Studies on meta effect appears in dioxetane chemiluminescence and ester photolysis

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Photolysis of m- and p-methoxybenzyl acetates (I_m and I_p) in methanol has been investigated and found the followings : (a) Photochemical C-O bond fission of I_m occurs from the excited singlet state, while that of I_p occurs from the triplet state ; (b) Φ_r apparently lower for I_m photolysis than for I_p is due to internal return of intimate pairs, I.e., the C-O bond fission quantum yield is 1 for I_m while 0.8 for I_p. c C-O bond cleavage of the excited singlet state of I_m occurs so readily that photolysis produce a singlet radical pair and ion pair origin products accompanied by extremely low fluorescence. Because C-O bonding splits slowly in the excited singlet state of I_p, intersystem crossing becomes the major pathway for excited singlet state reactions, resulting in a moderate yield of fluorescence and a high yield of the triplet state, and eventually yielding free radical origin products. The major factor determining I photolysis products is the relative rate of C-O bond cleavage in the excited singlet state versus intersystem crossing. The reason why C-O bond cleavage of excited singlet state of I_m is very fast while that of I_p is slow has been studied by quantum mechanical theory.When 3-methoxy-3-(w- and p-methoxyphenyl)-4,4-adamantiliden-l ,2-dioxetane (II_m, II_p) is photolyzed in a glassy matrix at 77K, adamantanone adjacent to S_l state of methyl methoxybenzoate plays as an external machine to alter electron spin multiplicity of the ester from S_l to T_l, by the energy return, mechanism, p-orbital overlapping between the ester and adamantanone is necessary to work the energy return mechanism between methyl m-methoxybenzoate and adamantanone. Intimate pair ofmethyl p-ethoxybenzoate and adamantanone generated through photolysis of II_p in glassy matrix at 77K forms exciplex. Energy transfer business that occurs immediately after the II_p fragmentation between two carbonyl fragments determines fate of ehemiluminescenee.