Free-base Analogue Research Articles

Singlet Oxygen Generation in Peripherally Modified Platinum and Palladium Porphyrins: Effect of Triplet Excited State Lifetimes and meso-Substituents on 1 O2 Quantum Yields.

A series of meso-substituted with aromatic (=tolyl, pyrenyl, fluorenyl, naphthyl, and triphenylamine) substituents, platinum (Pt), and palladium (Pd) porphyrins have been synthesized and characterized by spectroscopic and single-crystal X-ray diffraction studies to probe structure-reactivity aspects on the electrochemical redox potentials, and phosphorescence quantum yields and lifetimes. In the X-ray structures, the aromatic meso-substituents were rotated to some extent from the planarity of the porphyrin ring to minimize steric hindrance. Both Pt and Pd porphyrins revealed higher electrochemical redox gaps as compared to their free-base porphyrin analogs as a result of the harder oxidation and reduction processes. The ability of both Pt and Pd porphyrins to generate singlet oxygen was probed by monitoring the photoluminescence of 1 O2 at 1270 nm. Higher quantum yields for both triplet sensitizers compared to their free-base analogs were witnessed. Singlet oxygen quantum yields close to unity were possible to achieve in the case of Pt and Pd porphyrins bearing triphenylamine substituents at the meso-position. The present study brings out the importance of different meso-substituents on the triplet porphyrin sensitizers in governing singlet oxygen quantum yields; a key property of photosensitizers needed for photodynamic therapy, chemical synthesis, and other pertinent applications.

Crystal Structure, Hirschfeld Surface Analysis, and Absorption Spectra of a New Polymorph of Highly Symmetric Tetraphenylporphyrinato Zn(II), ZnTPP

The crystal and molecular structure of a new polymorph of four-coordinated Zn(II) tetraphenylporphyrin, with formula ZnTPP (TPP = 5,10,15,20-tetraphenyl porphyrin) has been determined from 3D X-ray diffraction data. The new polymorph crystallizes in the tetragonal space group, I4/m with cell dimensions a = 13.4206(9), c = 9.7210(16) A, V = 1750.9(3) A3, and Z = 4. The previous polymorph was crystallized in triclinic space group, P $$\bar {1}$$ . The single crystal of toluene solvate, a pseudopolymorph of tetraphenylporphinatozinc(II), ZnTPP ⋅ 2C7H8 was also crystallized in the triclinic space group, P $$\bar {1}$$ . The porphyrin cores of all the three centrosymmetric crystalline forms adopt a planar conformation which is attributed to the equivalent size of Zn2+ cations and that of porphyrin cores. In contrast to the two previous triclinic structures, which have two crystallography unique pyrrole rings, the tetragonal structure in the present form has all the pyrrole rings symmetric in nature. Further the symmetric orientations of phenyl rings about the mean N4 plane in the present form, unlike the two previous triclinic forms in which phenyl rings makes two types of dihedral angles with mean N4 plane reflects that the tetragonal structure of ZnTPP is the highly ordered geometry. All the four Zn–N bond distances in the present form are equal i.e., 2.048(2) A and is slightly longer than the average Zn–N bond distances found in the previous triclinic forms. The 3D packing of present form features sandwich type or face to face stacked layers that are bridged by a body centred molecule. The crystal packing is mainly stabilised by C–H⋅⋅⋅C and H⋅⋅⋅H intermolecular forces. Further the role and nature of various intermolecular interactions were quantitatively analysed by 3D Hirschfeld surface analysis and associated 2D fingerprint plots. The optical absorption spectra of ZnTPP and its free base analog, H2TPP have been further analysed in pure organic solvents of CH2Cl2 and CH3OH at different ionic strengths of the media and their results have been discussed in detail.

Photophysical Properties and Electronic Structure of Zinc(II) Porphyrins Bearing 0-4 meso-Phenyl Substituents: Zinc Porphine to Zinc Tetraphenylporphyrin (ZnTPP).

Six zinc(II) porphyrins bearing 0-4 meso-phenyl substituents have been examined spectroscopically and theoretically. Comparisons with previously examined free base analogues afford a deep understanding of the electronic and photophysical effects of systematic addition of phenyl groups in porphyrins containing a central zinc(II) ion versus two hydrogen atoms. Trends in the wavelengths and relative intensities of the absorption bands are generally consistent with predictions from time-dependent density functional theory calculations and simulations from Gouterman's four-orbital model. These trends derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital. The calculations reveal additional insights, such as a progressive increase in oscillator strength in the violet-red (B-Q) absorption manifold with increasing number of phenyls. Progressive addition of 0-4 phenyl substituents to the zinc porphyrins in O2-free toluene engenders a reduction in the measured lifetime of the lowest singlet excited state (2.5-2.1 ns), an increase in the S1 → S0 fluorescence yield (0.022-0.030), a decrease in the yield of S1 → T1 intersystem crossing (0.93-0.88), and an increase in the yield of S1 → S0 internal conversion (0.048-0.090). The derived rate constants for S1 decay reveal significant differences in the photophysical properties of the zinc chelates versus free base forms. The unexpected finding of a larger rate constant for internal conversion for zinc chelates versus free bases is particularly exemplary. Collectively, the findings afford fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins, which are widely used as benchmarks for tetrapyrrole-based architectures in solar energy and life sciences research.

Synthesis and spectroscopic properties of axial phenoxide and para amino phenoxide incorporated indium (III) porphyrins

Tetrakis-4-methoxyphenyl indium(III) porphyrins containing phenolic and para amino phenolic moieties in the apical sites have been successfully prepared and characterized spectroscopically. The phases and structure evolutions of prepared porphyrins were deduced from powder X-ray diffraction (PXRD). The axial coordination of phenolic moieties induces slight shift of the 1H NMR signals of indium porphyrin macrocycles. The measurements of UV–Visible absorptions of porphyrins in CH2Cl2 and CH3COOC2H5 reflects that axially ligated metal chelates at neutral pH display hyperchromicity of absorption bands compared to unligated metal porphyrin. The most striking feature was the structural stability of indium porphyrins under basic (pH = 10) as well as acidic media (pH = 4). On the other hand, the metal free ligand 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP (I) undergoes J type aggregation in acidic media (pH = 4). The robustness of indium porphyrins especially in acidic media are in strong contrary to other sitting atop (SAT) or out of plane (OOP) complexes which are usually demetallized under acidic pH. All the prepared macro rings exhibit negative solvatochromism reflected by the blue shift of absorption bands in CH3COOC2H5 (more polar) than in CH2Cl2 (less polar). The photoluminescence emissions in indium(III) species are significantly quenched relative to metal free ligand (I) due to internal heavy atom effect and distortion of ligand plane. The quenching of fluorescence bands are more pronounced in axially ligated metal porphyrins. The lesser excited state lifetimes of indium(III) porphyrins relative to free base analog (I) may be ascribed to spin orbit coupling induced by indium(III) ion in the respective metal chelates.

Crystallographic elucidations of indium(III) porphyrin conformations, morphology and aggregation behaviour: Comparative optical study of free base porphyrins and their indium(III) derivatives at varying pH

The supramolecular structural architectures of 5, 10, 15, 20-Tetrakis-(4-methoxyphenylporphyrinato) indium (III) chloride (I) and 5, 10, 15, 20-Tetrakis-(4-methylphenylporphyrinato) indium (III) chloride (II) have been determined from their three dimensional X-ray diffraction data. Complex I crystallizes in the orthorhombic space group, Pbca with a = 23.0083 (4) Å, b = 15.2447 (3) Å, c = 23.3737 (4) Å, α = β = γ = 90°, V = 8198.5 (3) Å3 and Z = 8, whereas complex II crystallizes in the triclinic space group, P-1 with a = 12.0737 (5) Å, b = 12.1000 (5) Å, c = 14.1682 (6) Å, α = 88.560 (3)°, β = 78.730 (3)°, γ = 78.280 (3)° V = 1987.40 (14) Å3 and Z = 2. The geometries around In+3 ions in both the complexes are square pyramidal giving dome shaped architectures to the metalloporphyrins. In their respective crystal lattices, the monomeric units of complex I and II are packed in J-type (head-to-tail) supramolecular aggregation. These monomers are linked into their three dimensional supramolecular networks by various non covalent forces. Further the packing of monomers of each complex in their respective unit cells leaves significant void volumes (porosities). The optical absorption spectra of complex I and II and their free base analogues i.e. 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP and 5, 10, 15, 20-Tetrakis-(4-methylphenyl) porphyrin, H2TTP have also been investigated in CH2Cl2 at different ionic strengths of the media. The investigations revealed that both the free bases exist as simple monomers, j-aggregates and dianionic monomers at neutral (pH = 7), acidic (pH = 4 and lower) and basic (pH = 10) media conditions. Also at high ionic strength of the media, the free bases undergo charge transfer electronic transitions. The two In3+ porphyrin exists as simple monomers at pH = 7. However at pH = 5 and lower, the In3+ ions in complex II is remarkably easily replaced by the protons of the acid to generate the corresponding protonated free base monomers which simultaneously aggregate in J-type stacking mode. In contrast to complex II that exhibits low stability in acidic media, the complex I remain intact in such media reflecting extremely high stability of the complex. The insertion of In3+ ions into the porphyrinic cores of free bases causes bathochromic shift followed by increase in the intensities of absorption bands. The appearance of two set of doublets for ortho phenyl protons in the 1H NMR spectra of each complex is attributed to the magnetic inequivalence of their ortho phenyl protons caused by the pulling of In+3 ions above the mean porphyrin planes.

Towards tuning PDT relevant photosensitizer properties: comparative study for the free and Zn2+ coordinated meso-tetrakis[2,6-difluoro-5-(N-methylsulfamylo)phenyl]porphyrin

The spectroscopic, photochemical, and biological studies of 5,10,15,20-tetrakis[2,6-difluoro-5(N-methylsulfamylo)phenyl]porphyrinate Zn(II) (ZnF2PMet) were carried out including absorption and fluorescence spectra, fluorescence quantum yields, triplet absorption spectra, triplet lifetimes, singlet oxygen quantum yield, and reactive oxygen species (ROS) detection under biological conditions and compared with its free-base analog (F2PMet). Zinc coordination into the porphyrin ring results in decrease of hydrophobicity and in higher cellular uptake. F2PMet localized specifically in endoplasmic reticulum and mitochondria while the ZnF2PMet is more diffused all over the cell, bonded to membrane proteins, as assessed by fluorescence microscopy. Zn-porphyrin exhibits greater singlet oxygen quantum yield than its free-base analog. Studies with fluorescent probes confirm that the ZnF2PMet produces mostly singlet oxygen, whereas F2PMet generates more hydroxyl radicals as the ROS. F2PMet is a more effective photosensitizer in vitro than its zinc complex, thus, the final photodynamic effect depends more on the nature of ROS than on the higher cellular uptake.

An in vitro enzymatic assay to measure transcription inhibition by gallium(III) and H3 5,10,15-tris(pentafluorophenyl)corroles.

Chemotherapy often involves broad-spectrum cytotoxic agents with many side effects and limited targeting. Corroles are a class of tetrapyrrolic macrocycles that exhibit differential cytostatic and cytotoxic properties in specific cell lines, depending on the identities of the chelated metal and functional groups. The unique behavior of functionalized corroles towards specific cell lines introduces the possibility of targeted chemotherapy. Many anticancer drugs are evaluated by their ability to inhibit RNA transcription. Here we present a step-by-step protocol for RNA transcription in the presence of known and potential inhibitors. The evaluation of the RNA products of the transcription reaction by gel electrophoresis and UV-Vis spectroscopy provides information on inhibitive properties of potential anticancer drug candidates and, with modifications to the assay, more about their mechanism of action. Little is known about the molecular mechanism of action of corrole cytotoxicity. In this experiment, we consider two corrole compounds: gallium(III) 5,10,15-(tris)pentafluorophenylcorrole (Ga(tpfc)) and freebase analogue 5,10,15-(tris)pentafluorophenylcorrole (tpfc). An RNA transcription assay was used to examine the inhibitive properties of the corroles. Five transcription reactions were prepared: DNA treated with Actinomycin D, triptolide, Ga(tpfc), tpfc at a [complex]:[template DNA base] ratio of 0.01, respectively, and an untreated control. The transcription reactions were analyzed after 4 hr using agarose gel electrophoresis and UV-Vis spectroscopy. There is clear inhibition by Ga(tpfc), Actinomycin D, and triptolide. This RNA transcription assay can be modified to provide more mechanistic detail by varying the concentrations of the anticancer complex, DNA, or polymerase enzyme, or by incubating the DNA or polymerase with the complexes prior to RNA transcription; these modifications would differentiate between an inhibition mechanism involving the DNA or the enzyme. Adding the complex after RNA transcription can be used to test whether the complexes degrade or hydrolyze the RNA. This assay can also be used to study additional anticancer candidates.

Zinc porphyrin-Re(I) bipyridyl-fullerene triad: synthesis, characterization, and kinetics of the stepwise electron-transfer processes initiated by visible excitation.

A new triad system featuring one zinc porphyrin and one fullerene moieties attached to a central redox-active Re(I) connector was obtained in remarkable yield by cleverly exploiting a facile two-step synthesis. Detailed description and discussion on the characterization of this multicomponent system and of its parent free-base analogue are presented, along with a kinetic study of the stepwise electron-transfer processes occurring upon visible excitation.

Co-Sensitization of Zinc and Free-Base Porphyrins with an Organic Dye for Efficient Dye-Sensitized Solar Cells

Co-sensitization of an efficient zinc porphyrin sensitizer (LD14) with its free-base analogue (H2LD14) and an organic dye (AN-3) improves the efficiency of the dye-sensitized solar cells. The superior performance of the co-sensitized system is attributed to complementary absorptions of the three dyes, resulting in panchromatic incident photon-to-current conversion efficiency responses at the expense of only slightly lowered open-circuit voltage.

"Spider"-shaped porphyrins with conjugated pyridyl anchoring groups as efficient sensitizers for dye-sensitized solar cells.

Two novel "spider-shaped" porphyrins, meso-tetraaryl-substituted 1PV-Por and zinc-metalated 1PV-Zn-Por, bearing four oligo(p-phenylenevinylene) (oPPV) pyridyl groups with long dodecyloxy chains on the phenyl groups, have been synthesized. The presence of four pyridyl groups in both porphyrins, which allow them to act as anchoring groups upon coordination to various Lewis acid sites, the conjugated oPPV bridges, which offer the possibility of electronic communication between the porphyrin core and the pyridyl groups, and the dodecyloxy groups, which offer the advantage of high solubility in a variety of organic solvents of different polarities and could prevent porphyrin aggregation, renders porphyrins 1PV-Por and 1PV-Zn-Por very promising sensitizers for dye-sensitized solar cells (DSSCs). Photophysical measurements, together with electrochemistry experiments and density functional theory calculations, suggest that both porphyrins have frontier molecular orbital energy levels that favor electron injection and dye regeneration in DSSCs. Solar cells sensitized by 1PV-Por and 1PV-Zn-Por were fabricated, and it was found that they show power conversion efficiencies (PCEs) of 3.28 and 5.12%, respectively. Photovoltaic measurements (J-V curves) together with incident photon-to-electron conversion efficiency spectra of the two cells reveal that the higher PCE value of the DSSC based on 1PV-Zn-Por is ascribed to higher short-circuit current (Jsc), open-circuit voltage (Voc), and dye loading values. Emission spectra and electrochemistry experiments suggest a greater driving force for injection of the photogenerated electrons into the TiO2 conduction band for 1PV-Zn-Por rather than its free-base analogue. Furthermore, electrochemical impedance spectroscopy measurements prove that the utilization of 1PV-Zn-Por as a sensitizer offers a high charge recombination resistance and, therefore, leads to a longer electron lifetime.

Novel Lipophilic Lanthanide Bis-Phthalocyanines Functionalized by Pentadecylphenoxy Groups: Synthesis, Characterization and UV-Photostability

Novel sandwich-type phthalocyanines containing a rare earth metal core (Pr, Nd, Eu–Lu) and macrocycles peripherally substituted by pentadecylphenoxy groups were synthesized using a cardanol-based phthalonitrile precursor and the respective lanthanide acetate. Additionally, the metal free-base analog compound was studied for comparison. The purified reaction products were all found to be thick and viscous substances at room temperature, showing liquid crystalline behavior with a distinct increase in fluidity at ca. 40 °C. The complexes are readily soluble in chloroalkyl solvents and dissolve fairly well in DMF with some tendency to form aggregates. Besides they are strongly hydrophobic and reveal a peculiar affinity for lipophilic media. The compounds have been characterized by UV-Vis (absorption and emission), FTIR, MS and DSC methods. Photochemical activity in the liquid phase (dimethylformamide, dichloromethane, mineral oil) and the degree of photodegradation demonstrated under constant UV-irradiation (λ = 352 nm) have been analyzed and discussed in terms of photostability.

Conformational and Electronic Consequences in Crafting Extended, π-Conjugated, Light-Harvesting Macrocycles

The synthesis of a new series of free-base, Ni(II) and Zn(II) 2,3,12,13-tetra(ethynyl)-5,10,15,20-tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3-diethynyl-5,20-diphenylpiceno[10,11,12,13,14,15-jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free-base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully-cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15-fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc-pVTZ(-f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol(-1) higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin-enediynes with ethene- and H-functionality substitutions at the meso-phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol(-1) in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.

Synthesis and Photophysical Characterization of Stable Indium Bacteriochlorins

Bacteriochlorins have wide potential in photochemistry because of their strong absorption of near-infrared light, yet metallobacteriochlorins traditionally have been accessed with difficulty. Established acid-catalysis conditions [BF(3)·OEt(2) in CH(3)CN or TMSOTf/2,6-di-tert-butylpyridine in CH(2)Cl(2)] for the self-condensation of dihydrodipyrrin-acetals (bearing a geminal dimethyl group in the pyrroline ring) afford stable free base bacteriochlorins. Here, InBr(3) in CH(3)CN at room temperature was found to give directly the corresponding indium bacteriochlorin. Application of the new acid catalysis conditions has afforded four indium bacteriochlorins bearing aryl, alkyl/ester, or no substituents at the β-pyrrolic positions. The indium bacteriochlorins exhibit (i) a long-wavelength absorption band in the 741-782 nm range, which is shifted bathochromically by 22-32 nm versus the analogous free base species, (ii) fluorescence quantum yields (0.011-0.026) and average singlet lifetime (270 ps) diminished by an order of magnitude versus that (0.13-0.25; 4.0 ns) for the free base analogues, and (iii) higher average yield (0.9 versus 0.5) yet shorter average lifetime (30 vs 105 μs) of the lowest triplet excited state compared to the free base compounds. The differences in the excited-state properties of the indium chelates versus free base bacteriochlorins derive primarily from a 30-fold greater rate constant for S(1) → T(1) intersystem crossing, which stems from the heavy-atom effect on spin-orbit coupling. The trends in optical properties of the indium bacteriochlorins versus free base analogues, and the effects of 5-OMe versus 5-H substituents, correlate well with frontier molecular-orbital energies and energy gaps derived from density functional theory calculations. Collectively the synthesis, photophysical properties, and electronic characteristics of the indium bacteriochlorins and free base analogues reported herein should aid in the further design of such chromophores for diverse applications.

Spin−Spin Interactions in Porphyrin-Based Monoverdazyl Radical Hybrid Spin Systems

The spin-spin interactions in a complex consisting of a metalloporphyrin with a verdazyl radical attached at one of the beta positions of the porphyrin ring are investigated. The X-ray crystal structure of the copper porphyrin complex shows that the plane of the verdazyl moiety is oriented such that it is nearly perpendicular to the plane of the porphyrin ring so that weak magnetic interactions between the metal and radical are expected. Consistent with this expectation, magnetic susceptibility and continuous-wave electron paramagnetic resonance (EPR) measurements of the copper (d(9)) and vanadyl (d(1)) versions of the porphyrin show that the metal and radical are weakly antiferromagnetically coupled. Thus, the ground state is a singlet, but the triplet state is thermally accessible above approximately 5 K. Spin-polarized transient EPR measurements of the free-base analogue show that its lowest excited state is a quartet, indicating that the verdazyl radical couples ferromagnetically to the triplet excited state of the porphyrin. Low-temperature transient EPR measurements on the vanadyl porphyrin reveal that the lowest excited quintet state is populated. This implies that the antiferromagnetic coupling between the metal and radical observed in the ground state is switched to a ferromagnetic arrangement in the excited state by the presence of the unpaired electrons in the pi and pi* orbitals of the porphyrin.

Trans-2 Addition Pattern to Power Charge Transfer in Dendronized Metalloporphyrin C60 Conjugates

Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.

Regioselective Bromination Tactics in the de Novo Synthesis of Chlorophyll b Analogues

The ability to introduce substituents at designated sites about the perimeter of the chlorin or 13(1)-oxophorbine macrocycle is essential for fundamental studies related to chlorophylls. A chlorin is a dihydroporphyrin, whereas a 13(1)-oxophorbine is a chlorin containing an annulated oxopentano ring spanning positions 13 and 15. 13(1)-Oxophorbines bearing auxochromes at the 7-position of the macrocycle are valuable targets given their resemblance to chlorophyll a or b, which contains the 13(1)-oxophorbine skeleton and bears a 7-methyl or 7-formyl group, respectively. A rational route to 7-substituted 13(1)-oxophorbines was developed that relies on a new method for regioselective bromination. Under neutral conditions, a 13-acetyl-10-mesitylchlorin (FbC-M(10)A(13)) undergoes bromination (with 1 molar equiv of NBS in THF) both in ring B (7-position) and at the 15-position (42% versus 28% isolated yield), thereby thwarting installation of the isocyclic ring (ring E, spanning the 13-15 positions). Under acidic conditions (10% TFA in CH(2)Cl(2)), ring B is deactivated, and bromination occurs preferentially at the 15-position (87% yield). The capability for preferential 15-bromination is essential to install the isocyclic ring, after which bromination can be directed to the 7-position of ring B (neutral conditions, 86% yield). The ability to suppress bromination in ring B (under acidic media) has been exploited in syntheses of sparsely substituted analogues of chlorophyll b. The analogues contain a 7-substituent (acetyl, formyl, or TIPS-ethynyl), a 10-mesityl group, and the 18,18-dimethyl group as the only substituents in the 13(1)-oxophorbine skeleton. The three analogues exhibit absorption spectral features that closely resemble those of free base analogues of chlorophyll b. Taken together, the facile access to chlorins and 13(1)-oxophorbines bearing substituents at distinct sites should enable fundamental spectroscopic studies and diverse applications.

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.

Characterization of Porphyrins, Chlorins, and Bacteriochlorins Formed via Allomerization of Bacteriochlorophylla. Synthesis of Highly Stable Bacteriopurpurinimides and Their Metal Complexes

Allomerization of bacteriochlorophyll a (Bchl a) was studied under various reaction conditions. Bchl a on stirring with KOH/propanol produced an "unstable bacteriochlorin", which decomposed in acidic conditions to give a complex mixture containing bacteriopurpurin a as a principal component. The yields of other compounds varied and were found to be dependent on reaction condition. The structures of the isolated porphyrins, chlorins, and bacteriochlorins, related to Bchl a, were assigned on the basis of 1D, 2D NMR (ROESY), and mass spectroscopy analyses. The presence of fused anhydride rings in porphyrin, chlorin, and bacterichlorin systems showed a significant influence on their optical properties. Compared to bacteriochlorophyll a and bacteriopheophytin, the related structurally modified analogues, e.g., the bacteriopurpurin a, 13(1)/15(1)-N-alkyl isoimide, and the imide analogues were found to be more stable with a significant difference in spectroscopic properties. Bacteriochlorins containing anhydride, imide, or isoimide cyclic rings demonstrated a significant bathochromic shift of their Q bands in their electronic absorption spectra. Under basic conditions the formation of the 12-hydroxymethyl, 12-formyl, and 12-methylene analogues as byproducts from the 12-methyl-bacteriopurpurin-N-hexylimide could be due to subsequent oxidation of the vinylogous enolate intermediates. To investigate the effect of the central metal in the electronic spectra, the stable bacteriopurpurin-18-N-hexylimide was converted to a series of metal complexes [Zn(II), Cd(II), and Pd(II)] by following the direct or transmetalation approaches. Compared to the free-base analogue, these complexes showed a remarkable shift in their electronic absorption spectra.

Photoinduced electron transfer in a symmetrical diporphyrin–fullerene triad

Two triad molecules consisting of either two zinc, or two free-base porphyrins symmetrically joined to a fullerene via phenyleneethynylene-containing linkages have been synthesized, and their photochemistry investigated. In the zinc form of the triad, PZn–C60–PZn, excitation of a zinc porphyrin in 2-methyltetrahydrofuran solution is followed by photoinduced electron transfer to the fullerene with a time constant of 20 ps. The resulting PZn˙+–C60˙−–PZn charge-separated state is formed with a quantum yield of 98% and has a lifetime of 820 ps. The first excited singlet state of the free-base analog gives the P2H˙+–C60˙−–P2H charge-separated state with a time constant of 200 ps and a yield of 98%. The charge-separated state decays with a lifetime of 2.8 ns. The difference in the rates of photoinduced electron transfer is consistent with reaction in the normal region of the Marcus–Hush relationship of transfer rate and driving force, and charge recombination is consistent with Marcus–Hush inverted behavior. The presence of the two porphyrin electron donors in these triads enhances the absorption cross section for light collection, and the molecular framework employed could be used to prepare molecules with enhanced energy conversion or optoelectronic properties.

Spectral Diffusion Experiment with a Denatured Protein

Spectral diffusion broadening of cytochrome c carrying the free-base analogue of heme is investigated in its unfolded state and compared with the corresponding broadening in the native state. Spectral diffusion is much larger in the unfolded state, in comparison to the native state. Interestingly, the time law that governs spectral diffusion changes as the aging time increases, from a power-law behavior in the native state to an apparent logarithmic behavior in the unfolded state.