atomic nuclei and bonding orbitals. As discussed
in Section 1.2.1.2 , electrons are described within a particular electronic state by their spin
quantum number ( s ), with values ms = +½ or -½. In Figure 1.4 the different spin states are