Porphyrin Boxes Constructed by Homochiral Self-Sorting Assembly: Optical Separation, Exciton Coupling, and Efficient Excitation Energy Migration

Photophysical and Computational Insights into Ag(I) Complexation of Porphyrinic Covalent Cages Equipped with Triazoles-Incorporating Linkers

The photophysical characterization of four supramolecular complexes based on covalent cages 2H-S-2H, 2H-L-2H, Zn-S-2H, and Zn-L-2H, consisting in either two free-base porphyrins or one Zn(II) porphyrin and one free-base porphyrin connected by four flexible linkers of different lengths incorporating triazole binding sites, and their Ag(I) complexation are reported. The complexation processes have been followed by means of absorption and emission spectroscopies, and a comprehensive computational study explains the behavior of the free-base porphyrin-containing cages. Absorption and emission features have been interpreted on the bases of conformational changes, metalation processes, and modification of energy transfer efficiencies occurring in the different cases. In all cages, except 2H-L-2H, the coordination of four Ag(I) ions to the lateral triazole groups of the linkers leads to the enlargement of their cavity. Only for 2H-L-2H is a different behavior observed, where the process of silver metalation of the porphyrins’ core prevails.

1,2,3-Tri(9-anthryl)benzene: Photophysical Properties and Solid State Intermolecular Interactions of Radially Arranged, Congested Aromatic π-Planes

We report the Negishi coupling based synthesis of 1,2,3-tri(9-anthryl)benzene derivatives, containing three radially arranged anthracenes in a π-cluster. In the crystalline state of the unsubstituted derivative, intermolecular π-π and CH-π interactions between the anthracene units drive the formation of a two-dimensional packing structure. Owing to though-space π-conjugation between anthracene units, the substances have unique electronic properties. The excited state dynamic behavior occurring between the three radially arranged anthracene moieties, such as exciton localization/delocalization, was elucidated by means of transient absorption measurements and quantum chemical calculations. Interestingly, even though the three anthracenes are closely oriented with a ca. 3.0 Å distances between their C-9 positions, exciton localization on two anthracene units is energetically favorable because of the flexible nature of the radially arranged aromatic rings.

Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells

To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3-6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

Tetrameric and Hexameric Porphyrin Nanorings: Template Synthesis and Photophysical Properties

Hexameric and tetrameric porphyrin nanorings, Z6·T6 and Z4·T4, were synthesized in 53% and 14% yields, respectively, by the Sonogashira-type self-oligomerization of porphyrin monomer 1 using hexadentate template T6 and tetrapyridylporphyrin template T4. Template-free nanorings Z6 and Z4 were also prepared. The femtosecond transient absorption measurements revealed fast excitation energy hopping (EEH) along these nanorings with hopping rates of 2-5 ps. Treatment of Z6 with chiral template CT6 gave Z6·CT6 showing circular dichroism (CD) and circularly polarized luminescence (CPL) in the absorption and fluorescence regions of Z6, respectively, which indicates chirality transfer from CT6 to Z6.

A Five-layer π-Aromatic Structure Formed through Self-assembly of a Porphyrin Trimer and Two Aromatic Guests

In this study we synthesized two- and four-armed porphyrins - bearing two carboxyl and four 2-aminoquinolino functionalities, respectively, at their meso positions - as a complementary hydrogen bonding pair for the self-assembly of a D₂ -symmetric porphyrin trimer host. Two units of the two-armed porphyrin and one unit of the four-armed porphyrin self-assembled quantitatively into the D₂ -symmetric porphyrin trimer, stabilized through ammidinium-carboxylate salt bridge formation, in CH₂ Cl₂ and CHCl₃ . The porphyrin trimer host gradually bound two units of 1,3,5-trinitrobenzene between the pair of porphyrin units, forming a five-layer aromatic structure. At temperatures below -40 °C, the rates of association and dissociation of the complexes were slow on the NMR spectroscopic time scale, allowing the 1 : 1 and 1 : 2 complexes of the trimer host and trinitrobenzene guest(s) to be detected independently when using less than 2 eq of trinitrobenzene. Vis titration experiments revealed the values of K₁ (2.1±0.4×10⁵  M^-1 ) and K₂ (2.2±0.06×10⁴  M^-1 ) in CHCl₃ at room temperature.

Chiral Self-Sorting Effects in the Self-Assembly of Metallosupramolecular Aggregates Comprising Ligands Derived from Tröger's Base

Five ligands with either nitrile or isonitrile metal binding motifs have been synthesized based on the 2,8- or 3,9-disubstituted Tröger's base scaffold, respectively. These ligands self-assemble into dinuclear cyclic metallosupramolecular aggregates upon coordination to [(dppp)Pd(OTf)₂ ] in a highly diastereoselective manner, by heterochiral self-sorting in a chiral self-discriminating manner as shown by ESI mass spectrometry, NMR spectroscopy, and single crystal XRD analysis. This observation is in contrast to earlier studies with ligands derived from Tröger's base that have larger metal binding motifs and bis(nitrile) and bis(isonitrile) ligands based on other rigid dissymmetric cores such as [2.2]paracyclophanes. Thus, the combination of these slim metal binding motifs with the rigid v-shaped 2,8- or 3,9-disubstituted Tröger's base scaffolds seems to be especially well preorganized to ensure high-fidelity social self-sorting behavior.

Nucleosides as organizing agents

In the present review, we discuss the results of the research we have done over the last decade on multi-porphyrinic architectures bearing a nucleotidic backbone, compare all the data, offer a novel overview, and emphasize common capabilities of nucleosides appearing in various systems, providing a new insight on the pre-organizing capabilities of nucleosidic backbones. The efficiency of nucleosides as pre-organizing agents was investigated through the synthesis and study of various bis-porphyrins bearing nucleosidic linkers, as well as through the investigation of the conformation of linear and arborescent multi-porphyrins constructed on a nucleosidic backbone. The capacity of these molecules to complex guests with a high association constant was used as a tool to evaluate their degree of pre-organization, as well as the investigation of the electronic coupling existing between their chromophores and their photo-chemical capacities. Such an overview of one decade of scientific investigations documents the fact that rigid linkers between chromophores are not necessary for their spatial pre-organization, opening new routes to the faster synthesis of flexible highly pre-organized molecular architectures avoiding the long and tedious synthesis of rigid tweezers, especially for the preparation of rigid linkers which may bring solubility and stability problems.

Influencing the Self-Sorting Behavior of [2.2]Paracyclophane-Based Ligands by Introducing Isostructural Binding Motifs

Two isostructural ligands with either nitrile (Lnit ) or isonitrile (Liso ) moieties directly connected to a [2.2]paracyclophane backbone with pseudo-meta substitution pattern have been synthesized. The ligand itself (Lnit ) or its precursors (Liso ) were resolved by HPLC on a chiral stationary phase and the absolute configuration of the isolated enantiomers was assigned by XRD analysis and/or by comparison of quantum-chemical simulated and experimental electronic circular dichroism (ECD) spectra. Surprisingly, the resulting metallosupramolecular aggregates formed in solution upon coordination of [(dppp)Pd(OTf)2 ] differ in their composition: whereas Lnit forms dinuclear complexes, Liso exclusively forms trinuclear ones. Furthermore, they also differ in their chiral self-sorting behavior as (rac)-Liso undergoes exclusive social self-sorting leading to a heterochiral assembly, whereas (rac)-Liso shows a twofold preference for the formation of homochiral complexes in a narcissistic self-sorting manner as proven by ESI mass spectrometry and NMR spectroscopy. Interestingly, upon crystallization, these discrete aggregates undergo structural transformation to coordination polymers, as evidenced by single-crystal X-ray diffraction.

Thermoresponsive Micellar Assembly Constructed from a Hexameric Hemoprotein Modified with Poly(N-isopropylacrylamide) toward an Artificial Light-Harvesting System

Artificial protein assemblies inspired by nature have significant potential in development of emergent functional materials. In order to construct an artificial protein assembly, we employed a mutant of a thermostable hemoprotein, hexameric tyrosine-coordinated heme protein (HTHP), as a building block. The HTHP mutant which has cysteine residues introduced on the bottom surface of its columnar structure was reacted with maleimide-tethering thermoresponsive poly(N-isopropylacrylamide), PNIPAAm, to generate the protein assembly upon heating. The site-specific modification of the cysteine residues with PNIPAAm on the protein surface was confirmed by SDS-PAGE and analytical size exclusion chromatography (SEC). The PNIPAAm-modified HTHP (PNIPAAm-HTHP) is found to provide a 43 nm spherical structure at 60 °C, and the structural changes observed between the assembled and the disassembled forms were duplicated at least five times. High-speed atomic force microscopic measurements of the micellar assembly supported by cross-linkage with glutaraldehyde indicate that the protein matrices are located on the surface of the sphere and cover the inner PNIPAAm core. Furthermore, substitution of heme with a photosensitizer, Zn protoporphyrin IX (ZnPP), in the micellar assembly provides an artificial light-harvesting system. Photochemical measurements of the ZnPP-substituted micellar assembly demonstrate that energy migration among the arrayed ZnPP molecules occurs within the range of several tens of picoseconds. Our present work represents the first example of an artificial light-harvesting system based on an assembled hemoprotein oligomer structure to replicate natural light-harvesting systems.

Investigation by NMR spectroscopy of the structural characteristics of modified oligo-nucleotides with pendant porphyrins

During the past years, we focused on exerting control over the position and distance of porphyrins along our specifically designed oligonucleotidic scaffold. Indeed, in naturally occurring light-harvesting complexes, biopolymer scaffolds hold pigments at intermolecular distances that optimize photon capture, electronic coupling, and energy transfer. To this end, four uridine-porphyrin conjugates (a monomer, a dimer, a tetramer and an octamer) were subjected to a comprehensive conformational analysis by using NMR spectroscopy. The collected NOE NMR data highlighted characteristic and strong interactions indicating that the glycosidic angle between the ribose and uracil base is anti. In order to further investigate the conformation of this family of molecules, NMR experiments were carried out at variable temperatures. At low temperature, the signals of the porphyrinic protons decoalesce, showing two sets of [Formula: see text]-pyrrolic protons. Similar observations are made for signals corresponding to sugar moieties and especially the H1′ protons, indicating molecular motions within our porphyrin-uridin arrays. These results testify in favor of the existence of a dynamic process between C3′-endo and C2′-endo conformations.

Photophysical properties of porphyrinic covalent cages endowed with different flexible linkers

In-depth photophysical studies of four flexible covalent cages bearing either two free-base porphyrins or one free-base porphyrin and one Zn(II) porphyrin, connected by linkers of different lengths, are reported. In the case of the cages with two free-base porphyrins, exciton coupling between the porphyrins is evidenced by large and split Soret bands in the absorption spectra, but the different length of the linkers has only a slight effect on their emission properties. Strong electronic interactions between the porphyrins are also evidenced for the cages that incorporate a free-base porphyrin and a Zn(II) porphyrin, with a more pronounced splitting of the Soret band for the system with longer linkers. In these cages, following excitation of the Zn-porphyrin component, an almost quantitative energy transfer to the free-base unit occurs, with a rate 1.4 times faster in the cage with longer linkers (1.4 × 10[Formula: see text] s[Formula: see text] vs. 1.0 × 10[Formula: see text] s[Formula: see text]. This difference might reflect the more flattened conformation adopted by the cage equipped with longer and more flexible linkers, the latter allowing for a shorter interplanar distance between the porphyrins. The results are discussed in terms of classical and short-range energy transfer mechanisms.

A thiazolo[5,4-d]thiazole-bridged porphyrin organic framework as a promising nonlinear optical material

We report a porous thiazolo[5,4-d]thiazole-bridged porphyrin organic framework, Por-TzTz-POF, with promising nonlinear optical (NLO) activity.

Chiral self-sorting behaviour of [2.2]paracyclophane-based bis(pyridine) ligands

[2.2]Paracyclophane-based bis(pyridine) ligands form dinuclear complexes upon coordination to palladium(ii) ions, however, with distinct differences concerning their chiral self-sorting ability.

Supramolecular Hemoprotein Assembly with a Periodic Structure Showing Heme-Heme Exciton Coupling

A supramolecular assembly of units of cytochrome b₅₆₂ with externally attached heme having intermolecular linkages formed via the heme-heme pocket interaction was investigated in an effort to construct a well-defined structure. The engineered site for surface attachment of heme at Cys80 in an N80C mutant of cytochrome b₅₆₂ provides the primary basis for the formation of the periodic assembly structure, which is characterized herein by circular dichroism (CD) spectroscopy and high-speed atomic force microscopy (AFM). This assembly represents the first example of the observation of a split-type Cotton effect by heme-heme exciton coupling in an artificial hemoprotein assembly system. Molecular dynamics simulations validated by simulated CD spectra, AFM images, and mutation experiments reveal that the assembly has a periodic helical structure with 3 nm pitches, suggesting the formation of the assembled structure is driven not only by the heme-heme pocket interaction but also by additional secondary hydrogen bonding and/or electrostatic interactions at the protein interfaces of the assembly.

Peripherally Metalated Porphyrins with Applications in Catalysis, Molecular Electronics and Biomedicine

Porphyrins are conjugated, stable chromophores with a central core that binds a variety of metal ions and an easily functionalized peripheral framework. By combining the catalytic, electronic or cytotoxic properties of selected transition metal complexes with the binding and electronic properties of porphyrins, enhanced characteristics of the ensemble are generated. This review article focuses on porphyrins bearing one or more peripheral transition metal complexes and discusses their potential applications in catalysis or biomedicine. Modulation of the electronic properties and intramolecular communication through coordination bond linkages in bis-porphyrin scaffolds is also presented.

Axially substituted phosphorous(v) corrole with polycyclic aromatic hydrocarbons: syntheses, X-ray structures, and photoinduced energy and electron transfer studies

Excited state energy and electron transfer processes in naphthalene and pyrene appended phosphorous(v) corroles.

A Structural Model of Nitro-Porphyrin Dyes Based on Spectroscopy and Density Functional Theory

Nitro-porphyrins are an important class of commercial dyes with a range of potential applications. The nitro group is known to dramatically affect the photophysics of the porphyrin, but there are few systematic investigations of the contributing factors. To address this deficiency, we present spectroscopic studies of a series of nitro-porphyrins, accompanied by density functional theory calculations to elucidate their structures. In particular, we explore how the positions of the substituents affect the energy levels and nuclear geometry. As expected, nitro groups on the meso-phenyl rings cause small changes to the orbital energies by induction, while those at the β-pyrrole positions more strongly conjugate into the aromatic system. In addition, however, we find evidence that β-pyrrole nitro groups distort the porphyrin, creating two non-planar conformations with distinct properties. This unexpected result helps explain the anomalous photophysics of nitro-porphyrins reported throughout the literature, including inhomogeneous line broadening and biexponential fluorescence decay. © 2017 Wiley Periodicals, Inc.

Efficiency of Dinucleosides as the Backbone to Pre-Organize Multi-Porphyrins and Enhance Their Stability as Sandwich Type Complexes with DABCO

Flexible linkers such as uridine or 2′-deoxyuridine pre-organize bis-porphyrins in a face-to-face conformation, thus forming stable sandwich complexes with a bidentate base such as 1,4-diazabicyclo[2.2.2]octane (DABCO). Increased stability can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constant by one to two orders of magnitude when compared to the association constant of DABCO with a reference porphyrin. Comparison with rigid tweezers shows a better efficiency of nucleosidic dimers. Thus, the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices.

Making a Right or Left Choice: Chiral Self-Sorting as a Tool for the Formation of Discrete Complex Structures

This review discusses chiral self-sorting-the process of choosing an interaction partner with a given chirality from a complex mixture of many possible racemic partners. Chiral self-sorting (also known as chiral self-recognition or chiral self-discrimination) is fundamental for creating functional structures in nature and in the world of chemistry because interactions between molecules of the same or the opposite chirality are characterized by different interaction energies and intrinsically different resulting structures. However, due to the similarity between recognition sites of enantiomers and common conformational lability, high fidelity homochiral or heterochiral self-sorting poses a substantial challenge. Chiral self-sorting occurs among natural and synthetic molecules that leads to the amplification of discrete species. The review covers a variety of complex self-assembled structures ranging from aggregates made of natural and racemic peptides and DNA, through artificial functional receptors, macrocyles, and cages to catalytically active metal complexes and helix mimics. The examples involve a plethora of reversible interactions: electrostatic interactions, π-π stacking, hydrogen bonds, coordination bonds, and dynamic covalent bonds. A generalized view of the examples collected from different fields allows us to suggest suitable geometric models that enable a rationalization of the observed experimental preferences and establishment of the rules that can facilitate further design.

Optically Active Porphyrin and Phthalocyanine Systems

This review highlights and summarizes various optically active porphyrin and phthalocyanine molecules prepared using a wide range of structural modification methods to improve the design of novel structures and their applications. The induced chirality of some illustrative achiral bis-porphyrins with a chiral guest molecule is introduced because these systems are ideal for the identification and separation of chiral biologically active substrates. In addition, the relationship between CD signal and the absolute configuration of the molecule is analyzed through an analysis of the results of molecular modeling calculations. Possible future research directions are also discussed.

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.

Exciton coupling dynamics in syn- and anti-type β–β linked Zn(ii) porphyrin linear arrays

The photophysical properties of molecular arrays are strongly dependent on a variety of structural factors: the constituent chromophores, dihedral angle, linkage length, linkage position, the center-to-center distance between chromophores, and the linker itself.

A Porphyrin Coordination Cage Assembled from Four Silver(I) Triazolyl-Pyridine Complexes

The synthesis of a zinc(II) porphyrin 1 with four appended triazolyl-pyridine chelates is reported. Complexation of the porphyrin peripheral ligands with Ag(I) ions in a 1:2 binding stoichiometry afforded quantitatively the coordination cage [Ag4 (1)2 ](4+) . The assembly and disassembly processes of the cage were investigated in solution using UV/Vis spectroscopy. The mathematical analysis of the data obtained in the UV/Vis titration of 1 with Ag(I) confirmed the assembly in CH2 Cl2 /MeOH (90:10) solution of a species having a 1:2 porphyrin/silver stoichiometry and assigned to it an overall stability constant of 5.0×10(26)  M(-5) . The use of a model system allowed an independent assessment of a microscopic binding constant value (Km ) for the interaction between the triazolyl-pyridine ligand and Ag(I) . The coincidence that existed between the Km values extracted from the model system and the titration of 1 provided an indication of the quality and fit of the data analysis. It also allowed the calculation of the average effective molarity (EM) value for the three intramolecular processes that led to the cage assembly as 2.6 mM. Simulated speciation profiles supported the conclusion that at millimolar concentration and working under strict stoichiometric control of the silver/porphyrin ratio, the cage [Ag4 (1)2 ](4+) was the species exclusively assembled in solution. On the other hand, when the concentration of added Ag(I) was approximately 2.6 mM, 50 % of the coordination cage disassembled into open aggregates.

Chirality- and sequence-selective successive self-sorting via specific homo- and complementary-duplex formations

Self-recognition and self-discrimination within complex mixtures are of fundamental importance in biological systems, which entirely rely on the preprogrammed monomer sequences and homochirality of biological macromolecules. Here we report artificial chirality- and sequence-selective successive self-sorting of chiral dimeric strands bearing carboxylic acid or amidine groups joined by chiral amide linkers with different sequences through homo- and complementary-duplex formations. A mixture of carboxylic acid dimers linked by racemic-1,2-cyclohexane bis-amides with different amide sequences (NHCO or CONH) self-associate to form homoduplexes in a completely sequence-selective way, the structures of which are different from each other depending on the linker amide sequences. The further addition of an enantiopure amide-linked amidine dimer to a mixture of the racemic carboxylic acid dimers resulted in the formation of a single optically pure complementary duplex with a 100% diastereoselectivity and complete sequence specificity stabilized by the amidinium–carboxylate salt bridges, leading to the perfect chirality- and sequence-selective duplex formation.

The recognition and self-sorting of chiral molecules is a vital feature of many biomolecules. Here, the authors report chirality- and sequence-specific self-sorting of organic strands containing carboxylic acid or amidine groups, leading to selective duplex formation.

Pre-organized dinucleosides with pendant porphyrins for the formation of sandwich type complexes with DABCO with high association constants

We report herein the synthesis of a dinucleotide bearing pendant porphyrins dedicated to adopt a pre-organized coformation with face-to-face porphyrins, and capable to self-organize in a stable sandwich type complexe with bidentate base such as DABCO. Earlier studies demonstrated that a peptidic linker does not provide sufficient pre-organization to enhance significantly the association constant with bidentate bases such as DABCO on the contrary of some other flexible linkers such as uridine or 2′-deoxyuridine. We document herein that the gain in stability for the formation of sandwich type host–guest complex with DABCO can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constants by one to two orders of magnitude when compared to the association constants of the same bidentate ligands with a reference Zn(II) porphyrin. Comparison of these results with those obtained for rigid tweezers shows a better efficiency of the flexible nucleosidic dimers. We thus document the fact that the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and that some well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices. Furthermore, the chirality and enantio-purity of the nucleosidic linkers paves the way toward the selective complexation of enantio-pure bidentate guests and the resolution of racemates.

Self-sorting behavior of a four-component host–guest system and its incorporation into a linear supramolecular alternating copolymer

Self-sorting behavior of a four-component host–guest system was found, which could be utilized to construct a linear supramolecular alternating copolymer.