Page 661 - Carrahers_Polymer_Chemistry,_Eighth_Edition

624 Carraher’s Polymer Chemistry

one of the chief modes of weathering by materials. Our focus here will be on positive changes
effected by the absorption of light. For many years, absorption of light has intentionally resulted
in cross-linking and associated insolubilization. This forms the basis for coatings and negative-
lithographic resists. Light-induced chain breakage is the basis for positive-lithographic resists.
Photoconductivity forms the basis for photocopying and photovoltaic effects and is the basis for
solar cells being developed to harvest light energy.
It is important to remember that the basic laws governing small and large molecules are the
same.
The Grotthus–Draper law states that photophysical/photochemical reactions only occur when a
photon of light is absorbed. This forms the basis for the First Law of Photochemistry, that is, only
light that is absorbed can have a photophysical/photochemical effect.
We can write this as follows:

P + light → P* (19.24)

where P* is P after it has taken on some light energy that it has acquired energy during a photo-
chemical reaction. The asterisk is used to show that M is now in an excited state.
There are two kinds of spectra, namely excitation (or emission) and absorption. The absorption
and excitation spectra are distinct but usually overlap, sometimes to the extent that they are nearly
indistinguishable. The excitation spectrum is the spectrum of emitted light by the material as a
function of the excitation wavelength. The absorption spectrum is the spectrum of absorbed light
by the material as a function of wavelength. The origin of the occasional discrepancies between the
excitation and absorption spectra are due to the differences in structures between the ground and
excited states, or the presence of photo reactions, or the presence of nonradiative processes that
relax the molecule to the ground state without passing through the luminescent states.
Visible color is normally a result of changes in the electron states. Molecules that reside in the
lowest energy level are said to be in the ground state or unexcited state. We will restrict our attention
to the electrons that are in the highest occupied molecular orbital (HOMO) and the lowest unoccu-
pied molecular orbital (LUMO). These orbitals are often referred to as the frontier orbitals.
Excitation of photons results in the movement of electrons from the HOMO to the LUMO. This
is pictured in Figure 19.4.
Photon energies can vary. Only one photon can be accepted at a time by an orbital. This is stated
in the Stark-Einstein Law also known as the Second Law of Photochemistry—if a species absorbs
radiation, then one particle (molecule, ion, atom, etc.) is excited for each quantum of radiation (pho-
ton) that is absorbed.

Remember that a powerful lamp will have a greater photon flux than a weaker lamp. Further,
photons enter a system one photon at a time. Thus, every photon absorbed does not result in bond

new homo
LUMO

Energy Gap

HOMO

Electron

FIGURE 19.4 Representation of a photon being absorbed by a single molecule of chromophore.

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