Inner-assembly singlet energy transfer in naphthalene-anthracene system linked by 2-ureido-4{1H}-pyrimidinone binding module

Naphthalene-naphthalene, anthracene-anthracene, and naphthalene-anthracene assemblies 1.1, 2.2, and 1.2 linked by 2-ureido-4{1H}-pyrimidinone binding module were synthesized. Fluorescence quenching and lifetime measurements demonstrate that the inner-assembly singlet energy transfer from naphthalene to anthracene in 1.2 occurs with the efficiency of ca. 89% and rate constant of ca. 9.8 × 10⁸ s⁻¹. Föster energy transfer mechanism operates in this energy transfer process.     

Preferential formation of cis-4,5-dihydrooxazole derivatives via photoinduced electron transfer-initiated cyclization of N-acyl-α-dehydroarylalanine alkyl esters

The alkyl, aryl, and acyl substituent effects on the photoinduced electron transfer-initiated cyclization reaction of the title compounds (1) were investigated in polar solvents from mechanistic and synthetic points of view. The irradiation of (Z)-1 in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (cis-2 and trans-2). In addition to thermodynamic considerations for electron transfer and fluorescence quenching in the presence of TEA, acyl and aryl substituent effects on the emission intensity and photoreactivity of 1 confirmed the involvement of consecutive electron transfer reactions that form (E)-arylmethylene radical anion and (E)-N-acyl radical anion intermediates. It was also confirmed that the cyclization of the latter intermediate eventually leads to 2. On the basis of the finding that the selectivity for cis-2 is greatly increased with increasing the steric bulkiness of alkyl and aryl substituents in 1, it was concluded that steric hindrance of these substituents toward hydrogen shift in the cyclized biradical intermediate, precursor of 2, is responsible for the kinetically controlled hydrogen shift in this intermediate. A product composition analysis showed that the protic polar solvent, methanol, of hydrogen-bonding solvation ability is a most suitable solvent for the photocyclization reactions examined.     

Optical properties of oligo(2,3-dioxyfunctionalized)naphthalenes

The properties of two series of oligo(2,3-dioxyfunctionalized)naphthalenes which are connected at the 1,4-positions, that is, methoxy derivatives 1-4 and derivatives that possess two pyrene groups on the central scaffolding oxygen functions 5-8, are described. In 1-4, the fluorescence quantum yields increased by about 20-80% as the number of naphthalene units increased. The intramolecular energy transfer quantum yields of 5-8 were around 20% regardless of the number of naphthalene units.     

Novel chemosensor for alkaline earth metal ion based on 9-anthryl aromatic amide using a naphthalene as a TICT control site and intramolecular energy transfer donor

Novel fluorescent chemosensors 1 and 2 with two different fluorophores (naphthalene and anthracene) at the both ends of polyether was synthesized. These compounds based on 9-anthryl aromatic amide adopted a naphthalene as a TICT controller and an intramolecular energy transfer source. Compound 1 shows high fluorescence efficiency upon complexation with metal ion, and the fluorescence efficiency of 2 is regulated by metal ionic size.     

Donor-acceptor-donor triads incorporating tetrathiafulvalene and perylene diimide units: synthesis, electrochemical and spectroscopic studies

Three donor-acceptor-donor triads 1-3 consisting of tetrathiafulvalene units attached to perylene diimides by flexible and rigid spacers were synthesized and characterized. UV/vis spectroscopic and cyclic voltammetric results indicate that they all show negligible intramolecular charge transfer interaction in their ground states. As compared to the reference compound 21, triads 1-3 display reduced fluorescence and their fluorescence lifetimes are shortened, which is probably owing to the photoinduced electron transfer interactions between the PI units and TTF units. The different photophysical behaviors between 1 and 2 (and 3) might be due to their difference in the spatial separation of TTF and PI units. It is preliminarily found that the steric hindrance of the groups attached to TTF units can affect their photostability.     

Tetrathiafulvalene-flavin dyads: electron transfer promoted by metal cations

Tetrathiafulvalene-flavin dyads 1 and 2 are reported. Both absorption and ESR spectral studies show that the intramolecular electron transfer occurs from TTF to flavin units in dyads 1 and 2 in the presence of Pb²⁺/Sc³⁺. But, the electron transfer is more efficient for dyad 1 in the presence of Pb²⁺/Sc³⁺. Electrochemical studies manifest that coordination of dyads 1 and 2 with Pb²⁺/Sc³⁺ play an important role in facilitating the electron transfer within dyads 1 and 2.     

Continuous intra- and intermolecular energy transfer in light-harvesting gels from natural amino acids-based dendrons

The gels and co-gels from glycine (Gly) and glutamic acid (Glu)-based dendrons with either tyrosine (Tyr) or tryptophan (Trp), two of the luminescent amino acid residues in natural proteins, at the focal point were prepared (Tyr-AB₄ and Trp-AB₄). It was found that such gels, especially the co-gels from Tyr-AB₄/Trp-AB₄, showed a high efficient energy transfer (ET) and light-harvesting behaviors. Moreover, luminescent gels with tunable emission ranging from blue to green were also observed owing to the cascade intra- and intermolecular ET from dendritic gelators to the guest molecules (PDNS) in the host-guest gel sample (co-gel with PDNS as the guest molecule), which mimicked the natural light-harvesting systems.     

Highly sensitive chemosensor for detection of PPi with improved detection limit

We have developed a new chemosensor 1·2Zn for the detection of PPi in an aqueous solution by photo-induced electron transfer. Two coumarin units were introduced to enhance the fluorescent signals. Consequently, this chemosensor showed an improved detection limit as compared to the previously reported chemosensor 2·2Zn.     

Selective and efficient transformation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters into 4,5-dihydrooxazole derivatives via photoinduced electron transfer

The irradiation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters (1) in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (2), which were described as being formed via electron transfer from TEA to the excited-state (E)-1 followed by kinetically-controlled cyclization of the (E)-1-derived anion radical. A product composition analysis showed that the cis-2/trans-2 composition ratio is greatly varied depending on the stereoelectronic properties of the substituents, the polarity of protic solvents and the concentration of TEA.     

Binaphthalene with substituted tetrathiafulvalene and trichloroquinone: a new example of metal ion-promoted electron transfer

A new binaphthalene (R)-1 with two substituted TTF and trichloroquinone units is reported. Both absorption and ESR spectral studies show that electron transfer occurs between TTF and trichloroquinone units of (R)-1 in the presence of metal ions (Pb²⁺, Sc³⁺, Zn²⁺, and Ca²⁺). We also propose a possible mechanism for this electron transfer process. But, the CD spectral change of (R)-1 in the presence of metal ions is rather small.     

Difluoroethylene as a chain transfer agent during ring-opening metathesis polymerization (ROMP) of norbornene by a ruthenium alkylidene complex: A computational study

The polynorbornene chain transfer reaction pathways to ethylene (2a), trans-1,2-difluoroethylene (2b) and trans-1,4-dichloro-2-butene (2c) by (1,3-diphenyl-4,5-dihydroimidazol-2-ylidene) (PCy₃)Cl₂RuCHPh (I) have been studied at B3LYP/LACVP^∗ level of theory. The calculations show that the free Gibbs activation energy of metathesis reaction is dependent on the volume of substituents directly linked to the double bond of an olefin. Highest activation energy is observed for 2c with highest molecular volume. The activation energy is lower for 2a with small molecular volume. Compared to 2a and 2c, fluorinated olefin 2b binds more strongly to the 14 electron Ru-alkylidene catalyst to form tighter transition state. Therefore, sterical factor is the most important contribution to the activation energy for Ru-alkylidene mediated olefin metathesis.     

Two new biscembranes with unprecedented carbon skeleton and their probable biogenetic precursor from the Hainan soft coral Sarcophyton latum

A new cembranolide diterpene, isosarcophytonolide D (1), and two biscembranes with an unprecedented fused carbon skeleton, bislatumlides A (3) and B (4), were isolated from the Hainan soft coral Sarcophyton latum. Their structures and relative stereochemistry were established by 1D and 2D NMR spectroscopic techniques. Compounds 3 and 4 should be obtained by Diels-Alder addition of two cembranoid units. Compound 1 could be one of the biosynthetic precursors. Both 3 and 4 showed mild cytotoxicity toward A549, HT-29, and P388 cell lines.     

Spectrokinetic studies on new bi-photochromic molecules containing two naphthopyran entities

A range of new bi-photochromic molecules containing two identical (3a-d) or two distinct naphthopyran units (6a-d), linked through the phenyl substituents located on the sp³ hybridised pyran ring carbon atom, using conjugated and non-conjugated spacers, have been synthesised from bis-propynols and (substituted)naphthols. Study of the spectrokinetic properties of these compounds under near UV-vis continuous irradiation conditions revealed that the two naphthopyran units are stimulated independently leading to open forms with higher colourabilities but without affecting the individual bleaching kinetics. Compared to the individual photochromic components and to model mono-photochromes it was observed that the nature of the bridge has a small effect on the photochromic properties of each system.     

Photochemistry of a naphthalene-thymine dyad in the presence of acetone

Irradiation of dyad 1 in aqueous acetone leads to the introduction of an acetonyl substituent at the naphthalene 5-position, to give photoproduct 2. The proposed reaction mechanism involves electron transfer from the naphthalene excited singlet state to the ketone. Neither thymine dimers, nor acetone photoadducts involving the thymine ring were detected. These photoproducts would arise from the thymine triplet excited state, which in dyad 1 must be efficiently depopulated via a fast intramolecular energy transfer to the naphthalene chromophore, due to the lower energy of its excited triplet state.     

A highly selective ratiometric chemosensor for Hg based on the anthraquinone derivative with urea groups

Anthraquinone derivatives with different substituents 1-3 were synthesized by introducing the urea group. Their cations' binding properties were investigated by UV-vis absorption spectroscopy. Compared with 1 and 2, 3 with electron-withdrawing group (-NO₂) showed a remarkable absorption change for Hg²⁺ over the other metal ions. The anthraquinone moiety and the N-H fragment of the urea moiety played key roles in sensing Hg²⁺. The different acidity of N-H fragments of the urea moiety, caused by electron push-pull properties of the substituents on the phenyl para position, is the main reason for recognition.     

Adenine-based urea receptors in fluorescent recognition of iodide

Adenine-based receptors 1 and 2 are designed and synthesized for selective sensing of iodide over the other halides and carboxylate anions. Both the receptors 1 and 2 use the urea motif for binding carboxylates and halides. Emissions of the naphthalene and the anthracene in 1 and 2, respectively, are monitored in CHCl₃ in detecting the anions. While carboxylates, fluoride, chloride, and bromide increase the emissions of naphthalene and anthracene in both the receptors 1 and 2 during complexation, iodide quenches the emission. Such selective quenching allows iodide to be discriminated from other halides and carboxylate anions.     

Imino Diels-Alder reaction of boronates: A new route to 3,4-dihydroquinolines

The synthesis of a series of 3,4-dihydroquinolines (3b-e, 3h and 3i) by Imino Diels-Alder reactions involving sulfolene and boronates 2a-j derived from Schiff bases is described. The reactions are regioselective leading to 4-substituted dihydroquinolines with a cis relative stereochemistry between the phenyl group on the boron atom and the vinyl substituent at position 4, as established by X-ray diffraction analyses of 3b, 3e, 3h and 3i.Boronates 2 containing substituents meta to the imino fragment lead to 4-ethenyl-dihydroquinolines while para substituted derivatives are more reactive and lead to 4-cyclohexenylquinolines 4 formed by a second Diels-Alder reaction with excess of butadiene. The results show that boronates derived from Schiff bases are electron deficient species that react with sulfolene providing a new route to 3,4-dihydroquinolines. Moreover, polarization of the imine bond activates these system toward cycloaddition.     

Synthesis, antitumor and DNA photocleaving activities of novel naphthalene carboxamides: effects of different thio-heterocyclic rings and aminoalkyl side chains

Two kinds of thio-heterocyclic fused naphthalene carboxamides, 3a-b, 4a-b, were designed, synthesized and quantitatively evaluated as efficient antitumor and DNA photocleaving agents. Compound 3a or 3b, having the thiophene ring, intercalated into DNA more strongly than compound 4a or 4b, having the thioxanthene ring. Compound 4a or 4b, photocleaved DNA more efficiently than 3a or 3b via superoxide anion. Compound 4a was the strongest inhibitor for P388 (murine leukemia cell), while 3a was the most cytotoxic one against A549 (human lung cancer cell). Each new compound showed stronger DNA photocleaving activity than corresponding naphthalimide.     

Rhodium complexes with chiral counterions: achiral catalysts in chiral matrices

The neutral complexes [Rh(I)(NBD)((1S)-10-camphorsulfonate)] (2) and [Rh(I)((R)-N-acetylphenylalanate)] (4) reacted with bis-(diphenylphosphino)ethane (dppe) to form the cationic Rh(I)(NBD)(dppe) complexes, 5 and 6, respectively, accompanied by their corresponding chiral counteranions. Analogously, 4 reacted with 4,4^′-dimethylbipyridine to yield complex 7. Complexes 5 and 6 disproportionated in aprotic solvents to form the corresponding bis-diphosphine complexes 8 and 9, respectively. 8 was characterized by an X-ray crystal structure analysis. In order to form achiral Rh(I) complexes bearing chiral countercations new sulfonated monophosphines 13-16 with chiral ammonium cations were synthesized. Tris-triphenylphosphinosulfonic acid (H₃TPPS, 11) was used to protonate chiral amines to yield chiral ammonium phosphines 14-16. Thallium-tris-triphenylphosphinosulfonate (Tl₃TPPS, 12) underwent metathesis with a chiral quartenary ammonium iodide to yield the proton free chiral ammonium phosphine 13. Phosphines 15 and 16 reacted with [Rh(NBD)₂]BF₄ to afford the highly charged chiral zwitterionic complexes [Rh(NBD)(TPPS)₂][(R)-N,N-dimethyl-1-(naphtyl)ethylammonium]₅ (17) and [Rh(NBD)(TPPS)₂][BF₄][(R)-N,N-dimethyl-phenethylammonium]₆ (18), respectively. Complexes 5, 6, and 18 were tested as precatalysts for the hydrogenation of de-hydro-N-acetylphenylalanine (19) and methyl-(Z)-(α)-acetoamidocinnamate (MAC, 20) under homogeneous and heterogeneous (silica-supported and self-supported) conditions. None of the reactions was enantioselective.     

Tweezers-like aromatic molecules and their luminescent properties depending on the structures

Di(hydroxyphenyl)pyrimidine with two anthryl substituents 1a was synthesized and characterized by X-ray crystallography and NMR spectroscopy. The molecule prefers ‘U’-shaped conformation supported by intramolecular O-H···N hydrogen bonds in the solid state and in solution. The CHCl₃-solvated compound binds two CHCl₃ molecules between the two parallel anthryl planes. Hexylation of the OH groups of 1a produces 1c whose diarylpyrimidine core contains these aromatic planes with O···H-C interaction and helical alignment. Compound 1c shows strong emission from the anthryl groups (410 nm, φ = 0.39), while luminescence is not observed for 1a partly due to quenching via PET (photo-induced electron transfer) process.