Synthesis, Characterization, Photophysics and Photochemistry of Pyrylogen Electron Transfer Sensitizers

Abstract

A series of new dicationic sensitizers that are hybrids of pyrylium salts and viologens has been synthesized. The electrochemical and photophysical properties of these "pyrylogen" sensitizers are reported in sufficient detail to allow rationale design of new photoinduced electron transfer reactions. The range of their reduction potentials (+0.37-+0.05 V vs SCE) coupled with their range of singlet (48-63 kcal mol(-1)) and triplet (48-57 kcal mol(-1)) energies demonstrate that they are potent oxidizing agents in both their singlet and triplet excited states, thermodynamically capable of oxidizing substrates with oxidation potentials as high as 3.1 eV. The pyrylogens are synthesized in three steps from readily available starting materials in modest overall 11.4-22.3% yields. These sensitizers have the added advantages that: (1) their radical cations do not react on the CV timescale with oxygen bypassing the need to run reactions under nitrogen or argon and (2) have long wavelength absorptions between 413 and 523 nm well out of the range where competitive absorbance by most substrates would cause a problem. These new sensitizers do react with water requiring special precautions to operate in a dry reaction environment.

A Cyclodextrin Polymer as Supramolecular Matrix for Scalable Green Photooxygenation of Hydrophobic Substrates in Homogeneous Phase

“…Several publications are available on the photochemical preparation of endoperoxides from anthracene and naphthalene derivatives [10,11] . The reaction follows a [4+2] cycloaddition mechanism between the lowest excited electronic state of molecular oxygen, 1 Δ g (herein referred as 1 O 2 ), and the ground state of the polycyclic aromatic substrates.…”

“…Several publications are available on the photochemical preparation of endoperoxides from anthracene and naphthalene derivatives. [10,11] The reaction follows a [4 + 2] cycloaddition mechanism between the lowest excited electronic state of molecular oxygen, 1 Δ g (herein referred as 1 O 2 ), and the ground state of the polycyclic aromatic substrates. Selective irradiation of a PS absorbing visible light is used to produce 1 O 2 , precluding direct substrate excitation potentially leading to the formation of byproducts.…”

“…Polycyclic aromatic hydrocarbons are cheap and versatile substrates able to reversibly bind and release O 2 upon suitable triggering (Figure 1A). [10] The endoperoxides obtained after reaction with 1 O 2 have been widely studied for several decades, [11] along which many insights regarding their preparation and reactivity have been obtained. [12] However, an approach towards the optimization of the synthesis envisaging industrial scale up of a sustainable and circular production process is still lacking.…”