Artificial, molecular-based light-harvesting antenna systems made of metal dendrimers and multibodipy species

Reaching strong absorption up to 700 nm with new benzo[g]quinoxaline-based heteroleptic copper(I) complexes for light-harvesting applications

Heteroleptic copper(I) complexes, with a diimine as a chromophoric unit and a bulky diphosphine as an ancillary ligand, have the advantage of a reduced pseudo Jahn-Teller effect in their excited state over the corresponding homoleptic bis(diimine) complexes. Nevertheless, their lowest absorption lies generally between 350 to 500 nm. Aiming at a strong absorption in the visible by stable heteroleptic Cu(I) complexes, we designed a novel diimine based on 4-(benzo[g]quinoxal-2'-yl)-1,2,3-triazole derivatives. The large π-conjugation of the benzoquinoxaline moiety shifted bathochromically the absorption with regard to other diimine-based Cu(I) complexes. Adding another Cu(I) core broadened the absorption and extended it to considerably longer wavelengths. Moreover, by fine-tuning the structure of the dichelating ligand, we achieved a panchromatic absorption up to 700 nm with a high molar extinction coefficient of 8000 M^-1 cm^-1 at maximum (λ = 570 nm), making this compound attractive for light-harvesting antennae.

Experimental and computational characterisation of an artificial light harvesting complex

Photosynthesis has been shown to be a highly efficient process for energy transfer in plants and bacteria. Like natural photosynthetic systems, the artificial light harvesting complex (LHC) BODIPY pillar[5]arene exhibits Förster resonance energy transfer (FRET). However, extensive characterisation of the BODIPY pillar[5]arene LHC to determine its suitability as an artificial LHC has yet to occur. In this paper we experimentally and computationally investigate the photophysical properties of the LHC by comparing the light absorption of the BODIPY LHC to individual BODIPY chromophores. Our results show evidence for quantum coherence, with oscillation frequencies of 100 cm^-1 and 600 cm^-1, which are attributable to vibronic, or exciton-phonon type coupling. Computational analysis suggests strong couplings of the molecular orbitals of the LHC resulting from the stacking of neighbouring BODIPY chromophore units. Interestingly, we find a 40% reduction in the absorbance of light for the BODIPY LHC compared to the individual chromophores which we attribute to electronic interactions between the conjugated π-systems of the BODIPY chromophores and the pillar[5]arene backbone.

Self-assembled systems for artificial photosynthesis

The last few decades have seen an impressive development in molecular-based artificial photosynthesis, thanks to the design of integrated light-harvesting antennae, charge separation systems, and catalysts for water oxidation or hydrogen production based on covalently linked subunits. However, in recent years, self-assembly and spontaneous aggregation of components emerged - sometimes also through serendipity - for the preparation of multicomponent systems aimed to perform the basic processes needed for artificial photosynthesis. Here we critically discuss some key articles that have recently shown the potential of self-assembly for artificial photosynthesis, ranging from self-assembly of antennae and charge separation systems to integrated antenna/catalyst assemblies, to planned co-localization of various components into restricted environments. It is evident that self-assembly can generate emerging properties with respect to the non-aggregated species, and such emerging properties can be quite convenient for designing efficient photocatalytic systems.

Synthesis and Photophysics Characterization of Boronic Styril and Distyryl BODIPYs for Water-Based Dye-Sensitized Solar Cells

In this study, two boronic acid BODIPYs are obtained through a microwave-assisted Knoevenagel reaction. The aim is to use them for the first time as dyes in a photosensitized solar cell (DSSC) to mimic chlorophyll photosynthesis, harvesting solar light and converting it into electricity. The microwave-assisted Knoevenagel reaction is a straightforward approach to extending the molecular conjugation of the dye and is applied for the first time to synthesize BODIPY’s boronic acid derivatives. These derivatives have proved to be very useful for covalent deposition on titania. This work studies the photo-physical and electrochemical properties. Moreover, the photovoltaic performances of these two new dyes as sensitizers for DSSC are discussed. Experimental data show that both dyes exhibit photosensitizing activities in acetonitrile and water. In particular, in all the experiments, distyryl BODIPY was more efficient than styryl BODIPY. In this study, demonstrating the use of a natural component as a water-based electrolyte for boronic BODIPY sensitizers, we open new possibilities for the development of water-based solar cells.

Bodipy-carbohydrate systems: synthesis and bio-applications

Luminescent BODIPY-sugar probes have stimulated the attention of researchers for the potential applications of such molecular systems in bio-imaging. The presence of carbohydrate units confers unique structural and biological features, beside enhancement of water solubility and polarity. On the other hand, BODIPY (BOronDiPYrromethene) derivatives represent eclectic and functional luminescent molecules because of their outstanding photophysical properties. This article provides a review on the synthesis and applications of BODIPY-linked glycosyl probes in which the labelling of complex carbohydrates with BODIPY allowed the disclosing of their in vivo behaviour or where the sugar constitutes a recognition element for specific targeting probes, or, finally, in which the stereochemical characteristics of the carbohydrate hydroxyl groups play as structural elements for assembling more than one photoactive subunit, resulting in functional supramolecular molecules with modulable properties. We describe the methods we have used to construct various multiBODIPY molecular systems capable of functioning as artificial antennas exhibiting extremely efficient and fast photo-induced energy transfer. Some of these systems have been designed to allow the modulation of energy transfer efficiency and emission color, and intensity dependent on their position within a biological matrix. Finally, future perspectives for such BODIPY-based functional supramolecular sugar systems are also highlighted.

Ru(ii) water oxidation catalysts with 2,3-bis(2-pyridyl)pyrazine and tris(pyrazolyl)methane ligands: assembly of photo-active and catalytically active subunits in a dinuclear structure

Two mononuclear Ru(ii) complexes, i.e. [RuCl(κ³N-terpy)(κ²N-dpp)]PF₆ ([1]PF₆; terpy = 2,2':6',2''-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(κ³N-tpm)(κ²N-dpp)]Cl ([2]Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru₂Cl(κ³N-tpm)(μ-κ²N:κ²N-dpp)Ru(κ²N-bpy)₂][PF₆]₃ ([3][PF₆]₃; bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (³⁵Cl NMR and conductivity measurements) study aimed to elucidate the nature of [1]⁺ and [2]⁺ in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of [2]⁺ in the ground state, replacing one tpm nitrogen. Conversely, in the case of [1]⁺, water coordination, assumed to be needed for the water oxidation process, presumably occurs following the oxidation of the metal. For all complexes, a catalytic wave has been detected in acetonitrile/water 1 : 1 (v/v) solution in the range 1.4-1.7 V vs. SCE. In all cases, water oxidation (investigated at pH < 8) takes place initially via a proton-coupled two-electron, two-proton process with the formation of an Ru(iv)[double bond, length as m-dash]O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range of 16-33 and 1.3-2.2 h^-1, respectively) of the complexes are higher than those of the benchmark [Ru(LLL)(LL)(OH₂)]²⁺-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), which is [Ru(terpy)(bpm)(OH₂)]²⁺.

Early photophysical events of a ruthenium(ii) molecular dyad capable of performing photochemical water oxidation and of its model compounds

The early photophysical events occurring in the dinuclear metal complex [(ttb-terpy)(I)Ru(μ-dntpz)Ru(bpy)2]3+ (2; ttb-terpy = 4,4',4''-tri-tert-butyl-terpy; bpy = 2,2'-bipyridine; dntpz = 2,5-di-(1,8-dinaphthyrid-2-yl)pyrazine) - a species containing the chromophoric {(bpy)2Ru(μ-dntpz)}2+ subunit and the catalytic {(I)(ttb-terpy)Ru(μ-dntpz)}+ unit, already reported to be able to perform photocatalytic water oxidation - have been studied by ultrafast pump-probe spectroscopy in acetonitrile solution. The model species [Ru(bpy)2(dntpz)]2+ (1), [(bpy)2Ru(μ-dntpz)Ru(bpy)2]4+ (3), and [(ttb-terpy)(I)Ru((μ-dntpz)Ru[(ttb-terpy)(I)]2+ (4) have also been studied. For completeness, the absorption spectra, redox behavior of 1-4 and the spectroelectrochemistry of the dinuclear species 2-4 have been investigated. The usual 3MLCT (metal-to-ligand charge transfer) decay, characterized by relatively long lifetimes on the ns timescale, takes place in 1 and 3, whose lowest-energy level involves a {(bpy)2Ru(dntpz)}2+ unit, whereas for 2 and 4, whose lowest-energy excited state involves a 3MLCT centered on the {(I)(ttb-terpy)Ru(μ-dntpz)}+ subunit, the excited-state lifetimes are on the ps timescale, possibly involving population of a low-lying 3MC (metal-centered) level. Compound 2 also exhibits a fast process, with a time constant of 170 fs, which is attributed to intercomponent energy transfer from the MLCT state centered in the {(bpy)2Ru(μ-dntpz)}2+ unit to the MLCT state involving the {(I)(ttb-terpy)Ru(μ-dntpz)}+ unit. Both the intercomponent energy transfer and the MLCT-to-MC activation process take place from non-equilibrated MLCT states.

Synthesis of Re(I) Rings Comprising Different Re(I) Units and Their Light-Harvesting Abilities

Trimethylamine N-oxide (Me₃NO) could selectively remove only one CO ligand from fac-[Re(N^N)(CO)₃(PR₂R')]⁺ (N^N = diimine ligand), whereby only the CO ligand in the trans position to the phosphorus ligand was selectively removed to give cis,trans-[Re^I(N^N)(CO)₂(PR₂R')(L)] ⁿ⁺ in good yields. This decarbonylation reaction using Me₃NO was found to be especially useful for synthesizing biscarbonyl Re(I) complexes with electron-withdrawing groups in the diimine ligand, which could not be synthesized or were obtained only in low yields by the photochemical method. Me₃NO also selectively removed the carbonyl ligands in the trans position to the phosphorus ligands from the edge Re(I) complex units, which have the fac-[Re(N^N)(CO)₃(PR₂R')]⁺ structure, in linear-shaped Re(I) multinuclear complexes. This reaction was successfully applied to synthesize a novel precursor with ring-shaped multinuclear Re complexes (Re-rings) comprising different kinds of Re(I) units. The newly synthesized Re-rings, which consist of one Re unit with a 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (CF₃bpy) ligand and one or two Re unit(s) with a 2,2'-bipyridine (bpy) ligand, showed almost quantitative excitation-energy harvesting ability from the Re unit(s) with bpy to that with CF₃bpy.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Linear dichroism of visible-region chromophores using M13 bacteriophage as an alignment scaffold

It is a challenge within the field of biomimetics to recreate the properties of light-harvesting antennae found in plants and photosynthetic bacteria. Attempts to recreate these biological structures typically rely on the alignment of fluorescent moieties via attachment to an inert linear scaffold, e.g. DNA, RNA or amyloid fibrils, to enable Förster resonance energy transfer (FRET) between attached chromophores. While there has been some success in this approach, refinement of the alignment of the chromophores is often limited, which may limit the efficiency of energy transfer achieved. Here we demonstrate how linear dichroism spectroscopy may be used to ascertain the overall alignment of chromophores bound to the M13 bacteriophage, a model linear scaffold, and demonstrate how this may be used to distinguish between lack of FRET efficiency due to chromophore separation, and chromophore misalignment. This approach will allow the refinement of artificial light-harvesting antennae in a directed fashion.

Perspectives of ruthenium(ii) polyazaaromatic photo-oxidizing complexes photoreactive towards tryptophan-containing peptides and derivatives

This review focuses on recent advances in the search for Ru^II polyazaaromatic complexes as molecular photoreagents for tryptophan-containing peptides and proteins, in view of future biomedical applications.

Going against the flow: Os(ii)-to-Ru(ii) energy transfer in rod-like polypyridyl chromophore

The judicious design of a unique mixed-metal Ru-Os molecular rod led to the first photoinduced energy transfer from osmium-to-ruthenium in a polypyridyl complex. The absorbed light is directed from the osmium metal center to the peripheral ruthenium moieties, where only the low energy luminescence from the heteroleptic ruthenium phen-hpp complex was observed.

A heptanuclear light-harvesting metal-based antenna dendrimer with six Ru(ii)-based chromophores directly powering a single Os(ii)-based energy trap

Quantitative Dexter energy transfer occurs in a novel metal dendrimer 1, from the peripheral Ru(ii) centres to the Os(ii) core, at a rate constant of 9.1 × 10¹⁰ s⁻¹.