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photochemistry

The concept proposed by Giacomo Chamician at the end of the 19th century

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

The definition of photochemistry has different expressions. C. H. Wells believes that what photochemistry studies is "absorption ultraviolet or visible light The molecules of Chemical behavior and physical process ”。 N. J. Turro thinks that "photochemistry studies Electron excited state Chemical behavior and physical process of molecules ". Because the excited state of the electron is usually absorbed by the molecule, ultraviolet light or visible light formation , so the essence of the above two definitions is the same.^[1]

Photochemistry is a branch of chemistry that studies the permanent chemical effects caused by the interaction between light and matter. Due to historical and experimental technical reasons, photochemistry involves light Wavelength range 100~1000 nm, i.e. from UV to Near infrared band. Shorter than UV wavelength electromagnetic radiation , such as X or Gamma ray Caused by Photoionization And related chemistry belong to the category of radiation chemistry. As for far-infrared or longer wavelength electromagnetic wave , it is generally believed that Photon energy It is not enough to cause photochemical process, so it does not belong to the research category of photochemistry. It is observed that some chemical reactions can be Infrared laser Is triggered, but it is attributed to infrared Laser chemistry Of category 。^[2]

Chinese name: photochemistry
Foreign name: photochemistry
Presenter: Giacomo Chamician

Main laws: Photochemical activity principle, photochemical equivalent law
Establishment time: Late 19th century
Famous scientist: Giacomo Chamician, Draper

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Photochemical law

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First law

First law of photochemistry first laW of photochemistry

The law that only the light absorbed by the substance can cause photochemical reaction, also known as the principle of photochemical activation or Groesses Draper Grotthus Draper's law, 1818. Facts show that the first law of photochemistry is also true in biological photochemical reaction, such as in vision Dark adaptation Relative spectral brightness curve of ambient vision and Rhodopsin Absorption of spectrum It is consistent that the spectrum of photosynthesis corresponds to the spectrum of absorption such as chlorophyll, which explains this problem.^[2]

Second law

Einstein proposed in 1905 that Photochemical reaction The number of molecules activated during the process (or Atomic number ）Equals Absorb light Of quantum Number, or molecular absorption of light is a single photon process (electron excited state The life of the molecule is very short, and the probability of absorbing the second molecule is very small), that is, the primary process of the photochemical reaction starts from the absorption of photons by the molecule, which is also called Einstein photochemistry Law of equivalence 。^[2]

E=hv= hc/λ

λ - light quantum wavelength

H -- Planck constant

C -- speed of light

——Avogadro constant

λ=400nm，E=299.1kJ/mol 　λ=700nm，E=170.9kJ/mol

Because of the common chemical bond Bond energy It is greater than 167.4kJ/mol, so light with wavelength greater than 700nm cannot cause photochemical dissociation.

classification

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American ace glass Photochemical reaction The system photochemical process can be divided into primary process and Secondary process 。 The primary process is that molecules absorb photons to excite electrons ground state Promote to excited state The lifetime of excited molecules is generally short. Photochemistry is mainly related to low excited state, and excited state molecules may occur dissociation Or react with adjacent molecules, or transition to a new excited state. These are primary processes, and any subsequent processes are called secondary processes. for example Oxygen molecule Photolysis to generate two oxygen atoms is its primary process; The reaction that oxygen atoms and oxygen molecules combine to form ozone is Secondary process This is the formation of upper atmosphere ozone layer Of Photochemical process 。 When the molecule is in the excited state, because the electronic excitation can cause the change of the bonding mode of the valence bond in the molecule, the geometric configuration of the excited molecule acidity , color, reaction activity or reaction mechanism may be very different from the ground state, so Photochemical reaction It is more colorful than thermochemical reaction.

Photochemical reaction has been widely used in synthetic chemistry. Since the absorption of photons of a given wavelength is often the property of a certain group in the molecule, photochemistry provides the best means to make a reaction take place at a specific position in the molecule. Photochemical reaction is more valuable for systems that lack selectivity in thermochemical reaction or whose reactants may be destroyed. The air pollution process also contains extremely rich and complex photochemical processes, such as Freon Isofluoride carbide Photolysis products may be destroyed in the upper atmosphere ozone layer , producing "holes" in the ozone layer.^[3]

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Absorption of Electromagnetic Radiation Energy and Excited States of Molecules

Semiconductor photochemistry

The primary process of photochemistry is that molecules absorb photons to excite electrons ground state Promote to excited state 。 The electronic state, vibration and rotation state of molecules are quantized, that is, the energy change between adjacent states is discontinuous. Therefore, the required Photon energy They are also different, and their energy values should match as much as possible. Because the energy of photon ε=hv=hc/λ (where h is Planck constant; v is the frequency of light; λ is the wavelength of light; C is the speed of light), so the energy matching is reflected in the matching of light wavelength.

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Molecules in General conditions It is in a stable state with lower energy, called the ground state. After being irradiated by light, if the molecule can absorb photons, it can be promoted to a higher energy state, called an excited state. If molecules can absorb electromagnetic radiation of different wavelengths, they can reach different excited states. According to its energy level, from ground state It is called from the top First excited state , Second excited state wait; All excited states higher than the first excited state are collectively called highly excited states. The life of excited state molecules is generally short, and the higher the excited state is, the shorter its life is, so that there is no time for chemical reaction. Therefore, photochemistry is mainly related to the low excited state. The electromagnetic radiation energy absorbed by molecules during excitation has two main dissipation First, it is combined with the thermal effect of photochemical reaction; The other is to transform into other forms of energy through optical physical processes. Photophysical processes can also be divided into: ① radiation Relaxation process , that is, all or part of the excess energy radiant energy Form of dissipation Drop the molecule back ground state , such as emitting fluorescence or phosphorescence ；② In the non radiative relaxation process, all the excess energy is dissipated in the form of heat, and the molecule returns to the ground state (see Jablons ground state diagram ）。

If the electrons in the molecule are one-to-one（ electron spin In the opposite direction), this state is spectroscopy The upper part is called the singlet (line) state (represented by superscript 1 in the upper left corner of the molecular formula, such as 1A, or S, and represented by S0, S1,... respectively according to the energy from low to high). If there are two electrons in the molecule spin Parallel, this state is called triple (line) state (represented by 3A or T1, T2,...). Singlet state Of excited state The service life is very short, generally in the order of 10-8~10-9 seconds. When the ground state is singlet, it is excited Triplet state Its life is generally long and can reach the order of 10-3~100 seconds. So the photochemistry of organic compounds is mostly triplet photochemistry.

When the molecule is in the excited state, the electronic excitation can cause the change of the bonding mode of the valence bond in the molecule (for example, the electrons are bonded by π orbitals transition π * orbitals to anti bonds, denoted as (π, π *); Or the transition from the non bond n orbital to the anti bond π * orbital, recorded as (n, π *), etc.), so that the geometric configuration of the excited state molecule acidity , color, reactivity or reaction mechanism may be ground state Photochemistry is more colorful than ground state (thermal) chemistry.^[4]

quantum yield

Also called quantum efficiency or Quantum yield 。 Photochemistry is important Basic quantity one of. Assuming that the reaction is A+hv → B quantum yield Defined as:

If excited state At the same time when the A molecule becomes B, other photochemical and photophysical processes occur in parallel, so the quantum yield of this primary process will be affected by the "quantum yield" of other competing parallel processes. Since each molecule can only absorb one photon under the condition of general light intensity, the total quantum yield of all primary processes should be equal to 1.

The measurement of quantum efficiency includes absolute measurement and Relative determination 。 The relative method refers to the method of comparing with a system whose absolute mass production rate is known. The absolute rule requires the direct establishment of the functional relationship between the quantum yield of the reaction and the wavelength, temperature, light intensity and the concentration of various ions (especially hydrogen ions). Such systems that have been studied now include gas systems (e.g Nitrous oxide 、 carbon dioxide 、 Hydrogen bromide 、 acetone Etc.); Liquid phase system (such as Ferric oxalate (III) Potassium solution uranyl oxalate Solution Diphenyl ketone - Diphenyl carbinol 、 2-hexanone 、 azobenzene 、 Benzoic acid Etc.]; Solid phase system (such as nitrobenzene Formaldehyde, diphenyl ketone diphenyl methanol, etc.). The instruments used in these methods are collectively called chemical dew meters.^[2]

Secondary Steps

If a excited state Instead of returning directly to its lowest energy state, a molecule must undergo the following processes: dissociation (Generated free electron , atom, free radical or molecular fragment); React with adjacent same or different molecules; Transition to a new excited state. These processes can occur in parallel, or only one or several of them can occur, but these are primary photochemical processes. Any subsequent steps are called secondary steps. For example, the generation of two oxygen atoms after photodegradation of oxygen molecules is its primary process; The importance that will occur in pure oxygen Secondary process Are oxygen atoms and Oxygen molecule Combine as ozone Reaction; In typical urban atmosphere, oxygen and ozone can react with hydrocarbons in a series of ways, all of which can be called secondary steps.^[1]

molecular rearrangement reaction

The reaction of atoms moving from one part of the molecule to another is called molecular rearrangement reaction 。 many Organic molecule The rearrangement process after light excitation is also a secondary step. If benzene becomes methylene Cyclopentadiene Reaction of:

The first step is just benzene ring Six more free conjugates in π electron (generally only one electron is excited), which has little effect on the hydrocarbon bond in benzene molecule; In the second step, due to the rearrangement of atoms, products with completely different structures are generated.

Sometimes, the primary photochemical process can be used as a tool to study secondary reactions, such as photosensitization. For example, mercury atom can effectively absorb Mercury lamp The emitted light is excited, and then the absorbed energy is transferred through collision with other molecules. For example:

Hg+hv→Hg*

Hg*+N two O→Hg+N two +O

Oxygen atoms can react with other substances in the system, and the number of oxygen atoms generated can be calculated from the amount of nitrogen released.

If the primary photochemical step is the photodissociation of molecules into two free radicals (molecular fragments with single or unpaired electrons), usually the secondary step is chain reaction 。 The reaction of hydrogen and chlorine is a well-known example, and its process is as follows:

hv+Cl two →2Cl

Cl+H two →HCl+H

H+Cl two →HCl+Cl

In chain reaction, each quantum Multiple product molecules can be produced, so the total quantum yield of such reactions may not only be greater than 1, but sometimes can reach hundreds or even thousands. Therefore, when the quantum yield is greater than 1, the chain reaction mechanism can be generally considered.

Decide on one Photochemical reaction The real way to achieve this goal often requires establishing several hypothetical models corresponding to different mechanisms, finding out the kinetic equations between each model system and concentration, light intensity and other relevant parameters, and then investigating which is the most consistent with the experimental results to determine which is the most likely reaction path. Common experimental methods for studying reaction mechanism, except Tracer atom Besides the labeling method, the quenching method, which is the earliest used in photochemistry, is still a very effective method. This method studies the photochemical reaction mechanism through the kinetic measurement of the fluorescence emitted by the excited molecule being quenched by other molecules. It can be used to determine whether the molecule is in the electronic state excited state The reaction rate of molecular dimerization and the rate of long-range energy transfer. Quenching is a bimolecular process, such as the original excited molecule is A *, Quenching agent The molecule is Q, and the process is:

A*+Q→A+Q*

Obviously, quenching process is also a sensitization Process. Q can be regarded as the quenching agent of A *, and A can also be regarded as Q Sensitizer 。^[2]

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Application in Synthetic Chemistry

Since the absorption of photons of a given wavelength is often the property of a group in the molecule, photochemistry provides the best means to make a reaction take place at a specific position in the molecule. It is more valuable for systems that lack selectivity in thermochemical reactions or may be destroyed by reactants. Photochemical reaction The other characteristic of is that the photon is used as the reagent. Once absorbed by the reactant, it will not leave other new impurities in the system, so it can be regarded as the "purest" reagent. If the reactants are fixed in the solid lattice, photochemical synthesis can take place under the expected conformation (or configuration), which is often difficult for thermochemical reactions. For example, maleic acid and Fumaric acid Solid state photosynthesis of dimer of, and crown ether And β - cyclodextrin.

Photochemistry in the atmosphere

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Atmospheric phenomena of the earth and planets, such as atmospheric composition aurora 、 Radiation shielding And climate, which are consistent with the chemical composition of the atmosphere irradiation The situation is relevant. The earth's atmosphere is mainly composed of nitrogen And oxygen form. However, the composition of atoms and molecules in the atmosphere at high altitude is quite different, mainly due to absorption solar radiation Post Photochemical reaction of The air pollution process contains extremely rich and complex chemical processes, which are used to describe these processes Integrated model It contains many photochemical processes. As brown Nitrogen dioxide The high-energy molecules excited by sunlight are oxygen and carbon hydride chain reaction The initiator of. Also like fluorine carbide The relationship between photolysis in the upper atmosphere and the change of ozone shielding layer is based on photochemistry (see environmental photochemistry).^[3]

difference

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Photochemical process is one of the most common and important processes on the earth. Photosynthesis of green plants, vision of animals, photoinduced denaturation of coatings and polymer materials, photography Photolithography Organic chemical reaction Photocatalysis All are related to photochemical processes. Photoinduced separation of isotopes and similar elements, synthesis and application of light controlled functional systems, which have received extensive attention in recent years, reflect that photochemistry is a very active field. However, in terms of theory and experimental technology, photochemistry is still immature in various fields of chemistry.^[5]

Photochemical reaction Compared with general thermochemical reactions, there are many differences, mainly as follows: ① When heating to activate molecules, the distribution of molecular energy in the system obeys Boltzmann Distribution; When the molecule is activated by light, it can be selectively excited in principle (the choice of energy jump value, electron excited state The molecular energy distribution in the system belongs to non-equilibrium distribution. So the path and product of photochemical reaction are often ground state Thermochemical reactions are different. ② As long as the wavelength of light is appropriate and can be absorbed by substances, even at very low temperatures, photochemical reactions can still be carried out.

Related disciplines

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Chemistry, inorganic chemistry, organic chemistry, analytical chemistry, physical chemistry chemical kinetics 、 Chemical thermodynamics 、 Structural chemistry , quantum chemistry, electrochemistry Nuclear chemistry Polymer Chemistry, Radiochemistry Isotope chemistry Radiation chemistry.

Professional books

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Author: Jiang Yueshun Li Tie Tianjin Publishing House: Chemical Industry Press Publication date: January 2005

ISBN : 7-5025-6135-8 Format: 16 Format

Category: Physical Chemistry, Material Science, Other Directions Page: 356

Introduction: This book introduces the basic knowledge of photochemistry, photophysics and photobiology. Specific contents include: molecular orbital and absorption spectrum ； molecule excited state The fate of photophysics and photochemical processes; Organic Photochemical reaction ； Photochemistry and Photochemistry of Inorganic and Semiconductor Materials Photoelectrochemistry ； Laser chemistry And molecular dynamics; Femtosecond chemistry ； Organic molecule Photoelectron transfer catalysis; Supramolecular Photophysical and photochemical processes of the assembly system; Light and surface and interface chemistry; Photography Photosensitive material Chemistry; light Information storage Materials and technology; Nanocrystalline Photoelectrochemical solar energy conversion; photosynthesis.

history

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Italian chemists were the first scholars to carry out photochemical research G. L. Ciamician From 1886, he and Italian chemist Paolo Silber jointly completed“ Benzoquinone towards hydroquinone And“ nitrobenzene In alcohol photochemical action ”He can also be considered as Solar panel Father. At the 8th International Conference on Applied Chemistry in 1912, he gave a speech on the topic of "The future of photochemistry", looking forward to the important role that photochemistry may play in the future.