β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications

Abstract

The singlet and triplet excited states of 9-phenylphenalenones undergo β-phenyl quenching (BPQ) via addition of the carbonyl oxygen to the ortho position of the phenyl substituent. This reaction leads to the formation of naphthoxanthenes , which, in the absence of quenchers, undergo a very rapid electrocyclic ring opening reaction reverting to within a few microseconds. Naphthoxanthene contains a remarkably weak C-H bond, which enables efficient hydrogen transfer reactions to suitable acceptors, giving rise to the production of the naphthoxanthenyl radical or the naphthoxanthenium cation, depending on the solvent polarity. The study uncovers a number of new aspects of BPQ and suggests an excited state-mediated metabolic pathway in the biosynthesis of plant fluorones.

Triphenylphosphonium cation: A valuable functional group for antimicrobial photodynamic therapy

“…Phenylphenalenones are abundant in nature, for example, as plant phytoalexins and phytoanticipins , and we have previously demonstrated that phenylphenalenones isolated from Fusarium oxysporum ‐infected banana plants showed enhanced antifungal activity under exposure to UVA, which led us to propose them as a light‐induced component in the defense armor of these plants against fungal pathogens . Details of their photochemistry have recently been clarified . Given the excellent photosensitising properties of phenalenone, it is not surprising that cationic derivatives have been prepared appending ammonium, anilinium, pyridinium (SAPYR), or guanidinium (SAGUA) moieties to its structure.…”

Photolysis of acetophenone derivatives with α‐cyclopropyl substituents

“…[11][12][13][14][15][16][17][18][19][20] The mechanism for β-quenching, however, has not been fully elucidated until recently. [21,22] Bucher has proposed that the intramolecular quenching of these β-phenyl aryl ketones takes place through addition of the carboxyl oxygen to the ipso carbon atom in the β-phenyl ring to form a triplet biradical that intersystem crosses to reform the starting material (Scheme 3). [23] This hypothesis was elegantly supported by theoretical calculations [23] and reinforced experimentally by investigating β-quenching in 9-phenylphenalenone derivatives.…”

“…[23] This hypothesis was elegantly supported by theoretical calculations [23] and reinforced experimentally by investigating β-quenching in 9-phenylphenalenone derivatives. [21,22] We have studied the photochemistry of cyclopropyl derivatives 1 and 6 that have a built-in triplet sensitizer, via product studies, density functional theory (DFT) calculations and nanosecond transient spectroscopy to better understand how cyclopropyl substituents affect the reactivity.…”

“…Interestingly, the ketone chromophores in 1 and 2 have a β‐phenyl group; the triplet excited state of β‐phenyl aryl ketones with (n, π*) configuration are generally unusually short‐lived because they are efficiently quenched by an intramolecular process . The mechanism for β‐quenching, however, has not been fully elucidated until recently . Bucher has proposed that the intramolecular quenching of these β‐phenyl aryl ketones takes place through addition of the carboxyl oxygen to the ipso carbon atom in the β‐phenyl ring to form a triplet biradical that intersystem crosses to reform the starting material (Scheme ) .…”

Interaction of Triplet Excited States of Ketones with Nucleophilic Groups: (π,π*) and (n,π*) versus (σ*,π*) States. Substituent-Induced State Switching in Triplet Ketones

“…[12,13] If it is phenyl-substituted in 9-position, photochemical addition of the carbonyl oxygen atom on the ortho-position of the 9-phenyl substituent occurs, resulting in the formation of a strongly reducing quinoid intermediate that is oxidised to a phenalenyl-type cation or radical with great ease. [14][15][16][17] Aiming at a more variable and convenient access to 9-substituted phenalenones, we recently prepared 9-trifluoromethanesulfonyloxy-and 9-iodophenalenone. [18] I now report on quantum chemical calculations on the lowest triplet excited state of 9-iodophenalenone, as well as of a series of other 9-substituted phenalenones, in a study aimed at elucidating the intramolecular interaction of ketone triplet excited states with nucleophiles.…”