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Achieving solar light harvesting followed by efficient charge separation and transport is an essential objective of molecular-based artificial photosynthesis. Architectures that afford strong absorption across the near-UV to near-infrared region, namely panchromatic absorptivity, are critically important given the broad spectral distribution of sunlight. A tetrapyrrole-perylene pentad array was synthesized and investigated as a means to integrate panchromatic light harvesting and intramolecular charge separation. The pentad consists of three moieties: (1) a panchromatically absorbing triad, in which a porphyrin is strongly coupled to two perylene-monoimides via ethyne linkages; (2) a perylene-diimide electron acceptor; and (3) a chlorin hole-trapping unit. Integrating the three components with diphenylethyne linkers generates moderate electronic coupling for intramolecular energy and hole/electron transfer. The construction of the array relies on a stepwise strategy for incorporating modular pigment building blocks. The key building blocks include a trans-A2BC porphyrin, a chlorin, a perylene-monoimide, and a perylene-diimide, each bearing appropriate (halo, ethynyl) synthetic handles for Pd-catalyzed Sonogashira coupling reactions. One target pentad, three tetrads, four triads, and four monomeric benchmark compounds were synthesized from six building blocks (three new, three reported) and 10 new synthetic intermediates. Four of the tetrapyrrole-containing arrays are zinc chelated, and four others are in the free base form. Absorption and fluorescence spectra and fluorescence quantum yields were also measured. Collectively, investigations of the arrays reveal insights into principles for the design of novel reaction centers integrated with a panchromatic antenna for artificial photosynthetic studies.

Phyllobilins, derived from the breakdown of chlorophyll in plants, are acyclic tetrapyrroles that bear the signature ring E of the parent macrocycle. Few synthetic studies have concerned members of this emergent class of natural products, which now number >70. A prior synthesis of southern rim structures relied on Knoevenagel and Nazarov reactions to construct a dipyrromethane bearing the annulated ring E. Here, the synthesis has been extended to linear tetrapyrroles containing ring E as well as an appended dicyanovinyl unit. The latter serves as an optical marker with clearly observable absorption near 400 nm, rendering all compounds yellow, orange or red, thereby facilitating isolation and reaction monitoring. Knoevenagel condensation was carried out with a DC half (gem-dimethyl-substituted dihydrodipyrrin-1-carboxaldehyde bearing a 9-carboethoxy group) and an AB half (dipyrromethane equipped with a β-ketoester at the 8-position and the dicyanovinyl group at the 1-position). The resulting propenone is blocked by the ester and dicyanovinyl groups from reactions at the respective pyrrolic termini. Subsequent Yb(OTf)3-catalyzed Nazarov reaction accompanied by unexpected prototropic rearrangement afforded the isolated phyllobilin (8) wherein conjugation shifted from the DC dihydrodipyrrin moiety to the CB southern rim, thereby resembling the conjugation of a native phyllolumibilin. The expected direct products of Nazarov cyclization, the cis and trans products (pre-8-cis , pre-8-trans) due to the configuration of substituents in ring E, were not observed. Calculations by density functional theory showed the stability order as 8 > pre-8-trans > pre-8-cis. Access to phyllobilin analogues may open opportunities for fundamental studies pertaining to the native constituents, which are germane to plant sciences, nutrition, and ecology. [ABSTRACT FROM AUTHOR]

Enzyme-instructed self-assembly (EISA) has been explored for many applications in the life sciences including imaging and therapeutics. To date, use of the strategy is limited by the relatively high concentration – often in the mM regime – required for assembly. Here, a porphyrin–peptide conjugate (1) was designed as a water-soluble, EISA substrate. Compound 1 is built around a trans-AB porphyrin chassis that is equipped with a monodisperse 1 kDa polyethylene glycol (PEG) linker (a surrogate for a potential targeting agent) and an enzymatically triggerable self-assembly motif. The latter is comprised of two identical self-assembly peptides (also known as hydrogelators) attached to the 2,6-positions of an aryl group. The peptides have sequence GffY and in protected form contain tyrosine as an O-phosphoester GffY(p). Treatment of 1 with alkaline phosphatase (ALP) gave rise to the self-assembly (or aggregation) process as characterized by absorption spectroscopy. A control compound without the phosphoester trigger was synthesized and examined for the self-assembly process using fluorescence and absorption spectroscopy. A concentration-dependent study showed that at 10 nM approximately 50% of porphyrin–peptide conjugate was in the aggregated form. The nanomolar assembly suggests possible applications of EISA substrates in chemotherapy, although a further decrease in concentration to the picomolar regime may be required for use in targeted molecular radiotherapy, where very low mass dosing is typical. [ABSTRACT FROM AUTHOR]

Molecular designs that achieve solubility of porphyrins in aqueous media are attractive for diverse applications. The presence of 4-sulfophenyl or 4-N-methylpyridinium groups at the four meso-positions is effective, but the use of fewer substituents is desirable for custom tailoring. Here, five target porphyrins (along with selected copper or zinc chelates) were prepared bearing PEG groups to understand how distinct designs affect aqueous solubility (where "PEG" refers to an oligoethylene glycol unit). One objective was to employ only one or two pegylated meso-aryl groups so that other meso-positions would be open for synthetic elaboration while retaining a compact structure. The key design features examined include (i) 2,6- versus 3,5-dipegylated aryl groups; (ii) one versus two 2,6-dipegylated aryl groups; (iii) a nonpolar versus ionizable terminus of the PEG moiety; and (iv) length of the PEG moiety. In each case, the PEG groups were attached on a porphyrin scaffold bearing one or two bis(2-propynyloxy)aryl groups. Assessment entailed octanol–aqueous solution partitioning (log P values) and aggregation over the concentration range of 0.2–200 μM. A trans-A2 free base porphyrin bearing two 2,6-dialkoxyphenyl groups equipped with methyl-terminated PEG6 groups gave ∼3 : 1 partitioning in water versus octanol but high overall solubility (at least 12 mM) in water alone; the analogous porphyrin with carboxylic acid-terminated PEG6 groups gave >100 : 1 partitioning in phosphate-buffered saline (PBS, pH 7.4). A trans-AB porphyrin bearing a single 2,6-dialkoxyphenyl group equipped with carboxylic acid-terminated PEG6 groups gave 43 : 1 partitioning in PBS versus octanol but was prone to self-aggregation at 20–200 μM in PBS alone. The results point to new molecular designs of potential value in the life sciences. [ABSTRACT FROM AUTHOR]

The development of chromophores that absorb in the near-infrared (NIR) region beyond 1000 nm underpins numerous applications in medical and energy sciences, yet also presents substantial challenges to molecular design and chemical synthesis. Here, the core bacteriochlorin chromophore of nature's NIR absorbers, bacteriochlorophylls, has been adapted and tailored by annulation in an effort to achieve absorption in the NIR-II region. The resulting bacteriochlorin, Phen2,1-BC, contains two annulated naphthalene groups spanning meso,β-positions of the bacteriochlorin and the 1,2-positions of the naphthalene. Phen2,1-BC was prepared via a new synthetic route. Phen2,1-BC is an isomer of previously examined Phen-BC, which differs only in attachment via the 1,8-positions of the naphthalene. Despite identical π-systems, the two bacteriochlorins have distinct spectroscopic and photophysical features. Phen-BC has long-wavelength absorption maximum (912 nm), oscillator strength (1.0), and S1 excited-state lifetime (150 ps) much different than Phen2,1-BC (1292 nm, 0.23, and 0.4 ps, respectively). These two molecules and an analogue with intermediate characteristics bearing annulated phenyl rings have unexpected properties relative to those of non-annulated counterparts. Understanding the distinctions requires extending concepts beyond the four-orbital-model description of tetrapyrrole spectroscopic features. In particular, a reduction in symmetry resulting from annulation results in electronic mixing of x- and y-polarized transitions/states, as well as vibronic coupling that together reduce oscillator strength of the long-wavelength absorption manifold and shorten the S1 excited-state lifetime. Collectively, the results suggest a heuristic for the molecular design of tetrapyrrole chromophores for deep penetration into the relatively unutilized NIR-II region. [ABSTRACT FROM AUTHOR]

The photosynthetic tetrapyrroles and their catabolic products (phyllobilins) share similar structural features in the southern rim encompassing rings C–E with variation in the degree of saturation including a dihydrodipyrrin (chlorophyll a and bacteriochlorophyll a), a dipyrromethane (phylloleucobilin), and a dipyrrin (chlorophyll c). Here, a direct, two-step route to southern rim structures was examined. The Knoevenagel condensation of a pyrrole bearing a β-ketoester at the 3-position with a pyrrole-2-carboxaldehyde formed the dipyrrole-substituted propenone, which upon Nazarov cyclization gave the dipyrromethane-type southern rim containing the annulated, isocyclic ring. In this manner, three southern rim structures (51–80%) were obtained that vary in the nature of the pyrrole substituents (H, methyl, carbomethoxy). Characterization by 1H NMR spectroscopy in each case revealed an expected mixture of trans/cis isomers (∼9 : 1 ratio). Several derivatization processes (pyrrole acylation with trichloroacetic anhydride, ketone reduction with borohydrides) were examined. Analysis of one southern rim compound by single-crystal X-ray diffraction showed the trans configuration for the expected pair of enantiomers in the unit cell; a second compound gave the trans enantiomers as a conglomerate. The Knoevenagel–Nazarov route from relatively accessible pyrroles may provide new scaffolds for constructing tetrapyrrole architectures. [ABSTRACT FROM AUTHOR]