Energy-Transfer-Mediated Photocatalysis by a Bioinspired Organic Perylenephotosensitizer HiBRCP

Abstract

Energy transfer plays a special role in photocatalysis by utilizing the potential energy of the excited state through indirect excitation, in which a photosensitizer determines the thermodynamic feasibility of the reaction. Bioinspired by the energy-transfer ability of natural product cercosporin, here we developed a green and highly efficient organic photosensitizer HiBRCP (hexaisobutyryl reduced cercosporin) through structural modification of cercosporin. After structural manipulation, its triplet energy was greatly improved, and then, it could markedly promote the efficient geometrical isomerization of alkenes from the E-isomer to the Z-isomer. Moreover, it was also effective for energy-transfer-mediated organometallic catalysis, which allowed realization of the cross-coupling of aryl bromides and carboxylic acids through efficient energy transfer from HiBRCP to nickel complexes. Thus, the study on the relationship between structural manipulation and their photophysical properties provided guidance for further modification of cercosporin, which could be applied to more meaningful and challenging energy-transfer reactions.

Bioinspired Photocatalyzed Organic Synthetic Transformations. The Use of Natural Pigments and Vitamins in Photocatalysis

“…The fluorescence emission spectrum of HiBRCP* is quenched by addition of incremental amounts of Ni complex I, by a Dexter-type EnT process. Based on the results obtained, the authors [32] postulate a plausible mechanism such as that shown in Scheme 13.…”

“…Zhang, Rao and colleagues [32] have synthesized an analogue of the naturally‐occurring Cercosporin ( vide supra ), the hexa iso butyryl reduced cercosporin (HiBRCP), where the core perylenequinonoid moiety is converted to perylene by a reduction and acylation process (structure in Scheme 12), to catalyze through EnT a series of coupling reactions between benzoic acid derivatives and aryl halides, employing NiBr 2 and 4,4’‐di ter tbutyl‐2,2’‐bipyridine (dtbbpy) as ligand in the presence of di‐ iso propylethylamine (DIPEA) as sacrificial donor in MeCN as solvent, irradiating with a 23 W CFL. The scope of the transformation is depicted in Scheme 12.…”

“…The authors [32] investigated the reaction mechanism (Scheme 13). Their findings show that a nitrogen atmosphere is needed, that photocatalyst HiBRCP, NiBr 2 , ligand dtbbpy, DIPEA and light are necessary for the reaction to take place.…”

“…Based on the favorable triplet energy of HiBRCP (E T HiBRCP = 54.2 kcal mol À 1 , structure in Scheme 12, vide supra and Scheme 39, vide infra), Zhang, Rao and colleagues [32] have very recently inspected the possibility of geometrical trans-cis isomerization of E-olefins by EnT utilizing HiBRCP as photosensitizer. The kinetics of geometrical isomerization are ruled by the thermodynamic feasibility, that is ΔE T = E T (alkene) -E T photosensitizer < 0.…”

“…Cercosporin CP has found applications in photodynamic therapy as well, [31] due to its capacity for singlet oxygen generation. Also, its triplet state energy of 40.8 kcal/mol [32] turns it into an excellent photocatalyst for triplet energy transfer (EnT) sensitization. [33] EnT photocatalysis [34,35] is a mild and selective manner to access triplet states by means of indirect excitation.…”