Naphthalene Spacers Research Articles

Intramolecular through-space charge transfer between benzofuran and ynone groups on a naphthalene spacer.

We have developed a new scaffold that exhibits an efficient intramolecular through-space charge transfer (CT). In this design, electron-rich benzofuran and electron-deficient ynone groups are placed strategically in proximity via a naphthalene spacer. Charge transfer is supported by distinct CT bands in the visible region (>500 nm) in their UV-vis absorption and emission spectra.

Regioisomers containing triarylboron-based motifs as multi-functional photoluminescent materials: from dual-mode delayed emission to pH-switchable room-temperature phosphorescence.

Triarylboron compounds have been established as promising candidates in optoelectronic applications. However, realizing multi-functional properties in triaryl boron-based materials remains challenging. Herein, we present two regioisomers, 1 and 2, designed judiciously by connecting a dimethylamino donor and a dimesitylboryl acceptor at 1,4 and 2,6-positions of the naphthalene spacer, respectively. Both compounds 1 and 2 display simultaneous, delayed fluorescence and persistent room-temperature phosphorescence (580 nm, τ av = 168 ms, Φ = 76% for 1; 550 nm, τ av = 129 ms, Φ = 88% for 2 in 1 wt% PMMA), with the delayed fluorescence bands being sensitive to doping concentration (in PMMA). Notably, compound 1 in 1 wt% PMMA films demonstrates a reversibly switchable single-molecule phosphorescence from orange (580 nm) to green (λ Ph = 550 nm, τ av = 42 ms) in response to pH, which can be utilized for anti-counterfeiting applications. These results were further corroborated by studying the respective cationic salts 1-OTF and 2-OTF. Moreover, 1 and 2 exhibited blue-shifted fluorescence in response to mechanical pressure. Compound 2 also showed three-photon (σ3P) absorption properties which were better compared to those of compound 1.

Molecular engineering of naphthalene spacers in low-dimensional perovskites

A series of hybrid organic–inorganic perovskites, based on naphthalene ammonium cations have been investigated. All materials exhibit edge- or face-sharing 1D chain structures and broadband light emission arising from the self-trapped excitons.

Molecular engineering of an inverse hexacopper(II) coordination complex with a photoactive metallacyclophane centroligand as prototype of a magnetic photoswitch

A primary goal of molecular spintronics is detecting the response of the spins from a single magnetic molecule to an external stimulus and, eventually, being able to implement logic quantum memory capabilities for information data storage and processing applications in quantum computation. Having this in mind, a novel hexanuclear oxamatocopper(II) complex of formula {[Cu2(naba)2][Cu(pmdien)]4}(ClO4)4·8H2O (2) [naba = N,N'-1,5-naphthalenebis(oxamate) and pmdien = N,N,N’,N”,N”-pentamethyldiethylenetriamine] has been prepared by the reaction of stoichiometric amounts of (nBu4N)4[Cu2(naba)2] · 4H2O (1) (nBu4N+ = tetra-n-butylammonium cation) and [Cu(pmdien)](ClO4)2 (1:4 molar ratio) in water. 2 exhibits a ladder-like “dimer-of-trinuclears” structure featuring a photoactive dicopper(II) naphthalenophane core acting as inverse coordination centre (centroligand) towards four peripheral [Cu(pmdien)]2+ groups through the outer amidate/carboxylate oxygen atoms of the oxamato fragments. 2 undergoes a unique magnetic photoswitching ON/OFF behaviour upon UV light irradiation because of an intramolecular (“pseudo-bimolecular”) photocycloaddition of the two facing naphthalene spacers through the more reactive carbon atoms at the 1,4 and 1’,4’ positions of the benzene rings to afford the putative [4 + 4] cycloaddition product c-2. Hence, the two oxamato-bridged S = 1/2 CuII3 linear units antiferromagnetically coupled within the CuII6 precursor (2, ON state) become magnetically uncoupled in the corresponding [4 + 4] photocycloaddition product (c-2, OFF state) [jeff = –2.13 (2) and 0 cm−1 (c-2); H = – jeffSA.SB with SA = SB = 1/2]. This new example of inverse coordination complex constitutes thus a potential candidate to molecular magnetic photoswitch in the “bottom-up” approach to nanometer-scale spintronic and quantum computing devices.

Copper complexes of multidentate carboxamide ligands

The copper coordination chemistry of two multidentate carboxamido ligands derived from HL1 (offering two quinolyl and one carboxamide donor) and H4L2 (with two pyridine(dicarboxamido) units linked by naphthalene spacers) was explored. The former was chosen because upon deprotonation it would provide a monoanionic mer-coordinating N-donor set that would model the putative deprotonated form of the His-brace in copper monooxygenases, while the latter was designed to bind two copper ions and enable comparisons to other systems with different ligand spacers. Upon reaction with Cu(I)-mesityl, HL1 yielded a symmetric dimer (L1Cu)2 in which each bis(quinolyl)amide ligand binds via two N-donors to one Cu(I) ion and via the third to the other Cu(I) center. Monomeric Cu(II) complexes [L1Cu(H2O)2](OTf) and L12Cu were also characterized. Treatment of H4L2 with Cu(OTf)2 and excess Me4NOH (in CH3CN, pyridine/H2O, or MeOH) yielded complexes with anions of general formula [L2Cu2(X)]n−, where X = CH3CONH– (n = 1), CO32– (n = 2), or MeO− (n = 1). X-ray structures of these complexes revealed the (L2)4− ligand binding to two Cu(II) ions in an open paddle-wheel geometry, with an additional bridging ligand (X) completing the square planar coordination sphere of each metal ion. The open paddlewheel motif differs from the more ‘open’ puckered geometry seen with related ligands with different spacer units.

Mixed‐Valent Molecular Triple Deckers

AbstractTwo phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ‐arene‐PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis‐NIR absorption, EPR spectroscopy, and computational studies, the one‐electron oxidized forms of the resulting compounds behave as class II organic mixed‐valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed‐valent triple‐decker compounds with possible electron‐transfer pathways directly across a stacked structure, and they illustrate the potential of oligo‐naphthalene building blocks for long‐range electron transfer and a future molecular electronics technology.

Mixed-Valent Molecular Triple Deckers.

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Green synthesis of luminescent blue emitters based on bistriazines with naphthalene as a π-conjugated spacer

A new series of luminescent blue emissive bistriazines is described. Bistriazines with a naphthalene spacer were prepared using a green methodology that involved microwave irradiation, solvent free conditions and a reaction time of only 10 min. The synthesis was followed by a simple purification procedure. D-σ-A-σ-π-σ-A-σ-D systems were constructed using 1,5-naphthylidene as a planar π-bridge and different donor substituents were attached to the triazine ring. UV–vis and fluorescence spectroscopy showed that the increased conjugation in bistriazines with respect to monotriazines resulted in a 40-fold increase in the photoluminescence quantum yield (ΦF). The bistriazines were efficient blue emitters with ΦF values up to 0.87. The aggregation behavior and the optical and thermal properties of these systems have been studied. The bistriazines showed good thermal stability in conjunction with high ΦF values and they are therefore very promising materials for use in optoelectronic devices.

Exploring the limits of nucleobase expansion: computational design of naphthohomologated (xx-) purines and comparison to the natural and xDNA purines

The properties of doubly-expanded (xx-) purine analogues are compared to the natural and singly-expanded (x-) purines using quantum chemical (B3LYP, MP2) methods. Purine expansion upon incorporation of a benzene or naphthalene spacer affects the preferred orientation of the base about the glycosidic bond in the corresponding nucleoside to a similar extent. Although the natural purines preferentially adopt the anti orientation with respect to the 2'-deoxyribose moiety, the syn and anti conformations are almost isoenergetic in the case of the expanded analogues. However, the anti/syn rotational barrier either remains similar to or is slightly elevated upon purine expansion, and therefore the expanded analogues will likely display similar rigidity with respect to rotation about the glycosidic bond as the natural purines. The A:T Watson-Crick hydrogen-bond strength is slightly enhanced, while the G:C interaction energy decreases slightly, upon incorporation of either expanded purine. As expected, the largest effect of base expansion occurs on the stacking energies. Specifically, the maximum (most negative) stacking energies in isolated purine dimers formed by aligning the nucleobase centers of mass can be increased by up to 34% by including a single x-purine and by up to 51% by including an xx-purine. Although increases in the purinepurine intrastrand stacking interactions are found even when a simplified duplex model composed of two stacked (hydrogen-bonded) base pairs is considered, the purinepyrimidine stacking energies decrease upon purine expansion. However, the total stability (sum of all hydrogen-bonding and stacking interactions) of natural duplex models is up to 10% and 22% greater for duplexes containing expanded x- and xx-purines respectively, which is mainly due to enhanced inter and intrastrand stacking interactions. Thus, unidirectional expansion in the size of the nucleobase spacer can continuously enhance the stability of expanded duplexes.

Effect on Charge Transfer and Charge Recombination by Insertion of a Naphthalene‐Based Bridge in Molecular Dyads Based on Borondipyrromethene (Bodipy)

The photophysical properties of two related dyads based on a N,N-dimethylaniline donor coupled to a fully-alkylated boron dipyrromethene (Bodipy) acceptor are described. In one dyad, BD1, the donor unit is attached directly to the Bodipy group, whereas in the second dyad, BD2, a naphthalene spacer separates the two units. Cyclic voltammograms recorded for the two dyads in deoxygenated MeCN containing a background electrolyte are consistent with the reversible one-electron oxidation of the N,N-dimethylaniline group and the reversible one-electron reduction of the Bodipy nucleus. There is a reasonable driving force (ΔG(CT)) for photoinduced charge transfer from the N,N-dimethylaniline to the Bodipy segment in MeCN. The charge-transfer state is formed for BD1 extremely fast (1.5 ps), but decays over 140 ps to partially restore the ground state. On the other hand, the charge-transfer state for BD2 is formed more slowly, but it decays extremely rapidly. Charge recombination for both dyads leads to a partial triplet formation on the Bodipy group. The naphthalene spacer group is extremely efficient at promoting back electron transfer.

Fluorescent Charge-Assisted Halogen-Bonding Macrocyclic Halo-Imidazolium Receptors for Anion Recognition and Sensing in Aqueous Media

The synthesis and anion binding properties of a new family of fluorescent halogen bonding (XB) macrocyclic halo-imidazolium receptors are described. The receptors contain chloro-, bromo-, and iodo-imidazolium motifs incorporated into a cyclic structure using naphthalene spacer groups. The large size of the iodine atom substituents resulted in the isolation of anti and syn conformers of the iodo-imidazoliophane, whereas the chloro- and bromo-imidazoliophane analogues exhibit solution dynamic conformational behavior. The syn iodo-imidazoliophane isomer forms novel dimeric isostructural XB complexes of 2:2 stoichiometry with bromide and iodide anions in the solid state. Solution phase DOSY NMR experiments indicate iodide recognition takes place via cooperative convergent XB-iodide 1:1 stoichiometric binding in aqueous solvent mixtures. (1)H NMR and fluorescence spectroscopic titration experiments with a variety of anions in the competitive CD(3)OD/D(2)O (9:1) aqueous solvent mixture demonstrated the bromo- and syn iodo-imidazoliophane XB receptors to bind selectively iodide and bromide respectively, and sense these halide anions exclusively via a fluorescence response. The protic-, chloro-, and anti iodo-imidazoliophane receptors proved to be ineffectual anion complexants in this aqueous methanolic solvent mixture. Computational DFT and molecular dynamics simulations corroborate the experimental observations that bromo- and syn iodo-imidazoliophane XB receptors form stable cooperative convergent XB associations with bromide and iodide.

Synthesis, structure and properties of copper(II) and manganese(II) coordination polymers with 1,5-dinitronaphthalene-3,7-dicarboxylate

Two new metal–organic coordination polymers with 1,5-dinitronaphthalene-3,7-dicarboxylate (NNDC), [Cu 2(NNDC) 2(DMF) 1.8(DMSO) 2.2(H 2O) 2]·H 2O (1) and [Mn 3(NNDC) 3(DMSO) 4]·2DMSO (2) have been synthesized under solvothermal conditions and characterized by single crystal X-ray diffraction and Thermogravimetric Analysis (TGA). The structure of compound 1 consists of one-dimensional chains with copper ions being linked by the dicarboxylate ligands. The coordination chains are associated into ladder-like double chains through O–H⋯O hydrogen bonds and π–π interactions, and the ladders are packed in a cross fashion through further π–π interactions to give the three-dimensional structure. The Mn(II) compound exhibits a 3D framework with the pcu topology, in which [Mn 3(COO) 6] clusters as octahedral secondary building blocks are linked by the naphthalene spacers. Magnetic analyses were carried out based on both temperature- and field-dependent data, consistently suggesting relatively weak antiferromagnetic interactions within the carboxylate bridged [Mn 3(COO) 6] cluster.

Poly[μ2-aqua-μ4-naphthalene-1,8-dicarboxylato-manganese(II)

The asymmetric unit of the title complex, [Mn(C12H6O4)(H2O)]n, contains one MnII ion, one 1,8-naphthalene­dicarboxyl­ate (1,8-NDC) ligand and one water mol­ecule. The MnII ion is six-coordinated within a distorted octa­hedral coordination geometry, in which the equatorial sites are occupied by four carboxyl­ate O atoms from four different 1,8-NDC ligands, while the axial positions are occupied by two O atoms of two coordinated water mol­ecules. Adjacent MnII centres are bridged by one coordinated water and two carboxyl­ate groups in a syn–syn mode to form infinite chains along the b axis, which are further cross-linked by the naphthalene spacers of the 1,8-NDC ligands to produce a two-dimensional extended network.

Langmuir and Langmuir−Blodgett Monolayers of Molecular Tweezers and Clips

Molecular clips and tweezers are able to selectively bind electron-deficient aromatic and aliphatic substrates. By means of pressure-area isotherms and Brewster angle microscopy (BAM), the self-association process and phase behavior of dimethylene-bridged molecular clips and tetramethylene-bridged molecular tweezers each substituted with two acetoxy groups as polar head groups were investigated. In a series of experiments, we observed that the molecular surface area of the clips and tweezers only depended on the skeletal structure and not on the polar groups. The measured areas agreed with the effective molecular diameters of the molecules if the aromatic side walls of the clips or tweezers were assumed to be aligned perpendicularly to the water surface. We compared the phase behavior of the pure molecular clips and tweezers with that of the host-guest complexes of these molecules, which were formed with 1,2,4,5-tetracyanobenzene (TCNB) as the guest molecule. For the clips with a central benzene (I) and naphthalene spacer unit (II), the complex formation with TCNB had no measurable influence on the phase diagrams of the films. We observed, however, a dramatic difference in the BAM images and pi-A isotherms between the pure molecular tweezers III and its complex with TCNB (TCNB@III). In addition to the pi-A isotherms, we used the surface potential (V)-area (A) isotherms to compare the pure tweezers III with the corresponding complex (TCNB@III). There was a strong difference in the maximum surface potential value for the pure tweezers (450 mV) and that for the complex (300 mV). In additional experiments, we prepared LB layers of such molecules, which were investigated by fluorescence spectroscopy. In comparison to the pure tweezers III, a luminescence emission of charge-transfer (CT) origin was observed for the host-guest complex (TCNB@III) fixed on the solid substrate. It turned out that the spectra were in good agreement with the results observed in chloroform solution.

Linear and macrocyclic ruthenium(II) complexes containing bis(2,2′:6′,2″-terpyridine) ligands with flexible, naphthalene-centred spacers

The syntheses and characterization of six ditopic ligands which contain polyethyleneoxy-substituted naphthalene spacers connecting two 2,2′:6′,2″-terpyridine metal-binding domains are described. The ligands, L, have been used to prepare two series of ruthenium(II) complexes: linear complexes of type [(tpy)Ru(L)Ru(tpy)] 4+ were prepared as models for polymeric species, and metallomacrocyclic species [Ru n L n ] 2 n+ ( n = 2 or 3). Single crystal structures of a representative ligand and macrocyclic Ru(II) complex are reported.

Synthesis and magnetic property of porous naphthalenetetracarboxylate manganese complex, the first (μ 2-aqua)bis(μ 2-carboxylate)-bridged manganese polymer

A manganese coordination polymer [Mn 2(NTC)(H 2O) 2] n , where NTC is 1,4,5,8-naphthalenetetracarboxylate anion, has been synthesized by slow diffusion of manganese chloride and H 4NTC. Mn(II) ions are bridged by one coordinated water and two carboxylate groups in syn– syn mode to form infinite chains, which are further cross-linked by the naphthalene spacers of the NTC to produce 3-D extended network with open-channels. An antiferromagnetic coupling interaction between adjacent manganese ions dominates the magnetic property of the compound.

Synthesis and supramolecular properties of trimethylene-bridged clips.

The novel trimethylene-bridged clips 3 and 4 have been synthesized by using repetitive stereoselective Diels-Alder reactions of the benzo- and naphthobismethylenenorbornenes 8 and 19 as dienes and norbornadiene 9 as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts 10 and 20 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips 3 and 4 serve as receptors for a variety of electron-deficient neutral and cationic aromatic substrates, comparable to the molecular tweezers 1 and 2. The thermodynamic parameters of the complex formation, K(a) and DeltaG, were determined by (1)H NMR titration experiments and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titration microcalorimetry. The finding that clip 4 forms more stable complexes than 3 can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in 4 compared to the corresponding benzene systems in 3. In the complexes with 4 as receptor, the plane of each aromatic substrate molecule is calculated to be oriented almost parallel to the naphthalene sidewalls. However, in the complexes of tweezers 2, the substrate is usually oriented parallel to the central naphthalene spacer unit. Due to the more open topology of 4, most complexes were calculated to consist of two or more equilibrating noncovalent conformers.

Through-bond interaction between sulfonium and sulfenyl sulfur atoms having anthracene and naphthalene spacers

The reaction of 9-(methylsulfinyl)-10-(methylthio)anthracence with trifluoroacetic anhydride followed by quenching with aqueous NaHCO 3 gives 9,9-bis(methylthio)-10-anthraquinone. The dithia dication and/or the corresponding carbodication via through-bond interaction between sulfonium and sulfenyl sulfur atoms is proposed as an intermediate.

Synthesis, X-ray Structure, and Electrochemical Oxidative Coupling Reactions of 1,5- and 2,6-Bis(1,4-dithiafulven-6-yl)naphthalenes.

Novel pi-extended tetrathiafulvalene (TTF) derivatives (12a-c, 13a-c, 15a-c) in which the two 1,3-dithiole units are connected through a naphthalene spacer have been prepared in high yields by Wittig-Horner olefination reaction from dialkoxy-substituted diformylnaphthalenes (11a,b, 14) and differently substituted phosphonate esters (10a-c). The electrochemical study revealed a similar behavior for the novel electron donor molecules (12a-c, 13a-c, 15a-c) regardless of the position of the 1,3-dithiole rings on the naphthalene core. The extended donors undergo an efficient electrooxidation process affording new oligomeric extended TTF species which exhibit lower oxidation potential values than their precursor donors. EPR experiments confirm the presence of the cation radical derived from the oligomeric TTF vinylogues and support an ECE process. The structural study has been carried out by X-ray analysis of 12a and semiempirical PM3 calculations and reveals a distorted geometry from the planarity with the naphthalene moiety forming an angle of approximately 35 degrees with the thiafulvalene rings. A good agreement was found between the experimental and calculated values, thus validating the PM3 method. The chemical oxidation of the synthesized donors (12a-c, 13a-c, 15a-c) with strong electron acceptors give rise to charge transfer complexes (CTC) which were characterized by UV-vis, FTIR, and EPR spectroscopy.

Radical Chemistry under Diffusional Constraints: Impact of Spacer Molecules on the Thermolysis of Surface-Immobilized Bibenzyl

Silica nanoparticle surfaces have been chemically modified to contain a molecular probe, 1,2-diphenylethane (bibenzyl), as well as a second spacer molecule of variable structure. Thermolysis kinetics and products at 400 °C have been determined for bibenzyl, and the impact of diffusional constraints and spacer molecular structure on the multipathway radical chemistry has been analyzed relative to fluid phases. Unimolecular homolysis rate constants, k = (7−9) × 10-6 s-1, are found to be independent of spacer molecular structure (naphthalene, diphenylmethane, tetralin) and similar to values measured in fluid phases, indicating the lack of a cage effect on the silica surface. However, the total rate of bibenzyl thermolysis and the resulting product selectivities are profoundly affected by both surface immobilization and the structure of the spacer molecule. Naphthalene spacers behave as “molecular walls” serving as physical barriers to bimolecular hydrogen transfer steps on the surface and augmenting the effects of diffusional constraints. In contrast, diphenylmethane spacers are found to serve as hydrogen transfer, radical relay catalysts that translocate radical sites across the surface and diminish the impact of diffusional constraints. Tetralin spacers undergo significant reaction with free-radical intermediates, selectively producing the isomeric methylindan via a chain pathway that is promoted by the restrictions on diffusion.