Sparsely substituted chlorins as core constructs in chlorophyll analogue chemistry. III. Spectral and structural properties

Fluorinated chlorin chromophores for red-light-driven CO2 reduction

The utilization of low-energy photons in light-driven reactions is an effective strategy for improving the efficiency of solar energy conversion. In nature, photosynthetic organisms use chlorophylls to harvest the red portion of sunlight, which ultimately drives the reduction of CO2. However, a molecular system that mimics such function is extremely rare in non-noble-metal catalysis. Here we report a series of synthetic fluorinated chlorins as biomimetic chromophores for CO2 reduction, which catalytically produces CO under both 630 nm and 730 nm light irradiation, with turnover numbers of 1790 and 510, respectively. Under appropriate conditions, the system lasts over 240 h and stays active under 1% concentration of CO2. Mechanistic studies reveal that chlorin and chlorinphlorin are two key intermediates in red-light-driven CO2 reduction, while corresponding porphyrin and bacteriochlorin are much less active forms of chromophores.

Tolyporphins-Exotic Tetrapyrrole Pigments in a Cyanobacterium-A Review

Tolyporphins were discovered some 30 years ago as part of a global search for antineoplastic compounds from cyanobacteria. To date, the culture HT-58-2, comprised of a cyanobacterium-microbial consortium, is the sole known producer of tolyporphins. Eighteen tolyporphins are now known-each is a free base tetrapyrrole macrocycle with a dioxobacteriochlorin (14), oxochlorin (3), or porphyrin (1) chromophore. Each compound displays two, three, or four open β-pyrrole positions and two, one, or zero appended C-glycoside (or -OH or -OAc) groups, respectively; the appended groups form part of a geminal disubstitution motif flanking the oxo moiety in the pyrroline ring. The distinct structures and repertoire of tolyporphins stand alone in the large pigments-of-life family. Efforts to understand the cyanobacterial origin, biosynthetic pathways, structural diversity, physiological roles, and potential pharmacological properties of tolyporphins have attracted a broad spectrum of researchers from diverse scientific areas. The identification of putative biosynthetic gene clusters in the HT-58-2 cyanobacterial genome and accompanying studies suggest a new biosynthetic paradigm in the tetrapyrrole arena. The present review provides a comprehensive treatment of the rich science concerning tolyporphins.

Four Routes to 3-(3-Methoxy-1,3-dioxopropyl)pyrrole, a Core Motif of Rings C and E in Photosynthetic Tetrapyrroles

The photosynthetic tetrapyrroles share a common structural feature comprised of a β-ketoester motif embedded in an exocyclic ring (ring E). As part of a total synthesis program aimed at preparing native structures and analogues, 3-(3-methoxy-1,3-dioxopropyl)pyrrole was sought. The pyrrole is a precursor to analogues of ring C and the external framework of ring E. Four routes were developed. Routes 1-3 entail a Pd-mediated coupling process of a 3-iodopyrrole with potassium methyl malonate, whereas route 4 relies on electrophilic substitution of TIPS-pyrrole with methyl malonyl chloride. Together, the four routes afford considerable latitude. A long-term objective is to gain the capacity to create chlorophylls and bacteriochlorophylls and analogues thereof by facile de novo means for diverse studies across the photosynthetic sciences.

Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin

SignificanceThe chemical reduction of unsaturated bonds occurs by hydrogenation with H2 as the reductant. Conversely, in biology, the unavailability of H2 engenders the typical reduction of unsaturated bonds with electrons and protons from different cofactors, requiring olefin hydrogenation to occur by proton-coupled electron transfer (PCET). Moreover, the redox noninnocence of tetrapyrrole macrocycles furnishes unusual PCET intermediates, including the phlorin, which is an intermediate in tetrapyrrole ring reductions. Whereas the phlorin of a porphyrin is well established, the phlorin of a chlorin is enigmatic. By controlling the PCET reactivity of a chlorin, including the use of a hangman functionality to manage the proton transfer, the formation of a chlorinphlorin by PCET is realized, and the mechanism for its formation is defined.

Photodynamic activity of Sn(IV) tetrathien-2-ylchlorin against MCF-7 breast cancer cells

A new readily-synthesized Sn(iv) tetraarylchlorin with thien-2-yl substituents (SnC) has been prepared and fully characterized by various spectroscopic techniques and its photophysical and photochemical properties, such as the singlet oxygen quantum yield (ΦΔ), fluorescence quantum yield (ΦF), triplet lifetime (τT) and photostability, have been evaluated. SnC has an unusually high ΦΔ value of 0.89 in DMF. Studies on the photodynamic activity against MCF-7 breast cancer cells exhibited a very low IC50 value of 0.9 μM and high phototoxicity (dark versus light) indices of >27.8 after irradiation with a 660 nm Thorlabs LED (280 mW cm-2). The results demonstrate that Sn(iv) tetraarylchlorins of this type are suitable candidates for further in-depth PDT studies.

Bioconjugatable synthetic chlorins rendered water-soluble with three PEG-12 groups via click chemistry

Chlorins provide many ideal features for use as red-region fluorophores but require molecular tailoring for solubilization in aqueous solution. A chlorin building-block bearing 18,18-dimethyl, 15-bromo and 10-[2,4,6-tris(propargyloxy)phenyl] substituents has been transformed via click chemistry with CH3(OCH2CH[Formula: see text]-N3 followed by Suzuki coupling with 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)propanoic acid, thereby installing a water-solubilization motif and a bioconjugatable handle, respectively. In toluene, [Formula: see text]-dimethylformamide (DMF) or water, the resulting facially encumbered free base chlorin exhibits characteristic chlorin absorption ([Formula: see text] [Formula: see text]412, 643 nm) and fluorescence ([Formula: see text] [Formula: see text]645 nm) spectra with only modest variation in fluorescence quantum yield ([Formula: see text] values (0.24, 0.25 and 0.19, respectively). The zinc chlorin derived therefrom exhibits similar spectral constancy ([Formula: see text] [Formula: see text]414 and 613 nm, [Formula: see text] [Formula: see text]616 nm) and [Formula: see text] 0.094, 0.10 and 0.086 in the three solvents. The results together indicate the viability of the molecular design and synthetic methodology to create red-region fluorophores for use in diverse applications.

Investigation of the demetallation of 10-aryl substituted synthetic chlorins under acidic conditions

The acidic demetallation of a series of sparsely substituted Zn(II) chlorins is reported. The chlorins were functionalized in the 10-position with substituents ranging from strongly electron donating mesityl and p-methoxyphenyl to electron-withdrawing p-nitrophenyl and pentafluorophenyl groups. The demetallation kinetics were investigated using UV-Visible absorption spectroscopy. Demetallation was carried out by exposing the metallochlorins dissolved in CH₂Cl₂ to an excess of trifluoroacetic acid. Reasonable correlation was found between the Hammett constant of the 10-substituent and the rate constant of the loss of the metal ion. The largest differences were observed between the p-methoxyphenyl and p-nitrophenyl-substituted Zn(II) chlorins, undergoing loss of Zn(II) with pseudo first order rate constants of 0.0789 × 10^-3 and 3.70 × 10^-3 min^-1, respectively. Taken together, these data establish the dramatic influence even subtle changes can have in altering the electronic properties of chlorins, which in turn impacts metallochlorin function.

Peppermint (Mentha piperita L.) extract effectively inhibits cytochrome P450 3A4 (CYP3A4) mRNA induction in rifampicin-treated HepG2 cells

Interaction between foods and drugs is an important consideration in pharmaceutical therapy. Therefore, here, we examined the suppressive effects of the extracts from seven edible herbs on the induction of CYP3A4 gene expression in rifampicin-treated HepG2 cells. We evaluated the structure and suppressive activity of the most effective active compound isolated from dried peppermint (Mentha piperita L.). The structure of the compound was identified as that of pheophorbide a based on spectroscopic data. It suppressed the induction of CYP3A4 mRNA expression by rifampicin in a dose-dependent manner. Quantitative high-performance liquid chromatography showed that 2 g of dry leaves 0.43 mg in one cup of peppermint tea. These findings demonstrate that pheophorbide a suppresses the induction of CYP3A4 mRNA expression in rifampicin-treated HepG2 cells. Pheophorbide is known to cause photosensitivity. However, the effective dose of pheophorbide a that had a suppressive effect was very low, indicating a high safety margin. Abbreviations: DAD: diode array detector; DMEM: Dulbecco's modified Eagle's medium; ELISA: enzyme-linked immunosorbent assay; HPLC: high-performance liquid chromatography; PCR: polymerase chain reaction; PXR: pregnane X receptor; CAR: constitutive androstane receptor; AHR: aryl hydrocarbon receptor; TLC: thin-layer chromatography.

Structure and conformation of photosynthetic pigments and related compounds. 15. Conformational analysis of chlorophyll derivatives – implications for hydroporphyrinsin vivo

Controlling the function of chlorophylls depends in part on their 3D conformation. The NSD program presents a powerful tool to identify the distortion modes in phytochlorins.

Symmetrical and Nonsymmetrical Meso-Meso Directly Linked Hydroporphyrin Dyads: Synthesis and Photochemical Properties

A series of a rigid meso-meso directly linked chlorin-chlorin, chlorin-bacteriochlorin, and bacteriochlorin-bacteriochlorin dyads, including free bases as well as Zn(II), Pd(II), and Cu(II) complexes, has been synthesized, and their absorption, emission, singlet oxygen (¹O₂) photosensitization, and electronic properties have been examined. Marked bathochromic shifts of the long-wavelength Q _y absorption band and increase in fluorescence quantum yields in dyads, in comparison to the corresponding monomers, are observed. Nonsymmetrical dyads (except bacteriochlorin-bacteriochlorin) show two distinctive Q _y bands, corresponding to the absorption of each dyad component. A nearly quantitative S₁-S₁ energy transfer between hydroporphyrins in dyads, leading to an almost exclusive emission of hydroporphyrin with a lower S₁ energy, has been determined. Several symmetrical and all nonsymmetrical dyads exhibit a significant reduction in fluorescence quantum yields in solvents of high dielectric constants; this is attributed to the photoinduced electron transfer. The complexation of one macrocycle by Cu(II) or Pd(II) enhances intersystem crossing in the adjacent, free base dyad component, which is manifested by a significant reduction in fluorescence and increase in quantum yield of ¹O₂ photosensitization.

Exploring function activated chlorins using MCD spectroscopy and DFT methods: design of a chlorin with a remarkably intense, red Q band

Fictive β-substituted fluorene-thiophene Zn chlorins exhibit exceptionally intense, red Q bands with unusually small ΔHOMO.

Furan- and Thiophene-Based Auxochromes Red-shift Chlorin Absorptions and Enable Oxidative Chlorin Polymerizations

The de novo syntheses of chemically stable chlorins with five-membered heterocyclic (furane, thiophene, formylfurane and formylthiophene) substituents in selected meso- and β-positions are reported. Heterocycle incorporation in the 3- and 13-positions shifted the chlorin absorption and emission to the red (up to λem =680 nm), thus these readily incorporated substituents function analogously to auxochromes present in chlorophylls, for example, formyl and vinyl groups. Photophysical, theoretical and X-ray crystallographic experiments revealed small but significant differences between the behavior of the furan- and the thiophene-based auxochromes. Four regioisomeric bis-thienylchlorins (3,10; 3,13, 3,15 and 10,15) were oxidatively electropolymerized; the chlorin monomer geometry had a profound impact on the polymerization efficiency and the electrochemical properties of the resulting material. Chemical co-polymerization of 3,13-bis-thienylchlorin with 3-hexylthiophene yielded an organic-soluble red-emitting polymer.

Bioconjugatable, PEGylated Hydroporphyrins for Photochemistry and Photomedicine. Narrow-Band, Red-Emitting Chlorins

Chromophores that absorb and emit in the red spectral region (600-700 nm), are water soluble, and bear a bioconjugatable tether are relatively rare yet would fulfill many applications in photochemistry and photomedicine. Here, three molecular designs have been developed wherein stable synthetic chlorins - analogues of chlorophylls - have been tailored with PEG groups for use in aqueous solution. The designs differ with regard to order of the installation (pre/post-formation of the chlorin macrocycle) and position of the PEG groups. Six PEGylated synthetic chlorins (three free bases, three zinc chelates) have been prepared, of which four are equipped with a bioconjugatable (carboxylic acid) tether. The most effective design for aqueous solubilization entails facial encumbrance where PEG groups project above and below the plane of the hydrophobic disk-like chlorin macrocycle. The chlorins possess strong absorption at ~400 nm (B band) and in the red region (Q_y band); regardless of wavelength of excitation, emission occurs in the red region. Excitation in the ~400 nm region thus provides an effective Stokes shift of >200 nm. The four bioconjugatable water-soluble chlorins exhibit Q_y absorption/emission in water at 613/614, 636/638, 698/700 and 706/710 nm. The spectral properties are essentially unchanged in DMF and water for the facially encumbered chlorins, which also exhibit narrow Q_y absorption and emission bands (full-width-at-half maximum of each <25 nm). The water-solubility was assessed by absorption spectroscopy over the concentration range ~0.4 μM - 0.4 mM. One chlorin was conjugated to a mouse polyclonal IgG antibody for use in flow cytometry with compensation beads for proof-of-principle. The conjugate displayed a sharp signal when excited by a violet laser (405 nm) with emission in the 620-660 nm range. Taken together, the designs described herein augur well for development of a set of spectrally distinct chlorins with relatively sharp bands in the red region.

Directly linked hydroporphyrin dimers

The synthesis and chemical, photophysical and electrochemical characterisation of directly meso–meso- or meso-β-linked hydroporphyrin (chlorin) dimers is reported.

Self-Assembled Light-Harvesting System from Chromophores in Lipid Vesicles

Lipid vesicles are used as the organizational structure of self-assembled light-harvesting systems. Following analysis of 17 chromophores, six were selected for inclusion in vesicle-based antennas. The complementary absorption features of the chromophores span the near-ultraviolet, visible, and near-infrared region. Although the overall concentration of the pigments is low (~1 μM for quantitative spectroscopic studies) in a cuvette, the lipid-vesicle system affords high concentration (≥10 mM) in the bilayer for efficient energy flow from donor to acceptor. Energy transfer was characterized in 13 representative binary mixtures using static techniques (fluorescence-excitation versus absorptance spectra, quenching of donor fluorescence, modeling emission spectra of a mixture versus components) and time-resolved spectroscopy (fluorescence, ultrafast absorption). Binary donor-acceptor systems that employ a boron-dipyrrin donor (S0 ↔ S1 absorption/emission in the blue-green) and a chlorin or bacteriochlorin acceptor (S0 ↔ S1 absorption/emission in the red or near-infrared) have an average excitation-energy-transfer efficiency (ΦEET) of ~50%. Binary systems with a chlorin donor and a chlorin or bacteriochlorin acceptor have ΦEET ∼ 85%. The differences in ΦEET generally track the donor-fluorescence/acceptor-absorption spectral overlap within a dipole-dipole coupling (Förster) mechanism. Substantial deviation from single-exponential decay of the excited donor (due to the dispersion of donor-acceptor distances) is expected and observed. The time profiles and resulting ΦEET are modeled on the basis of (Förster) energy transfer between chromophores relatively densely packed in a two-dimensional compartment. Initial studies of two ternary and one quaternary combination of chromophores show the enhanced spectral coverage and energy-transfer efficacy expected on the basis of the binary systems. Collectively, this approach may provide one of the simplest designs for self-assembled light-harvesting systems that afford broad solar collection and efficient energy transfer.

Progress Towards Synthetic Chlorins with Graded Polarity, Conjugatable Substituents, and Wavelength Tunability

Advances in chlorin synthetic chemistry now enable the de novo preparation of diverse chlorin-containing molecular architectures. Five distinct molecular designs have been explored here, including hydrophobic bioconjugatable (oxo)chlorins; a hydrophilic bioconjugatable chlorin; a trans-ethynyl/iodochlorin building block; a set of chlorins bearing electron-rich (methoxy, dimethylamino, methylthio) groups at the 3-position; and a set of ten 3,13-disubstituted chlorins chiefly bearing groups with extended π-moieties. Altogether 23 new chlorins (17 targets, 6 intermediates) have been prepared. The challenge associated with molecular designs that encompass the combination of "hydrophilic, bioconjugatable and wavelength-tunable" chiefly resides in the nature of the hydrophilic unit.

Effects of substituents on synthetic analogs of chlorophylls. Part 4: How formyl group location dictates the spectral properties of chlorophylls b, d and f

Photosynthetic organisms are adapted to light characteristics in their habitat in part via the spectral characteristics of the associated chlorophyll pigments, which differ in the position of a formyl group around the chlorin macrocycle (chlorophylls b, d, f) or no formyl group (chlorophyll a). To probe the origin of this spectral tuning, the photophysical and electronic structural properties of a new set of synthetic chlorins are reported. The zinc and free base chlorins have a formyl group at either the 2- or 3-position. The four compounds have fluorescence yields in the range 0.19-0.28 and singlet excited-state lifetimes of ca 4 ns for zinc chelates and ca 8 ns for the free base forms. The photophysical properties of the 2- and 3-formyl zinc chlorins are similar to those observed previously for 13-formyl or 3,13-diformyl chlorins, but differ markedly from those for 7-formyl analogs. Molecular-orbital characteristics obtained from density functional theory (DFT) calculations were used as input to spectral simulations employing the four-orbital model. The analysis has uncovered the key changes in electronic structure engendered by the presence/location of a formyl group at various macrocycle positions, which is relevant to understanding the distinct spectral properties of the natural chlorophylls a, b, d and f.

Synthesis and characterisation of lanthanide-hydroporphyrin dyads

Hydroporphyrin-linked lanthanide complexes with variations in tetrapyrrole structure, linker length and mode of attachment are reported.

NMR spectral properties of 16 synthetic bacteriochlorins with site-specific 13C or 15N substitution

The 1 H , 13 C , and 15 N nuclear magnetic resonance (NMR) spectral properties have been examined of a family of synthetic bacteriochlorins wherein each member incorporates a pair of 13 C or 15 N atoms. The atom locations span the inner core of the macrocycle: (1) 15 N at the 21,23- or 22,24-positions; (2) 13 C at the meso- (5,15- or 10,20-) positions; (3) 13 C at the pyrrole α-positions (1,11- or 4,14-positions); and (4) 13 C at the pyrroline α-positions (6,16- or 9,19-positions). Each bacteriochlorin lacks peripheral substituents other than a geminal dimethyl group at the 8- and 18-positions to preclude adventitious dehydrogenation. In total, eight free base and eight zinc bacteriochlorin isotopologs were examined to directly assign 1 H , 13 C and 15 N resonances of the macrocycle skeleton. Complete and unambiguous assignments, including those for all tertiary and quaternary carbons, were accomplished chiefly by direct inspection of 1D NMR spectra of each isotopolog. Coupling constants (1 H –1 H , 13 C –1 H , 15 N –1 H , 13 C –13 C and 15 N –13 C ), which are rarely reported for tetrapyrroles, also were extracted. The 1 H and 13 C chemical shifts were then compared to those of unsaturated analogs (chlorin, porphyrin) and natural bacteriochlorophylls. The comprehensive set of NMR spectroscopic properties of sparsely substituted bacteriochlorins provides valuable information for understanding substitution effects and aromaticity in structurally more elaborate counterparts.

Complexes of Chlorin e6 with Pluronics and Polyvinylpyrrolidone: Structure and Photodynamic Activity in Cell Culture

Polymeric carriers are extensively used in photodynamic therapy (PDT) for increase of efficacy of photosensitizers. Here, we report the influence of nine Pluronic copolymers on phototoxicity of chlorin e6 (Ce6), in particular 5- to 7-fold rise in the phototoxicity caused by hydrophilic Pluronics F127, F108, F68 and F87 and practically no influence on Ce6 of more hydrophobic polymers. The revealed value of 0.2 mg mL(-1) of Pluronic F127 concentration sufficient for half-of-maximal increase of Ce6 photodynamic activity proved to be close to 0.16 mg mL(-1) inherent in well-documented carrier poly(N-vinylpyrrolidone) (PVP). The dissociation constants of Ce6 complexes with Pluronic F127 and PVP that were estimated from UV spectra were 0.252 and 0.036 mg mL(-1) , respectively, indicating higher stability of Ce6 complex with PVP. According to the results of (1) H-NMR studies of Ce6 complexes, the porphyrin interacts not only with hydrophobic regions but also with hydrophilic sides of both polymers.

Synthesis of chlorin-sensitized near infrared-emitting lanthanide complexes

Lanthanide (Yb(3+), Nd(3+)) complexes equipped with red-absorbing hydroporphyrin (chlorin) antennae were synthesized and characterized. The syntheses are scalable, highly modular, and enable the introduction of different chlorins functionalized with a single reactive group (COOH or NH(2)). Absorption maxima were dependent on chlorin substitution pattern (monomeso aryl or dimeso aryl) and metalation state (free base or zinc chelate). The complexes benefit from dual chlorin (610-639 nm) and lanthanide (980 or 1065 nm for Yb- or Nd-complexes, respectively) emission in the biologically relevant red and near IR region of the spectrum.

meso-arylporpholactones and their reduction products

The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO(4)(-)-mediated oxidations of the corresponding meso-tetraaryl-2,3-dihydroxychlorins (7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2-oxachlorin (11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q(x) band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC-CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.

meso-Aryl-3-alkyl-2-oxachlorins

The formal replacement of a pyrrole moiety of meso-tetraarylporphyrin 1 by an oxazole moiety is described. The key step is the conversion of porpholactones 4 (prepared by a known two-step oxidation procedure from 1) by addition of alkyl Grignard reagent to form meso-tetraaryl-3-alkyl-2-oxachlorins 9 (alkyloxazolochlorins; alkyl = Me, Et, iPr). Hemiacetal 9 can be converted to an acetal, reduced to an ether, or converted to bis-alkyloxazolochlorins 11. The optical properties (UV-visible and fluorescence spectroscopy) are described. The chlorin-like optical properties of the alkyloxazolochlorins are compared to regular chlorins, such as 2,3-dihydroxychlorins and nonalkylated oxazolochlorins made by reduction from porpholactone 4. The conformations of the mono- and bis-alkylated 2-oxachlorins, as determined by single crystal X-ray diffractometry, are essentially planar, thus proving that their optical properties are largely due to their intrinsic electronic properties and not affected by conformational effects. The mono- and bis-3-alkyl-2-oxachlorins are a class of readily prepared and oxidatively stable chlorins.

Stepwise conversion of two pyrrole moieties of octaethylporphyrin to pyridin-3-ones: synthesis, mass spectral, and photophysical properties of mono and bis(oxypyri)porphyrins

Free-base octaethylporphyrin (OEP) was converted in two steps (beta,beta'-dihydroxylation and oxidative diol cleavage with concomitant aldol condensation) to the corresponding oxypyriporphyrin. This conversion was previously described to be applicable only to the Ni(II) complex of OEP. Modified diol cleavage conditions made this reaction sequence now applicable to free-base OEP. The single-crystal structure of the resulting free-base oxypyriporphyrin was determined, proving its near-perfect planarity. The reaction sequence can also be applied to oxypyriporphyrin itself, generating the unprecedented bacteriochlorin-type bis(oxypyri)porphyrin as two separable isomers. The ground-state (UV/Vis and fluorescence spectroscopies) and excited-state (transient triplet-triplet absorption, triplet lifetimes, and triplet EPR spectroscopy) photophysical properties of all chromophores are compared with those of OEP, chlorins, and oxochlorins. The pyridone-modified porphyrins possess unique spectroscopic signatures that distinguish them from regular porphyrins or chlorins. The presence of the pyridone moiety alters the ESI(+) collision-induced fragmentation properties of these oxypyriporphyrins only to a minor degree when compared with those of OEP or chlorins, confirming their stability.

Chlorin-bacteriochlorin energy-transfer dyads as prototypes for near-infrared molecular imaging probes: controlling charge-transfer and fluorescence properties in polar media

The photophysical properties of two energy-transfer dyads that are potential candidates for near-infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC-FbB and ZnC-FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2-dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of approximately (5-10 ps)(-1) and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Phif=0.19) and singlet excited-state lifetimes (tau approximately 5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited-state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC-FbB than for ZnC-FbB in a given solvent. For example, the Phif and tau values for FbC-FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC-FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge-transfer states, as assessed by ground-state redox potentials and supported by molecular-orbital energies derived from density functional theory calculations. Controlling the extent of excited-state quenching in polar media will allow the favorable photophysical properties of the chlorin-bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC-FbB, 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (lambdaf=760 nm), long bacteriochlorin excited-state lifetime (approximately 5.5 ns), and narrow (<or=20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity- and lifetime-imaging techniques.

Effects of substituents on synthetic analogs of chlorophylls. Part 2: Redox properties, optical spectra and electronic structure

The optical absorption spectra and redox properties are presented for 24 synthetic zinc chlorins and 18 free base analogs bearing a variety of 3,13 (beta) and 5,10,15 (meso) substituents. Results are also given for a zinc and free base oxophorbine, which contain the keto-bearing isocyclic ring present in the natural photosynthetic pigments such as chlorophyll a. Density functional theory calculations were carried out to probe the effects of the types and positions of substituents on the characteristics (energies, electron distributions) of the frontier molecular orbitals. A general finding is that the 3,13 positions are more sensitive to the effects of auxochromes than the 5,10,15 positions. The auxochromes investigated (acetyl>ethynyl>vinyl>aryl) cause a significant redshift and intensification of the Qy band upon placement at the 3,13 positions, whereas groups at the 5,10,15 positions result in much smaller redshifts that are accompanied by a decrease in relative Qy intensity. In addition, the substituent-induced shifts in first oxidation and reduction potentials faithfully track the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The calculations show that the LUMO is shifted more by substituents than the HOMO, which derives from the differences in the electron densities of the two orbitals at the substituent sites. The trends in the substituent-induced effects on the wavelengths and relative intensities of the major features (By, Bx, Qx, Qy) in the near-UV to near-IR absorption bands are well accounted for using Gouterman's four-orbital model, which incorporates the effects of the substituents on the HOMO-1 and LUMO+1 in addition to the HOMO and LUMO. Collectively, the results and analysis presented herein and in the companion paper provide insights into the effects of substituents on the optical absorption, redox and other photophysical properties of the chlorins. These insights form a framework that underpins the rational design of chlorins for applications encompassing photomedicine and solar-energy conversion.

Toward a general synthesis of chlorins

Recently, we described a new synthesis of C,D-ring symmetric chlorins 11, involving 2 + 2 condensation of bis-formyl-dihydrodipyrrins 9 with symmetrically substituted dipyrromethane diacids 10 (Method I). However, while versatile in many aspects, Method I was unsuited to the broader goal of synthesizing fully non-symmetric chlorins of general structure 15, which requires regioselective control over the reacting centers in the A,B- and C,D-ring components. In this paper, we describe four new 2 + 2 strategies that accomplish this differentiation (Methods II-V). Of these, Method V, which combines operational simplicity with moderate to high product yields, proved to be the most effective route, exploiting reactivity differences between the two formyl groups of A,B-rings 9 to impart excellent regioselectivity. Methods II-IV are also useful alternatives to Method V, although in some cases, the appropriately functionalized precursors are less readily available. All four approaches generate single regioisomers of diversely substituted chlorins, and in every case, the 2 + 2 condensation is accomplished in a simple, one-flask procedure without need for additives such as oxidizing agents or metals. Taken together, these methodologies provide expanded access to an array of chlorins for SAR studies that may advance the effectiveness of PDT and other applications.