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Physical Chemistry 336

Types of electronic transition

Electronic transitions arise from different types of electron rearrangement inthe
molecule, or groups of atoms in a molecule. A group of atoms that gives rise to a
characteristic optical absorption is called a chromophore.
The C=O and C=C double bonds of organic molecules are common chromophores.
The excitation of a bonding π electron in a C=C bond into an anti-bonding π* orbital by
absorption of a photon is called a π−π* transition. For an unconjugated double bond the
energy of this transition corresponds to absorption of ultraviolet light at about 180 nm.
When several double bonds form a conjugated chain, the energies of the extended π and
π* orbitals lie closer together and the absorption transition shifts into the visible.
A similar, but weaker, electronic transition occurs in a carbonyl (C=O) group where
one of the Ione pair electrons on the O atom is excited into the anti-bonding π* molecular
orbital of the carbonyl group. The absorption that promotes this n−π* transition occurs
at about 300 nm in the near ultraviolet. Carbonyl groups can also conjugate with C=C
bonds and this again shifts the absorption towards the visible. The colors of many natural
objects and synthetic dyes are due to π−π* and n−π* absorptions in conjugated systems,
e.g. the carotene compounds responsible for the reds and yellows in vegetation.
Another common type of electronic transition, responsible for the intense color of
many transition metal complexes and inorganic pigments, is a charge-transfer transition.
In these transitions an electron transfers from the d orbitals of the metal to one of the
ligands, or υice υersa. An example of charge-transfer occurs in the permanganate ion,
−
MnO 4 . Absorption in the 420–700 nm range (responsible for the intense violet color) is
associated with the redistribution of charge accompanying an electron transfer from an O
atom to the Mn atom.

Fluorescence and phosphorescence

All electronically excited states have a finite lifetime. In most cases, particularly for large
molecules in solids and liquids, the energy of excitation is dissipated into the disordered
thermal motion of its surroundings. However, a molecule may also lose energy by
radiative decay, with the emission of a photon as the electron transfers back into its
lower energy orbital. There are two modes of radiative decay:

(i) fluorescence: the rapid spontaneous emission of radiation immediately following
absorption of the excitation radiation;
(ii) phosphorescence: the emission of radiation over much longer timescales (seconds or
even hours) following absorption of the excitation radiation. The delay in
phosphorescence is a consequence of energy storage in an intermediate, temporary
reservoir.
A Jablonski diagram (Fig. 3) illustrates the relationship between fluorescence and
phosphorescence and a typical arrangement of molecular electronic and vibrational
energy levels. Therefore, the absorption of radiation promotes the molecule from the
ground electronic state, S 0, to vibrationally excited levels in an upper electronic state, S 1
Therefore, the absorption spectrum shows structure (if any) characteristic of the