Page 659 - Carrahers_Polymer_Chemistry,_Eighth_Edition

622 Carraher’s Polymer Chemistry

requirement is the absence of an inversion center requiring the presence of asymmetric centers and/
or poling. Poling is the application of a high-voltage field to a material that orients some or all of

the molecules generally in the direction of the field. The most effective poling is found when the

polymers are poled above the T (that allows a better movement of chain segments) and then cooled
g
to lock in the “polled” structure. Similar results are found for polymers that contain side chains that
are easily poled. Again, cooling helps lock in the polled structure. At times, cross-linking is also
employed to help lock in the polled structure.
Third-order NLO behavior generally involves three photons resulting in affects similar to those
obtained for second-order NLO behavior. Third-order NLO behavior does not require the presence
of asymmetrical structures.
Polymers that have already been found to offer NLO behavior include polydiacetylenes and
a number of polymers with liquid crystal side chains. Polymers are also employed as carriers of
materials that themselves are NLO materials. Applications include communication devices, routing
components, and optical switches.

19.3 PHOTOPHYSICS AND PHOTOCHEMISTRY—BASICS

Photophysics involves the absorption, transfer, movement, and emission of electromagnetic, light,
energy without chemical reactions. By comparison, photochemistry involves the interaction of elec-
tromagnetic energy that results in chemical reactions. Let us briefl y review the two major types of
spectroscopy with respect to light. In absorption the detector is placed along the direction of the
incoming light and the transmitted light measured. In emission studies, the detector is placed at
o
some angle, generally 90 , away from the incoming light. Remember that energy, E = hv.
When absorption of light occurs, the resulting polymer, P*, contains excess energy and is said
to be excited.
P + hν → P* (19.19)

The light can be simply reemitted.
P* → hν + P (19.20)

Of much greater interest is the migration, either along the polymer backbone or to another chain,
of the light. This migration allows the energy to move to a site of interest. Thus, for plants, the site
of interest is chlorophyll. These “light-gathering” sites are referred to as antennas. Natural antennas
include chlorophyll, carotenoids, and special pigment-containing proteins. These antenna sites har-
vest the light by absorbing the light photon and storing it in the form of an electron that is promoted
to an excited singlet energy state by the absorbed light.
Bimolecular occurrences can occur leading to an electronic relaxation called quenching. In this

approach, P* finds another molecule or part of the same chain, A, transferring the energy to A.
P* + A → P + A* (19.21)

Generally, the quenching molecule or site is in its ground state.
Eliminating chemical rearrangements, the end results for quenching are most likely electronic
energy transfer, complex formation, or increased nonradioactive decay. Electronic energy transfer
involves an exothermic process where part of the energy is absorbed as heat energy, and part is
emitted as fluorescence or phosphorescence radiation. Polarized light is taken on in fl uorescence

depolarization also known as luminescence anisotropy. Thus, if the chain segments are moving at
about the same rate as the reemission, part of the light is depolarized. The extent of depolarization
is then a measure of the segmental motions.

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