Chemistry began to learn about artificial atoms, elementary particles (electrons, protons, neutrons, neutrinos, mesons, and so on), new molecules, new states of matter (plasma), and finally new means of action on chemical reactions in the first half of the twentieth century. We must first mention electromagnetic and corpuscular radiation among these new means of action. Photosynthesis is initiated on the surface of the Earth by the electromagnetic waves (photon flux) that the Sun emits. using chlorophyll's role as a catalyst. The importance of the chemical reactions promoted by the action of electrons, neutrons, alpha (α) particles, and gamma (γ) rays increases as the technical applications for atomic energy do as well.
A number of significant differences distinguish the chemical effects of visible or ultraviolet radiation carrying small amounts of energy from powerful gamma radiation, fast flows of electrons and other particles carrying large amounts of energy.
Photochemistry:
The study of photochemistry focuses on how matter is impacted by visible light and ultraviolet rays with wavelengths longer than 250 Å. The molecules are typically sufficiently excited by these radiations which then participate in different reactions. Dissociation from these radiations can only occur very rarely. Without any directional effect being noticed, the excited molecules form uniformly across the mass of the radiation-absorbing body. Both the excitation and dissociation of molecules are possible effects of corpuscular radiation.
Photochemical process:
When a flow of high energy particles, such as alpha particles or electrons, passes through a body, the reaction products are formed all along their trajectories, and the trace of the particles is then marked by these ionization products. The primary act and the secondary or subsequent reactions can be considered the two stages of the moderately energetic photons' action. The main action, known as electron excitation, is essentially the transfer of electrons to higher energy levels. There are numerous ways to recover the energy that the molecule absorbs when a photon strikes it.
- La forme la plus simple de restitution d'énergie se produit lorsqu'un atome ou une molécule émet le quantum de lumière absorbé dans l'espace, le ramenant à son état initial:
A+ hv === A* (excitement).
A* === A+ hv (radiation).
The molecule is only in the excited state for a very brief period of time (about 10 power minus 10 s for the helium atom). Fluorescence is the name given to this process.
When the molecule collides with another molecule, it can deactivate, or lose its excess energy:
A* +A== 2A
Or: A*+ B== A+ B.
Another possibility is that the excited molecule A* changes into an energetic byproduct without emitting any radiation:
A*== P.
The molecule can also undergo photolysis, which results in radical formation and the rupturing of single bonds:
AB+ hv=== A° + B°.
Excited singlet state:
The ground state of most molecules is a singlet, and most molecules have paired electrons. The molecule is said to have entered an excited singlet state when the excited electron moves to a different orbital while maintaining its antiparallel spin to that of its partner.
Triplet state
The spin conservation law is then in default and a state with parallel spins can appear in more complex molecules where we observe an interaction of spin with orbital angular momentum. Triplet state is the name of such a state. It has a long life and can be identified when paramagnetic properties start to appear.
The energy of a triplet state is lower than that of a singlet state. The triplet state exhibits an excellent chemical reaction due to its long lifetime and single electron.
The characteristics of a biradical and having a high reaction affinity describe a triplet state molecule. The triplet state might experience additional excitation, which would lead to a change to another triplet state. Biradicals are the cause of a lot of photochemical reactions in organic chemistry.
References:
[Smail Meziane: Livre Chimie générale- Structure de la matiére. Berti edition, Alger, 2006]
[Principles & Applications of Photochemistry, Brian Wardle,Wiley, ISBN 0470014938]
[R. OUAHES et B. DEVALLEZ- Chimie générale- Office des publacations universitaires- Alger]