No abstract

A new dibrominated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is reported as a new metal-free photocatalyst. This BODIPY showed similar optoelectronic, electrochemical, and performance properties to those of Ru­(bpy)3Cl2, one of the most common photocatalysts in a known radical–ionic transformation, such as the formation of 1,4-dicarbonyl compounds. Moreover, additional sequences in which the generated oxonium ion is trapped by an internal nucleophile were developed using this BODIPY photocatalyst. These new sequences allowed the straightforward preparation of γ-alkoxylactones, monoprotected 1,4-ketoaldehydes, and dihydrofurans. This new catalyst, the methodology, and the forged functional groups could be important tools in organic synthesis.

The synthesis of two A2B2 porphyrins, {5,15-bis-[4-(octyloxy)phenyl]-porphyrinato}zinc(ii) () and {5,15-bis-(carbazol-3-yl-ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-porphinato}-zinc(ii) (), is reported. Their photophysical properties were studied by steady-state absorption and emission. Substituting the carbazolylethynyl moieties at two of the meso positions results in a large bathochromic shift of all the absorption bands, a notable increase in the absorption coefficient of the Q(0,0) band, and higher fluorescence quantum yield compared to porphyrin , with two unsubstituted meso positions. Cyclic voltammetry and digital simulation show that electrogenerated radical ions of are more stable than those of . The lack of substituents at the meso positions of leads to dimerization reactions of the radical cation. Despite this, the annihilation reaction of and produces very similar electrogenerated chemiluminescence (ECL) intensity. Spectroelectrochemical experiments demonstrate that the electroreduction of leads to a strong absorption band that might quench the ECL.

The electrochemical reduction mechanisms of diprotonated tetraphenylporphyrin (H2TPP) and mono- and diprotonated octaethylporphyrin (H2OEP) were studied in tetrabutylammonium perchlorate/benzonitrile. The diprotonated forms of both porphyrins undergo two one-electron reversible reduction processes forming isophlorin. Contrastingly, monoprotonated H2OEP is reduced in a single process involving a two-electron one-proton transfer that yields two types of short-lived intermediates, isophlorin and neutral phlorin. The existence of intermolecular proton transfer reactions, from the parent protonated porphyrin to the isophlorin or neutral phlorin, to form phlorin cation species (isophlorin protonated at the meso-position) was demonstrated. In-situ UV–vis spectroelectrochemical experiments allowed us to identify the absorption of the isophlorin species of H2TPP but not of H2OEP. These results show that the lack of phenyl substituents increases the rate of protonation at the meso-position. Finally, it was demonstrated that the protonation of the porphyrin macrocycle not only lowers the reduction potentials but also increases the reactivity of the electrogenerated species.