Surface photochemistry: generation of benzyl radical pairs on dry silica gel

. Can. J. Chem. 71, 1340 (1993).The photochemistry of several substituted a-(aryloxy)acetoveratrones that are models for chromophores in lignin has been studied using a combination of laser flash photolysis and product studies in solution and on silica, Na-X zeolite, and cellulose. The lifetimes of the triplet ketones vary substantially with the electron-donating ability of the substituents in the a-aryloxy ring, with values ranging from 30 ns for the 4-methoxy derivative to 5.5 ps for the 4-cyano ketone in acetonitrile. The triplet ketones are considerably longer lived on a silica surface than in solution and do not decay with first-order kinetics; however, they are all shorter lived than triplet acetoveratrone under the same conditions, indicating that P-phenyl quenching and (or) P-cleavage dominate their decay. The same triplets show quite different kinetic behavior in Na-X or cellulose and there is no indication in these systems for the importance of either P-phenyl quenching or P-cleavage in determining the rates of triplet decay. Product studies for a-guaiacoxyacetoveratrone indicate that Pcleavage from the singlet excited state is responsible for product formation; in contrast to solution results, the phenacyl-phenoxyl radical pairs react exclusively by geminate recombination to give rearranged ketones resulting from ortho and para ring coupling of the two radicals. Product formation is very inefficient for this ketone in cellulose, indicating either a low singlet reactivity or efficient geminate recombination of the initial radical pair to regenerate starting material. Utilisant une combinaison de photolyse Cclair au laser et une Ctude des produits en solution, sur de la silice, sur une zColite Na-X et sur de la cellulose, on a CtudiC la photochimie de plusieurs a-(aryloxy)acCtovCratrones qui sont des modkles pour les chromophores de la lignine. Les temps de vie dcs cCtones triplet varient beaucoup avec les capacitts des substituants du cycle aryloxy ?i donner des Clectrons; les valeurs varient de 30 ns pour le dCrivC 4-mtthoxy 5 5,5 ps pour la 4-cyanocCtone dans I'acCtonitrile. Les cCtones triplet ont des temps de vie beaucoup plus ClevCs sur une surface de silice qu'en solution et leur dCgCnCrescence n'est pas du premier ordre; toutcfois, leurs temps de vie sont tous plus courts que celui du triplet de I'acCtovCratrone dans les mCmes conditions et ces rksultats indiquent que la relaxation P-phknyle et (ou) un clivage P dominent leur dCgCnCrescence. Les mCmes triplets prtsentent un comporternent cinCtique trks diffCrent avec Na-X ou la cellulose et il ne semble pas que, dans ces systi-mes, la relaxation P-phCnyle ou un clivage P joue un r6le important dans la determination de la vitesse de dCgCnCrescence de I'Ctat triplet. Des etudes de produit pour l'a-guai'acoxyacttovCratrone indiquent que le clivage P B partir de 1'Ctat singulet excitC est responsable pour la formation des produits; par opposition aux rksultats obtenus en solution, Ics paires de radicaux phenacyle-phCnoxyle rkagissent exclusivement p...

Section: Discussionmentioning

confidence: 95%

Photochemistry of lignin model compounds on solid supports

Hurrell¹,

Johnston²,

Mathivanan³

et al. 1993

“…When dibenzyl ketone is photolysed in either micelles (6) or on silica gel (18), an isomeric product, 1-phenyl-4'-methylacetophenone, 2, is produced. This isomer is formed by geminate recombination of the initial radical pair and is thus expected to be formed more often from I3C ketone molecules than from "C molecules.…”

Section: Discussionmentioning

confidence: 99%

“…When the ketone was to be adsorbed on silica gel dried at 200°C at I Torr for ? 5 h. the previously described method and apparatus were used (18). For samples on silica gel which had not been predried, a methylene chloride solution of I was added to a preweighed silica sample in a round bottom flask.…”

Section: Methodsmentioning

confidence: 99%

Surface photochemistry: ¹³C enrichment by photolysis on silica gel

Johnston¹,

Wong²

1984

Self Cite

The enrichment of the 13C isotope by partial photolysis of dibenzyl ketone on silica gel has been demonstrated. The 13C enrichment is substantial and is parallel to the increase in the probability of geminate radical recombination on a silica surface as compared with that in solution. A substantial 13C enrichment was also observed for the rearranged starting material, 1-phenyl-4′-methylacetophenone. The single-stage isotope separation factors are different for the two ketones although both are isomers derived from the same benzyl–phenylacetyl radical pairs.

“…In many cases it has been demonstrated that adsorption on solids such as silica and alumina and inclusion in zeolites can often modify the normal solution photochemistry of a given species. In particular, a variety of benzyl radical pairs have been used to examine the effects of zeolites and silicas on the pathways available to organic molecules adsorbed on the various surfaces (1,2,4). These studies have investigated the effects of the additives, temperature, and modifications of the silica or zeolite properties upon the observed chemistry of the radicals.…”

mentioning

confidence: 99%

Intrazeolite photochemistry. VI. Diffuse reflectance laser flash photolysis and product studies of diphenylmethyl radicals on solid supports

Kelly¹,

Willsher²,

Wilkinson³

et al. 1990

. J. Chem. 68, 812 (1990).Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows A, , at -335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. On a produit des radicaux diphknylmtthyles en excitant, l'aide d'un laser optrant a 266 nm, de la 1,1,3,3-tktraphknylacttone adsorbte sur du gel de silice ou incluse dans des ztolithes de NaX et de Silicalite et on les a ttudits en faisant appel aux techniques de photolyse <> a l'aide de laser i rtflectance diffuse. Le spectre du radical prksente un A, , a environ 335 nm dans chacun des trois supports et il est semblable a celui observt en solution. Les radicaux se dtcomposent a des vitesses qui varient de quelques centikmes de nanosecondes a des minutes; certaines indications nous laissent croire que certains radicaux pourraient se dkcomposer a des vitesses plus rapides que celles que l'on peut mesurer. L'efficacitt du pikgeage par l'oxygkne augmente lorsqu'on passe de la Silicalite au NaX au gel de silice; cet ordre est en accord avec la plus grande disponibilitk de l'oxygkne sur les pores du gel de silice par comparaison avec les canaux Ctroits qui existent sur la Silicalite. Pour divers supports, on a aussi examink les effets de la dose du laser ainsi que de la quantitC de cttone utiliste. On discute aussi des applications potentielles d'un traitement cinttique des donntes bast sur la cinttique de rkactions dispersives cornrne moyen de rksoudre le problkme de la cinktique de dtcompositions qui se produisent a des vitesses trks diffkrentes.