Spectroscopic Insights of an Emissive Complex between 4'-N,N-Diethylaminoflavonol in Octa-Acid Deep-Cavity Cavitand and Rhodamine 6G.

Excited-state chemistry relies on the communication between molecules, making it a crucial aspect of the field. One important question that arises is whether intermolecular communication and its rate can be modified when a molecule is confined. To explore the interaction in such systems, we investigated the ground and excited states of 4'-N,N-diethylaminoflavonol (DEA3HF) in an octa acid-based (OA) confined medium and in ethanolic solution, both in the presence of Rhodamine 6G (R6G). Despite the observed spectral overlap between the flavonol emission and the R6G absorption, as well as the fluorescence quenching of the flavonol in the presence of R6G, the almost constant fluorescence lifetime at different amounts of R6G discards the presence of FRET in the studied systems. Steady-state and time-resolved fluorescence indicate the formation of an emissive complex between the proton transfer dye encapsulated within water-soluble supramolecular host octa acid (DEA3HF@(OA)2) and R6G. A similar result was observed between DEA3HF:R6G in ethanolic solution. The respective Stern-Volmer plots corroborate with these observations, suggesting a static quenching mechanism for both systems.

Anticancer Agent with Inexplicable Potency in Extreme Hypoxia: Characterizing a Light-Triggered Ruthenium Ubertoxin.

Tumor hypoxia renders treatments ineffective that are directly (e.g., radiotherapy and photodynamic therapy) or indirectly (e.g., chemotherapy) dependent on tumor oxygenation. This study introduces a ruthenium compound as a light-responsive anticancer agent that is water-soluble, has minimal dark cytotoxicity, is active at concentrations as low as 170 pM in ∼18.5% O2 normoxia and near 10 nM in 1% O2 hypoxia, and exhibits phototherapeutic indices as large as >500,000 in normoxia and >5,800 in 1% O2 hypoxia using broadband visible and monochromatic blue light treatments. These are the largest values reported to date for any compound class. We highlight the response in four different cell lines to improve rigor and reproducibility in the identification of promising clinical candidates.

Revisiting Dinuclear Ruthenium Water Oxidation Catalysts: Effect of Bridging Ligand Architecture on Catalytic Activity.

An attractive catalytic pathway for the conversion of water to oxygen would involve two metal oxide centers combining in a constructive sense to make O═O. This prospect makes the study of certain dinuclear transition metal complexes particularly attractive. In this work, we describe the design and synthesis of two symmetrical bis-tridentate polypyridine ligands 6 and 12 that bind two RuII centers at a separation of 3.6 Šin 7 and 5.7 Šin 13. In the presence of CeIV at pH = 1, these systems oxidize water with the system having the more proximal metals being more reactive. In the case of the more proximal metal centers, the bridging ligand is a 3,6-disubstituted pyridazine which, under the influence of CeIV, cleaves into two [Ru(bpc)(pic)2CH3CN]+ fragments (14) which then function as the actual catalyst (bpc = 2,2'-bipyridine-6-carboxylate, pic = 4-methylpyridine). The second dinuclear catalyst contains a central pyrimidine ring which is less sensitive to oxidative decay and hence less reactive. Caution is advised in the use of CeIV as a sacrificial electron acceptor due to unexpected oxidative decay of the catalyst.

Supramolecular Control of Singlet Oxygen Generation.

Singlet oxygen (1O2) is the excited state electronic isomer and a reactive form of molecular oxygen, which is most efficiently produced through the photosensitized excitation of ambient triplet oxygen. Photochemical singlet oxygen generation (SOG) has received tremendous attention historically, both for its practical application as well as for the fundamental aspects of its reactivity. Applications of singlet oxygen in medicine, wastewater treatment, microbial disinfection, and synthetic chemistry are the direct results of active past research into this reaction. Such advancements were achieved through design factors focused predominantly on the photosensitizer (PS), whose photoactivity is relegated to self-regulated structure and energetics in ground and excited states. However, the relatively new supramolecular approach of dictating molecular structure through non-bonding interactions has allowed photochemists to render otherwise inactive or less effective PSs as efficient 1O2 generators. This concise and first of its kind review aims to compile progress in SOG research achieved through supramolecular photochemistry in an effort to serve as a reference for future research in this direction. The aim of this review is to highlight the value in the supramolecular photochemistry approach to tapping the unexploited technological potential within this historic reaction.

Correction: Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors.

Correction for 'Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors' by Fabiano S. Santos et al., Photochem. Photobiol. Sci., 2017, 16, 840-844.

Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors.

Efficient photoinduced electron transfer from proton transfer dyes encapsulated within water soluble supramolecular host octa acid to electron acceptors present outside the capsule was observed in aqueous solution. 4,4'-Dimethylviologen dichloride was found to be the best acceptor compared to N-methylpyridinium iodide, most likely due to its better binding with the exterior of the host octa acid walls [corrected].

Supramolecular Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on a Clay Nanosheet Surface.

Three coumarin derivatives (7-propoxy coumarin, coumarin-480, and coumarin-540a, 2, 3, and 4, respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium-functionalized cavitand octaamine (OAm, 1). Guests 2, 3, and 4 absorb in ultraviolet, violet, and blue regions and emit in violet, blue, and green regions, respectively. Energy transfer between the above three coumarin@(OAm)2 complexes assembled on the surface of a saponite clay nanosheet was investigated by steady-state and time-resolved emission techniques. Judging from their emission and excitation spectra, we concluded that the singlet-singlet energy transfer proceeded from 2 to 3, from 2 to 4, and from 3 to 4 when OAm-encapsulated 2, 3, and 4 were aligned on a clay surface as two-component systems. Under such conditions, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were calculated to be 33, 36, and 50% in two-component systems. When all three coumarins were assembled on the surface and 2 was excited, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were estimated to be 32, 34, and 33%. A comparison of energy transfer efficiencies of the two-component and three-component systems revealed that excitation of 2 leads to emission from 4. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three-component dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in an artificial light-harvesting phenomenon.

Room temperature phosphorescence from a guest molecule confined in the restrictive space of an organic-inorganic supramolecular assembly.

Stable room-temperature phosphorescence of guest aromatic molecules was achieved by the effective suppression of oxygen quenching. The organic capsule (first wall) suppressed static oxygen quenching by enclosing a guest molecule, and dynamic quenching via the capsule opening-closing process was well suppressed and manipulated by the intercalation of this capsule into the restrictive space between clay nanosheets (second wall).

Sequential energy and electron transfer in a three-component system aligned on a clay nanosheet.

To achieve the goal of energy transfer and subsequent electron transfer across three molecules, a phenomenon often utilized in artificial light harvesting systems, we have assembled a light absorber (that also serves as an energy donor), an energy acceptor (that also serves as an electron donor) and an electron acceptor on the surface of an anionic clay nanosheet. Since neutral organic molecules have no tendency to adsorb onto the anionic surface of clay, a positively charged water-soluble organic capsule was used to hold neutral light absorbers on the above surface. A three-component assembly was prepared by the co-adsorption of a cationic bipyridinium derivative, cationic zinc porphyrin and cationic octaamine encapsulated 2-acetylanthracene on an exfoliated anionic clay surface in water. Energy and electron transfer phenomena were monitored by steady state fluorescence and picosecond time resolved fluorescence decay. The excitation of 2-acetylanthracene in the three-component system resulted in energy transfer from 2-acetylanthracene to zinc porphyrin with 71% efficiency. Very little loss due to electron transfer from 2-acetylanthracene in the cavitand to the bipyridinium derivative was noticed. Energy transfer was followed by electron transfer from the zinc porphyrin to the cationic bipyridinium derivative with 81% efficiency. Analyses of fluorescence decay profiles confirmed the occurrence of energy transfer and subsequent electron transfer. Merging the concepts of supramolecular chemistry and surface chemistry we realized sequential energy and electron transfer between three hydrophobic molecules in water. Exfoliated transparent saponite clay served as a matrix to align the three photoactive molecules at a close distance in aqueous solutions.

Photophysical studies of an encapsulated neutral guest intercalated into the 2-dimensional space of α-Zr(IV) phosphate.

Our long-range objective of developing surface anchored supramolecular assemblies as artificial light harvesting systems led us to explore the intercalation of guest molecules confined within octaamine hydrochloride (OAm·HCl) in the 2-dimensional galleries of layered inorganic material α-Zr(IV)phosphate (α-ZrP). Photophysical properties of 4,4'-dimethylbenzil, camphorthione, 4,4'-dimethylstilbene, pyrene and coumarin-1 were used to probe the intercalation behavior of OAm capsules within the galleries of α-ZrP. (1)H NMR and emission spectral investigations suggested the inclusion of guests within OAm and also confirmed the stability of host-guest complexes under acidic conditions in water. Stirring guest encapsulated OAm capsule with exfoliated α-ZrP nanosheets resulted in intercalation of the host-guest assembly as a whole in the case of 4,4'-dimethylbenzil, camphorthione, and 4,4'-dimethylstilbene as guests. According to powder X-ray diffraction and emission data these capsules are stable in the galleries of α-ZrP. The fact that the capsules are stable and can be included in α-ZrP nanosheets opens up further opportunities to explore inclusion of two different capsules, one with a donor and the other with an acceptor, and study the energy and electron transfer phenomenon between neutral molecules in α-ZrP galleries.

Excited state chemistry of flavone derivatives in a confined medium: ESIPT emission in aqueous media.

The excited state behavior of two flavone derivatives 3-hydroxyflavone and 4'-N,N-diethylaminoflavonol in a confined medium indicates that supramolecular effects could alter the nature of the fluorescence emission. Within the octa acid host the neutral unionized species of these two dyes are present showing large Stokes shifted emission due to intramolecular proton transfer, a pattern different from that in aqueous medium.

Excited state chemistry of capsular assemblies in aqueous solution and on silica surfaces.

Synthesis and encapsulation properties of two new water-soluble resorcinol-capped organic cavitands (tetra acid and octa acid; RTA and ROA) are reported in this Letter. Organic guest molecules template the formation of capsular assembly of the above cavitands in water. Depending upon the guest, either 1:2 (guest to host) or 2:2 capsular assemblies were formed. The excited state properties of guests such as anthracene, camphorthione, and 4,4'-dimethyl benzil were distinctly different within a capsular assembly from those when they were free in a solution. Importantly, the host-guest complexes of the above two hosts (RTA and ROA) as well as octa acid (OA) could be transferred to a silica surface. The excited state behavior of host-guest assemblies on silica surface resembled that in solution. The high cage effect in the decarbonylation products and high yield of rearrangement product obtained upon photolysis of 1-phenyl-3-tolyl-2-propanone included within RTA, ROA, and OA both in solution and on silica surface supported the conclusion that capsular assemblies of these hosts are stable on silica surface.

Insights into enantioselective separations of ionic metal complexes by sub/supercritical fluid chromatography.

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO2, even with additives and organic modifiers. Recently, a large amount of research has been centered on octahedral complexes of Ru(II) and Os(II) with bidentate polypyridyl ligands due to their ability to serve in cancer treatment and other biological activities. These compounds exist as the delta (Δ) and lambda (Λ) enantiomers. Previously, similar compounds have been enantiomerically separated using HPLC and capillary electrophoresis, but never with SFC. Cyclofructan-6 (CF6) derivatized with (R)-naphthyl ethyl (RN) groups has been proven to be an effective chiral stationary phase for these separations in HPLC. This column chemistry was expanded to SFC to provide the first chiral separation of a wide variety (23 complexes in total) of ionic octahedral polypyridyl complexes. Unexpected behavior for mixing methanol and acetonitrile as the organic modifier will be discussed, along with the effects of additives. Enantioselectivity on CF6-RN chemistry is shown to be dependent on the conjugation level and rigidity of the metal complexes. Mass transfer kinetic behavior is also shown, and high-efficiency baseline resolved rapid separations are shown for fast screening or quantitation of representative coordination complexes.

Fine-Feature Modifications to Strained Ruthenium Complexes Radically Alter Their Hypoxic Anticancer Activity†.

In an earlier study of π-expansive ruthenium complexes for photodynamic and photochemo-therapies, it was shown that a pair of structural isomers differing only in the connection point of a naphthalene residue exhibited vastly different biological activity. These isomers are further explored in this paper through the activity of their functionalized derivatives. In normoxia, the inactive 2-NIP isomer (5) can be made as photocytotoxic as the active 1-NIP isomer (1) by functionalizing with methyl or methoxy groups, while methoxy variants of the 1-NIP isomer became inactive. In all cases, the singlet oxygen sensitization quantum yield was below 1%. Hypoxic photocytotoxicity was attenuated, with only three of the series showing any activity, notwithstanding the photodissociative ligands. The results here are consistent with the earlier findings in that seemingly minor structural modifications on the non-strained ligand can dramatically modulate the normoxic and hypoxic activity of these strained compounds and that these changes appear to exert a greater influence on photocytotoxicity than singlet oxygen sensitization or rates of photosubstitution in cell-free conditions would suggest.

Ruthenium Photosensitizers for NIR PDT Require Lowest-Lying Triplet Intraligand (3IL) Excited States.

A family of complexes of the type [Ru(tpbn)(IP-R)(4-pic)]Cl2 (tbpn=2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(1,8-napthyridine); 4-pic=4-picoline; IP-R=imidazo[4,5-f][1,10]phenanthroline attached to an aromatic group R for 2-8 and H for 1) were prepared as near-infrared (NIR) absorbing coordination complexes to test whether triplet intraligand excited states (3IL) of higher energy than the lowest-lying triplet metal-to-ligand charge transfer excited states (3MLCT) could effectively generate cytotoxic singlet oxygen (1O2) and elicit in vitro photodynamic therapy (PDT) effects. Aromatic groups ranged from benzene to anthracene, with corresponding triplet state energies that were all significantly higher (approximately 3.7-1.8 eV) than the 3MLCT state estimated at 1.5 eV. Complexes 1-8 absorbed NIR light, with their longest-wavelength peak maxima occurring near 725 nm that extended out to 800 nm. The 1O2 quantum yields for the aromatic-containing compounds were extremely small (ΦΔ=0.07), with correspondingly modest in vitro photocytotoxicities. All compounds were nontoxic without a light trigger, with dark EC50 values >60 µM and most values closer to 100 or greater. EC50 values with visible light were 5-6 (PI=15-20), 7-10 (PI=8-11), and 10-15 µM (PI=6-8) in SKMEL28, A375, and B16F10 cancer cell lines, respectively. With NIR light, these values were even less: 11-16 (PI=5-9), 16-50 (PI=2-6), and 15-19 µM (PI=4-6) in SKMEL28, A375, and B16F10 cancer cell lines, respectively. While measurable, the modest activities and absence of any trend between the 3IL energies and values for ΦΔ or PI demonstrate that 3IL states with energies above the lowest-lying 3MLCT states do not contribute to the overall excited state dynamics responsible for potent PDT effects in previous studies. Lowest-lying 3MLCT states in this family of NIR-absorbing photosensitizers do not produce the requisite 1O2 for effective in vitro photocytotoxic effects, underscoring the need to install 3IL states that are lower in energy than the lowest-lying 3MLCT states in order the create potent NIR-activatable Ru(II) complexes for PDT.

Supramolecular-Surface Photochemistry: Assembly and Photochemistry of Host-Guest Capsules on Silica Surface.

Host cavitands and organic guest molecules independently adsorbed on silica particles when mixed and shaken in the presence of a few drops of water underwent intra- and interparticle migration to form capsular complexes that were not formed either in water or organic solvents. Importance of cavitand migration and tumbling on silica surface was established by demonstrating that covalently linked cavitands do not form capsular complexes. The encapsulated guests exhibited selective photochemistry as they do within an organic capsule in solution.