Investigations On The Photochemical Reaction Mechanisms Of Selected Dibenzoylmethane Compounds

Dibenzoylmethane(DBM)has attracted considerable attention for its applications in a variety of fields,especially in biomedicine and display devices.As a representative of dicarbonyl compounds,DBM exists as ketone and enol tautomers.In recent years,Sket and coworkers synthesized several DBM derivatives with halogen substitutions and performed steady-state spectrum measurements.These halogen substitutions of DBM derivatives blocked the keto-enol transformation,allowing study on the behavior of the pure diketo tautomer.It is demonstrated a new photocyclization occurs for substituted DBM producing flavone derivatives,and the products strongly depend on the type of halogen substitutions banded at C2 and also the position of electron-donor groups in the phenyl rings.To get a detailed understanding of the photocyclization reaction mechanisms,selected DBM derivatives of 1a,1b,1c and 1d were studied using ultrafast transient absorption spectroscopy and density functional theory calculations.It is concluded that the cyclization reaction mechanisms for 1a and 1b are similar.Both undergo a C-Cl heterolysis yielding a α-ketocation intermediate,which follows cyclization to give rise the cation species.On the other hand,1c and 1d undergo dechlorination first producing a radical species,and followed by a cyclization reaction.The dominant factor leading to the different reaction mechanisms is the involvement of a fluorine atom bonded at C2 of 1c and 1d.Due to the meta-effect,the p-methoxy substitution on the benzene ring inhibits the photocyclization reaction,and reduces the yield of photocyclization.This work unravels the influence of fluorine substituents and meta-effects on the photocyclization reaction mechanisms of DBM derivatives,and provides useful guidance for designing DBM derivatives with higher photocyclization yield.The thesis consists of three chapters.The first chapter is the introduction part,which summarizes the significance of photochemistry,β-dicarbonyl compound,dibenzoylmethane derivatives,time-resolved spectroscopy,density functional theory and related research background.The second chapter studies the photocyclization reaction mechanism of the DBM derivatives(1a and 1b)with only chlorine substitution on α-C,and clarifies the effect of the meta-methoxy substitution on the benzene ring on the photocyclization yield.The third chapter explores the photocyclization reaction mechanism of DBM derivatives(1c and 1d)substituted by both fluorine and chlorine atoms on α-C.Reveals the influence of fluorine atom substituents on the mechanism of photocyclization reaction.The effect of the substitution of the para-methoxy group on the benzene ring on the photocyclization yield is clarified.