Structural impacts on the photodissociation dynamics of aromatic endoperoxides in solution

Reversible Binding of Oxygen to Aromatic Compounds

“…Cycloreversion quantum yield Q C increases for many nonsymmetrical EPOs stepwise with decreasing λ, indicating that cycloreversion occurs from several excited S n (ππ*) ( n = 2, 3...) states. , Cycloreversion must be very fast to compete efficiently with internal conversion. Several groups, however, measured surprisingly long PAC rise times τ 2 ranging for eight EPOs from 40 ( 24f ) to 95 ps ( 11c ). − These results indicate a two-step reaction with a much faster first step. Experiments with 24f demonstrated that τ 2 does not depend on solvent polarity and viscosity, indicating a fast homolytic cleavage of a C−O bond of the peroxide bridge as first step .…”

New Assignment of the Electronically Excited States of Anthracene-9,10-endoperoxide and Its Derivatives: A Critical Experimental and Theoretical Study

“…In recent times, several picosecond time-resolved studies have been reported 2b,9 even using femtosecond pulses, where unusually long raise times (40−95 ps) for the cycloreversion of aromatic endoperoxides have been observed, except in one case . These observations have been interpreted as being due to the formation of a biradical intermediate through a nonconcerted cleavage of one of the C−O bonds 2b,9,10 and due to a barrier which slows down the cycloreversion during the second C−O bond cleavage.…”

“…These observations have been interpreted as being due to the formation of a biradical intermediate through a nonconcerted cleavage of one of the C−O bonds 2b,9,10 and due to a barrier which slows down the cycloreversion during the second C−O bond cleavage. If it were to be a concerted and barrierless S n ( n ≥ 2) photocycloreversion, then a much shorter raise time (<3 ps) is expected. 2b,, Thus, presently it is believed that the photocycloreversion of endoperoxides occurs from higher excited singlet states (S n , n ≥ 2) in a nonconcerted fashion with a barrier at the last step of the reaction.…”

Elucidation of the Electron Transfer Reduction Mechanism of Anthracene Endoperoxides

“…Anthracene endoperoxides exhibit interesting thermal and photochemical properties. − Their potential for reversible oxygen storage and as a chemical source of singlet oxygen, 1 O 2 ( 1 Δ g ), a species responsible for causing physiological damage, has made these compounds the subject of many fundamental investigations. − Typically, anthracenyl-type endoperoxides decompose thermally by two distinct reaction pathways. One pathway regenerates the parent aromatic hydrocarbon and oxygen in either the singlet or triplet state from a formal cycloreversion reaction, and the other generates products formed via homolytic cleavage of the O−O bond (Scheme ). ,,− The relative yields of these two reaction processes have been studied as a function of structure and experimental conditions to ascertain the mechanism and activation parameters for a number of systems.…”

“…The first step in their proposed mechanism involves the barrierless C−O cleavage to form a biradical in competition with internal conversion. Next, the second C−O cleavage occurs to extrude 1 O 2 in the rate-determining step. ,− ,, Recent computational and experimental investigations of this mechanism by Klein and co-workers dispute Brauer assignments of the excited states involved in the competing processes . This was followed with some discussion from both sides of this issue in the literature. , Currently, the assignment of the excited states is open to debate, and an excellent account of the reversible binding of oxygen to aromatic compounds has recently appeared …”