One-Photon Excitation Followed by a Three-Step Sequential Energy–Energy–Electron Transfer Leading to a Charge-Separated State in a Supramolecular Tetrad Featuring Benzothiazole–Boron-Dipyrromethene–Zinc Porphyrin–C60

A panchromatic triad, consisting of benzothiazole (BTZ) and BF₂-chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using ¹H NMR spectroscopy, ESI-MS, UV-visible, steady-state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ-BODIPY-ZnP, and dyads, BTZ-BODIPY and BODIPY-ZnP, along with the reference compounds BTZ-OMe, BODIPY-OMe, and ZnP-OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ-BODIPY-ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ-BODIPY-ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from ¹BTZ^* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the ¹BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two-step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ-BODIPY-ZnP:ImC₆₀, was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC₆₀), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from ¹ZnP* to ImC₆₀. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, ¹BTZ^*-BODIPY-ZnP:ImC₆₀→BTZ-¹BODIPY^*-ZnP:ImC₆₀→BTZ-BODIPY-¹ZnP^*:ImC₆₀→BTZ-BODIPY-ZnP^.+:ImC₆₀^.− in the supramolecular tetrad. The evaluated rate of energy transfer, k_EnT, was found to be 3–5×10¹⁰ s⁻¹, which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC₆₀. The rate constants for charge separation and recombination, k_CS and k_CR, respectively, calculated by monitoring the rise and decay of C₆₀^.− were found to be 5.5×10¹⁰ and 4.4×10⁸ s⁻¹, respectively, for the BODIPY-ZnP:ImC₆₀ triad, and 3.1×10¹⁰ and 4.9×10⁸ s⁻¹, respectively, for the BTZ-BODIPY-ZnP:ImC₆₀ tetrad. Initial excitation of the tetrad, promoting two-step energy transfer and a final electron-transfer event, has been successfully demonstrated in the present study.     

Excited State Mixed-Valence Complexes: From the Special Pair to the Creutz–Taube Ion and Beyond

Compounds and complexes with mixed-valence electronic ground states, such as the Creutz–Taube ion, have proven to be excellent vehicles through which to study intramolecular electron-transfer processes. In a recent contribution by Cadranel and co-workers, time-resolved pump-probe spectroscopy reveals photo-induced metal-to-bridge charge transfer within the homovalent analogue of the Creutz–Taube ion, [{(NH₃)₅Ru}(μ-pz){Ru(NH₃)₅}]⁴⁺, giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (τ=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer-lived species demonstrate analogies with the photo-induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed-valency, and a framework from which to explore photocatalytic reactions.     

Hierarchical Self-Assembly of Curved Aromatics: From Donor–Acceptor Porphyrins to Triply Periodic Minimal Surfaces

A saddle-shaped π-extended zinc porphyrin containing a peripheral pyridyl ligand undergoes quantitative self-assembly into a cyclic trimer. The trimer has a prismatic structure with negatively curved side walls, which promote the formation of supramolecular organic frameworks stabilized by dispersion interactions. The first framework type, UWr-1 , has the npo topology, with a hexagonal structure analogous to the Schwartz H triply periodic minimal surface. Co-crystallization of the trimer with either C₆₀ and C₇₀ produces the isomorphous cubic UWr-2 and UWr-3 phases, characterized by the ctn network topology and a structural relationship to the Fischer-Koch minimal surface S. All three phases contain complex labyrinths of solvent-filled channels, corresponding to very large probe-accessible volumes (68 % to 76 %). The UWr-2 network could be partly desolvated while retaining its long range dimensional order, indicating remarkable strength of the dispersion interactions in the crystal. A theoretical analysis of noncovalent interactions shows the role of geometrical matching between the negatively curved porphyrin units and positively curved fullerenes.     

Kinetic Evidence for Methanol Trimer Assisted Proton Transfer: Transient Absorption Study on Excited Triplet State of 4-Hydroxy-4′-nitrobiphenyl

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4'-nitrobiphenyl (NO₂-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T₁) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T₁ state of NO₂-Bp-OH (³NO₂-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between ³NO₂-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.     

Samarium Diiodide-Induced Reductive Cleavage of Se-St Bond in Arylselenotrimethylsilane Leading to the Formation of Samarium Arylselenolate: Synthesis of β-Arylseleno Estsrs and β-Arylseleno Nitriles

Theapplicationofsamariumdiiodideinorganicsynthesishasreceivedmoreandmoreattenhoninthelastdecadel4.ItisapoWerfuloneelectrontransferreductant.OurpreviousworkonthereduchvecleavageofC-S,S-S,Se-Se,Te-TeandS-StbondwithSInly-'hasledustoinvershgatethereduchvecleavageofSe-StbondbySInI2.Organoseleniumcompoundshaveattractedconsiderableinterestasreagentsandintermedieatesinorganicsynthesisrecently'-".Somemethodsforthepreparationof0selenoesterand6-selenonitrilehavebeenreported.AusefulapproachistheMi…     

Enantioselective synthesis of β-hydroxy carboxylic acids: direct conversion of β-oxocarboxylic acids to enantiomerically enriched β-hydroxy carboxylic acids via neighboring group control

β-Oxocarboxylic acids can be reduced to the corresponding β-hydroxy carboxylic acids employing DIP-Cl^™ as a reducing agent. The β-carboxylic substituent exerts a remarkable neighboring group effect on the reduction. The reaction presumably proceeds in an intramolecular fashion through a `rigid' bicyclic transition state assembly, which produces enantioselectivities approaching 99%.     

Erratum to: Synthesis of Macrocyclic Depsipeptides via the Passerini–Click Reaction Strategy

Russian Journal of Organic Chemistry - Erratum     

First iodocyclization reaction of allene–thioureas: an efficient approach to bicyclic β-lactams

The regioselective iodocyclization reaction of allene–thioureas is described, for the first time, for the synthesis of bicyclic β-lactams. The substitution at the allenyl part heavily influenced the iodocyclization reaction. The iodocyclization reaction of the unsubstituted allene–thioureas afforded six-membered 3-thia-1-dethiacephems whereas, the substituted allene–thiourea afforded seven-membered thiazepines along with five-membered isopenams.     

Co/Mn-mediated oxidative cross-coupling of indoles with β-keto esters via dioxygen activation: an efficient access to ketonization-olefination of indoles

The Co/Mn-mediated oxidative cross-coupling of indoles with β-keto esters via dioxygen activation under mild conditions leads to ketonization-olefination of indoles. This reaction furnishes the highly functionalized products with excellent selectivity in good yields.     

Palladium-catalyzed oxidative arylation of chromones via a double C–H activation: an expedient approach to flavones

A palladium-catalyzed oxidative arylation of chromones was examined. A regioselective 2-arylation of chromone was carried out via a double C–H activation process. The procedure provided an expedient approach for the preparation of flavone derivatives.     

Transfer Reactions of Rydberg (Quasi-Rydberg Molecular) Electrons in Condensed Media: V. Large Proton Displacement in the N–H Bond upon Photoionization of Diphenylamine via Highly Excited Triplet State T* in Methanol at 77 K

The mechanism of large proton displacements in polarized R–H⁺ bonds, which involves the formation of a triplet complex R···H* (H* is the excited hydrogen atom) in the T* state upon the capture of an excited * electron onto the p, d, and f quasi-Rydberg proton orbitals in H⁺ (part IV) is further substantiated. It is assumed that the proton displaced upon photoionization of added diphenylamine molecules (Ph₂NH) via the complex Ph₂N···H* in methanol at 77 K can be fixed by the polarization of the medium in the vicinity of the diphenylaminyl radical (Ph₂N···H⁺ _sol) formed. When the solution is thawed up to 106 K, the protons returns to Ph₂N to afford the radical cation Ph₂NH⁺.     

Transformation of β-Nitro Alcohols to the Corresponding Nitro Imines with Lithium Hexamethyldisilazide (LHMDS) via Sequential Retro Nitro-Aldol–Nitro-Mannich Reaction

β-Nitro alcohols were converted to the corresponding nitro imines in the presence of lithium hexmethyldisilazide (LHMDS) in one pot. The formal transformation of the hydroxy to imino group proceeded in a sequence of the retro nitro-aldol reaction of β-nitro alcohols and the double nitro-Mannich reaction of the resulting aldimines and nitro alkane dianions.     

Catalyst-free aza-Michael addition of azole to β,γ-unsaturated-α-keto ester: an efficient access to C–N bond formation

An efficient aza-Michael addition of azoles to β,γ-unsaturated-α-keto esters under room temperature conditions has been developed. In this conjugate addition, no additional catalyst is employed. Azole reacts with β,γ-unsaturated-α-keto ester smoothly to afford new C–N bond adducts in good to excellent yields (up to 96%).     

DMAP-promoted cascade C–S/C–N bonds formation approach to 1,3-thiazolidin-4-ones via annulation of β-ketothioamides with α-halocarboxylic acids at room temperature

An operationally simple and efficient one-pot straightforward method for the construction of 1,3-thiazolidin-4-ones has been developed by cyclocondensation of β-ketothioamides and an in situ generated acid anhydride from α-halocarboxylic acid in the presence of DCC at room temperature. The attractive features of this strategy include metal-free mild reaction conditions, short reaction time, and efficacy of forming consecutive C–S and C–N bonds and one ring in a single synthetic operation. Noteworthy, a Z-stereochemistry was observed for the exocyclic α-oxoketene moiety at 2-position of 1,3-thiazolidine ring.     

Isomerization and functionalization of 2:1 Diels–Alder adducts of cyclopentadiene and p-benzoquinone: Applications to polycycles via ring-closing metathesis and ring-opening metathesis as key steps

Five stereochemically different 2:1 Diels–Alder adducts were prepared starting with cyclopentadiene and p-benzoquinone. Separately, these adducts were treated with an excess amount of base and allyl bromide to deliver two different tetra-allylated products. These allyl derivatives gave the corresponding mono-propellane derivatives on metathesis sequence. Five Diels–Alder adducts on treatment with Grubbs ruthenium catalysts delivered the corresponding mono- and di-ring-opened metathesis products. Structures of three functionalized adducts were determined by single-crystal X-ray diffraction studies.     

Asymmetric synthesis of β-substituted γ-lactams via rhodium/diene-catalyzed 1,4-additions: application to the synthesis of (R)-baclofen and (R)-rolipram

An efficient rhodium/diene-catalyzed asymmetric addition of arylboronic acids to α,β-unsaturated γ-lactams has been developed. The power of this methodology is further demonstrated by the concise synthesis of (R)-baclofen and (R)-rolipram.