1.11 photo physics

COMPILED BY TANVEER AHMED

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

of light.


2

Excited states and energy deactivation processes

The concept of the energy level diagram can be extended to illustrate

someof the energy deactivation and reaction pathways

open to an electronically excitedmolecule.

Such a diagram is known as a Jablonski diagram (Figure 1.42).


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.


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 emissionspectrum of anthracene in solution.

reverse emission transition takes place

from the zero vibrational level to a range of vibrationallevels in the ground (S0) state.


5

Since the emission transitions are of lower energies,

the emission spectrum is shiftedto lower frequencies or longer wavelengths.

The mirror-image appearance of the anthracene Absorption and fluorescence emission spectra arises

from the similarity of thevibrational energy level spacing in the ground (S0) and first excited (S1) states.


6

Photoreactions from the excited states


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 returnto 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 undergoingsuch 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 or less).


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 thespectral changes shown in Figure 1.44(a).


9

It is believed that the dye reacts via

the triplet state in which the lone-pair electronsin the carbonyl chromophore become relatively less electronegative

(the long-wavelengthn p* transition moves electrons towards the aromatic ring system)

and pick up hydrogen atoms from reducible solvent or substrate species,

resulting in the formationof the fully reduced quinol ring structure.


10

If dyed polymer film is irradiated in the presence of oxygen the photoreaction observedis quite different (Figure 1.44(b))

and is initiated with the UV portion of theirradiating 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 thedye on polyester substrate has been shown to be 1–2. The requirement of the photoreactionfor UV irradiation suggests that it is initiated through one of the higher-energysinglet excited states, such as S2 or S3.


11

If dyed polymer film is irradiated in the presence of oxygen the photoreaction observedis quite different (Figure 1.44(b))

and is initiated with the UV portion of theirradiating 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 thedye on polyester substrate has been shown to be 1–2. The requirement of the photoreactionfor UV irradiation suggests that it is initiated through one of the higher-energysinglet excited states, such as S2 or S3.


12

Light-fastness measurementsMeasurement 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).


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.


14


• 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


15


• 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).


16


• 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.


17


• 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


18


• 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.


19

Backdrops:

- These are full sized backdrops, just scale them up!

- Can be Copy-Pasted out of Templates for use anywhere!

Title Backdrop Slide Backdrop Transitional Backdrop Print Backdrop

www.animationfactory.com

Additional Graphics:- Scale them up or down!

- .JPG clipart can be scaled up and take up little file space.

- .PNG clipart can be scaled unusually large without distortion.

http://www.animationfactory.com/

http://www.animationfactory.com/

1.11 photo physics

Education

Transcript of 1.11 photo physics