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2.1.1 Photochemical Concepts, Laws, and Rules

DOI: 10.1055/sos-SD-229-00002

Gaida, F.; Griesbeck, A. G.; Vollmer, M.Science of Synthesis: Photocatalysis in Organic Synthesis, (201814.

The photons (i.e., electromagnetic radiation) that are generally applied are visible or ultraviolet photons (wavelengths between 250 and 800 nm). Infrared radiation, high-energy vacuum UV, and γ-rays can also give rise to chemical reactions that can additionally be termed “infrared photochemistry” or “radiation chemistry”, respectively. The prefix “photo” indicates that photons play a crucial role in one or more steps of a chemical reaction or a catalysis process. It does not reveal, however, what specific role the photons have. This becomes clear from the IUPAC definitions of photochemistry[‌24‌] [the branch of chemistry concerned with the chemical effects of light (far ultraviolet to infrared)] and photocatalysis [change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance (the photocatalyst) that absorbs light and is involved in the chemical transformation of the reaction partners]. Also note that the traditional role of the prefix in words such as organocatalysis and metal catalysis does not hold here: the photon cannot function as the catalytic species because it is consumed during all chemical processes. In fact, photons are often described as traceless reagents because they disappear from the reaction vessel if not consumed.[‌25‌] The term “consumption” corresponds to the absorption of photons, and this is also reflected in the first law of photochemistry: The Grotthuss–Draper law (also called the principle of photochemical activation) states that only light that is absorbed by a system can initiate a chemical change.[‌26‌] Thus, light transmission, reflection, scattering, and emission are not able to initiate the chemistry of a given species, e.g. A in Scheme 2. This appears trivial at a first glance, but is useful to bear in mind upon designing photochemical processes that are initiated by light absorption to give the excited species of interest, e.g. A* in Scheme 2.

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Meeeee 8 Meeeeeeee Meeeeeee: (e) Meeeeeeeee (Meeeeeeee Meeeeeeeee); (e) Meeeeeee (Meeeeeeee Meeeeeeeee); (e) Meeeeee (Meee Mee Meeeeeee Meeeeeeeeeee Meeee)

Meeee 8 Meeeeeeeee ee Meeeeeeeeeeee Meeeeeeee[‌8‌,‌8‌]

Meeeeee Meeeeeeee (e) Mee
eeeeeeeeee 88−88 ee 88−88 [‌8‌,‌8‌]
eeeeeeeeeee eeeeeeeeee 88−88 ee 88−88 [‌8‌,‌8‌]
eeeeeeeeeeee 88−88 ee 88−8 [‌8‌,‌8‌]
eeeeeeeeeeeeeee 88−8 ee 88−8 [‌8‌,‌8‌]
eeeeeeeeeee eeeeeeee (eeeeeee → eeeeeee) 88−88 ee 88−8 [‌8‌,‌8‌]
eeeeeeee eeeeeeeeee 88−88 ee 88−88 [‌8‌,‌8‌]

Meeeee 8 Meeeee-Meeeee Meeeeeeeee eee (Meeeeeeeee) Meeeeeeeeeee Meeeeee eeee Meeeeeeeeeeee ee eee Meeeee Meeeeeee Meeeeeeeee (M88) ee Meeeeeee Meeee ee eee Meee Meeee (λeeeMeeeM) Meeeee

References


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