Photoelectrochemistry for Measuring the Photocatalytic Activity of Soluble Photosensitizers

Abstract

We introduce a rapid method to test the photocatalytic activity of singlet‐oxygen‐producing photosensitizers using a batch cell, a LED laser and a conventional potentiostat. The strategy is based on coupling of photo‐oxidation of hydroquinone and simultaneous electrochemical reduction of its oxidized form at a carbon electrode in an organic solvent (methanol). This scheme gives an immediate response and avoids complications related to long‐term experiments such as oxidative photo‐degradation of photosensitizers and singlet oxygen traps by reactive oxygen species (ROS). Among the tested compounds, a fluoro‐substituted subphthalocyanine showed the highest photocurrent and singlet oxygen quantum yield (ΦΔ) in comparison to phenoxy‐ and tert‐butyl‐substituted analogues, whereas the lowest photocurrents and yields were observed for aggregated and dimeric phthalocyanine complexes. The method is useful for fast screening of the photosensitizing activity and represents the first example of one‐pot coupling of electrochemical and photocatalytic reactions in organic media.

Photoelectrochemical Behavior of Phthalocyanine-Sensitized TiO2 in the Presence of Electron-Shuttling Mediators

“…Unlike phenol, HQ is oxidized reversibly to benzoquinone (BQ) via an electron transition process without obvious side reactions, Figure A. It is also known that HQ readily oxidizes to BQ by 1 O 2, − also noted in our previous studies with solubilized 1 O 2 -producing photosensitizers . At potentials lower than 0 V, BQ is reduced back to HQ, as per the cyclic voltammogram shown in Figure A.…”

“…A new absorbance shoulder at >750 nm appears. The changes correspond to the behavior of Pcs in solid-state films and have been ascribed to molecular distortions and intermolecular interactions through stacking and aggregation. − These interactions and aggregations are of concern for PEC measurements due to possible energy dissipation in photoexcited singlet and triplet states, typically leading to decreased quantum yields and PEC responses, as demonstrated previously for a partially aggregated Al phthalocyanine and its stable dimer …”

“…To additionally prove that the generation of 1 O 2 does not result in noticeable photocurrents in the presence of one-electron donors, we tested both t- Bu 4 PcZn and F 64 PcZn in methanol solution, using the previously reported setup . In the absence of TiO 2 , t- Bu 4 PcZn dissolved in methanol is an efficient 1 O 2 producing photosensitizer, , similar to F 64 PcZn. , In solutions with the same optical density 0.2 a.u. at the laser wavelength 659 nm, t- Bu 4 PcZn and F 64 PcZn show the same photocurrents (1.0 ± 0.1 μA) in the presence of 1 mM HQ and only negligible photocurrents (0.01–0.02 μA) in the presence of 1 mM ferrocenemethanol (Table S3).…”

Electrochemical and spectroelectrochemical studies of tert-butyl-substituted aluminum phthalocyanine

“…Aluminum (Al 3+ ) with a high charge density due to its small size is of special interest because of its strong impact on charge distribution in the Pc macrocycle and the axial ligand. Aluminum phthalocyanine complexes were shown to be promising in the field of catalytic synthesis of cyclic carbonates from carbon dioxide [5,6] and other carbonylation reactions [7], photodynamic therapy of cancer [8], photocatalysis [9], bulkheterojunction solar cells [10,11] and as molecular switches [12,13]. The ability to coordinate an axial ligand plays a crucial role in preparation of ordered Pc thin films [14] with improved photoelectrochemical properties [15] and in synthesis of hybrid nanostructures due to the axial ligand substitution reaction [16].…”

Resonant Plasmon‐Enhanced Absorption of Charge Transfer Complexes in a Metal–Organic Monolayer

“…[ 2 ] Phthalocyanines constitute a very rich family of compounds enabling coordination with more than 70 elements of the periodic table in their central coordination site alongside with numerous possible substituents at the periphery. Possessing an 18 π‐electron conjugation system in their neutral redox state, phthalocyanines demonstrate remarkable optical properties which are extensively studied for potential applications in organic and hybrid photovoltaics (PV), optical memory devices, [ 3 ] photodynamic therapy (PDT), and fluorescence diagnostics of cancer, [ 4 ] optical gas sensors, [ 5,6 ] and field‐effect transistors, [ 7,8 ] photocatalysis, [ 9,10 ] and light‐harvesting elements in artificial photosynthetic systems. [ 11,12 ] At the same time, radical forms of phthalocyanines are of great interest for organic magnetism and molecular spintronics.…”