Causes of dye fading under the sun

When a dye molecule absorbs the energy of a photon, it will cause the outer valence electron of the molecule to transition from the ground state to the excited state. According to different structures, the dye molecules can have different excitation processes under the effect of light waves of different wavelengths, including π→π*, n→π*, CT (charge transfer), S→S (single state), S→T ( Triplet state), ground state→first excited state and ground state→second excited state, etc. 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. The corresponding three-line states are indicated by T0, T1, and T2.

In the process of intensification, the dye molecules are excited into electronic excited states of various oscillating energy levels. Their oscillating energy levels will drop rapidly, converting energy into heat and dissipating. This process of decreasing energy levels is called oscillation passivation. In the process of oscillation passivation, the S2 excited state with a low oscillation energy level will also be transformed into an S1 excited state with a higher oscillation energy level, and the oscillation passivation will continue. In this way, the S2 excited state with a higher energy level is quickly transformed into the S1 excited state with the lowest oscillation energy level. The transformation between S2 and S1 electronic energy states under the condition of equal energy intersection does not include the change of electron spin multiplicity, which is called internal transformation. There will also be a transition between singlet and triplet states, from S1 to T1 excited state. This kind of electron energy state transformation under the condition of equal energy intersection with electron spin multiplicity is called intersystem crossing. Because of the "forbidden" of the electron spin selection law, the speed of intersystem crossing is generally relatively low.

Factors affecting the light fastness of dyes

1. The wavelength of the light source and the shining light;

2. Environmental factors;

3. The chemical properties and arrangement structure of the fiber;

4. Bonding strength between dye and fiber;

5. The chemical structure of the dye;

6. Dye concentration and aggregation state;

7. The influence of artificial sweat on the fading of dye;

8. The influence of additives.