Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 116
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Am Chem Soc ; 2021 Jan 12.
Artigo em Inglês | MEDLINE | ID: mdl-33434435

RESUMO

Osmium(II) polypyridines are a well-known class of complexes with luminescent metal-to-ligand charge-transfer (MLCT) excited states that are currently experiencing a revival due to their application potential in organic photoredox catalysis, triplet-triplet annihilation upconversion, and phototherapy. At the same time, there is increased interest in the development of photoactive complexes made from Earth-abundant rather than precious metals. Against this background, we present a homoleptic Mo(0) complex with a new diisocyanide ligand exhibiting different bite angles and a greater extent of π-conjugation than previously reported related chelates. This new design leads to deep red emission, which is unprecedented for homoleptic arylisocyanide complexes of group 6 metals. With a 3MLCT lifetime of 56 ns, an emission band maximum at 720 nm, and a photoluminescence quantum yield of 1.5% in deaerated toluene at room temperature, the photophysical properties are reminiscent of the prototypical [Os(2,2'-bipyridine)3]2+ complex. Under 635 nm irradiation with a cw-laser, the new Mo(0) complex sensitizes triplet-triplet annihilation upconversion of 9,10-diphenylanthracene (DPA), resulting in delayed blue fluorescence with an anti-Stokes shift of 0.93 eV. The photorobustness of the Mo(0) complex and the upconversion quantum yield are high enough to generate a flux of upconverted light that can serve as a sufficiently potent irradiation source for a blue-light-driven photoisomerization reaction. These findings are relevant in the greater contexts of designing new luminophores and photosensitizers for use in red-light-driven photocatalysis, photochemical upconversion, light-harvesting, and phototherapy.

2.
Chemistry ; 2020 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-33274791

RESUMO

One-electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so-called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof of concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin-correlated radical pair whose (unproductive) recombination is strongly spin-forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.

3.
Inorg Chem ; 59(20): 14627-14628, 2020 Oct 19.
Artigo em Inglês | MEDLINE | ID: mdl-33070615
4.
Chemistry ; 2020 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-33111994

RESUMO

Transition metal catalyzed cross-coupling reactions are important in chemical synthesis for the formation of C-C and C-heteroatom bonds. Suitable catalysts are frequently based on palladium or nickel, and lately the cheaper and more abundant first-row transition metal element has been much in focus. The combination of nickel catalysis with photoredox chemistry has opened new synthetic possibilities, and in some cases electronically excited states of nickel complexes play a key role. This is a remarkable finding, because photo-excited metal complexes are underexplored in the context of organic bond-forming reactions, and because the photophysics and the photochemistry of first-row transition metal complexes are underdeveloped in comparison with their precious metal-based congeners. Consequently, there is much potential for innovation at the interface of synthetic-organic and physical-inorganic chemistry. This Minireview highlights recent key findings in light-driven nickel catalysis and identifies essential concepts for the exploitation of photoactive nickel complexes in organic synthesis.

5.
Chemistry ; 26(50): 11451-11461, 2020 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-32780914

RESUMO

Resorcin[4]arene cavitands, equipped with diverse quinone (Q) and [Ru(bpy)2 dppz]2+ (bpy=2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazine) photosensitizing walls in different configurations, were synthesized. Upon visible-light irradiation at 420 nm, electron transfer from the [Ru(bpy)2 dppz]2+ to the Q generates the semiquinone (SQ) radical anion, triggering a large conformational switching from a flat kite to a vase with a cavity for the encapsulation of small guests, such as cyclohexane and heteroalicyclic derivatives, in CD3 CN. Depending on the molecular design, the SQ radical anion can live for several minutes (≈10 min) and the vase can be generated in a secondary process without need for addition of a sacrificial electron donor to accumulate the SQ state. Switching can also be triggered by other stimuli, such as changes in solvent, host-guest complexation, and chemical and electrochemical processes. This comprehensive investigation benefits the development of stimuli-responsive nanodevices, such as light-activated molecular grippers.

6.
Inorg Chem ; 59(15): 10430-10438, 2020 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-32687331

RESUMO

Herein is presented a molecular dyad comprised of a [Ru(bpy)3]2+ photosensitizer and an anthraquinone (AQ) acceptor coupled by an ethynyl linker ([Ru(bpy)2(bpy-cc-AQ)]2+) in which activation/deactivation of photoinduced electron-transfer from the [Ru(bpy)3]2+ photosensitizer to the AQ acceptor is achieved and characterized as a function of the dielectric constant and hydrogen-bond donating ability of the solvent used. It is demonstrated that the rate of photoinduced electron-transfer can be modulated over several orders of magnitude (105-1011 s-1) by choice of solvent. Nanosecond transient absorption spectra are dominated by MLCT signals and exhibit identical decay kinetics to the corresponding emission signals. Ultrafast transient absorption and time-resolved infrared spectroscopies provide direct evidence for the formation of the charge-separated (CS) state and rapid (on the order of a few picoseconds) establishment of an excited-state pseudoequilibrium.

7.
Artigo em Inglês | MEDLINE | ID: mdl-32588869

RESUMO

Direct excitation of the commercially available super-electron donor tetrakis(dimethylamino)ethylene (TDAE) with light-emitting diodes at 440 or 390 nm provides a stoichiometric reductant that is able to reduce aryl chlorides and fluorides. The method is very simple and requires only TDAE, substrate, and solvent at room temperature. The photoactive excited state of TDAE has a lifetime of 17.3 ns in cyclohexane at room temperature and an oxidation potential of ca. -3.4 V vs. SCE. This makes TDAE one of the strongest photoreductants able to operate on the basis of single excitation with visible photons. Direct substrate activation occurs in benzene, but acetone is reduced by photoexcited TDAE and substrate reduction takes place by a previously unexplored solvent radical anion mechanism. Our work shows that solvent can have a leveling effect on the photochemically available redox power, reminiscent of the pH-leveling effect that solvent has in acid-base chemistry.

8.
J Am Chem Soc ; 142(23): 10468-10476, 2020 06 10.
Artigo em Inglês | MEDLINE | ID: mdl-32412242

RESUMO

Sensitized triplet-triplet annihilation (sTTA) is the most promising mechanism for pooling the energy of two visible photons, but its applications in solution were so far limited to organic solvents, with a current maximum of the excited-singlet state energy of 3.6 eV. By combining tailor-made iridium complexes with naphthalenes, we demonstrate blue-light driven upconversion in water with unprecedented singlet-state energies approaching 4 eV. The annihilators have outstanding excited-state reactivities enabling challenging photoreductions driven by sTTA. Specifically, we found that an aryl-bromide bond activation can be achieved with blue photons, and we obtained full conversion for the very energy-demanding decomposition of a persistent ammonium compound as typical water pollutant, not only with a cw laser but also with an LED light source. These results provide the first proof-of-concept for the usage of low-power light sources for challenging reactions employing blue-to-UV upconversion in water and pave the way for the further development of sustainable light-harvesting applications.

9.
Inorg Chem ; 2020 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-32142275

RESUMO

The development of new solar-to-fuel scenarios is of great importance, but the construction of molecular systems that convert sunlight into chemical energy represents a challenge. One specific issue is that the molecular systems have to be able to accumulate redox equivalents to mediate the photodriven transformation of relevant small molecules, which mostly involves the orchestrated transfer of multiple electrons and protons. Disulfide/dithiol interconversions are prominent 2e-/2H+ couples and can play an important role for redox control and charge storage. With this background in mind, a new photosensitizer [Ru(S-Sbpy)(bpy)2]2+ (12+) equipped with a disulfide functionalized bpy ligand (S-Sbpy, bpy = 2,2'-bipyridine) was synthesized and has been comprehensively studied, including structural characterization by X-ray diffraction. In-depth electrochemical studies show that the S-Sbpy ligand in 12+ can be reduced twice at moderate potentials (around -1.1 V vs Fc+/0), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion (E2 > E1) and allowed to derive kinetic parameters for the sequential electron-transfer processes. However, reduction at room temperature also triggers the ejection of one sulfur atom from 12+, leading to the formation of [Ru(Sbpy)(bpy)2]2+(22+). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy)3]2+, 12+ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the S-Sbpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 12+ on the basis of ground-state Wigner sampling and using charge-transfer descriptors has shown that bpy modification with a peripheral disulfide moiety leads to an energy splitting between charge-transfer excitations to the S-Sbpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 12+ is photostable and shows an emission from a 3MLCT state in deoxygenated acetonitrile with a lifetime of 109 ns. This work demonstrates a rationally designed system that enables future studies of photoinduced multielectron, multiproton PCET chemistry.

10.
Angew Chem Int Ed Engl ; 59(24): 9659-9668, 2020 06 08.
Artigo em Inglês | MEDLINE | ID: mdl-32166853

RESUMO

Chiral eniminium salts, prepared from α,ß-unsaturated aldehydes and a chiral proline derived secondary amine, underwent, upon irradiation with visible light, a ruthenium-catalyzed (2.5 mol %) intermolecular [2+2] photocycloaddition to olefins, which after hydrolysis led to chiral cyclobutanecarbaldehydes (17 examples, 49-74 % yield), with high diastereo- and enantioselectivities. Ru(bpz)3 (PF6 )2 was utilized as the ruthenium catalyst and laser flash photolysis studies show that the catalyst operates exclusively by triplet-energy transfer (sensitization). A catalytic system was devised with a chiral secondary amine co-catalyst. In the catalytic reactions, Ru(bpy)3 (PF6 )2 was employed, and laser flash photolysis experiments suggest it undergoes both electron and energy transfer. However, experimental evidence supports the hypothesis that energy transfer is the only productive quenching mechanism. Control experiments using Ir(ppy)3 showed no catalysis for the intermolecular [2+2] photocycloaddition of an eniminium ion.

11.
Nat Chem ; 12(4): 323-324, 2020 04.
Artigo em Inglês | MEDLINE | ID: mdl-32203443
12.
Artigo em Inglês | MEDLINE | ID: mdl-31945241

RESUMO

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

13.
Chemistry ; 26(14): 3119-3128, 2020 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-31794079

RESUMO

The development of molecular materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. While copper(I)-bis(diimine) complexes [CuI (L)2 ]+ are considered as potent substitutes for [RuII (bpy)3 ]2+ , they exhibit limited structural integrity as ligand loss by substitution can occur. In this article, we present a new concept to stabilize copper bis(phenanthroline) complexes by macrocyclization of the ligands which are preorganized around the CuI ion. Using oxidative Hay acetylene homocoupling conditions, several CuI complexes with varying bridge length were prepared and analyzed. Absorption and emission properties are assessed; rewardingly, the envisioned approach was successful since the flexible 1,4-butadiyl-bridged complex does show enhanced MLCT absorption and emission, as well as improved photostability upon irradiation with a blue LED compared to a reference complex.

14.
Chemistry ; 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31674695

RESUMO

Atropisomeric 1,2-naphthylene scaffolds provide access to donor-acceptor compounds with helical oligomer-based bridges, and transient absorption studies revealed a highly unusual dependence of the electron-transfer rate on oligomer length, which is due to their well-defined secondary structure. Close noncovalent intramolecular contacts enable shortcuts for electron transfer that would otherwise have to occur over longer distances along covalent pathways, reminiscent of the behavior seen for certain proteins. The simplistic picture of tube-like electron transfer can describe this superposition of different pathways including both the covalent helical backbone, as well as noncovalent contacts, contrasting the wire-like behavior reported many times before for more conventional molecular bridges. The exquisite control over the molecular architecture, achievable with the configurationally stable and topologically defined 1,2-naphthylene-based scaffolds, is of key importance for the tube-like electron transfer behavior. Our insights are relevant for the emerging field of multidimensional electron transfer and for possible future applications in molecular electronics.

15.
J Am Chem Soc ; 141(36): 14394-14402, 2019 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-31464429

RESUMO

Newly discovered tris(diisocyanide)molybdenum(0) complexes are Earth-abundant isoelectronic analogues of the well-known class of [Ru(α-diimine)3]2+ compounds with long-lived 3MLCT (metal-to-ligand charge transfer) excited states that lead to rich photophysics and photochemistry. Depending on ligand design, luminescence quantum yields up to 0.20 and microsecond excited state lifetimes are achieved in solution at room temperature, both significantly better than those for [Ru(2,2'-bipyridine)3]2+. The excited Mo(0) complexes can induce chemical reactions that are thermodynamically too demanding for common precious metal-based photosensitizers, including the widely employed fac-[Ir(2-phenylpyridine)3] complex, as demonstrated on a series of light-driven aryl-aryl coupling reactions. The most robust Mo(0) complex exhibits stable photoluminescence and remains photoactive after continuous irradiation exceeding 2 months. Our comprehensive optical spectroscopic and photochemical study shows that Mo(0) complexes with diisocyanide chelate ligands constitute a new family of luminophores and photosensitizers, which is complementary to precious metal-based 4d6 and 5d6 complexes and represents an alternative to nonemissive Fe(II) compounds. This is relevant in the greater context of sustainable photophysics and photochemistry, as well as for possible applications in lighting, sensing, and catalysis.

16.
Dalton Trans ; 48(31): 11690-11705, 2019 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-31265019

RESUMO

We report on the photophysical properties of three dyads that combine a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a Pt(PEt3)2 fragment. σ-Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a multitude of (potentially) emissive states. This culminates in the presence of four different emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the 3PB-CT state is observed with a quantum yield of up to 15%. Excitation into the BDP-based 1ππ* or the pyrene-based 1ππ* band additionally trigger fluorescence and phosphorescence emissions from the BDP-centred 1ππ* and 3ππ* states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence emissions from the KBDP ligand. The assignments of the various excited states and the deactivation cascades were probed by absorption and emission spectroscopy, transient absorption spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are able to generate singlet oxygen.

17.
Chem Sci ; 10(21): 5624-5633, 2019 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-31293747

RESUMO

Charge-separated states (CSSs) are key intermediates in photosynthesis and solar energy conversion. However, the factors governing the formation efficiencies of CSSs are still poorly understood, and light-induced electron-hole recombinations as deactivation pathways competing with desired charge accumulations are largely unexplored. This greatly limits the possibility to perform efficient multi-electron transfer, which is essential for artificial photosynthesis. We present a systematic investigation of two donor-sensitizer-acceptor triads (with different donor-acceptor distances) capable of storing as much as 2.0 eV in their CSSs upon the absorption of a visible photon. Using quantitative one- and two-pulse laser flash photolysis, we provide deep insights into both the CSS formation quantum yield, which can reach up to 80%, and the fate of the CSS upon further (secondary) excitation with green photons. The triad with shorter intramolecular distances shows a remarkable excitation wavelength dependence of the CSS formation quantum yield, and the CSS of this triad undergoes more efficient light-induced charge recombination than the longer equivalent by about one order of magnitude, whilst thermal charge recombination shows the exact opposite behavior. The unexpected results of our detailed photophysical study can be rationalized by detrimental singlet charge transfer states or structural considerations, and could significantly contribute to the future design of CSS precursors for accumulative multi-electron transfer and artificial photosynthesis.

18.
Chem Commun (Camb) ; 55(28): 4004-4014, 2019 Apr 02.
Artigo em Inglês | MEDLINE | ID: mdl-30810148

RESUMO

The conversion of CO2, H2O, or N2 to energy-rich compounds such as CH3OH, H2 or NH3 requires the properly orchestrated transfer of multiple electrons and protons. Artificial photosynthetic systems therefore must be able to synchronize the rapid primary photoinduced transfer of single electrons to the slower catalytic (multi-electron) turnover of substrates, and this generates a need for temporary accumulation and storage of redox equivalents. This is a very difficult task, particularly in absence of sacrificial reagents. Toward this end, proton-coupled multi-electron transfer (PCMET) driven by light is now receiving increased attention. This invited Feature article considers recent pertinent studies of donor-sensitizer-acceptor compounds and inorganic-organic hybrid systems, as well as some recent photoredox catalysis studies of proton-coupled multi-electron reductions. Key principles for successful light-driven accumulation and storage of redox equivalents are discussed, and the relevance of PCMET for the formation of solar fuels and for photoredox catalysis is emphasized.

19.
J Am Chem Soc ; 141(5): 2122-2127, 2019 02 06.
Artigo em Inglês | MEDLINE | ID: mdl-30672694

RESUMO

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

20.
Chemistry ; 25(24): 6043-6052, 2019 Apr 26.
Artigo em Inglês | MEDLINE | ID: mdl-30615242

RESUMO

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example, in lighting, sensing, solar cells, and photoredox catalysis. There is a long-standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long-lived excited state with pronounced charge-transfer character between the ruthenium metal and ligands. These metal-to-ligand charge-transfer (MLCT) excited states can be luminescent, with typical lifetimes in the range of 100 to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes, the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal-centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than that of commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research into photoactive earth-abundant metal complexes. After outlining some of the fundamental challenges, key strategies used so far to enhance the photophysical and photochemical properties of iron complexes are discussed and recent conceptual breakthroughs are highlighted in this invited Concept article.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA