Aluminium(iii) porphyrins as supramolecular building blocks

Cation-Induced Fibrillation of Microbial Glycolipid Biosurfactant Probed by Ion-Resolved In Situ SAXS

Biological amphiphiles are molecules with a rich phase behavior. Micellar, vesicular, and even fibrillar phases can be found for the same molecule by applying a change in pH or by selecting the appropriate metal ion. The rich phase behavior paves the way toward a broad class of soft materials, from carriers to hydrogels. The present work contributes to understanding the fibrillation of a microbial glycolipid, glucolipid G-C18:1, produced by Starmerella bombicola ΔugtB1 and characterized by a micellar phase at alkaline pH and a vesicular phase at acidic pH. Fibrillation and prompt hydrogelation is triggered by adding either alkaline earth, Ca²⁺, or transition metal, Ag⁺, Fe²⁺, Al³⁺, ions to a G-C18:1 micellar solution. A specifically designed apparatus coupled to a synchrotron SAXS beamline allows the performing of simultaneous cation- and pH-resolved in situ monitoring of the morphological evolution from spheroidal micelles to crystalline fibers, when Ca²⁺ is employed, or to wormlike aggregates, when Fe²⁺ or Al³⁺ solutions are employed. The fast reactivity of Ag⁺ and the crystallinity of Ca²⁺-induced fibers suggest that fibrillation is driven by direct metal-ligand interactions, while the shape transition from spheroidal to elongated micelles with Fe²⁺ or Al³⁺ rather suggest charge screening between the lipid and the hydroxylated cation species.

Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC[Formula: see text] to form a new series of donor–acceptor constructs. A 1:2 complex formation with ImC[Formula: see text] was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants [Formula: see text] and [Formula: see text] for step-wise addition of ImC[Formula: see text] to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M[Formula: see text], respectively. For [(TPA)4P]Co:ImC[Formula: see text], the measured [Formula: see text] values was found to be 6.48 × 104 M[Formula: see text], slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC[Formula: see text] indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC[Formula: see text], and electron transfer from Im1C[Formula: see text]* to cobalt(II) porphyrin resulting into the formation of PCo[Formula: see text]:ImC[Formula: see text] charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was [Formula: see text] 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC[Formula: see text] due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.

Decelerating Charge Recombination Using Fluorinated Porphyrins in N,N-Bis(3,4,5-trimethoxyphenyl)aniline-Aluminum(III) Porphyrin-Fullerene Reaction Center Models

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, BTMPA-Im→AlPorF_n-Ph-C₆₀ (n = 0, 3, 5), were constructed by linking the fullerene (C₆₀) and bis(3,4,5-trimethoxyphenyl)aniline (BTMPA) to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF₃, or AlPorF₅) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF₃), or 2,3,4,5,6-pentafluorophenyl (PhF₅) groups in its meso positions. The C₆₀ and BTMPA units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge separated state, BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•-, which lies energetically 1.50 eV above the ground state. Despite the fact that the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•- radical pair was found to be very different in each of them, that is, 1240, 740, and 56 ns for BTMPA-Im→AlPorF₅-Ph-C₆₀, BTMPA-Im→AlPorF₃-Ph-C₆₀, and BTMPA-Im→AlPor-Ph-C₆₀, respectively. These results clearly suggest that the charge recombination is an activated process that depends on the midpoint potential of the central aluminum(III) porphyrin (AlPorF_n).

Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY-aluminum(iii) porphyrin-fullerene supramolecular triads

Three, broad band capturing, vertically aligned supramolecular triads, R2-BDP-AlPorF3←Im-C60 [R = H, styryl (C2H2-Ph), C2H2-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph2 or BDP-TPA2), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3) and fullerene (C60) entities. The C60 and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C60 through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF3 dyad, selective excitation of BDP resulted in singlet-singlet energy transfer to AlPorF3 (kEnT = 1.0 × 1010 s-1). On the other hand, excitation of the AlPorF3 entity in the BDP-AlPorF3←Im-C60 triad revealed charge separation leading to the BDP-(AlPorF3)˙+-(C60)˙- charge separated state (kCS = 2.43 × 109 s-1). In the case of the Ph2-BDP-AlPorF3 dyad, energy transfer from 1AlPorF3* to 1(Ph2-BDP)* was witnessed (kEnT = 1.0 × 1010 s-1); however, upon assembling the supramolecular triad, (Ph2-BDP)-AlPorF3←Im-C60, electron transfer from 1AlPorF3* to C60 (kCS = 3.35 × 109 s-1), followed by hole shift (kHS = 1.00 × 109 s-1) to Ph2-BDP, was witnessed. Finally, in the case of the TPA2-BDP-AlPorF3←Im-C60 triad, only electron transfer leading to the (TPA2-BDP)˙+-AlPorF3←Im-(C60)˙- charge separated state, and no energy transfer, was observed. The facile oxidation of Ph2-BDP and TPA2-BDP compared to AlPorF3 in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.

Self-Assembled Ruthenium(II)Porphyrin-Aluminium(III)Porphyrin-Fullerene Triad for Long-Lived Photoinduced Charge Separation

A very efficient metal-mediated strategy led, in a single step, to a quantitative construction of a new three-component multichromophoric system containing one fullerene monoadduct, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin. The Al(III) monopyridylporphyrin component plays the pivotal role in directing the correct self-assembly process and behaves as the antenna unit for the photoinduced processes of interest. A detailed study of the photophysical behavior of the triad was carried out in different solvents (CH₂Cl₂, THF, and toluene) by stationary and time-resolved emission and absorption spectroscopy in the pico- and nanosecond time domains. Following excitation of the Al-porphyrin, the strong fluorescence typical of this unit was strongly quenched. The time-resolved absorption experiments provided evidence for the occurrence of stepwise photoinduced electron and hole transfer processes, leading to a charge-separated state with reduced fullerene acceptor and oxidized ruthenium porphyrin donor. The time constant values measured in CH₂Cl₂ for the formation of charge-separated state Ru-Al⁺-C₆₀^- (10 ps), the charge shift process (Ru-Al⁺-C₆₀^- → Ru⁺-Al-C₆₀^-), where a hole is transferred from Al-based to Ru-based unit (75 ps), and the charge recombination process to ground state (>5 ns), can be rationalized within the Marcus theory. Although the charge-separating performance of this triad is not outstanding, this study demonstrates that, using the self-assembling strategy, improvements can be obtained by appropriate chemical modifications of the individual molecular components.

Self-Assembly of Russian Doll Concentric Porphyrin Nanorings

Electronic communication between concentric macrocycles with wave functions that extend around their circumferences can lead to remarkable behavior, as illustrated by multiwalled carbon nanotubes and photosynthetic chlorophyll arrays. However, it is difficult to hold one π-conjugated molecular ring inside another. Here, we show that ring-in-ring complexes, consisting of a 6-porphyrin ring locked inside a 12-porphyrin ring, can be assembled by placing different metals in the two rings (zinc and aluminum). A bridging ligand with carboxylate and imidazole binding sites forms spokes between the two rings, resulting in a highly cooperative supramolecular self-assembly process. Excitation is transferred from the inner 6-ring to the outer 12-ring of this Russian doll complex within 40 ps. These complexes lead to a form of template-directed synthesis in which one nanoring promotes formation of a larger concentric homologous ring; here, the effective template is an eight-component noncovalent assembly. Russian doll templating provides a new approach to amplifying the size of a covalent nanostructure.

Six-coordinate Group 13 complexes: the role of d orbitals and electron-rich multi-center bonding

Bonding in six-coordinate complexes based on Group 13 elements (B, Al, Ga, In, Tl) is usually considered to be identical to that in transition-metal analogues. We herein demonstrate through sophisticated electronic-structure analyses that the bonding in these Group 13 element complexes is fundamentally different and better characterized as electron-rich hypervalent bonding with essentially no role for the d orbitals. This characteristic is carried through to the molecular properties of the complex.

Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(III) porphyrin and fullerene entities

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

Ultrafast charge separation and charge stabilization in axially linked ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ reaction center mimics

Sequential electron transfer leading to charge stabilization in newly synthesized vertically aligned ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ supramolecular triads is reported.

Axially bonded pentads constructed on the Sn(iv) porphyrin scaffold

Three pentads based on the Sn(iv) porphyrin scaffold were synthesized using three covalently linked dyads such as porphyrin–ferrocene, porphyrin–BODIPY and porphyrin–sapphyrin as axial units. The spectral, electrochemical and photophysical properties were studied for these dyads and pentads.

Aggregation of a model porphyrin within poly(ethylene glycol) (PEG): effect of water, PEG molecular weight, ionic liquids, salts, and temperature

Porphyrin J-aggregation is facilitated in 10 wt% water-added PEGs at pH 1; it increases with PEG MW and is controlled by RTILs.

Electrochemical and Spectroscopic Characterization of Aluminium(III)-para-methyl-meso-tetraphenylporphyrin Complexes Containing Substituted Salicylates as Axial Ligands

A series of aluminium(III)-p-methyl-meso-tetraphenylporphyrin (p-CH3TPP-Al(III)) containing axially coordinated salicylate anion [p-CH3TPP-Al-X)], where X = salicylate (SA), 4-chlorosalicylate (4-CSA), 5-chlorosalicylate (5-CSA), 5-flourosalicylate (5-FSA), 4-aminosalicylate (4-ASA), 5-aminosalicylate (5-ASA), 5-nitrosalicylate (5-NSA), and 5-sulfosalicylate (5-SSA), have been synthesized and characterized by various spectroscopic techniques including ultraviolet-visible (UV-vis), infrared (IR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy,13C NMR, and elemental analysis. A detailed study of electrochemistry of all the synthesized compounds has been done to compare their oxidation and reduction mechanisms and to explain the effect of axial coordination on their redox properties.