Photochromism refers to the phenomenon that when A compound (A) is irradiated by a certain wavelength of light, it can undergo a specific chemical reaction to obtain a product (B), which results in a significant change in its absorption harmonic due to a change in structure. Under the irradiation of another wavelength of light or the action of heat, it can be restored to the original form.
Most of the photochromic systems are built on the basis of single molecule reaction, and the change of potential energy curve more vividly and intuitively shows this photochromic process.
Curve a is the ground state potential energy curve or thermal isomerization potential energy curve. Compound A can be transformed into B after overcoming the barrier (E,) by thermal activation, and compound B can be transformed into B if the activation energy E is obtained. So it could become A again. If ER>E then A is thermodynamically stable, and vice versa; If E. And E are both large enough that a bistable state exists. Curve b is the excitation state energy curve of compound A, when compound A is excited by light (hv) can become B, curve c represents the excitation state energy curve of compound B, when B is excited by light (hv’), it can return to A. The photoisomerization reaction is not necessarily unidirectional, but more bidirectional, that is, the excited state formed by the compound A and B after the photoexcitation can be changed into B and can be transformed into A. Some photoreactions in the isomerization of excited states sometimes also have activation energy (E. * or Eb*), but compared with the ground state activation energy is much smaller, so it is often ignored.
Photochromism is a reversible chemical change, which is an important criterion for judging. Irreversible reactions that occur under the action of light, which can also lead to changes in color, can only fall within the general category of photochemistry and not within the category of photochromism discussed in this chapter. Secondly, in general,A is A colorless body, and the transformation from A to B is excited by light at the maximum absorption wavelength of species A (generally in the ultraviolet region). B is generally a color body, and its maximum absorption wavelength is in the visible light region.
With the development and deepening of scientific research, the definition of photochromism based on single molecule reaction system is obviously incomplete and needs to be supplemented. At present, in the field of photochromic research, the following three different reaction modes should also be included:
First, the multi-component reaction model
For two (A or B) or more (rarely) reaction components to produce one or more products (P) under the action of light, this reaction must also be reversible.
Second, cyclic reaction mode or multistable reversible reaction mode
This kind of reaction is more intentional than the bistable model discussed earlier.
Third, multi-photon photochromic reaction system
Some substances do not have photochromic reactions under the action of a single photon, and must be achieved by multi-photon excitation, while some photons cannot cause reactions due to energy reasons. Photochromic reaction can be achieved by continuous excitation of multiple photons.
In summary, photochromism can be summarized into the following systems:
(1) All-optical photochromic systems, including single-molecule and multi-molecule systems, whose color bodies can only return to the initial state through a light-induced reaction.
(2) photochromic thermoreversible system, the photochromic product is heated back to the initial state.
(3) photochromic light and heat are reversible systems, photochromic products can be returned to the initial state either by heat or by light excitation.
(4) Multi-photon photochromic system, the photochromic process is driven by at least two photons.
(5) In the reverse photochromic system, the initial state is absorbed in the long wave region, and the final state is absorbed in the short wave region.
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