Analysis of Photofading in Textile Dyes: Mechanisms and Stability

This report provides a comprehensive analysis of the photofading of textile dyes, focusing on the underlying photochemical mechanisms and factors influencing light stability. It begins by explaining the Jablonski diagram to illustrate electronic state transitions and energy loss pathways, including intersystem crossing, radiative emission, and photochemical reactions. The report discusses photofading via excited singlet states and photo-oxidation involving singlet oxygen, highlighting the roles of dye structure, experimental conditions, and sensitizers like methylene blue. It also addresses the complexities in elucidating reaction mechanisms due to complex reaction products and characterization challenges. The report references key studies and reviews in the field, providing a detailed overview of the current understanding of textile dye photofading.

Photofading of textile dyes
The Jablonski diagram shown on figure 1 is used to clearly demonstrate and analyses the effects
of photochemistry on the textile dyes, the diagram is used in demonstrating the fundamental
alterations that exists on the electronic states. From the diagram the low vibration energy levels
on every electronic state are demonstrated by using thick lines, while other light lines shows
higher vibrational energy levels that are related to every electronic state. The light that is
absorbed lightens the molecule to raise it from its ground electronic state (S0) to excited states (S1
or S2 for next level). Due to short existence of excited state the molecule will be returned back to
the original ground states. During the process of transitionally changing back from the excited
electronic state to ground electronic state this is as a result of loss in the energy that was
absorbed, where the process of loss of the energy will be as a result of;
a) Intersystem which results to lack of transition radiations
b) Release of the radiation energy
c) Reactions resulted from photochemical
Figure 1: Jablonski scheme

Photochemical reactions
The origin of photochemical reactions is traced from triplet state whose lifetime is within the
range of 100 ns to 10 ns. The single excited state has a shorter lifetime of the range between 1 –
1000 ps which will not support complete chemical reaction from taking place. Generally
relaxation of higher singlet excited state occurs at condensation phase, where thermal energy in
the process is loss to a low singlet excited state (τ −10−9) and in the process the chemical
reaction is rendered irrelevant. The existence of low singlet excited state of a long duration
results to degradation reaction, at this point intersystem transition of molecules takes place where
molecules are allowed to cross to their matching triplet state, which can exist for more duration
that may further result to production of photo-degradation. Based on the two degradation process
the molecules can be allowed to get into a higher level of vibrational ground state.
Photofading via excited singlet states
The photofading is regarded as a moderate inefficient process where the application of a short
existence of a singlet excited state process does not take place, in the process of where the
production of HD* occurs when photon is absorbed by the molecules. Distinctively, the fading
textile dye yields quantum of order ( Moore et al, 2001) that means that a ratio of one in hundred
thousand photons that is absorbed will lead to a process of photo-degradation, this is also
experienced on a high singlet excited state that has extreme shorter lifetime (10-12 s), that is azo
dyes ( Vinodgopal and Karnat , 1994) there will be chances of photofading process to take place
at a higher rate of intrinsic as shown in scheme 1, where HD represents the dye to indicate it has
An existence of hydrogen atom and also to show that azo dyes contains alkaline pKa values.
Figure 1 demonstrates that the singlet excited state has a higher energy level which means that it
has higher content of energy as compared to triplet state; this makes it to be more reactive. In
contrast there exist a long lifetime (Nav, Cozens and Scaiano, 1996) with the triplet state, which
means that it’s capable of reacting. The existence of reactive groups on the textile dyes fabric
enables it in the singlet excited state even though it a lifetime that is short. Furthermore, the
chances of the dye reacting with oxygen molecule are high at the triplet state, because oxygen is
able to undergo diffusion on the neighboring molecules of the dye.
HD → H D¿ → Decomposition
Scheme 1

Photo-oxidation via singlet oxygen
In the process of the dye undergoing triplet state, chances are in presence of oxygen it will
experience a triplet – triplet annihilation, which results to singlet oxygen as shown in scheme 2,
an initiation of destruction of dye is shown in scheme 3 below.
3HD* + 3O2 → HD+¿1O2
Scheme 2
1O2 + HD → Decomposition
Scheme 3
The degradation of dyes is through singlet oxygen where there exists a prevailing degradation
reaction if the singlet oxygen sensitisers will be present. Example of singlet oxygen generator is
methylene blue (Zollinger, 1991) , copper (II) and phthalocyanine (Giles and McKay, 1963)
where there efficiency will depend on the type of the solvent that will be used based on the
singlet oxygen lifetime. The senstitiser are in a position to transfer their own energies from a
lowest excited triplet state to an acceptor state, where the donors molecules of excited states are
regress to ground state whereas the acceptor are raised to the excited state.
The clear analysis of photosensitization principles through photo-oxidation has been done with
Griffiths and Hawkins (Griffiths and Hawkins, 1977) who explained the pathway of the azo dyes
photo – oxidation, which is also referred to ene reaction. The singlet oxygen sensister under the
condition of the presence of the methylene blue in solvent of alcohol, the pathway of the
photochemical proceeded through attack of the signet oxygen on the dye, by the intermediate
unsaturated azo hydro-peroxide as shown in figure 2 below. This is presented as a process called
type 2 photo – oxidation, because radicals are not involved on its primary reaction. The
degradation will be observed to occur even in the absence of the methylene blue.

Figure 2: Mechanism of oxidation of azo dyes by singlet oxygen

SUMMARY
The photo-degradation flow will depend on the dyes structures and the condition available for
the experiment. In many cases explanation and analysis has not been done on the reaction
mechanism, because of the complex products achieved from the reaction and also complexity to
characterize the reaction.

References
Giles C. H and McKay R.B, 1963. Textlies. Pp. 527.
Griffiths and Hawkins C, 1977. Chem. Soc., Perkin Trans, pp. 747.
Moore J. N, Abbot L, MacFaul P, Lindsay Smith J.R, Jansen L and Oakes J, 2001. Dyes Pigm,
pp. 49.
Vinodgopal K and Karnat P V, 1994. Photochem. Photobiol., A83, pp. 14.
Nav W M. Cozens F L and Scaiano J C, 1996. Am. Chem. soc., 118, pp. 2275.
Zollinger H, 1991. Color chemistry, 2nd Edn, Weinheim: VCH.