1. PHOTOPHYSICS, PHOTOCHEMISTRY
AND LIGHT FASTNESS
The energy content of visible radiation is in excess of 200 kJ mol–1, depending on wavelength
(section 1.2.2), which is sufficient in principle to break most chemical bonds.
It is therefore remarkable that commercially important dyes and pigments have
high light stability despite being designed to absorb light strongly.
To appreciate why this is so requires understanding of the processes
by which molecules dissipate the initially absorbed photon energy,
in particular the fast photophysical and photochemical processes
which the initially formed excited states undergo immediately after the absorption
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of light.

2. Excited states and energy
deactivation processes
The concept of
the energy level diagram
can be extended to illustrate
some
of the energy deactivation
and reaction pathways
open to an electronically excited
molecule.
Such a diagram is known
as a Jablonski diagram (Figure 1.42).
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3. Excited states and energy
deactivation processes
• An important consideration is the timescales over which the various photophysical
• processes take place,
– the light-absorption process itself occurring within a femtosecond.
– Most light-stable molecules return to the ground state
– within a few picoseconds (1 ps = 1 ´ 10–12 s)
– by efficient deactivation processes as discussed below.
• Some molecules, however, resist collisional deactivation processes
• and remain in the excited singlet state for up to a few nanoseconds,
• after which they either emit fluorescent radiation or undergo an electron spin change and
cross over to the longer-lived metastable triplet state
• (an excited state having two electrons with parallel spin in different orbitals).
• The lifetime of the triplet state can range from microseconds to milliseconds
• or longer, and in certain systems can lead to delayed phosphorescent emission.
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4. The energy changes that lead
to fluorescence,
usually on the long-wavelength
side of the lowest-energy
(S0 S1) absorption band,
are illustrated in some detail in
Figure 1.43,
which also shows
the typical UV absorption
and violet-blue
fluorescence emission
spectrum of anthracene in
solution.
reverse emission transition
takes place
from the zero vibrational level to
a range of vibrational
levels in the ground (S0) state.
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5. Since the emission transitions are
of lower energies,
the emission spectrum is
shifted
to lower frequencies
or longer wavelengths.
The mirror-image appearance of
the anthracene Absorption
and fluorescence emission
spectra arises
from the similarity of the
vibrational energy level spacing in
the ground (S0)
and first excited (S1) states.
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6. Photoreactions from the excited states
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7. As indicated above, it was formerly believed that
light-stable molecules passed on their excitation energy
by simple energy-transfer processes
during collision with surrounding solvent or substrate molecules.
Recent work with picosecond and femtosecond pulsed laser spectroscopy
suggests,
however, that molecules immediately change shape
on light absorption
and that chemical isomerisations
or fast reversible hydrogen atom transfers
(reduction/oxidation processes)
are involved in processes leading to a return
to the ground state with no overall change in the light-absorbing molecule .
Very occasionally side reactions in this process may lead to the destruction of
the dye molecule, but with dyes of light fastness greater than 5 the chance of a
molecule undergoing
such destruction is no more than about one in a million
(the photochemical process leading to dye destruction is said to have a
quantum yield of about 10–6 COMPILED BY TANVEER
or less). 7
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8. Certain dye systems are susceptible to
light degradation with much
higher quantum yields.
The reactions of CI Disperse Blue 14
(1,4-dimethyl-amino-anthraquinone)
will be used
 to illustrate this aspect of dye
photochemistry.
Irradiation of the dye
with UV/ visible light
in solution
or in nylon film,
in the absence of oxygen,
leads to significant photo-reduction
(partially reversible when oxygen is
admitted)
accompanied by the
spectral changes shown in Figure
1.44(a).
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9. It is believed that
the dye reacts via
the triplet state in which
the lone-pair electrons
in the carbonyl chromophore
become relatively less electronegative
(the long-wavelength
n  p* transition moves electrons
towards the aromatic ring system)
and pick up hydrogen atoms
from reducible solvent
or substrate species,
resulting in the formation
of the fully reduced quinol ring structure.
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10. If dyed polymer film is irradiated
in the presence of oxygen the photoreaction
observed
is quite different (Figure 1.44(b))
and is initiated with the UV portion of the
irradiating light.
The principal reaction is a de-alkylation of alkyl-
amino groups,
Leading to a reduction in the electron-donating
power of the auxochromic amino groups
And hence a blue (or hypso-chromic) shift in the
absorption band.
The light fastness of the
dye on polyester substrate has been shown to
be 1–2. The requirement of the photoreaction
for UV irradiation suggests that it is initiated
through one of the higher-energy
singlet excited states, such as S2COMPILED BY TANVEER
or S3. 10
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11. If dyed polymer film is irradiated
in the presence of oxygen the photoreaction
observed
is quite different (Figure 1.44(b))
and is initiated with the UV portion of the
irradiating light.
The principal reaction is a de-alkylation of alkyl-
amino groups,
Leading to a reduction in the electron-donating
power of the auxochromic amino groups
And hence a blue (or hypso-chromic) shift in the
absorption band.
The light fastness of the
dye on polyester substrate has been shown to
be 1–2. The requirement of the photoreaction
for UV irradiation suggests that it is initiated
through one of the higher-energy
singlet excited states, such as S2COMPILED BY TANVEER
or S3. 11
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12. Light-fastness measurements
Measurement of the light stability or light fastness of dyed and pigmented systems is
A prerequisite in assessing the overall quality of coloured materials.
The international specification for light-fastness testing (ISO 105 : B01 and B02 : 1988,
BS EN20105 : 1993)
details the exposure conditions for daylight behind glass (B01)
 and artificial lightÊ (xenon arc fading lamp test) (B02).
In both methods the samples to be tested are exposed alongside a set of blue-dyed wool
standards used to define light fastness on a scale from 1 (very low) to 8 (very high).
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13. Light-fastness measurements
• The dyes specified for the production of the blue wool standards were
chosen so
• that each standard in daylight tests requires roughly twice the exposure of
the next lower standard.
• This approximation does not hold for some of the standards, which show
varying rates of fading.
• The low-fastness standards (1 and 2) are anomalous in that they are
bleached by visible light whereas the others show their maximum sensitivity
over the UV region.
• A different set of blue wool standards is produced in America and the light-
fastness values derived using that series are prefixed with the letter L.
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14. Light-fastness measurements
• Since some of the specified dyes are no longer being manufactured (and
anyway batch-to-batch reproducibility has proved a problem), tests are
currently under way using
• a set of blue pigmented samples printed on card as replacements for the
blue wool standards.
• The first set of trial pigment standards is based on varying the ratio of
two pigments of low and high fastness
• along with titanium dioxide in a printing ink formulation
• to cover the 3–7 fastness range
• (the range into which most dyed textiles fall).
• Recent developments have been summarised in an interim report
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15. Light-fastness measurements
• The specification for the xenon arc used for fading tests under
standard B02 indicates
• that the lamp should have a correlated colour temperature of
5500 to 6500 K and
• that it should contain a light filter transmitting at least 90% in the
visible region above 380 nm and falling to zero between 310
and 320 nm.
• In this way the UV radiation is steadily reduced over the near-
UV region.
• IR heat filters are also used to minimise the heating effect of the
IR region (cf. Figure 1.10).
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16. Light-fastness measurements
• Existing light-fastness lamps are either water-cooled or air-
cooled, and the humidity and temperature conditions have to be
adjusted to values laid down in the appropriate standard.
• This is specified in terms of the maximum temperature recorded
in a black panel incorporated in the sample position racks, with
humidity control being determined
• by the fastness rating of a sample of cotton dyed with an azoic
red combination whose humidity sensitivity in light-fastness
testing has been calibrated.
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17. Light-fastness measurements
• The two basic light-fastness standards are supplemented
by standards B03 to B08,
• which cover:
• – B03 colour fastness to weathering: outdoor exposure
• – B04 colour fastness to weathering: xenon arc
• – B05 detection and assessment of photochromism
• – B06 colour fastness to artificial light at high temperatures: xenon arc
fading lamp test
• – B07 colour fastness to light of wet textiles
• – B08 quality control of light-fastness reference materials.
• PHOTOPHYSICS, PHOTOCHEMISTRY AND LIGHT FASTNESS
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18. Light-fastness measurements
• The related standard BS1006 includes a UK-only test, specifying the
use of mercury vapour
• fading lamps.
• The test B05 for photo-chromism is a test for change of colour (usually
at least partially
• reversible) caused by irradiation. Photo-chromism is usually
dependent on some
• reversible change in the chemical structure of the colorant induced
through the first
• excited state.
• Light-fastness testing is discussed further in section 4.5.
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