A first collaborative paper with the Agrios Group, UConn Department of Civil and Environmental Engineering and Center for Clean Energy Engineering (C2E2) was published: Hewage, N.; Bowen; Agrios, A. G.; Brückner, C. ‘Introduction of Carboxylic Ester and Acid Functionalities to meso‑Tetrakis(pentafluorophenyl)porphyrin and their Limited Electronic Effects on the Chromophore’ Dyes Pigm. 2015, 121, 159–169.

Alkyl- or aryl-carboxylic acid-functionalized porphyrinic dyes are sought after because of their propensity to adhere strongly to many metal oxide surfaces as required for their application as, for instance, sensitizers in dye-sensitized solar cells (DSSCs), in air purification, or chemosensing systems. The S_NAr reaction of the pentafluorophenyl group is a versatile method to introduce functionality into mesopentafluorophenyl- substituted porphyrins. The conditions to introduce one through four alkyl- or aryl-carboxyl functionalities using mercaptopropionate or 3,4-dihydroxybenzoate esters, respectively, are explored, and the regioisomeric products are spectroscopically characterized. Their saponification to the corresponding carboxylic acids was studied. By experimental determination of their optical properties (absorption and emission spectroscopy) and their frontier orbital positions by cyclic voltammetry, we demonstrate the minimal electronic influence this derivatization method has on the chromophore.

See also the Audioslide presentation of article.

Another paper resulting from the collaboration with the Bringmann Group at the University of Würzburg was published! Indaphyrins and indachlorins possess large chiral porphyrinoid π-systems with particularly long-wavelength absorption properties. All indaphyrin derivatives, including the indaphyrin M(II) complexes (M = Ni(II), Cu(II), Zn(II), and Pt(II)), adopt strongly ruffled and metal-dependent conformations incorporating a helimeric twist, thus forming two stereochemically stable helimeric enantiomers. Resolution of the racemic mixtures of the helimers of all derivatives Introduction was achieved by HPLC on a chiral phase and their absolute stereostructures were assigned. The much altered UV/Vis spectra of the indaphyrin derivatives, when compared to those of porphyrins, were rationalized using excited state calculations. The report forms the basis for future applications that exploit the chiral properties of the chromophores: Götz, D. C. G.; Gehrold, A.; Dorazio, S.; Samankumara, L.; Daddario, P.; Bringmann, G.; Brückner, C.; Bruhn, T. Eur. J. Org. Chem. 2015, 3913–3922.

Yellow Prussiate of Soda (YPS, Na₄[Fe(CN)₆]·10H₂O) is an approved anticaking agent in table salt. Given that it is a cyanide salt, its use as a food additive is surprising. In a recently accepted manuscript (Dorazio, S. J.; Brückner, C. J. Chem. Educ. 2015, accepted for publication) we highlight recent reports by Bode et al. (Cryst. Growth Des. 2012, 12, 1919. and  ibid. 5889.) on the mode of action of sub-monoatomic layers of YPS on NaCl crystals to act as an anticaking agent through nucleation inhibition . The molecular-level explanation reinforces the notion of how structure and charge affect the properties of matter. In addition, we report a simple naked eye analytical method of detecting YPS on store-bought table salt in the form of the pigment Prussian Blue (Fe₄[Fe(CN)₆]₃·xH₂O).

Treatment of the oxime of octaethyloxochlorin under Beckmann conditions provided not the expected lactam, but the unprecendented octaethyl-1,3-oxazinochlorin, a chlorin analogue in which a pyrrole moiety of the parent oxochlorin was expanded by an oxygen atom to an 1,3-oxazinone moiety. Its mechanism of formation was demonstrated to occur along an “abnormal Beckmann” pathway, followed by intramolecular ring closure and hydrolysis. The work expands the methodologies known to convert octaethylporphyrin to pyrrole-modified porphyrin analogues with altered optical properties compared to the parent compounds.

Meehan, E.; Li, R.; Zeller, M.; Brückner, C. ‘Octaethyl-1,3-oxazinochlorin: A ß-Octaethylchlorin Analogue Made by Pyrrole Expansion’ Org. Lett. 2015, 17, 2210–2213. DOI: 10.1021/acs.orglett.5b00800.

Congratulations to Eileen’s and Ruoshi’s first paper from our group!

The reduction of a free-base chlorin generally forms a bacteriochlorin (BC), while the reduction of the corresponding metallochlorin forms a metalloisobacteriochlorin (M-iBC). Nearly 50 years after its discovery, the regioselective reduction of chlorins was now explained: Bruhn, T.; Bückner, C. J. Org. Chem. 2015, 80, 4861–4868. (cover article 2015, 80(10))

A combination of DFT-calculated average local ionization energies (ALIEs), thermodynamics of the products, and the transition-state trajectories of reduction reactions of meso-tetraaryl- and β-octaethylchlorins, as their free bases and zinc complexes, now fully delineate the theoretical basis of the reduction regioselectivity. The reactions are kinetically controlled. Steric effects originating in the conformational flexibility of the chlorin macrocycle direct the reactions toward the formation of iBCs. Only when electronic effects are strong enough to override the steric effects are BCs formed. The practical value of this work lies in the presentation of a simple predictive method toward synthetic tetrahydroporphyrins by reduction of chlorins.

Following our synthetic papers on the the synthesis of porphyrin N‑ and dithiaporphyrin S‑oxides (Banerjee, S.; Zeller, M.; Brückner, C. ‘MTO/H₂O₂/Pyrazole-Mediated N-Oxidation of meso-Tetraarylporphyrins and –chlorins, and S-Oxidation of meso-Tetraaryldithiaporphyrin and ‑chlorin’ J. Org. Chem. 2009, 74, 4283–4288.), this theoretical paper illuminates the origin of the optical properties of these compounds. Bruhn, T.; Brückner, C. ‘Origin of the Absorption Spectra of Porphyrin N‑ and Dithiaporphyrin S‑Oxides in their Neutral and Protonated States’ Phys. Chem. Chem. Phys. 2015, 17, 3560–3569. DOI: 10.1039/c4cp04675g

A follow-up of our communication (Akhigbe, J.; Zeller, M.; Brückner, C. ‘Quinoline-annulated Porphyrins’ Org. Lett. 2011, 13, 1322–1325.), this full paper describes the synthesis, structures, and optical properties of the quinoline-annulated porphyrins, π-extended porphyrins with particularly long wavelengths absorption spectra. Akhigbe, J.; Luciano, M.; Zeller, M.; Brückner, C. ‘Mono- and Bisquinoline-annulated Porphyrins from Porphyrin β,β’-Dione Oximes’ J. Org. Chem. 2015, 80, 499–511. DOI: 10.1021/jo502511j

Congratulations to Mike’s first paper from our group!

A further development for porpholactone-based high pH sensors: They have become water-soluble and can be incorporated into Nation membranes. Worlinsky, J. L.; Halepas, S.; Ghandehari, M.; Khalil, G.; Brückner, C. ‘Oxazolochlorins 15. High pH Sensing with Water-soluble Porpholactone Derivatives and their Incorporation into a Nafion® Optode Membrane’ Analyst 2015, 140, 190–196. DOI: 10.1039/c4an01462f

The paper was selected as HOT article and discussed on the Analyst Blog: http://blogs.rsc.org/an/