Luz: um raro produto de reação

Abstract

Recebido em 18/6/10; publicado na web em 30/11/10 LIGHT: A RARE REACTION PRODUCT. The production of visible light by chemical reactions constitutes interesting and fascinating phenomena and several reaction mechanisms are discussed to rationalize excited state formation. Most efficient chemiluminescence reactions are thought to involve one or more electron transfer steps and chemiexcitation is believed to occur by radical annihilation. A brief introduction to the general principles of light production and the main known chemiexcitation mechanisms will be given here. Subsequently, recent results on the mechanistic elucidation of efficient chemiluminescence systems, as the peroxyoxalate reaction, the induced decomposition of phenoxy-substituted 1,2-dioxetanes and the catalyzed decomposition of new a-peroxylactones will be discussed.Keywords: chemiluminescence; excited states; organic peroxides. HISTÓRICO DAS REAÇÕES QUIMILUMINESCENTESA quimiluminescência ou, em outras palavras, o processo de emissão de luz como produto ou um dos produtos de uma reação química, é um dos fenômenos mais exuberantes da Química, ao mesmo tempo em que é um dos mais incompreendidos do ponto de vista do "como". Como uma reação química é capaz de "acender", uma vez que certos reagentes foram misturados? Tal tipo de reação ainda pode ocorrer em organismos vivos, sendo atribuído o nome de bioluminescência, fenômeno esse que foi capaz de atrair a atenção do próprio Aristóteles, que descreveu a observação de uma "luz fria" emitida por vaga-lumes.1 Inclusive, foram os próprios gregos os responsáveis por dar o nome de "vaga-lume" aos insetos que aparentavam ser faíscas perambulando noite adentro. 1Mesmo sabendo que observações da emissão de luz a partir de reações e seres vivos são tão antigas, não há informações acerca de quem descobriu tais eventos, ou mesmo aonde. Sabe-se, entretanto, que Wiedemann foi o primeiro a reportar de maneira documentada uma definição para quimiluminescência: "Das bei chemischen Prozessen auftretende Leuchten würde Chemilumineszenz genannt", 2 o mesmo proferiu em 1888, que pode ser traduzido como "a emissão observada durante processos químicos deve ser chamada de quimiluminescência". Wiedemann propôs tal classificação utilizando como base o trabalho realizado por Radziszewski, mais de uma década antes, que observou uma intensa emissão de luz proveniente da autoxidação da lofina (2,4,5-trifenilimidazol, 1, Esquema 1), sendo que tal transformação não ocorria quando o aquecimento era realizado na ausência de oxigênio.3 Importante notar que é na publicação de Wiedemann que o mesmo propõe uma diferenciação entre a quimiluminescência e o fenômeno de emissão de luz devido a um aumento de temperatura, a incandescência, um processo físico, não químico, decorrente da emissão de radiação pelo corpo negro.

The chemiluminescent peroxyoxalate system: state of the art almost 50 years from its discovery

“…Chemiluminescence reactions can be generally divided in three main steps: (i) formation of a high-energy intermediate (HEI), in one or more chemical transformations of ground state molecules; (ii) unimolecular decomposition of the HEI or its interaction with other reagents, leading to electronically excited state formation (chemiexcitation step); (iii) decay of this excited state to the ground state accompanied by fluorescence or phosphorescence emission, depending on the multiplicity of the excited state. 9,11 With the synthesis of cyclic organic peroxides like 1,2-dioxetanones (4) and 1,2-dioxetanes (5), which are nothing more than isolated HEI, and detailed studies on their chemiluminescent decomposition, two distinct general chemiexcitation mechanisms could be outlined: (i) unimolecular cleavage or rearrangement of molecules with high energy content forming excited states, as in the unimolecular thermal decomposition of 1,2-dioxetanes; 12 or (ii) catalyzed decomposition of the high energy peroxide by a suitable activator (ACT), forming the excited state of the ACT, a mechanism known as CIEEL (Chemically Initiated Electron Exchange Luminescence), initially proposed by Schuster 13 (Scheme 1). Studies on the chemiluminescence properties of 1,2-dioxetanones 14 and diphenoyl peroxide 15 (6) revealed that the observed light emission rate constants (k obs ), as well as the chemiluminescence quantum yields (Φ CL ), increased proportionally with the concentration of added ACT.…”

“…Furthermore, a dependency of the k obs and the emission intensities with the oxidation potential of the added ACT could be taken as a clearcut evidence for the occurrence of an electron transfer from the ACT to the cyclic peroxide in the rate-limiting step. 9,11,[13][14][15] On the basis of these and other experimental observations, Schuster and coworkers postulated the CIEEL mechanism, where the first step comprises formation of a charge-transfer type encounter complex (K CT ) between the peroxide and the ACT, inside the solvent cavity (Scheme 1). 7,[13][14][15] Elongation of the relatively weak O-O bond by thermal activation within the charge-transfer complex results in the lowering of the antibonding orbital energy (σ * ), permitting the occurrence of an endothermic electron transfer from the ACT to the peroxide (k ET ), which is accompanied by the cleavage of the O-O bond.…”

“…The quantum yield for the diphenoyl peroxide / perylene system was shown to be three orders of magnitude lower than the value initially determined, 17,18 and since this is the prototype system for the CIEEL proposal, the validity of this mechanism was itself questioned. 9,11 Contrarily, 1,2-dioxetanes containing substituents with low oxidation potentials, such as…”

“…However, in the former work it was also verified that activators bearing electron-withdrawing substituents, therefore possessing substantially high oxidation potentials, show high rateconstants for their interaction with HEI, therefore violating the linear free-energy correlation obtained with other activators. 73 This unexpected observation encouraged Bartoloni and coworkers 76 to perform a systematic study where the relative catalytic rate constants and singlet quantum yields in the peroxyoxalate system were measured using 9-chloro, 9,10-dichloro, 9-cyano and 9,10-dicyanoanthracene as activators. The linear free-energy relationship of the relative rate constants with the activators' reduction potentials (E red ) confirmed, for the first time, the occurrence of an inverse electron transfer in the chemiexcitation step; in this case the electron transfer occurs from the HEI to the activator.…”

“…In this inverse CIEEL process, once the pair of radical ions, with an inverse charge distribution as compared to the normal CIEEL mechanism, is formed, the electron back-transfer (from the radical anion of the ACT to a carbonyl radical cation) may also be capable singlet excited state formation, however, with low excitation quantum yields. 76 An additional evidence for the validity of the occurrence of the inverse CIEEL mechanism for electron acceptor-substituted activators has been obtained by the observed correlation between the measured excitation quantum yields and the energy liberated in the chemiexcitation step, calculated from the redox-potentials of the radical-ions involved, 76 in analogy to the correlation obtained before between the quantum yields in the "normal" CIEEL process and the energy liberated upon annihilation between the carbon dioxide radical anion and the radical cations of the commonly utilized activators. …”