Oxochlorin frameworks confining a β-hydroxyketone moiety

Nominally, meso-hydroxyoxochlorins, like known 5-hydroxy-7-oxo-octaethylchlorin (9), its nickel complex [5-hydroxy-7-oxo-octaethylchlorinato]nickel(II) (9Ni), or the novel 5-hydroxy-7,17-dioxo-octaethylbacteriochlorin (10), incorporate an acetylacetonate (acac)-moiety in the enol form in their chromophore structures. X-Ray diffraction studies of the compounds show the presence of a strong H-bond between the enol and flanking β-ketone. Like acac, the functionality can be deprotonated. However, unlike regular acac-like moieties, we did not find any indication that this functionality is competent in chelating any of the 3d or 4d transition metal ions tested. Evidently, the conjugation that contributes to the stability of acac as a ligand cannot be expressed in the meso-hydroxyoxochlorins since it would perturb the aromaticity of the porphyrinic chromophores; in other words, the metal binding energies do not offset the loss in aromaticity. The halochromic properties of the molecules provide some more insight into the location of the protonation/deprotonation sites. The interpretation of the findings is supported by computations.

Electronic Structures of an Annulated meso-Tetraphenylchlorin and a Related Chlorin Analogue Incorporating an 8-Membered Ring through MCD Spectroscopy and DFT Calculations

Herein, we compare the electronic structures of the metal-free and nickel(II) derivatives of an annulated meso-tetraphenyldihydroxychlorin with those of the (metallo)chlorin analogues derived by pyrroline β,β'-ring cleavage of the annulated (metallo)chlorins. These (metallo)chlorin analogues incorporate 8-membered heterocycles in place of the pyrroline, carry oxo-functionalities on the former pyrroline β-carbon atoms, and were previously shown to possess drastically ruffled (twisted) nonplanar conformations. The magnetic circular dichroism spectra of all chromophores investigated feature chlorin-like UV-vis spectra and correspondingly reversed (positive-to-negative in ascending energy) sign sequences in the Q-band region, indicative of ΔHOMO < ΔLUMO relationships. Density functional theory (DFT) calculations indicate that the HOMOs in all compounds are a1u-type molecular orbitals (in traditional for the porphyrin spectroscopy D4h point group). Time-dependent DFT calculations correlate well with the experimental spectra and indicate that Gouterman's four-orbital model can be applied to these chromophores. This work highlights to which degree synthetic chlorin analogues can deviate from the structural parameters of natural chlorins without losing their electronic chlorin characteristics.

Stepwise Reduction of β-Trioxopyrrocorphins: Collapse of the Oxo-Induced Macrocycle Aromaticity

The diatropic ring current that characterizes the unexpectedly aromatic octaethyltrioxopyrrocorphins gets drastically reduced upon chemical reduction of one and particularly two ketone moieties. With increasing reduction, the chromophores containing one pyrrole, one/two pyrrolinone, and one/two pyrrolines become more similar to regular, nonmacrocycle-aromatic pyrrocorphins (hexahydroporphyrins). Single-crystal diffraction analysis shows the reduction products to be idealized planar. With increasing reduction, their UV-vis spectroscopic signatures are those of conjugated but nonaromatic oligopyrroles. Their diatropic ring currents, as assessed by ¹H NMR spectroscopy, showed them to possess largely nonaromatic π-systems. Dihydroxylation of select β,β'-dioxobacteriochlorin and β,β'-dioxoisobacteriochlorins also resulted in the formation of equivalent mixed pyrrole/two pyrrolinone/pyrroline chromophores. Computations were able to reproduce the experimental trends of the diatropic ring currents and filled in the data for the regioisomers that could not be experimentally accessed. The work further highlights the electronic influence of the β-oxo-substituents and, more specifically, the origin of the aromaticity of the trioxopyrrocorphins. It also presents a series of chemically robust pyrrocorphins, a chromophore class for which many chemically very sensitive members have been reported.

Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-β-oxo-Substituted Porphyrins: Competing Effects of Spin-Vibronic and NH Tautomerism Relaxation Channels

We demonstrate that β-oxo-substitution provides effective fine-tuning of both steady-state and transient electronic properties of octaalkyl-β-mono-oxochlorin and all isomers of the β,β'-dioxo-substituted chromophores. The addition of a carbonyl group increases the Qy oscillator strength and red-shifts the absorption spectra. Each oxo-substitution results in a 2-fold increase in the singlet to triplet state intersystem crossing (ISC) rates, resulting in a 4-fold ISC rate increase for the dioxo-substituted chromophores. The effects of oxo-substitution on the ISC rate are thus additive. The progressive increase in the ISC rates correlates directly with the spin-vibronic channels provided by the C═O out-of-plane distortion modes, as evidenced by density functional theory (DFT) modeling. The triplet states, however, were not evenly affected by β-oxo-substitution, and reduction in the triplet lifetime seems to be influenced instead by the presence of NH tautomers in the dioxoisobacteriochlorins.

De Novo Synthesis of Bacteriochlorins Bearing Four Trideuteriomethyl Groups

Site-specific introduction of isotopes in tetrapyrrole macrocycles provides the foundation for probing physicochemical features germane to photosynthetic energy-transduction processes, but has chiefly been done with porphyrins rather than the more biologically relevant hydroporphyrin analogues of native photosynthetic pigments. A prior study incorporated pairwise 13C or 15N atoms in the skeleton of a bacteriochlorin containing a gem-dimethyl group in each pyrroline ring. Here, a complementary effort is reported that installs deuterium atoms in substituents at the perimeter of a bacteriochlorin. Thus, perdeuteriated 3-methyl-2,4-pentanedione was converted in an 8-step synthesis via the intermediacy of tert-butyl 5-formyl-3,4-bis(trideuteriomethyl)pyrrole-2-carboxylate to the 2,3,12,13-tetrakis(trideuteriomethyl)-8,8,18,18-tetramethylbacteriochlorin (BC-2). The fidelity of isotope substitution was maintained throughout the synthesis. Resonance Raman spectroscopy of the copper chelate (CuBC-2) revealed that addition of the four β-pyrrolic substituents alone is not sufficient to account for the vibronic complexity observed for the copper chelate of bacteriochlorophyll a (CuBChl a). The increased vibronic activity exhibited by the natural pigments and CuBChl a must arise from the increased structural complexity of the macrocycle.

Synthesis of bacteriochlorins bearing diverse β-substituents

Eleven bacteriochlorins have been prepared for surface attachment, bioconjugation, water-solubilization, vibrational studies, and elaboration into multichromophore arrays.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl-β,β'-dioxo-bacterio- and -isobacteriochlorin Series

It is well-known that treatment of β-octaethylporphyrin with H₂ O₂ /conc. H₂ SO₄ converts it to a β-oxochlorin as well as all five constitutional isomers of the corresponding β,β'-dioxo-derivatives: two bacteriochlorin-type isomers (β-oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin-type isomers (β-oxo-groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled β-oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X-ray structures of all free-base diketone isomers and a comparative description of their UV-vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying β-oxo functionalities. This comparative study lays the basis for their further study and utilization.

Stepwise Reduction of Octaethyl-β,β'-dioxochlorin Isomers: Access to Structurally and Electronically Diverse Hydroporphyrins

Di- and tetrahydroporphyrins (chlorins, bacteriochlorins and isobacteriochlorins, respectively) are key "pigments of life." They have been a major focus of attention in synthetic tetrapyrrole chemistry. A long-known one-pot epoxidation/epoxide ring-opening/pinacol-pinacolone rearrangement of octaethylporphyrin (OEP) generates a β-ketochlorin and all five β,β'-diketone isomers. We present herein the single and double reductions of all isomers of the β,β'-diketones, generating hydroxychlorin and β-hydroxy-β-ketodihydroporphyrin isomers, generally in regioselective manner, and sets of separable stereoisomeric dihydroxytetrahydroporphyrin regioisomers. The connectivity of the regio- and stereoisomers were determined spectroscopically and, in many cases, using single-crystal X-ray crystallography. The optical properties of the chlorin-, bacteriochlorin-, and isobacteriochlorin-type chromophores are described. They highlight general observations on the regiochemical effects of the β-oxo-auxochrome. This contribution thus delineates the formation of a range of regio- and stereoisomers of a family of chromophores with broadly varying optical properties from a single and readily available starting material (OEP) in two straightforward steps, albeit requiring extensive chromatography.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic C_β═C_β bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═C_β-O_β bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Rational Design of Modified Oxobacteriochlorins as Potential Photodynamic Therapy Photosensitizers

The modulation of the photophysical properties of a series of recently synthetized oxobacteriochlorins with the introduction of heavy atoms in the macrocycles, was investigated at density functional level of theory and by means of the time-dependent TDDFT formulation. Absorption frequencies, singlet-triplet energy gaps and spin-orbit coupling (SOC) constants values were computed for all the investigated compounds. Results show how the sulfur- selenium- and iodine-substituted compounds possess improved properties that make them suitable for application in photodynamic therapy (PDT).

Elucidation of the electronic structure of water-soluble quaternized meso-tetrakis(3-pyridyl)bacteriochlorin derivatives by experimental and theoretical methods

Three water-soluble quaternized metal-free tetra-(3-pyridyl)bacteriochlorins were synthesized and characterized by UV-vis, MCD, and NMR spectroscopy as well as elemental analysis. DFT calculations are indicative of the [Formula: see text]HOMO [Formula: see text] [Formula: see text]LUMO relationship, which correlates well with the experimentally observed by MCD spectroscopy for all bacteriochlorins “reversed” sign sequence in the Vis-NIR region. TDDFT calculations also correctly predict large splitting between the [Formula: see text] and [Formula: see text] bands as well as splitting of the Soret band observed experimentally in all bacteriochlorins.

The limited extent of the electronic modulation of chlorins and bacteriochlorins through chromene-annulation

Optical data (UV-vis absorption and fluorescence emission spectra, including fluorescence yields and lifetimes) and electrochemical measurements are used to quantify the modulation of the electronic properties of meso-tetrakis(pentafluorophenyl)-chlorin diol and -bacteriochlorin tetraols upon intramolecular chromene-annulation, including the investigation of regio- and stereoisomers. The small modulations of the frontier orbitals of the porphyrinoids are rationalized using DFT computations and can be traced to small electronic effects due to the co-planarized meso-aryl groups in combination with conformational effects.

Quantitation of Tolyporphins, Diverse Tetrapyrrole Secondary Metabolites with Chlorophyll-Like Absorption, from a Filamentous Cyanobacterium-Microbial Community

INTRODUCTION

Tolyporphins are unusual tetrapyrrole macrocycles produced by a non-axenic filamentous cyanobacterium (HT-58-2). Tolyporphins A-J, L, and M share a common dioxobacteriochlorin core, differ in peripheral substituents, and exhibit absorption spectra that overlap that of the dominant cyanobacterial pigment, chlorophyll a. Identification and accurate quantitation of the various tolyporphins in these chlorophyll-rich samples presents challenges.

OBJECTIVE

To develop methods for the quantitative determination of tolyporphins produced under various growth conditions relative to that of chlorophyll a.

METHODOLOGY

Chromatographic fractionation of large-scale (440 L) cultures afforded isolated individual tolyporphins. Lipophilic extraction of small-scale (25 mL) cultures, HPLC separation with an internal standard, and absorption detection enabled quantitation of tolyporphin A and chlorophyll a, and by inference the amounts of tolyporphins A-M. Absorption spectroscopy with multicomponent analysis of lipophilic extracts (2 mL cultures) afforded the ratio of all tolyporphins to chlorophyll a. The reported absorption spectral data for the various tolyporphins required re-evaluation for quantitative purposes.

RESULTS AND DISCUSSION

The amount of tolyporphin A after 50 days of illumination ranged from 0.13 nmol/mg dry cells (media containing nitrate) to 1.12 nmol/mg (without nitrate), with maximum 0.23 times that of chlorophyll a. Under soluble-nitrogen deprivation after 35-50 days, tolyporphin A represents 1/3-1/2 of the total tolyporphins, and the total amount of tolyporphins is up to 1.8-fold that of chlorophyll a.

CONCLUSIONS

The quantitative methods developed herein should facilitate investigation of the biosynthesis of tolyporphins (and other tetrapyrroles) as well as examination of other strains for production of tolyporphins. Copyright © 2017 John Wiley & Sons, Ltd.