Page 662 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Selected Topics 625

breakage or other possible measurable effect. The quantum yield, ϕ, is a measure of the effective-
ness for effecting the desired outcome, possibly bond cleavage and formation of free radicals.

φ= number of molecules of reactant consumed
number of photons consumed (19.25)

Quantum yields can provide information about the excited electronic states such as the rates of
radiative and nonradiative processes. Moreover, they can also find applications in the determination

of chemical structures and sample purity. The emission quantum yield can be defi ned as the frac-
tion of molecules that emits a photon after direct excitation by a light source. So emission quantum
yield is also a measure of the relative probability for radiative relaxation of the electronically excited
molecules.
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Quantum yields vary greatly from the photons being very ineffective (10 ) to being very effec-
6
tive (10 ). Values greater than 1 indicate that some chain reaction, such as in a polymerization,
occurred.
We often differentiate between the primary quantum yield that focuses on only the fi rst event
(here the quantum yield cannot be greater than 1), and secondary quantum yield that focuses on the
total number of molecules formed via secondary reactions and here the quantum yield can be high.
Luminescence is a form of cold body radiation. Older TV screens operated on the principle of
luminescence, where the emission of light occurs when they are relatively cool. Luminescence

includes phosphorescence and fluorescence. In the TV, electrons are accelerated by a large electron
gun sitting behind the screen. They are accelerated by a large voltage. In the black and white sets,
the electrons slam into the screen surface that is coated with a phosphor that emits light when hit
with an electron. Only the phosphor that is hit with these electrons gave off light. The same principle
operates in the old generation color TVs except the inside of the screen is coated with thousands of
groups of dots, each group consisting of three dots—red, green, and blue.
The kinetic energy of the electrons is absorbed by the phosphor and reemitted as visible light to
be seen by us.
Fluorescence involves the molecular absorption of a photon that triggers the emission of a photon
of longer wavelength (less energy; Figure 19.3). The energy difference ends up as rotational, vibra-
tional, or heat energy losses.
Here excitation is described as

P + hν → P (19.26)
o
ex
1
and emission as
P → hν + P (19.27)
o
1
em

where P is the ground state, P is the first excited state.
1
o
The excited state molecule can relax by a number of different, generally competing pathways.
One of these pathways is conversion to a triplet state that can subsequently relax through phos-
phorescence or some secondary nonradiative step. Relaxation of the excited state can also occur
through fluorescence quenching. Molecular oxygen is a particularly efficient quenching molecule

because of its unusual triplet ground state.
Watch hands that can be seen in the dark allow us to read the time in the dark. These watch

hands typically are painted with phosphorescent paint. Like fluorescence, phosphorescence is the
emission of light by a material previously hit by electromagnetic radiation. Unlike fl uorescence,
phosphorescence emission persists as an afterglow for some time after the radiation has stopped.
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The shorter end of the duration for continued light emission is 10 s but the process can persist for
hours and days.

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