Dual Role of a Novel Heteroleptic Cu(I) Complex in Visible-Light-Driven CO2 Reduction.

A novel mononuclear Cu(I) complex was synthesized via coordination with a benzoquinoxalin-2'-one-1,2,3-triazole chelating diimine and the bis[(2-diphenylphosphino)phenyl] ether (DPEPhos), to target a new and efficient photosensitizer for photocatalytic CO2 reduction. The Cu(I) complex absorbs in the blue-green region of the visible spectrum, with a broad band having a maximum at 475 nm (ϵ =4500 M-1 cm-1), which is assigned to the metal-to-ligand charge transfer (MLCT) transition from the Cu(I) to the benzoquinoxalin-2'-one moiety of the diimine. Surprisingly, photo-driven experiments for the CO2 reduction showed that this complex can undergo a photoinduced electron transfer with a sacrificial electron donor and accumulate electrons on the diimine backbone. Photo-driven experiments in a CO2 atmosphere revealed that this complex can not only act as a photosensitizer, when combined with an Fe(III)-porphyrin, but can also selectively produce CO from CO2. Thus, owing to its charge-accumulation properties, the non-innocent benzoquinoxalin-2-one based ligand enabled the development of the first copper(I)-based photocatalyst for CO2 reduction.

Flavin-induced charge separation in transmembrane model peptides.

Hydrophobic peptide models derived from the α-helical transmembrane segment of the epidermal growth factor receptor were synthetically modified with a flavin amino acid as a photo-inducible charge donor and decorated with tryptophans along the helix as charge acceptors. The helical conformation of the peptides was conserved despite the modifications, notably also in lipid vesicles and multibilayers. Their ability to facilitate photo-induced transmembrane charge transport was examined by means of steady-state and time-resolved optical spectroscopy. The first tryptophan next to the flavin donor plays a major role in initiating the charge transport near the N-terminus, while the other tryptophans might promote charge transport along the transmembrane helix. These artificially modified, but still naturally derived helical peptides are important models for studying transmembrane electron transfer and the principles of photosynthesis.

Peptide Self-Assembly Controlled Photoligation of Polymers.

Highly efficient chemical ligations that operate in water under mild conditions are the foundation of bioorthogonal chemistry. However, the toolbox of suitable reactions is limited. Conventional approaches to expand this toolbox aim at altering the inherent reactivity of functional groups to design new reactions that meet the required benchmarks. Inspired by controlled reaction environments that enzymes provide, we report a fundamentally different approach that makes inefficient reactions highly efficient within defined local environments. Contrasting enzymatically catalyzed reactions, the reactivity controlling self-assembled environment is brought about by the ligation targets themselves─avoiding the use of a catalyst. Targeting [2 + 2] photocycloadditions, which are inefficient at low concentrations and readily quenched by oxygen, short ß-sheet encoded peptide sequences are inserted between a hydrophobic photoreactive styrylpyrene unit and a hydrophilic polymer. In water, electrostatic repulsion of deprotonated amino acid residues governs the formation of small self-assembled structures, which enable a highly efficient photoligation of the polymer, reaching ∼90% ligation within 2 min (0.034 mM). Upon protonation at low pH, the self-assembly changes into 1D fibers, altering photophysical properties and shutting down the photocycloaddition reaction. Using the reversible morphology change, it is possible to switch the photoligation "ON" or "OFF" under constant irradiation simply by varying the pH. Importantly, in dimethylformamide, the photoligation reaction did not occur even at 10-fold higher concentrations (0.34 mM). The self-assembly into a specific architecture, encoded into the polymer ligation target, enables a highly efficient ligation that overcomes the concentration limitations and high oxygen sensitivity of [2 + 2] photocycloadditions.

Phase-Dependent Long Persistent Phosphorescence in Coumarin-Phosphine-Based Coinage Metal Complexes.

A coumarin functionalized aminodiphosphine has been introduced as a bidentate ligand in coinage metal chemistry. Mono-, di-, and trimetallic copper and silver complexes were synthesized with this ligand. The hybrid character of the ligand led to compounds with rich luminescence properties. These include coumarin-based blue fluorescence, observed as a sole emission in solution at room temperature, and green phosphorescence, which is efficient at low temperatures and dominates the spectra of the metal complexes. In the rigid environment of frozen solutions, the green phosphorescence shows an unusually long (for metal complexes) decay on the seconds timescale in high quantum yield. In addition, a red phosphorescence, which may be assigned to the triplet state localized in the phosphine-M3 Cl2 (M=Cu, Ag), is observed for the trinuclear complexes at low temperature. Neither the second-long phosphorescence nor the red emission is observed for the coumarin ligand, thus they must be a result of the coordination to coinage metal clusters. The excited states in these compounds were also investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations.

Excited State Dynamics of Alizarin Red S Nanoparticles in Solution.

Alizarin red S is a sulfonated, water-soluble derivative of alizarin. This work presents femtosecond studies of alizarin red S (ARS) nanoparticles in comparison to ARS in aqueous solution and to alizarin in DMSO. The femtosecond studies cover a probing spectral range of 350-750 nm using different excitation wavelengths, taking into account the variation of the absorption spectra with the pH values of the solvent. Stationary absorption spectra show slight differences between solution and nanoparticles. Excitation at 530 nm results in low and noisy responses, therefore, we additionally recorded transient spectra of the nanoparticles at λex = 267 nm. While the results in DMSO are comparable to previous studies in non-aqueous solvents, we report a relatively fast relaxation of 14 ps in [La(OH)2][ARS] nanoparticles in aqueous solution after excitation at 530 nm, which is similar to Na(ARS) solution (19 ps). The dynamics changed with lower pH, but still without significant differences between nanoparticles and solution. We propose [La(OH)2][ARS] nanoparticles as a suitable alternative to dissolved molecules with similar spectroscopic properties, for example, with regard to biomarker applications.

Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation.

We compare the intersystem crossing rate, kISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]2-), the singly deprotonated monoanion ([RB-H]-), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]- and [RB-2H + Cs]-, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S1) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopy─all with ca. 50 femtosecond temporal resolution. [RB-H]- shows an S1 lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]2-, has a much longer lifetime. Superimposed on S1 decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4-5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S1 state, initially populated in the S0 geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussed─also in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states.

Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence.

The enantiomerically pure ligand P,P-diphenyl-N,N'-bis((R)-1-phenylethyl)phosphinimidic amide (1; (R)-HPEPIA) was synthesized and subsequently deprotonated with alkali metal precursors to yield dimeric complexes [M2{(R)-PEPIA}2] (M = Li (2), Na (3), K (4), Rb (5)). The cesium compound [M{(R)-PEPIA}] (6) crystallized as a cocrystal composed of dimeric ([Cs2{(R)-PEPIA}2] (6d ) and 1D-polymeric ([Cs{(R)-PEPIA}] n ) (6p ) species in a 1 : 1 ratio. The coordination polymer 6p features a unique sinus-shaped configuration of repeating -Cs-N-P-N-Cs-N-P-N- units. Unusual photoluminescence (PL) properties were found for solid 1-6: in contrast to the fluorescent ligand 1, the alkali metal complexes show phosphorescence at low temperatures (<100 K) and thermally activated delayed fluorescence (TADF) above ∼150 K. The latter provides for PL quantum yields up to 36% (3) at ambient temperature. DFT calculations support that both 1 and 2-6d have similar singlet and triplet excited states with energy separations of a few tens of meV. The strongly enhanced intersystem crossing (ISC) in the metal complexes, resulting in TADF, is attributed to their dimeric structure. This suggests that the fluorophore dimerization may serve as a tool to effect ISC for the design of TADF emitters.