Christopher M. Lemon

The synthesis, reactivity, and peripheral functionalization of corroles

Corroles are unusual macrocycles that often exhibit chemical reactivity that is distinct from their closely related porphyrin cousins. Standard organic transformations with corroles often result in the formation of unexpected products. A survey of synthetic methods for the preparation of both meso-substituted and β-substituted corroles will give an overview of the different synthetic strategies. This review provides a comprehensive description of the chemical reactivity and functionalization of corroles, focusing especially on reactions at the periphery of the macrocycle. Formylation, nitration, bromination and chlorosulfonation reactions will be explored in detail. In addition, demetalation processes, reactivity of the N-pyrrolic nitrogens and the lability of the macrocycle ring toward expansion and ring-opening reactions will be discussed. Finally, the synthesis of super-structured (picket-fence, capped, and strapped) corroles and isocorroles will be surveyed.

Micelle-Encapsulated Quantum Dot-Porphyrin Assemblies as in Vivo Two-Photon Oxygen Sensors.

Micelles have been employed to encapsulate the supramolecular assembly of quantum dots with palladium(II) porphyrins for the quantification of O2 levels in aqueous media and in vivo. Förster resonance energy transfer from the quantum dot (QD) to the palladium porphyrin provides a means for signal transduction under both one- and two-photon excitation. The palladium porphyrins are sensitive to O2 concentrations in the range of 0-160 Torr. The micelle-encapsulated QD-porphyrin assemblies have been employed for in vivo multiphoton imaging and lifetime-based oxygen measurements in mice with chronic dorsal skinfold chambers or cranial windows. Our results establish the utility of the QD-micelle approach for in vivo biological sensing applications.

Electronic Structure of Copper Corroles

The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled Cu(II) corrole radical cation ground state. X-ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, Cu(II) isocorrole and [TBA][Cu(CF3)4] were studied as authentic Cu(II) and Cu(III) samples, respectively. In addition, the one-electron reduction and one-electron oxidation processes are both ligand-based, demonstrating that the Cu(II) centre is retained in these derivatives. These observations underscore ligand non-innocence in copper corrole complexes.

Photophysical Properties of β-Substituted Free-Base Corroles

Corroles are an emergent class of fluorophores that are finding an application and reaction chemistry to rival their porphyrin analogues. Despite a growing interest in the synthesis, reactivity, and functionalization of these macrocycles, their excited-state chemistry remains undeveloped. A systematic study of the photophysical properties of β-substituted corroles was performed on a series of free-base β-brominated derivatives as well as a β-linked corrole dimer. The singlet and triplet electronic states of these compounds were examined with steady-state and time-resolved spectroscopic methods, which are complemented with density functional theory (DFT) and time-dependent DFT calculations to gain insight into the nature of the electronic structure. Selective bromination of a single molecular edge manifests in a splitting of the Soret band into x and y polarizations, which is a consequence of asymmetry of the molecular axes. A pronounced heavy atom effect is the primary determinant of the photophysical properties of these free-base corroles; bromination decreases the fluorescence quantum yield (from 15% to 0.47%) and lifetime (from 4 ns to 80 ps) by promoting enhanced intersystem crossing, as evidenced by a dramatic increase in knr with bromine substitution. The nonbrominated dimer exhibits absorption and emission features comparable to those of the tetrabrominated derivative, suggesting that oligomerization provides a means of red-shifting the spectral properties akin to bromination but without decreasing the fluorescence quantum yield.

Gold Corroles as Near-IR Phosphors for Oxygen Sensing

The triplet state of gold(III) corroles is exploited for optical oxygen sensing. We report intense phosphorescence for gold(III) corroles in the near-IR, an optical window that is ideal for tissue transparency. Moreover, the triplet excited-state emission exhibits significant changes in intensity and lifetime over the 0-160 Torr O2 pressure range. This renders these compounds sensitive at biologically relevant pressures and overcomes the spectral limitations of palladium and platinum porphyrins for oxygen sensing in biology.

Natural and artificial photosynthesis: general discussion

Cornelia Bohne, Qing Pan, Paola Ceroni, Karl Börjesson, Jana Rohacova, Frederick Lewis, Antonin Vlcek, Dario M. Bassani, Frank Würthner, Andrea Sartorel, Amilra Prasanna de Silva, Dan Nocera, Franco Scandola, Christopher Lemon, Clémence Allain, Gary W. Brudvig, Silvia Marchesan, Villy Sundstrom, Sebastiano Campagna, Stafford W. Sheehan, Per-Arno Plötz, Filippo Monti, John M. Kelly, Elizabeth Gibson, Marcelino Maneiro, Anthony Harriman, Albert Ruggi, Elena Galoppini, Randolph Thummel, Julia Weinstein, Johannes Vos, Osamu Ishitani, Devens Gust and Alejandro Dı́az-Moscoso

Halogen Photoelimination from SbV Dihalide Corroles

Main-group p-block metals are ideally suited for mediating two-electron reactions because they cycle between M n and M n+2 redox states, as the one-electron state is thermodynamically unstable. Here, we report the synthesis and structure of an SbIII corrole and its SbVX2 (X = Cl, Br) congeners. SbIII sits above the corrole ring, whereas SbV resides in the corrole centroid. Electrochemistry suggests interconversion between the SbIII and SbVX2 species. TD-DFT calculations indicate a HOMO → LUMO+2 parentage for excited states in the Soret spectral region that have significant antibonding character with respect to the Sb-X fragment. The photochemistry of 2 and 3 in THF is consistent with the computational results, as steady-state photolysis at wavelengths coincident with the Soret absorption of SbVX2 corrole lead to its clean conversion to the SbIII corrole. This ability to photoactivate the Sb-X bond reflects the proclivity of the pnictogens to rely on the PnIII/V couple to drive the two-electron photochemistry of M-X bond activation, an essential transformation needed to develop HX-splitting cycles.

Comparison of self-assembled and micelle encapsulated QD chemosensor constructs for biological sensing

Whereas a variety of covalent conjugation strategies have been utilized to prepare quantum dot (QD)-based nanosensors, supramolecular approaches of self-assembly have been underexplored. A major advantage of self-assembly is the ability to circumvent laborious synthetic efforts attendant to covalent conjugation of a chemosensor to functionalized QDs. Here, we combine a CdSe/ZnS core-shell QD with gold(III) corroles using both self-assembly and micelle encapsulation to form QD nanosensors. Appreciable spectral overlap between QD emission and corrole absorption results in efficient Förster resonance energy transfer (FRET), which may be initiated by one- or two-photon excitation. The triplet state of the gold(III) corroles is quenched by molecular oxygen, enabling these constructs to function as optical O2 sensors, which is useful for the metabolic profiling of tumours. The photophysical properties, including QD and corrole lifetimes, FRET efficiency, and O2 sensitivity, have been determined for each construct. The relative merits of each conjugation strategy are assessed with regard to their implementation as sensors.

Luminescence sensing and imaging: General discussion

Paola Ceroni, Zoe Pikramenou, Luca Prodi, Qing Pan, Dave Adams, Julia Weinstein, Frederick Lewis, Cornelia Bohne, Antonin Vlcek, Dario M. Bassani, Amilra Prasanna de Silva, Cécile Moucheron, Dan Nocera, Alejandro Dı́az-Moscoso, Marc Padilla, Christopher Lemon, Sebastiano Campagna, Samuel Bradberry, Elena Galoppini, Per-Arno Plötz, John M. Kelly, Jana Rohacova, Anthony Harriman, Páraic Keane, Devens Gust, Johannes Vos, Matteo Mauro, Luisa De Cola, Saleesh Kumar Nambalan Sivaraman, Gilles Lemercier, Shani Osborne and Filippo Monti

Light activated molecular machines and logic gates: general discussion

A. Prasanna de Silva replied: This is a necessary consideration if small molecule-based human-level computations like edge detection are to proliferate. Repeated measurement on the same sample is how we carried out most of our exploratory experiments. Edge widths as thick as 4 mm were measured in this way aer the optimal writing time, even though the crucial partial drying was optimized. Once the optimal writing time is determined in this way, experiments can be run by maintaining an uninterrupted optimal writing time. This eliminates registration errors caused by repeated positioning of the mask on the paper. The thinnest edges that we have measured in this way are 1 mm.