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Photobleaching mechanism, influencing factors and improvement methods of dyes1

About the light fastness:

Light fastness refers to the ability of the dye to maintain its original color under sunlight. According to the general regulations, the determination of light fastness is based on sunlight. For ease of control in the laboratory, artificial light sources are generally used and corrected if necessary. The most commonly used artificial light source is xenon light, but also carbon arc light. Under the irradiation of light, the dye absorbs light energy, the energy level is increased, the molecule is in an intensified state, and the color system of the dye molecule changes or is destroyed, resulting in discoloration or discoloration of the dye.

1). The effect of light on dye production

 

When a dye molecule absorbs the energy of a photon, it will cause the outer valence electrons of the molecule to transition from the ground state to the excited state.

 

According to the structure, the dye molecules can undergo different activation processes under the action of different wavelengths of light, including π → π*, n → π*, CT (charge transfer), S → S (single line state), S → T ( The triplet state, the ground state → the first excited state and the ground state → the second excited state, and the like. The ground state of the singlet state is written as S0, and the first and second excited singlet states are written as S1 and S2, respectively. The corresponding triplet is represented by T0, T1, T2.

 

During the intensification process, the dye molecules are excited into electron-excited states of various vibrational levels, and their vibrational levels are rapidly reduced, converting energy into heat and dissipating. This process of lowering the energy level is called vibration passivation. During the vibration passivation process, the S2 excited state with low vibrational level is also transformed into the S1 excited state with higher vibrational level, and vibration passivation continues to occur. Thus, the S2 excited state with a higher energy level is rapidly converted into the S1 excited state of the lowest vibrational level. The conversion between the S2 and S1 electron energy states under equal energy intersection does not involve the change of electron spin multiplicity and is called internal transformation. Conversion occurs between the singlet and triplet states, from S1 to T1. This kind of electron spin state change accompanied by electron spin multiplicity changes is called intersystem crossing. Due to the "forbidden" of the electronic spin law, the rate of inter-system levitation is generally low.

 

The photochemical reaction between the excited dye molecules and other molecules leads to photofading of the dye and light brittleness of the fibers.

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