Photo-driven water oxidation performed by supramolecular photocatalysts made of Ru(II) photosensitizers and catalysts.

Two new supramolecular photocatalysts made of covalently linked Ru(II) polypyridine chromophore subunits ([Ru(bpy)3]2+-type species; bpy = 2,2'-bipyridine) and [RuL(pic)2] (L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline) water oxidation catalyst subunits have been prepared. The new species, 1 and 2, contain chromophore and catalyst subunits in the molecular ratios 1:1 and 1:2, respectively. The model chromophore species [Ru(bpy)2(L1)]2+ (RuP1; L1=4-[2-(4-pyridyl)-2-hydroxyethyl]-4-methyl-2,2'-bipyridine) and [Ru(bpy)2(L2)]2+ (RuP2; L2 = 4,4'-bis[2-(4-pyridyl)-2-hydroxyethyl]-2,2'-bipyridine) have also been prepared. The absorption spectra, oxidation behavior, and luminescent properties of 1 and 2 have been studied, and the results indicate that each subunit largely maintains its own properties in the supramolecular species. However, the luminescence of the chromophore subunits is significantly quenched in 1 and 2 in comparison with the luminescence of the respective model species. Both 1 and 2 exhibit catalytic water oxidation in the presence of cerium ammonium nitrate, exhibiting an I2M mechanism, with a better efficiency than the known catalyst [RuL(pic)2] under the same experimental conditions. Upon light irradiation, in the presence of persulfate as a sacrificial acceptor agent, 1 and 2 are more efficient photocatalysts than a system made of separated [Ru(bpy)3]2+ and [RuL(pic)2] species, highlighting the advantage of using multicomponent, supramolecular species with respect to isolated species. The O-O bond formation step is I2M, even in the photo-driven process. The photocatalytic process of 2 is more efficient than that of 1, with the turnover frequency reaching a value of 1.2 s-1. A possible reason could be an increased local concentration of catalytic subunits in the needed bimolecular assembly required for the I2M mechanism in 2 with respect to 1, a consequence of the presence of two catalytic subunits in each multicomponent species 2.

Synthesis of Bodipy-Tagged Galactoconjugates and Evaluation of Their Antibacterial Properties.

As a development of our research on biocompatible glycoconjugate probes and specifically multi-chromophoric systems, herein, we report the synthesis and early bactericidal tests of two luminescent glycoconjugates whose basic structure is characterized by two boron dipyrromethene difluoride (BODIPY) moieties and three galactoside rings mounted on an oligophenylene ethynylene (OPE) skeleton. BODIPY fluorophores have found widespread application in many branches of biology in the last few decades. In particular, molecular platforms showing two different BODIPY groups have unique photophysical behavior useful in fluorescence imaging. Construction of the complex architecture of the new probes is accomplished through a convergent route that exploits a series of copper-free Heck-Cassar-Sonogashira cross-couplings. The great emergency due to the proliferation of bacterial infections, in conjunction with growing antibiotic resistance, requires the production of new multifunctional drugs and efficient methods for their targeted delivery to control bacteria-associated diseases. Preliminary studies of the glycoconjugate properties as antibacterial agents against representatives of Gram-negative (P. aeruginosa) and Gram-positive (S. aureus) pathogens, which are associated with chronic infections, indicated significant bactericidal activity ascribable to their structural features.

Photophysical Properties of Homo- and Hetero-Aggregate Assemblies Made of N-Annulated Perylene Derivatives.

We report the absorption spectra and photophysical properties of homo and hetero-aggregate assemblies of a strongly emissive N-annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P-MV), in ethanol-water solutions. In homo-aggregate assemblies of P, the π-π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo-aggregate assemblies of P-MV, photoinduced charge separation, with formation of P+ -MV- species, occurs in 3 ps with a charge recombination of 20 ps. In hetero-aggregate P/P-MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P-MV unit is reached, charge separation occurs; however, excimer emission is present for P/P-MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero-aggregate systems extend over a limited number of P chromophores.

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.

Photoinduced Intercomponent Processes in Selectively Addressable Bichromophoric Dyads Made of Linearly Arranged Ru(II) Terpyridine and Expanded Pyridinium Components.

Three new linearly arranged bichromophoric systems 1-3 have been prepared, and their photophysical properties have been studied, taking also advantage of femtosecond pump-probe transient absorption spectroscopy. The three compounds contain the same chromophores, that is a Ru(II)-terpy-like species and a fused expanded bipyridinium (FEBP) unit, separated by three different, variously methylated biphenylene-type bridges. The chromophores have been selected to be selectively addressable, and excitation involving the Ru-based or the FEBP-based dyes results in different excited-state decays. Upon Ru-based excitation at 570 nm, oxidative photoinduced electron transfer (OPET) takes place in 1-3 from the 3MLCT state; however, the charge-separated species does not accumulate, indicating that the charge recombination rate constant exceeds the OPET rate constant. Upon excitation of the organic dye at 400 nm, the FEBP-based 1π-π* level is prepared, which undergoes a series of intercomponent decay events, including (i) electron-exchange energy transfer leading to the MLCT manifold (SS-EnT), which successively decays according to 570 nm excitation, and (ii) reductive photoinduced electron transfer (RPET), leading to the preparation of the charge-separated (CS) state. Reductive PET, involving the FEBP-based singlet state, is much faster than oxidative PET, involving the MLCT triplet state, essentially because of driving force reasons. The rate constant of CR is intermediate between the rate constants of OPET and RPET, and this makes 1-3 capable to selectively read the 400 nm excitation as an active input to prepare the CS state, whereas excitation at wavelengths longer than 480 nm is inefficient to accumulate the CS state. Moreover, intriguing differences between the rate constants of the various processes in 1-3 have been analyzed and interpreted according to the superexchange theory for electron transfer. This allowed us to uncover the role of the electron-transfer and hole-transfer superexchange pathways in promoting the various intercomponent photoinduced decay processes occurring in 1-3.

On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor-bridge-acceptor systems.

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.

Photochemical synthesis, characterization, and electrochemical sensing properties of CD-AuNP nanohybrids.

Among the existing nanosystems used in electrochemical sensing, gold nanoparticles (AuNPs) have attracted considerable attention owing to their intriguing chemical and physical properties such as good electrical conductivity, high electrocatalytic activity, and high surface-to-volume ratio. However, despite these useful characteristics, there are some issues due to their instability in solution that can give rise to aggregation phenomena and the use of hazardous chemicals in the most common synthetic procedures. With an aim to find a solution to these issues, recently, we prepared and characterized carbon dots (CDs), from olive solid wastes, and employed them as reducing and capping agents in photo-activated AuNP synthesis, thus creating CD-Au nanohybrids. These nanomaterials appear extremely stable in aqueous solutions at room temperature, are contemporary, and have been obtained using CDs, which are exclusively based on non-toxic elements, with an additional advantage of being generated from an otherwise waste material. In this paper, the synthesis and characterization of CD-Au nanohybrids are described, and the electrochemical experiments for hydroquinone detection are discussed. The results indicate that CD-Au acts as an efficient material for sensing hydroquinone, matching a wide range of interests in science from industrial processes to environmental pollution.

Molecular dyads of ruthenium(II)- or osmium(II)-bis(terpyridine) chromophores and expanded pyridinium acceptors: equilibration between MLCT and charge-separated excited states.

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Photoinduced electron transfer in Os(terpyridine)-biphenylene-(bi)pyridinium assemblies.

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied. The electronic structure and spectroscopic properties of a representative compound of the series (i.e., 2) have also been investigated at the theoretical level, performing Density Functional Theory (DFT)-based calculations. Time-dependent transient absorption spectra of 1-3 have also been recorded at room temperature. The results indicate that efficient photoinduced oxidative electron transfer takes place in the D-S-A systems at room temperature in fluid solution, for which rate constants (in the range 4 × 10(8)-2 × 10(10) s(-1)) depend on the driving force of the process and the spacer nature. In all the D-S-A systems, charge recombination is faster than photoinduced charge separation, in spite of the relatively large energy of the D(+)-S-A(-) charge-separated states (between 1.47 and 1.78 eV for the various species), which would suggest that the charge recombination occurs in the Marcus inverted region. Considerations based on superexchange mechanism suggest that the reason for the fast charge recombination is the presence of a virtual D-S(+)-A(-) state at low energy--because of the involvement of the easily oxidizable biphenylene spacer--which is beneficial for charge recombination via superexchange but unsuitable for photoinduced charge separation. To further support the above statement, we prepared a fourth D-S-A species, 4, analogous to 2 but with a (hardly oxidizable) single phenylene fragment serving as the spacer. For such a species, charge recombination (about 3 × 10(10) s(-1)) is slower than photoinduced charge separation (about 1 × 10(11) s(-1)), thereby confirming our suggestions.