Self-Assembly of Synthetic and Biological Polymeric Systems of ...

excitation; namely, a valance electron of a molecule is excited upon absorbing energy from the
electromagnetic radiation and is, thereby, transferred from one energy level to other more
energetic level. The spectra are electronic. In contrast, IR spectra describe the vibration of
atoms (not electrons) around a chemical bond, as previously described in certain detail.
An electron transition consists of the promotion of an electron from a molecular orbital in the
ground state to an unoccupied orbital by absorption of a photon. The molecule is, then, said to
be in an excited state. Let us recall first the various types of molecular orbitals. A orbital can
be formed either from two s atomic orbitals, from one s and one p, or from two p atomic
orbitals having a collinear symmetry axis. The bond formed in this way is called a bond. A
orbital is formed from two p atomic orbitals overlapping laterally. The resulting bond is called a
bond. For example, in ethylene (CH2=CH2) the two carbon atoms are linked by one and one
bond. Absorption of appropriate energy can promote, for example, one of the electrons to
an anti-bonding orbital denoted by . The transition is, then, called .
A molecule may also possess non-bonding electrons located on heteroatoms such oxygen or
nitrogen. The corresponding molecular orbitals are called orbitals. Promotion of a non-
bonding electron to an anti-bonding orbital is possible, and the associated transition is
denoted by . Hence, molecules containing a non-bonding electron, such as oxygen,
nitrogen, sulphur, or halogens, often exhibit absorption in the UV region. To illustrate these
energy levels, Figure 2.26 shows formaldehyde as an example with all their possible
transitions. In particular, the transition deserves further attention: upon excitation, an
electron is removed from the oxygen atom and goes into the orbital localized half on the
carbon atom and half on the oxygen atom. The excited state, thus, has a charge
transfer character, as shown by the increase observed in the dipole moment regarding the
ground state dipole moment of C=O (40)(53).
In summary, the electronic transitions observed in UV-Vis spectroscopy are ,
, , , , and . The energy of these electronic transitions is,
generally, in the following order: < < < < < . Of
the six transitions outlined, only the two lowest energetic ones ( and ) are
achieved by the energies available in the 200 to 800 nm. The last four types of electronic
transitions required higher energy inputs, below 200 nm, corresponding to the far ultraviolet
region of the electromagnetic spectrum. For instance, most transitions for individual
bonds take place below 200 nm and a compound containing only bonds is transparent (near
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