Construction of Segregated Arrays of Multiple Donor and Acceptor Units Using a Dendritic Scaffold: Remarkable Dendrimer Effects on Photoinduced Charge Separation

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.

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.

Coordination-Driven Folding in Multi-ZnII -Porphyrin Arrays Constructed on a Pillar[5]arene Scaffold

Pillar[5]arene derivatives bearing peripheral porphyrin subunits have been efficiently prepared from a deca-azide pillar[5]arene building block (17) and Zn^II -porphyrin derivatives bearing a terminal alkyne function (9 and 16). For the resulting deca-Zn^II -porphyrin arrays (18 and 20), variable temperature NMR studies revealed an intramolecular complexation of the peripheral Zn^II -porphyrin moieties by 1,2,3-triazole subunits. As a result, the molecules adopt a folded conformation. This was further confirmed by UV/Vis spectroscopy and cyclic voltammetry. In addition, we have also demonstrated that the coordination-driven unfolding of 18 and 20 can be controlled by an external chemical stimulus. Specifically, addition of an imidazole derivative (22) to solution of 18 or 20 breaks the intramolecular coordination at the origin of the folding. The resulting molecular motions triggered by the addition of the imidazole ligand mimic the blooming of a flower.

Dendrimers with the protocatechuic acid building block for anticancer drug delivery

Protocatechuic acid (3,4-dihydroxybenzoic acid; PCA) is a well-known antioxidant compound and a potential antitumor drug that is commonly found in fruits and vegetables.

A multicomponent molecular approach to artificial photosynthesis – the role of fullerenes and endohedral metallofullerenes

In this review article, we highlight recent advances in the field of solar energy conversion at a molecular level.

Photoelectrochemical properties of supramolecular composites of an anionic zinc chlorin and Li+@C60 on SnO2

An electron donor–acceptor supramolecular complex was formed between an anionic zinc chlorin carboxylate ( ZnCh -) and lithium-ion-encapsulated [60]fullerene ( Li +@ C 60) by an electrostatic interaction in benzonitrile ( PhCN ). Photoinduced electron transfer in the supramolecular complex of ZnCh -/ Li +@ C 60 resulted in the formation of the charge-separated state via electron transfer from the triplet excited state of ZnCh - to Li +@ C 60. We report herein photovoltaic cells using ZnCh -/ Li +@ C 60 nanoclusters, which are assembled on the optically transparent electrode (OTE) of nanostructured SnO 2 (OTE/ SnO 2). The photoelectrochemical behavior of the nanostructured SnO 2 film of supramolecular nanoclusters of ZnCh - and Li +@ C 60 denoted as OTE/ SnO 2/( ZnCh -/ Li +@ C 60)n is significantly higher than the single component films of ZnCh - or Li +@ C 60 clusters, denoted as OTE/ SnO 2/( ZnCh -)n or OTE/ SnO 2/( Li +@ C 60)n.

The impact of copper-catalyzed alkyne-azide 1,3-dipolar cycloaddition in fullerene chemistry

Click reactions largely cross the borders of organic synthetic chemistry and are now at the forefront of many interdisciplinary studies at the interfaces between chemistry, physics, and biology. As part of this research, our group is involved in a program on the development of clickable fullerene building blocks and their application in the preparation of a large variety of new advanced materials and bioactive compounds. Importantly, the introduction of the click chemistry concept in fullerene chemistry allowed us to produce compounds that would barely be accessible by using the classical tools of fullerene chemistry. This is particularly the case for the conjugation of fullerenes with other carbon nanoforms, such as carbon nanohorns and graphene. It is also the case for most of the sophisticated molecular ensembles constructed from clickable fullerene hexa-adduct building blocks. In this paper, we have summarized our ongoing progress in this particular field.

Self-assembly: from amphiphiles to chromophores and beyond

Self-assembly has been recognised as a ubiquitous aspect of modern chemistry. Our understanding and applications of self-assembly are substantially based on what has been learned from biochemical systems. In this review, we describe various aspects of self-assembly commencing with an account of the soft structures that are available by assembly of surfactant amphiphiles, which are important scientific and industrial materials. Variation of molecular design using rules defined by surfactant self-assembly permits synthesis of functional nanostructures in solution and at surfaces while increasing the strength of intermolecular interactions through π-π stacking, metal cation coordination and/or hydrogen bonding leads to formation of highly complex bespoke nanostructured materials exemplified by DNA assemblies. We describe the origins of self-assembly involving aggregation of lipid amphiphiles and how this subject has been expanded to include other highly advanced chemical systems.

A supramolecular photosynthetic model made of a multiporphyrinic array constructed around a C60 core and a C60-imidazole derivative

The photophysical properties of a supramolecular fullerene-porphyrin ensemble resulting from the self-assembly of a pyrrolidinofullerene-imidazole derivative (F1) with a multimetalloporphyrin array constructed around a hexasubstituted fullerene core (F(ZnP)12) have been investigated. The fullerene hexa-adduct core of the host system does not play any active role in the cascade of photoinduced events of the supramolecular ensemble, indeed no intercomponent photoinduced processes could be observed in host F(ZnP)12. In contrast, upon axial coordination with the monosubstituted fullerene guest F1, a quantitative quenching of the fluorescence signal of the metalloporphyrins was observed for the supramolecular complex [F(ZnP)12(F1)n] both in polar and nonpolar solvents. In toluene, the supramolecular ensemble exhibits a charge transfer emission centered around nm, suggesting the occurrence of intramolecular face-to-face interactions of F1 with neighboring metalloporphyrin moieties within the self-assembled photoactive array. This mechanism is supported by the fact that a one order of magnitude increase in the binding constant was observed for the supramolecular complex [F(ZnP)12(F1)n] when compared with a reference system lacking the pyrrolidinofullerene unit. In benzonitrile, a long-lived charge-separated state (τ=0.3 μs) has been detected for the supramolecular adduct.

Controlling porphyrin nanoarchitectures at solid interfaces

Two complementary examples of porphyrin nanoarchitectonics are presented. The fabrication of binary molecular monolayers using two different porphyrin molecules, tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin (1) and tetrakis(4-pyridyl)porphyrin (2), by deposition in ultrahigh vacuum was demonstrated. Two unusual heteromolecular monolayer structures were observed, with one exhibiting good separation of 1 molecules within the monolayer. Also, a synthetic nanoarchitectonic approach was used to prepare self-assembled molecular nanowires at a mica substrate. The nanowires could be observed to grow using atomic force microscopy (AFM), and the network structures of the nanowires could be influenced by manipulation using the AFM probe tip.

Advances in supramolecularly assembled nanostructures of fullerenes and porphyrins at surfaces

The ‘bottom-up’ strategy is an attractive and promising approach for the construction of nanoarchitectures. Supramolecular assemblies based on non-covalent interactions have been explored in an attempt to control surface properties. In this minireview, we focus on advances made in the past three years in the field of scanning tunneling microscopy (STM) on supramolecular assembly and the function of porphyrins, phthalocyanines, and fullerenes, non-covalently bound on metal single crystal surfaces. Well-defined adlayers, consisting of porphyrin and phthalocyanine for the design of supramolecular nanoarchitectures, supramolecular traps of C 60 on hydrogen bond networks, a unique approach for controlling molecular orientation by a 1:1 supramolecularly assembled film consisting of C 60 and the related derivatives and metallooctaethylporphyrins, and nanoapplications of fullerenes, either induced by tip manipulation or driven by thermal fluctuations at surfaces, were clearly visualized by STM.

Hydrogen-bonding induced cooperative effect on the energy transfer in helical polynorbornenes appended with porphyrin-containing amidic alanine linkers

Polynorbornenes appended with porphyrins containing a range of different linkers are synthesized. The use of bisamidic chiral alanine linkers between the pending porphyrins and the polymeric backbone has been shown to bring the adjacent porphyrin chromophores to more suitable orientation for exciton coupling owing to hydrogen bonding between the adjacent linkers. The hydrogen bonding between the adjacent pendants in these polymers may induce a cooperative effect and therefore render single-handed helical structures for these polymers. Such a cooperative effect is reflected in the enhancement of FRET efficiencies between zinc-porphyrin and free base porphyrin in random copolymers.

Self-association and electron transfer in donor-acceptor dyads connected by meta-substituted oligomers

The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor--C(60)--and the electron donor--pi-extended tetrathiafulvalene (exTTF)--are bridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOF spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature (1)H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5 greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays a decisive role in governing excited-state interactions in the corresponding electron donor-acceptor conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged by just one aromatic ring, displays--exclusively and independent of the concentration (10(-6) to 10(-4) M)--efficient intramolecular electron transfer events on the basis of a "through-bond" mechanism. On the contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10(-6) M) "through space" intramolecular electron transfer events. These are, however, quite ineffective and, in turn, lead to excited-state deactivations that are at high concentrations (10(-4) M) dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C(60) of another molecule, and that stabilize the resulting radical ion pair state with lifetimes reaching 4.0 micros.