Fotohemija

С Википедије, слободне енциклопедије
Photochemical immersion well reactor (50 mL) with a mercury-vapor lamp.

Fotohemija je podoblast hemije koja izučava hemijske reakcije koje se odvijaju uz apsorpciju svetla na atomima ili molekulima.[1] Generalno, ovaj izraz se koristi za opisivanje hemijske reakcije izazvane apsorpcijom ultraljubičastog (talasna dužina od 100 do 400 nm), vidljivog svetla (400–750 nm) ili infracrvenog zračenja (750–2500 nm).[2]

Primeri fotohemijskih reakcija iz svakodnevnog života su fotosinteza, degradacija plastike i formiranje Vitamina D uz pomoć sunčeve svetlosti.[3] Fotohemijske reakcije se odvijaju drugačije od reakcija vođenih temperaturom. Fotohemijski putevi omogućavaju pristup međuproizvodima visoke energije koji se ne mogu generirati toplotno, čime se prevladavaju velike aktivacione barijere u kratkom vremenskom periodu i ostvaruju reakcije koje inače nisu dostupne termičkim procesima. Fotohemija je takođe destruktivna, što ilustruje fotodegradacija plastike.

Koncept[уреди | уреди извор]

Grotthuss–Draper law and Stark-Einstein law[уреди | уреди извор]

Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state. The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. According to the second law of photochemistry, known as the Stark-Einstein law (for physicists Johannes Stark and Albert Einstein), for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield.[4][5]

Fluorescence and phosphorescence[уреди | уреди извор]

When a molecule or atom in the ground state (S0) absorbs light, one electron is excited to a higher orbital level. This electron maintains its spin according to the spin selection rule; other transitions would violate the law of conservation of angular momentum. The excitation to a higher singlet state can be from HOMO to LUMO or to a higher orbital, so that singlet excitation states S1, S2, S3… at different energies are possible.

Kasha's rule stipulates that higher singlet states would quickly relax by radiationless decay or internal conversion (IC) to S1. Thus, S1 is usually, but not always, the only relevant singlet excited state. This excited state S1 can further relax to S0 by IC, but also by an allowed radiative transition from S1 to S0 that emits a photon; this process is called fluorescence.

Jablonski diagram. Radiative paths are represented by straight arrows and non-radiative paths by curly lines.

Alternatively, it is possible for the excited state S1 to undergo spin inversion and to generate a triplet excited state T1 having two unpaired electrons with the same spin. This violation of the spin selection rule is possible by intersystem crossing (ISC) of the vibrational and electronic levels of S1 and T1. According to Hund's rule of maximum multiplicity, this T1 state would be somewhat more stable than S1.

This triplet state can relax to the ground state S0 by radiationless IC or by a radiation pathway called phosphorescence. This process implies a change of electronic spin, which is forbidden by spin selection rules, making phosphorescence (from T1 to S0) much slower than fluorescence (from S1 to S0). Thus, triplet states generally have longer lifetimes than singlet states. These transitions are usually summarized in a state energy diagram or Jablonski diagram, the paradigm of molecular photochemistry.

These excited species, either S1 or T1, have a half empty low-energy orbital, and are consequently more oxidizing than the ground state. But at the same time, they have an electron in a high energy orbital, and are thus more reducing. In general, excited species are prone to participate in electron transfer processes.[6]

Photochemistry in combination with flow chemistry[уреди | уреди извор]

Continuous flow photochemistry offers multiple advantages over batch photochemistry. Photochemical reactions are driven by the number of photons that are able to activate molecules causing the desired reaction. The large surface area to volume ratio of a microreactor maximizes the illumination, and at the same time allows for efficient cooling, which decreases the thermal side products.[7]

Principi[уреди | уреди извор]

Svetlost je tip elektromagnetne radijacije, izvora energije. Po Grotus–Drejperovom zakonu hemijska supstanca mora da absorbuje svetlost da bi došlo do fotohemijske reakcije. Jednim fotonom apsorbovane svetlosti se može aktivirati jedan molekul, ili manje, u zavisnosti od kvantnog prinosa.

In the case of photochemical reactions, light provides the activation energy. Simplistically, light is one mechanism for providing the activation energy required for many reactions. If laser light is employed, it is possible to selectively excite a molecule so as to produce a desired electronic and vibrational state.[8] Equally, the emission from a particular state may be selectively monitored, providing a measure of the population of that state. If the chemical system is at low pressure, this enables scientists to observe the energy distribution of the products of a chemical reaction before the differences in energy have been smeared out and averaged by repeated collisions.

Do hemijske reakcije dolazi kad molekuli poseduju neophodnu energiju aktivacije. Jednostavan primer je sagorevanje benzina (ugljovodonika) do ugljen dioksida i vode. U toj reakciji, energija aktivacije se unosi u obliku toplote ili varnice. Kod fotohemijskih reakcija svetlost pruža energiju aktivacije. Svetlost je jedan od načina davanja energije aktivacije neophodne za mnoge reakcije. Ako se koristi lasersko svetlo, moguće je selektivno pobuditi molekul tako da se proizvede željeno elektronsko i vibraciono stanje. Slično tome, emisija sa specifičnog stanja se može selektivno pratiti, što daje meru zastupljenosti tog stanja. Ako je hemijski sistem na niskom pritisku, moguće je dobiti uvid u distribuciju energije produkata hemijske reakcije pre nego što dođe do disipacije i usrednjavanja energije usled višestrukih molekulskih kolizija.

Apsorpcija svetlosnog fotona na reaktantnom molekulu može da omogući odvijanje reakcije ne samo pružanjem energije aktivacije, nego i promenom simetrije molekulske elektronske konfiguracije, omogućavajući odvijanje inače nepristupačnog reakcionog puta, u skladu sa Vudvord-Hofmanonom pravilima selekcije. 2+2 reakcija cikloadicije je jedan od primera periciklične reakcije koja se može analizirati koristeći ta pravila, ili putem srodnog pristupa molekulsko orbitalne teorije.

Fotohemijske reakcije uzrokuju elektronsku reorganizaciju iniciranu elektromagnetnom radijacijom. Te reakcije su za nekoliko redova veličine brže od termalnih reakcija. Njihova brzina može da bude 10−15 do 10−9 sekundi.

Spektralni regioni[уреди | уреди извор]

Fotohemijske rakcije se tipično izvode koristeći nekoliko specifičnih sekcija elektromagnetskog spektra. Među najčešće korišćenim sekcijama su:

Fotohemijske reakcije[уреди | уреди извор]

Primeri fotohemijskih reakcija[уреди | уреди извор]

Organska fotohemija[уреди | уреди извор]

Детаљније: Organska fotohemija

Examples of photochemical organic reactions are electrocyclic reactions, radical reactions, photoisomerization and Norrish reactions.[14][15]

Norrish type II reaction

Alkenes undergo many important reactions that proceed via a photon-induced π to π* transition. The first electronic excited state of an alkene lack the π-bond, so that rotation about the C-C bond is rapid and the molecule engages in reactions not observed thermally. These reactions include cis-trans isomerization, cycloaddition to other (ground state) alkene to give cyclobutane derivatives. The cis-trans isomerization of a (poly)alkene is involved in retinal, a component of the machinery of vision. The dimerization of alkenes is relevant to the photodamage of DNA, where thymine dimers are observed upon illuminating DNA to UV radiation. Such dimers interfere with transcription. The beneficial effects of sunlight are associated with the photochemically induced retro-cyclization (decyclization) reaction of ergosterol to give vitamin D. In the DeMayo reaction, an alkene reacts with a 1,3-diketone reacts via its enol to yield a 1,5-diketone. Still another common photochemical reaction is Zimmerman's Di-pi-methane rearrangement.

In an industrial application, about 100,000 tonnes of benzyl chloride are prepared annually by the gas-phase photochemical reaction of toluene with chlorine.[16] The light is absorbed by chlorine molecule, the low energy of this transition being indicated by the yellowish color of the gas. The photon induces homolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical:

Cl2 + hν → 2 Cl·
C6H5CH3 + Cl· → C6H5CH2· + HCl
C6H5CH2· + Cl· → C6H5CH2Cl

Mercaptans can be produced by photochemical addition of hydrogen sulfide (H2S) to alpha olefins.

Vidi još[уреди | уреди извор]

Reference[уреди | уреди извор]

  1. ^ Međunarodna unija za čistu i primenjenu hemiju. "photochemistry". Kompendijum Hemijske Terminologije Internet edition.
  2. ^ Međunarodna unija za čistu i primenjenu hemiju. "photochemistry". Kompendijum Hemijske Terminologije Internet edition.
  3. ^ Glusac, Ksenija (2016). „What has light ever done for chemistry?”. Nature Chemistry. 8 (8): 734—735. Bibcode:2016NatCh...8..734G. PMID 27442273. doi:10.1038/nchem.2582. 
  4. ^ Calvert, J. G.; Pitts, J. N. Photochemistry. Wiley & Sons: New York, US, 1966. Congress Catalog number: 65-24288
  5. ^ Photochemistry, website of William Reusch (Michigan State University), accessed 26 June 2016
  6. ^ Wayne, C. E.; Wayne, R. P. Photochemistry, 1st ed.; Oxford University Press: Oxford, United Kingdom, reprinted 2005. ISBN 0-19-855886-4.
  7. ^ Oelgemöller, Michael; Shvydkiv, Oksana (2011). „Recent Advances in Microflow Photochemistry”. Molecules. 16 (9): 7522—7550. PMC 6264405Слободан приступ. PMID 21894087. doi:10.3390/molecules16097522. 
  8. ^ Menzel, Jan P.; Noble, Benjamin B.; Lauer, Andrea; Coote, Michelle L.; Blinco, James P.; Barner-Kowollik, Christopher (2017). „Wavelength Dependence of Light-Induced Cycloadditions”. Journal of the American Chemical Society. 139 (44): 15812—15820. ISSN 0002-7863. PMID 29024596. doi:10.1021/jacs.7b08047. hdl:1885/209117Слободан приступ. 
  9. ^ Saunders, D. S. (2002-11-11). Insect Clocks, Third Edition. стр. 179. ISBN 0444504079. 
  10. ^ Dugave, Christophe (2006-10-06). Cis-trans Isomerization in BiochemistryСлободан приступ ограничен дужином пробне верзије, иначе неопходна претплата. стр. 56. ISBN 9783527313044. 
  11. ^ Protti, Stefano; Fagnoni, Maurizio (2009). „The sunny side of chemistry: Green synthesis by solar light”. Photochemical & Photobiological Sciences. 8 (11): 1499—516. PMID 19862408. doi:10.1039/B909128A. 
  12. ^ Peplow, Mark (17. 4. 2013). „Sanofi launches malaria drug production”. Chemistry World. 
  13. ^ Paddon, C. J.; Westfall, P. J.; Pitera, D. J.; Benjamin, K.; Fisher, K.; McPhee, D.; Leavell, M. D.; Tai, A.; Main, A.; Eng, D.; Polichuk, D. R. (2013). „High-level semi-synthetic production of the potent antimalarial artemisinin”. Nature (на језику: енглески). 496 (7446): 528—532. ISSN 0028-0836. PMID 23575629. doi:10.1038/nature12051Слободан приступ. 
  14. ^ Klán, Petr; Wirz, Jakob (2009-03-23). Photochemistry of Organic Compounds: From Concepts to Practice. ISBN 978-1405190886. 
  15. ^ Turro, Nicholas J.; Ramamurthy, V.; Scaiano, Juan C. (2010). Modern Molecular Photochemistry of Organic Molecules. ISBN 978-1891389252. 
  16. ^ Rossberg, Manfred; Lendle, Wilhelm; Pfleiderer, Gerhard; Tögel, Adolf; Dreher, Eberhard-Ludwig; Langer, Ernst; Rassaerts, Heinz; Kleinschmidt, Peter; Strack, Heinz; Cook, Richard; Beck, Uwe; Lipper, Karl-August; Torkelson, Theodore R.; Löser, Eckhard; Beutel, Klaus K.; Mann, Trevor (2006). „Chlorinated Hydrocarbons”. Ullmann's Encyclopedia of Industrial Chemistry. ISBN 3527306730. doi:10.1002/14356007.a06_233.pub2. 

Literatura[уреди | уреди извор]

Spoljašnje veze[уреди | уреди извор]