Monitoring the photochemistry of a formazan over 15 orders of magnitude in time

2,3,5-triphenyltetrazolium chloride (TTC) may convert into phenyl-benzo[c] tetrazolocinnolium chloride (PTC) and 1,3,5-triphenylformazan (TPF) under irradiation with light. The latter reaction, albeit enzymatically rather than photochemically, is used in so-called TTC assays indicating cellular respiration and cell growth. In this paper, we address the photochemistry of TPF with time-resolved spectroscopy on various time scales. TPF is stabilized by an intramolecular hydrogen bond and switches photochemically via an E-Z isomerization around an N=N double bond into another TPF-stereoisomer, from which further isomerizations around the C=N double bond of the phenylhydrazone group are possible. We investigate the underlying processes by time-resolved spectroscopy in dependence on excitation wavelength and solvent environment, resolving several intermediates over a temporal range spanning 15 orders of magnitude (hundreds of femtoseconds to hundreds of seconds) along the reaction path. In a quantum-chemical analysis, we identify 16 stable ground-state isomers and discuss which ones are identified in the experimental data. We derive a detailed scheme how these species are thermally and photochemically interconnected and conclude that proton transfer processes are involved

Excited‐State Proton Transfer Dynamics of a Super‐Photoacid in Acetone‐Water Mixtures

Super-photoacids, that is, photoacids with a negative urn:x-wiley:23670932:media:cptc202200041:cptc202200041-math-0001 value in the electronically excited state, can trigger an excited-state proton transfer (ESPT) to the solvent. For the neutral pyranine-derived super-photoacid studied here, even indications for ESPT in acetoneous solution are reported. The characteristics of ESPT in this environment, that is, which intermediates exist and what the impact of cosolvents is, remain unsettled though. In this work, we study ESPT in acetone-water mixtures by steady-state and time-resolved fluorescence spectroscopy. Various effects are observed: First, the addition of water supports the formation of a hydrogen-bonded ground-state complex comprising one water molecule and the photoacid, whose excitation triggers the formation of a hydrogen-bonded ion pair on a sub-ns time scale. Second, water has an overall accelerating effect on the fluorescence dynamics of the involved emitting species, whose contributions are disentangled in a global analysis scheme, enabling the identification of emission from the free photoacid, a photoacid-water complex, a hydrogen-bonded ion pair, and the deprotonated photoacid. At least two water molecules are necessary for ESPT in the environment. Third, additional acidification thwarts an efficient ground-state complex formation of the photoacid and water. However, upon excitation, complexation may occur on a timescale faster than the photoacid's excited-state lifetime, so that emission from a nascent complex emerges

Thermodynamic Driving Forces of Guest Confinement in a Photoswitchable Cage

Photoswitchable cages that confine small guest molecules inside their cavities offer a way to control the binding/unbinding process through irradiation with light of different wavelengths. However, a detailed characterization of the structural and thermodynamic consequences of photoswitching is very challenging to obtain by experiment alone. Thus, all-atom molecular dynamics (MD) simulations were carried out to gain insight into the relationship between structure and binding affinity. Binding free energies of the B12F122- guest were obtained for all photochemically accessible forms of a photoswitchable dithienylethene (DTE) based coordination cage. The MD simulations show that successive photo-induced closure of the four individual DTE ligands that form the cage gradually decreases the binding affinity. Closure of the first ligand already significantly lowers the unbinding barrier and the binding free energy, and therefore favours guest unbinding both kinetically and thermodynamically. Analysis of the different enthalpy contributions to the free energy shows that binding is enthalpically unfavourable and thus an entropy-driven process, in agreement with experimental data. Dissecting the enthalpy into the contributions from electrostatic, van der Waals, and bonded interactions in the force field shows that the unfavourable binding enthalpy is due to the bonded interactions being more favourable in the dissociated state, suggesting the presence of structural strain in the bound complex. Thus, the simulations provide microscopic explanations for the experimental findings and open a possible route towards the targeted design of switchable nanocontainers with modified binding properties

Steuerung der ultraschnellen Öffnungs‐ und Schließungsdynamik eines photochromen Koordinationskäfigs durch Gastmoleküle

Photochemische Studien über supramolekulare Wirte, die kleine Gastmoleküle einkapseln können, konzentrieren sich zumeist auf drei Aspekte: Die Photoschaltung des Käfigs, um den Gast freizusetzen oder einzufangen, die Wirkung der Käfigumgebung auf den Gast und die lichtinduzierte Exzitonen- oder Ladungsübertragung innerhalb der Käfigstruktur. Hier nutzen wir ultraschnelle Spektroskopie, um zu untersuchen, wie der Gast die Photoschaltcharakteristik des Käfigs verändert. Zu diesem Zweck werden die Auswirkungen von drei unterschiedlichen Gastmolekülen auf die Ringöffnung oder den Ringschluss eines Dithienylethen (DTE)-Liganden in einem photoschaltbaren Koordinationskäfig auf DTE-Basis einander gegenübergestellt. Der Gast moduliert sowohl das Ergebnis als auch die Zeitskala der Photodynamik des Käfigs durch ein Zusammenspiel von struktureller Wechselwirkung, dem Schweratomeffekt und einer Verstärkung von Ladungstransferprozessen, die der Gast auf den photoangeregten Käfig ausübt. Der Ansatz könnte sich als nützlich erweisen, um die Anwendbarkeit von photoschaltbaren Nanocontainern und gewünschten Gastverbindungen aufeinander abzustimmen

Steering the Ultrafast Opening and Closure Dynamics of a Photochromic Coordination Cage by Guest Molecules

Photochemical studies on supramolecular hosts that can encapsulate small guest molecules commonly focus on three aspects: photoswitching the cage to release or trap the guest, the effect of the confining environment on the guest, and light-induced exciton or charge transfer within the cage structure. Here, we exploit ultrafast spectroscopy to address how the guest alters the photoswitching characteristics of the cage. For this, the impacts of three disparate guest compounds on ring-opening or ring-closure of a dithienylethene (DTE) ligand in a photoswitchable DTE-based coordination cage are juxtaposed. The guest modulates both outcome and timescale of the cage's photodynamics, by an interplay of structural strain, heavy-atom effect, and enhancement of charge-transfer processes exercised by the guest on the photo-excited cage. The approach might prove beneficial for attuning the applicability of photoswitchable nanocontainers and desired guest compounds

Ultrafast transient absorption and solvation of a super-photoacid in acetoneous environments

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales

Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer

The photochemical deracemization of spiro[cyclopropane-1,3 '-indolin]-2 '-ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2-dichloro compound is configurationally labile upon direct irradiation at lambda=350 nm and upon irradiation at lambda=405 nm in the presence of achiral thioxanthen-9-one as the sensitizer. The triplet 1,3-diradical intermediate generated in the latter reaction was detected by transient absorption spectroscopy and its lifetime determined (tau=22 mu s). Using a chiral thioxanthone or xanthone, with a lactam hydrogen bonding site as a photosensitizer, allowed the deracemization of differently substituted chiral spirocyclopropyl oxindoles with yields of 65-98 % and in 50-85 %ee(17 examples). Three mechanistic contributions were identified to co-act favorably for high enantioselectivity: the difference in binding constants to the chiral thioxanthone, the smaller molecular distance in the complex of the minor enantiomer, and the lifetime of the intermediate 1,3-diradical

Hydrogen‐Bond‐Modulated Nucleofugality of SeIII Species to Enable Photoredox‐Catalytic Semipinacol Manifolds

Chemical bond activations mediated by H-bond interactions involving highly electronegative elements such as nitrogen and oxygen are powerful tactics in modern catalysis research. On the contrary, kindred catalytic regimes in which heavier, less electronegative elements such as selenium engage in H-bond interactions to co-activate C−Se σ-bonds under oxidative conditions are elusive. Traditional strategies to enhance the nucleofugality of selenium residues predicate on the oxidative addition of electrophiles onto SeII-centers, which entails the elimination of the resulting SeIV moieties. Catalytic procedures in which SeIV nucleofuges are substituted rather than eliminated are very rare and, so far, not applicable to carbon-carbon bond formations. In this study, we introduce an unprecedented combination of O−H⋅⋅⋅Se H-bond interactions and single electron oxidation to catalytically generate SeIII nucleofuges that allow for the formation of new C−C σ-bonds by means of a type I semipinacol process in high yields and excellent selectivity

Pseudo‐Octahedral Iron(II) Complexes with Near‐Degenerate Charge Transfer and Ligand Field States at the Franck‐Condon Geometry

Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's pi* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp)(2)](2+) (1(2+)) and [Fe(cpmp)(ddpd)](2+) (2(2+)) with the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy pi* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings. The push-pull ligand cpmp provides intra-ligand and ligand-to-ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 1(2+) and 2(2+) were accessed by X-ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X-ray emission spectroscopy, static and time-resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations