The photogeneration of charged intermediates in organic photochemistry: the effect of an internal cyclic array of p-orbitals




Krogh, Erik

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A series of structurally related compounds has been prepared to investigate how the number of cyclically conjugated π electrons affect photochemical reaction rates. To this end, photosolvolysis and photodecarboxylation reactions have been extended to a series of bridged diaryl methanol and diaryl acetic acid derivatives. Product studies have been carried out in a number of solvent systems and demonstrate that charged intermediates are involved in these reactions. Mechanistic probes, such as solvent, pH and substituent effects, support a mechanism involving heterolytic bond cleavage of the S1 state as the primary photochemical event. The bridging unit in the series of structurally related compounds has been varied to affect the number of π electrons in the internal cyclic array (ICA) of these photogenerated intermediates. Product quantum yields and fluorescence lifetimes were measured for all members of the series. The combined data were used to establish the rates of the excited state bond cleavage. The most photosolvolytically reactive system, 9-fluorenol, involves a 4 π electron carbocation intermediate, the 9-fluorenylium ion. The rate constant for excited state bond cleavage has been calculated to be ks ≈ 5 x 109 s-1, which was at least 500 times greater than that of 5-suberenol, which involves a 4n+2 π electron intermediate. The most reactive system towards photodecarboxylation was 5-suberenecarboxylic acid, (kdc ≈ 6 x 109 s-1) which involves a 4n π electron carbanion intermediate, the 5-suberenyl anion. Here again, the rate constant for fluorenecarboxylic acid, a 4n+2 π electron precursor. Thus, the charged intermediates most favoured in these photochemical reactions, (i.e., 4n systems), are those that are the most elusive in the ground state because of their formally antiaromatic character. The present study clearly shows a reversal of the ground state reactivity trends for both solvolysis and decarboxylation reactions upon photoactivation. Rate constants for all members of the series will be presented and the origin of the special structure-reactivity effects will be discussed.



Excited state chemistry, Organic photochemistry