A Kinetic Investigation of the Supramolecular Chiral Self-Assembling Process of Cationic Organometallic (2,2':6',2″-terpyridine)methylplatinum(II) Complexes with Poly(L-glutamic Acid).

The cationic platinum(II) organometallic complex [Pt(terpy)Me]+ (terpy = 2,2':6',2″-terpyridine) at mild acidic pH interacts with poly(L-glutamic acid) (L-PGA) in its α-helix conformation, affording chiral supramolecular adducts. Their kinetics of formation have been investigated in detail as a function of the concentrations of both reagents and changing pH, ionic strength, the length of the polymeric scaffold and temperature. After a very fast early stage, the kinetic traces have been analyzed as three consecutive steps, suggesting a mechanism based on the electrostatic fast formation of a not-organized aggregate that subsequently evolves through different rearrangements to form the eventual supramolecular adduct. A model for this species has been proposed based on (i) the attractive electrostatic interaction of the cationic platinum(II) complexes and the polyelectrolyte and (ii) the π-stacking interactions acting among the [Pt(terpy)Me]+ units.

Interaction of Aromatic Amino Acids with Metal Complexes of Tetrakis-(4-Sulfonatophenyl)Porphyrin.

The interaction of a series of metal derivatives of 5, 10, 15, 20-tetrakis(4-sulfonato-phenyl)porphyrin (MTPPS4, M = Cu(II), Pt(II), Ni(II), Zn(II) and Co(II)), including the metal free porphyrin (TPPS4), with the aromatic amino acids L-tryptophan (L-Trp), L-and D-phenylalanine (L-and D-Phe) and L-histidine (L-His) have been investigated through UV/Vis spectroscopy. The amino acid L-serine (L-Ser) has been included as reference compound. The spectroscopic changes induced by adding the amino acids have been exploited to evaluate the extent of interaction between the molecular components in the supramolecular adducts. The binding constants have been estimated for most of the investigated systems, assuming a simple 1:1 equilibrium. The bathochromic shifts of the B-bands, the extent of hypochromicity and the binding constants have been analyzed through two chemical descriptors. All the data point to the important role played by the steric hindrance introduced by axial ligands coordinated to the metal ions and to the degree of hydrophobicity and size of the aromatic moiety in the amino acids.

Absorption and Fluorescence Emission Investigations on Supramolecular Assemblies of Tetrakis-(4-sulfonatophenyl)porphyrin and Graphene Quantum Dots.

The one-pot synthesis of N-doped graphene quantum dots (GQDs), capped with a positively charged polyamine (trien), has been realized through a microwave-assisted pyrolysis on solid L-glutamic acid and trien in equimolar amounts. The resulting positively charged nanoparticles are strongly emissive in aqueous solutions and are stable for months. The interaction with the anionic tetrakis(4-sulphonatophenyl)porphyrin (TPPS4) has been investigated at neutral and mild acidic pH using a combination of UV/vis absorption spectroscopy together with static and time-resolved fluorescence emission. At pH = 7, the experimental evidence points to the formation of a supramolecular adduct mainly stabilized by electrostatic interactions. The fluorescence emission of the porphyrin is substantially quenched while GQDs remain still emissive. On decreasing the pH, protonation of TPPS4 leads to formation of porphyrin J-aggregates through the intermediacy of the charged quantum dots.

Kinetic Investigations on the Chiral Induction by Amino Acids in Porphyrin J-Aggregates.

The self-assembling kinetics of the 5,10,15,20-tetrakis(4-sulfonato-phenyl)porphyrin (TPPS4) into nano-tubular J-aggregates under strong acidic condition and in the presence of amino acids as templating chiral reagents have been investigated through UV/Vis spectroscopy. The ability of the chiral species to transfer its chiral information to the final J-aggregate has been measured through circular dichroism (CD) spectroscopy and compared to the spontaneous symmetry breaking process usually observed in these nano-aggregates. Under the experimental conditions here selected, including mixing protocol, we have observed a large difference in the observed aggregation rates for the various amino acids, those with a positively charged side group being the most effective. On the contrary, these species are less efficient in transferring their chirality, exhibiting a quite low or modest enhancement in the observed dissymmetry g-factors. On the other side, hydrophobic and some hydrophilic amino acids are revealed to be very active in inducing chirality with a discrete increase of intensity of the detected CD bands with respect to the spontaneous symmetry breaking.

Enhancement of the Rates for Insertion of Zinc(II) Ions into a Cationic Porphyrin Catalyzed by Poly(glutamate).

The self-assembly of porphyrins onto polyelectrolytes could lead to interesting changes in their reactivity with respect to the bulk solution. Here, we investigated the kinetics of Zn2+ incorporation into tetra-cationic water-soluble 5,10,15,20-tetrakis-(N-methylpyridinium-4-yl)porphyrin (TMpyP(4)) in the presence of poly(L-glutamic acid) (PGA) in a pH range from 4 to 6.5. Under these conditions, the porphyrin electrostatically interacted with the polymer, which gradually switched from an α-helical to a random coil structure. The profile of the logarithm of the observed rate constant (kobs) versus the pH was sigmoidal with an inflection point close to the pH of the conformation transition for PGA. At a pH of 5.4, when PGA was in its highly charged random coil conformation, an almost 1000-fold increase in the reaction rates was observed. An increase in the ionic strength of the bulk solution led to a decrease in the metal insertion rates. The role of the charged matrix was explained in terms of its ability to assemble both reagents in proximity, in agreement with the theory of counter-ion condensation around polyelectrolytes in an aqueous solution.

High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures.

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor-acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.

Supramolecular BODIPY based dimers: synthesis, computational and spectroscopic studies.

The synthetic procedures for the preparation of supramolecular BODIPY dimers decorated with complementary patterns able to induce the formation of a triple hydrogen bond through mutual interactions are here reported. The BODIPY and styryl-equipped BODIPY species have been suitably functionalized in meso position with 2,6-diacetamido-4-pyridyl and 1-butyl-6-uracyl moieties. Dimers and monomers have been subjected to computational and photophysical investigations in solvent media. Various peculiarities concerning the effects of the interaction geometry on the stability of the H-bonded systems have also been investigated. The combination of modelling and experimental data provides a paradigm for improving and refining the BODIPY synthetic pathway to have chromophoric architectures with a programmable supramolecular identity. Furthermore, the possibility of assembling dimers of different dyes through H-bonds could be appealing for a systematic investigation of the principal factors affecting the dynamics of the energy migration and possibly driving coherent transfer mechanisms. Our work highlights how the chemical versatility of these dyes can be exploited to design new BODIPY-based supramolecular architectures.

The effect of hydrogen bonds on the ultrafast relaxation dynamics of a BODIPY dimer.

The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.

Advances in the Chemistry of Porphyrins and Related Macrocycles.

Porphyrins and their analogues feature remarkably in nature, being prosthetic groups in a wide variety of primary metabolites playing a pivotal role in many biological processes [...].

Effects of the Mixing Protocol on the Self-Assembling Process of Water Soluble Porphyrins.

The hierarchical self-assembling kinetics of the porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS44-) into J-aggregates at high ionic strength under acidic conditions and eventually in the presence of an added chiral templating agent (tartrate) were investigated through UV/Vis spectroscopy, resonance light scattering, and circular dichroism (CD). The effect of changing the mixing order of the various components in the solution on the kinetic parameters and the expression of chirality on the final J-aggregates was evaluated. In this latter case, only when the chiral tartrate anion is premixed with the porphyrin, the resulting nano-architectures exhibit CD spectra that reflect the handedness of the chiral inducer. We discuss a general mechanistic scheme, with the involvement of ion pairs or dimers that offer an alternative pathway to the aggregation process.

Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin.

Under moderate acidic conditions, the cationic (+3) complexes ions tris(1,10-phenanthroline)cobalt(III), [Co(phen)3]3+, and hexamminecobalt(III), [Co(NH3)6]3+, efficiently promote the self-assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4) into J-aggregates. The growth kinetics have been analyzed according to a well-established autocatalytic model, in which the rate determining step is the initial formation of a nucleus containing m porphyrin units (in the range 2-3), followed by a stage whose rate constant kc evolves as a power of time. The observed catalytic rate constants and the extent of J-aggregation increase on increasing the metal complex concentration, with the phen complex being the less active. The UV/Vis extinction spectra display quite broad envelops at the J-band, especially for the amino-complex, suggesting that electronic dipolar coupling between chromophores is operative in these species. The occurrence of spontaneous symmetry breaking has been revealed by circular dichroism and the measured dissymmetry g-factor decreases on increasing the aggregation rates. The role of these metal complexes on the growth and stabilization of porphyrin nano-assemblies is discussed in terms of the different degree of hydrophilicity and hydrogen bonding ability of the ligands present in the coordination sphere around the metal center.

Controlling J-Aggregates Formation and Chirality Induction through Demetallation of a Zinc(II) Water Soluble Porphyrin.

Under acidic conditions and at high ionic strength, the zinc cation is removed from its metal complex with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) thus leading to the diacid free porphyrin, that subsequently self-organize into J-aggregates. The kinetics of the demetallation step and the successive supramolecular assembly formation have been investigated as a function of pH and ionic strength (controlled by adding ZnSO4). The demetallation kinetics obey to a rate law that is first order in [ZnTPPS4] and second order in [H+], according to literature, with k2 = 5.5 ± 0.4 M-2 s-1 at 298 K (IS = 0.6 M, ZnSO4). The aggregation process has been modeled according to an autocatalytic growth, where after the formation of a starting seed containing m porphyrin units, the rate evolves as a power of time. A complete analysis of the extinction time traces at various wavelengths allows extraction of the relevant kinetic parameters, showing that a trimer or tetramer should be involved in the rate-determining step of the aggregation. The extinction spectra of the J-aggregates evidence quite broad bands, suggesting an electronic coupling mechanism different to the usual Frenkel exciton coupling. Resonance light scattering intensity in the aggregated samples increases with increasing both [H+] and [ZnSO4]. Symmetry breaking occurs in these samples and the J-aggregates show circular dichroism spectra with unusual bands. The asymmetry g-factor decreases in its absolute value with increasing the catalytic rate kc, nulling and eventually switching the Cotton effect from negative to positive. Some inferences on the role exerted by zinc cations on the kinetics and structural features of these nanostructures have been discussed.

Kinetic Investigation on Tetrakis(4-Sulfonatophenyl)Porphyrin J-Aggregates Formation Catalyzed by Cationic Metallo-Porphyrins.

Under mild acidic conditions, various metal derivatives of tetrakis(4-N-methylpyridinium)porphyrin (gold(III), AuT4; cobalt(III), CoT4; manganese(III), MnT4 and zinc(II), ZnT4) catalytically promote the supramolecular assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4) into J-aggregates. The aggregation kinetics have been treated according to a well-established model that involves the initial formation of a critical nucleus containing m porphyrin units, followed by autocatalytic growth, in which the rate evolves as a power of time. An analysis of the extinction time traces allows to obtain the rate constants for the auto-catalyzed pathway, kc, and the number of porphyrins involved in the initial seeding. The aggregation kinetics have been investigated at fixed H2TPPS4 concentration as a function of the added metal derivatives MT4. The derived rate constants, kc, obey a rate law that is first order in [MT4] and depend on the specific nature of the catalyst in the order AuT4 > CoT4 > MnT4 > ZnT4. Both resonance light scattering (RLS) intensity and extinction in the aggregated samples increase on increasing [MT4]. With the exception of AuT4, the final aggregated samples obtained at the highest catalyst concentration exhibit a negative Cotton effect in the J-band region, evidencing the occurrence of spontaneous symmetry breaking. The role of the nature of the metal derivative in terms of overall charge and presence of axial groups will be discussed.

Supramolecular Adducts of Anionic Porphyrins and a Biocompatible Polyamine: Effect of Photodamage-on Human Red Blood Cells.

Supramolecular adducts obtained by interaction between the anionic porphyrin meso-tetrakis(4-carboxyphenyl)porphyrin (TPPC) or its zinc(II) derivative (ZnTPPC) with a biocompatible amino-terminated polypropylene or poly(ethylene oxide)s (Jeffamines) has been investigated. The interaction with the polymer allows the stabilization of the porphyrins in their monomeric form under physiological conditions. The photodynamic properties of the supramolecular adducts were explored by typical 1O2 indirect detection. Their photodynamic action were evaluated in vitro using human red blood cells (HRBCs) under different experimental conditions. The morphology of erythrocytes was investigated by optical microscopy after incubation with porphyrin compounds and light irradiation. The images show loss of their normal biconcave profile and an incoming spiny configuration with blebs evident on their surfaces.

Poly(carboxylic acid)-Cyclodextrin/Anionic Porphyrin Finished Fabrics as Photosensitizer Releasers for Antimicrobial Photodynamic Therapy.

In the development of new antibacterial therapeutic approaches to fight multidrug-resistant bacteria, antimicrobial photodynamic therapy (aPDT) represents a well-known alternative to treat local infections caused by different microorganisms. Here we present a polypropylene (PP) fabric finished with citrate-hydroxypropyl-ßCD polymer (PP-CD) entrapping the tetra-anionic 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine (TPPS) as photosensitizer-eluting scaffold (PP-CD/TPPS) for aPDT. The concept is based on host-guest complexation of porphyrin in the cavities of CDs immobilized on the PP fibers, followed by its sustained and controlled delivery in release medium and simultaneous photoinactivation of microorganisms. Morphology of fabric was characterized by optical (OM) and scanning electron microscopies (SEM). Optical properties were investigated by UV-vis absorption, steady- and time-resolved fluorescence emission spectroscopy. X-ray photoelectron spectroscopy (XPS) and FT-IR revealed the surface chemical composition and the distribution map of the molecular components on the fabric, respectively. Direct 1O2 determination allowed to assess the potential photodynamic activity of the fabric. Release kinetics of TPPS in physiological conditions pointed out the role of the CD cavity to control the TPPS elution. Photoantimicrobial activity of the porphyrin-loaded textile was investigated against both Gram-positive Staphylococcus aureus ATCC 29213 (S. aureus) and Gram-negative Pseudomonas aeruginosa ATCC 27853 (P. aeruginosa). Optical microscopy coupled with UV-vis extinction and fluorescence spectra aim to ascertain the uptake of TPPS to S. aureus bacterial cells. Finally, PP-CD/TPPS fabric-treated S. aureus cells were photokilled of 99.98%. Moreover, low adhesion of S. aureus cells on textile was established. Conversely, no photodamage of fabric-treated P. aeruginosa cells was observed, together with their satisfying adhesion.

Investigation of the Aggregation Properties of a Chiral Porphyrin Bearing Citronellal Meso Substituent Groups.

A new porphyrin bearing four R or S hydrogenated citronellal units directly bound to the meso positions of the porphyrin ring was synthesized and fully characterized through MALDI-TOF, NMR, UV/Vis absorption, and fluorescence emission spectroscopies. Both enantiomers exhibit a monomeric nature in a series of organic solvents. Acting on the polarity of the solvent, i.e., increasing the amount of water in mixture with acetone, aggregation occurs, as revealed by UV/Vis absorption, fluorescence emission, and resonance light scattering. The occurrence of both H- and J-type aggregates was suggested by fluorescence lifetime measurements. In contrast to the monomeric species, these aggregates exhibit CD spectra reflecting the chirality of the building blocks. AFM microscopy shows that micrometer ribbon-like structures form by the casting solution of these porphyrins in acetone/water onto a glass surface.

Kinetic control of chirality in porphyrin J-aggregates.

Detailed kinetic investigations demonstrate the fundamental role of kinetic parameters in the expression and transmission of chirality in supramolecular systems. The rate of the aggregation process leading to the formation of J-aggregates strongly affects the size of these nanoassemblies and the chiral induction.

Unusual Stepwise Protonation and J-Aggregation of meso-Tetrakis(N-methylpyridinium-4-yl)porphine on Binding Poly(sodium vinylsulfonate).

The binding of a tetracationic porphyrin to a highly charged polymer like poly(sodium vinylsulfonate) has been investigated over a wide pH range and under various experimental conditions. We present evidence that, depending on the pH, the high electrostatic field exerted by the polymer stabilizes the diprotonated form of the free base porphyrin at unusual pH values or otherwise causes the formation of H-type aggregates. In particular, at a low polymer concentration, lowering the pH at first allows the formation of the diacid species then it determines its reorganization in close-packed J-type aggregates. The employment of various metallo-derivatives of the title porphyrin enables a better insight into the nature of all the detected species.

Investigation of J-Aggregates of 2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl) Porphyrin in Aqueous Solutions.

The highly distorted water-soluble 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (Br8TPPS44-) is readily protonated under acidic pH, forming the diacid H2Br8TPPS42- and subsequently the zwitterionic H4Br8TPPS4, which eventually evolves into J-aggregates. These latter species exhibit a relevant bathochromic shift with respect to the monomer with a quite sharp band due to motional narrowing. The depolarization ratio measured in resonant light scattering spectra allows estimating a tilt angle of ~20° of the porphyrins in the J-aggregate. The kinetic parameters are obtained by applying a model based on the initial slow nucleation step, leading to a nucleus containing m monomers, followed by fast autocatalytic growth. The kc values for this latter step increase on decreasing the acid concentration and on increasing the porphyrin concentration, with a strong power-law dependence. No spontaneous symmetry breaking or transfer of chirality from chiral inducers is observed. Both Atomic Force Microscopy (AFM) and Dynamic Light Scattering (DLS) point to the presence, in both the solid and solution phases, of globular-shaped aggregates with sizes close to 130 nm. Density functional theory (DFT) calculations performed on simplified models show that (i) upon protonation, the saddled conformation of the porphyrin ring is slightly altered, and a further rotation of the aryl rings occurs, and (ii) the diacid species is more stable than the parent unprotonated porphyrin. Time-dependent DFT analysis allows comparing the UV/Vis spectra for the two species, showing a consistent red shift upon protonation, even if larger than the experimental one. The simulated Raman spectrum agrees with the experimental spectrum acquired on solid samples.

1,1'-Bis(diphenylphosphino)ferrocene Platinum(II) Complexes as a Route to Functionalized Multiporphyrin Systems.

In this study, the cationic complex [PtMe(Me2SO)(dppf)]CF3SO3 (PtFc) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) was exploited as a precursor to functionalize the multi-chromophoric system hexakis(pyridyl-porphyrinato)benzene (1). The final adduct [PtFc]18-1, containing eighteen platinum(II) organometallic [PtMe(dppf)] fragments, was prepared and characterized through UV/Vis absorption, 31P{1H}-NMR spectroscopy, and fluorescence emission. UV/vis and fluorescence titrations confirmed the coordination between the platinum(II) center and all the pyridyl moieties of the peripheral substituent groups of the porphyrin. The drop casting of diluted dichloromethane solution of [PtFc]18-1 onto a glass surface afford micrometer-sized emissive porphyrin rings.