Substituted heterocyclic naphthalene diimides with unexpected acidity. Synthesis, properties, and reactivity

Synthesis, Solution, and Solid State Properties of Homological Dialkylated Naphthalene Diimides—A Systematic Review of Molecules for Next-Generation Organic Electronics

This systematic study aimed at finding a correlation between molecular structure, solubility, self-assembly, and electronic properties of a homological series of N-alkylated naphthalene diimides (NDIs). NDIs are known for their n-type carrier mobility and, therefore, have potential in the field of organic electronics, photovoltaics, and sensors. For the purpose of this study, nine symmetrical N,N′-dialkylated naphthalene diimides (NDIC3-NDIC11) were synthesized in the reaction of 1,4,5,8-naphthalenetetracarboxylic dianhydride with alkylamines ranging from propyl- to undecyl-. The NDIs were characterized by spectroscopic (NMR, UV-Vis, FTIR), microscopic, and thermal methods (TGA and DSC), and X-ray diffraction (XRD). Our experimental study, extensively referring to findings reported in the literature, indicated that the NDIs revealed specific trends in spectroscopic and thermal properties as well as solubility and crystal morphology. The solubility in good solvents (chloroform, toluene, dichlorobenzene) was found to be the highest for the NDIs substituted with the medium-length alkyl chains (NDIC5–NDIC8). Systematic FTIR and XRD studies unraveled a distinct parity effect related to the packing of NDI molecules with odd or even numbers of methylene groups in the alkyl substituents. The NDIs with an even number of methylene groups in the alkyl substituents revealed low-symmetry (P1−) triclinic packing, whereas those with an odd number of carbon atoms were generally monoclinic with P21/c symmetry. The odd–even parity effect also manifested itself in the overlapping of the NDIs’ aromatic cores and, hence, the π-π stacking distance (dπ-π). The odd-numbered NDIs generally revealed slightly smaller dπ-π values then the even-numbered ones. Testing the NDIs using standardized field-effect transistors and unified procedures revealed that the n-type mobility in NDIC6, NDIC7, and NDIC8 was 10- to 30-fold higher than for the NDIs with shorter or longer alkyl substituents. Our experimental results indicate that N,N′-alkylated NDIs reveal an optimum range of alkyl chain length in terms of solution processability and charge transport properties.

Selective Binding and Redox-Activity on Parallel G-Quadruplexes by Pegylated Naphthalene Diimide-Copper Complexes

G-quadruplexes (G4s) are higher-order supramolecular structures, biologically important in the regulation of many key processes. Among all, the recent discoveries relating to RNA-G4s, including their potential involvement as antiviral targets against COVID-19, have triggered the ever-increasing need to develop selective molecules able to interact with parallel G4s. Naphthalene diimides (NDIs) are widely exploited as G4 ligands, being able to induce and strongly stabilize these structures. Sometimes, a reversible NDI-G4 interaction is also associated with an irreversible one, due to the cleavage and/or modification of G4s by functional-NDIs. This is the case of NDI-Cu-DETA, a copper(II) complex able to cleave G4s in the closest proximity to the target binding site. Herein, we present two original Cu(II)-NDI complexes, inspired by NDI-Cu-DETA, differently functionalized with 2-(2-aminoethoxy)ethanol side-chains, to selectively drive redox-catalyzed activity towards parallel G4s. The selective interaction toward parallel G4 topology, controlled by the presence of 2-(2-aminoethoxy)ethanol side chains, was already firmly demonstrated by us using core-extended NDIs. In the present study, the presence of protonable moieties and the copper(II) cavity, increases the binding affinity and specificity of these two NDIs for a telomeric RNA-G4. Once defined the copper coordination relationship and binding constants by competition titrations, ability in G4 stabilization, and ROS-induced cleavage were analyzed. The propensity in the stabilization of parallel topology was highlighted for both of the new compounds HP2Cu and PE2Cu. The results obtained are particularly promising, paving the way for the development of new selective functional ligands for binding and destructuring parallel G4s.

Regiocontrolled dimerization of asymmetric diazaheptacene derivatives toward X-shaped porous semiconductors

Conformationally rigid X-shaped PAHs are attracting interest due to their self-assembly into unique networks and as models to study through-space exciton and charge delocalization in one single molecule. We report here the synthesis of X-shaped PAHs by dimerization of diazaheptacene diimides. The diimide groups are employed to effectively direct the self-assembly into antiparallel dimer aggregates, which assist the compounds to undergo a regiocontrolled [4 + 4] dimerization, leading to an X-shaped conformation bearing electron-poor and -rich subunits. The resulting PAHs are found to pack in 2D layers with large open channels and infinite π⋯π arrays. Furthermore, these highly crystalline porous materials serve as electron-transporting materials in OFETs due to the long-range π-stacked arrays in the layers. This work presents a potentially generalizable strategy, which may provide a unique class of porous semiconductors for organic devices, taking advantage of their open channels.

The synthesis of conformationally rigid X-shaped PAHs by regiocontrolled cyclodimerization of diazaheptacene diimides is presented. The resulting porous materials exhibit enhanced semiconducting behaviors with large open channels.

Selective targeting of mutually exclusive DNA G-quadruplexes: HIV-1 LTR as paradigmatic model

Targeting of G-quadruplexes, non-canonical conformations that form in G-rich regions of nucleic acids, has been proposed as a novel therapeutic strategy toward several diseases, including cancer and infections. The unavailability of highly selective molecules targeting a G-quadruplex of choice has hampered relevant applications. Herein, we describe a novel approach, based on naphthalene diimide (NDI)-peptide nucleic acid (PNA) conjugates, taking advantage of the cooperative interaction of the NDI with the G-quadruplex structure and hybridization of the PNA with the flanking region upstream or downstream the targeted G-quadruplex. By biophysical and biomolecular assays, we show that the NDI-PNA conjugates are able to specifically recognize the G-quadruplex of choice within the HIV-1 LTR region, consisting of overlapping and therefore mutually exclusive G-quadruplexes. Additionally, the conjugates can induce and stabilize the least populated G-quadruplex at the expenses of the more stable ones. The general and straightforward design and synthesis, which readily apply to any G4 target of choice, together with both the red-fluorescent emission and the possibility to introduce cellular localization signals, make the novel conjugates available to selectively control G-quadruplex folding over a wide range of applications.

Recent progress in the usage of tetrabromo-substituted naphthalenetetracarboxylic dianhydride as a building block to construct organic semiconductors and their applications

The recent synthetic strategies and significant applications of TBNDA and their derivatives as promising building blocks to construct π-expanded semiconductors have been carefully summarized in this review.

Synthesis of Heterocyclic Core-Expanded Bis-Naphthalene Tetracarboxylic Diimides

A highly reactive bis-naphthalene tetracarboxylic diimide (bis-NDI) intermediate, TBrDNDI, was designed and synthesized for core-expanded NDIs. Based on this intermediate, we achieved 9- and 11-membered core-expanded bis-NDI derivatives. Through expanding the NDI core and introducing electron-donor or electron-acceptor groups, the frontier energy orbitals, optical and electrical properties of these bis-NDIs can be finely tuned to obtain air-stable ambipolar or n-type materials.

More is not always better: finding the right trade-off between affinity and selectivity of a G-quadruplex ligand

Guanine-rich nucleic acid sequences can fold into four-stranded G-quadruplex (G4) structures. Despite growing evidence for their biological significance, considerable work still needs to be done to detail their cellular occurrence and functions. Herein, we describe an optimized core-extended naphthalene diimide (c_ex-NDI) to be exploited as a G4 light-up sensor. The sensing mechanism relies on the shift of the aggregate-monomer equilibrium towards the bright monomeric state upon G4 binding. In contrast with the majority of other ligands, this novel c_ex-NDI is able to discriminate among G4s with different topologies, with a remarkable fluorescent response for the parallel ones. We investigate this sensing by means of biophysical methods, comparing the lead compound to a non-selective analogue. We demonstrate that mitigating the affinity of the binding core for G4s results in an increased selectivity and sensitivity of the fluorescent response. This is achieved by replacing positively charged substituents with diethylene glycol (DEG) side chains. Remarkably, the limit of detection values obtained for parallel G4s are more than one order of magnitude lower than those of the parallel-selective ligand N-methyl mesoporphyrin IX (NMM). Interestingly, the classical fluorescent intercalator displacement (FID) assay failed to reveal binding of c_ex-NDI to G4 because of the presence a ternary complex (G4-TO-c_ex-NDI) revealed by electrospray-MS. Our study thus provides a rational basis to design or modify existent scaffolds to redirect the binding preference of G4 ligands.

Naphthalene Diimides as Multimodal G-Quadruplex-Selective Ligands

G-quadruplexes are four-stranded nucleic acids structures that can form in guanine-rich sequences. Following the observation that G-quadruplexes are particularly abundant in genomic regions related to cancer, such as telomeres and oncogenes promoters, several G-quadruplex-binding molecules have been developed for therapeutic purposes. Among them, naphthalene diimide derivatives have reported versatility, consistent selectivity and high affinity toward the G-quadruplex structures. In this review, we present the chemical features, synthesis and peculiar optoelectronic properties (absorption, emission, redox) that make naphtalene diimides so versatile for biomedical applications. We present the latest developments on naphthalene diimides as G-quadruplex ligands, focusing on their ability to bind G-quadruplexes at telomeres and oncogene promoters with consequent anticancer activity. Their different binding modes (reversible versus irreversible/covalent) towards G-quadruplexes and their additional use as antimicrobial agents are also presented and discussed.

Tuning of the Electronic Levels of Oligothiophene-Naphthalimide Assemblies by Chemical Modification

Inversion of the connections of amidine linkers combined with controlled oligothiophene chain catenation in oligothiophene-naphthalimide assemblies provides an efficient method to tune the HOMO and LUMO values in this type of assemblies. This modification also suppresses the intramolecular charge transfer (ICT) band normally found in this type of derivatives, also delocalizing the frontier molecular orbitals over the whole conjugated skeleton. The resultant assemblies were used in the fabrication of field-effect transistors, which showed well-balanced ambipolar transport.

Synthesis, Binding and Antiviral Properties of Potent Core-Extended Naphthalene Diimides Targeting the HIV-1 Long Terminal Repeat Promoter G-Quadruplexes

We have previously reported that stabilization of the G-quadruplex structures in the HIV-1 long terminal repeat (LTR) promoter suppresses viral transcription. Here we sought to develop new G-quadruplex ligands to be exploited as antiviral compounds by enhancing binding toward the viral G-quadruplex structures. We synthesized naphthalene diimide derivatives with a lateral expansion of the aromatic core. The new compounds were able to bind/stabilize the G-quadruplex to a high extent, and some of them displayed clear-cut selectivity toward the viral G-quadruplexes with respect to the human telomeric G-quadruplexes. This feature translated into low nanomolar anti-HIV-1 activity toward two viral strains and encouraging selectivity indexes. The selectivity depended on specific recognition of LTR loop residues; the mechanism of action was ascribed to inhibition of LTR promoter activity in cells. This is the first example of G-quadruplex ligands that show increased selectivity toward the viral G-quadruplexes and display remarkable antiviral activity.

Naphthalene diimides as red fluorescent pH sensors for functional cell imaging

Water-soluble naphthalene diimides have been designed and synthesized as cell permeable pH “turned-on” fluorescent sensors for cellular applications.

Synthesis of monolateral and bilateral sulfur-heterocycle fused naphthalene diimides (NDIs) from monobromo and dibromo NDIs

Monolateral and bilateral sulfur-heterocycle fused naphthalene diimides (NDIs) are successfully synthesized from monobromo and dibromo NDIs via nucleophilic aromatic substitution reactions and the subsequent oxidative aromatization processes.

Strategies for the synthesis of functional naphthalene diimides

Naphthalene diimides, which have for a long time been in the shadow of their higher homologues the perylene diimides, currently belong to the most investigated classes of organic compounds. This is primarily due to the initial synthetic studies on core functionalization that were carried out at the beginning of the last decade, which facilitated diverse structural modifications of the naphthalene scaffold. Compounds with greatly modified optical and electronic properties that can be easily and effectively modulated by appropriate functionalization were made accessible through relatively little synthetic effort. This resulted in diverse interesting applications. The electron-deficient character of these compounds makes them highly valuable, particularly in the field of organic electronics as air-stable n-type semiconductors, while absorption bands over the whole visible spectral range through the introduction of core substituents enabled interesting photosystems and photovoltaic applications. This Review provides an overview on different approaches towards core functionalization as well as on synthetic strategies for the core expansion of naphthalene diimides that have been developed mainly in the last five years.

Targeting loop adenines in G-quadruplex by a selective oxirane

Caught in the oxirane: Naphthalene diimides conjugated to a quinone methide and an oxirane have been synthesized and investigated as selective DNA G-quadruplex alkylating agents. The oxirane derivative generates a stable adduct with a G-quadruplex and shows selective alkylation of the loop adenines, as illustrated.

Water soluble extended naphthalene diimides as pH fluorescent sensors and G-quadruplex ligands

Extended naphthalene diimides (NDIs) fused to 1,4-dihydropyrazine-2,3-dione, containing two solubilizing moieties, have been synthesized. Fluorescence spectra of the new NDIs were remarkably affected by pH, as the second deprotonation of the dihydropyrazinedione moiety (pK(a) 6.9) switched off the emission. Binding to a G-quadruplex folded oligonucleotide and stoichiometry were evaluated by FRET melting assay and CD analysis. G-quadruplex binding was strongly enhanced shifting from pH 7.4 to pH 6.0 as a consequence of the dihydropyrazinedione moiety protonation. Cytotoxicity studies using two human telomerase-positive cell lines (HT29 and A549) revealed that the best G-quadruplex ligand was very active against the colon cell line, with an EC(50) of 300 nM.

Naphthalene diimide scaffolds with dual reversible and covalent interaction properties towards G-quadruplex

Selective recognition and alkylation of G-quadruplex oligonucleotides has been achieved by substituted naphathalene diimides (NDIs) conjugated to engineered phenol moieties by alkyl-amido spacers with tunable length and conformational mobility. FRET-melting assays, circular dichroism titrations and gel electrophoresis analysis have been carried out to evaluate both reversible stabilization and alkylation of the G-quadruplex. The NDIs conjugated to a quinone methide precursor (NDI-QMP) and a phenol moiety by the shortest alkyl-amido spacer exhibited a planar and fairly rigid geometry (modelled by DFT computation). They were the best irreversible and reversible G-quadruplex binders, respectively. The above NDI-QMP was able to alkylate the telomeric G-quadruplex DNA in the nanomolar range and resulted 100-1000 times more selective on G-quadruplex versus single- and double-stranded oligonucleotides. This compound was also the most cytotoxic against a lung carcinoma cell line.

A bio-inspired design strategy: Organization of tryptophan-appended naphthalenediimide into well-defined architectures induced by molecular interactions

The chemistry of molecular assemblies involves weak yet complex non-covalent interactions, and the molecular organization of the π-conjugated material is crucial in determining the performance of an organic electronic device. Herein we demonstrate a bioinspired design strategy to tune the self-assembly of naphthalenediimides (NDIs) by minute structural variations, π-π stacking, hydrophobic interactions and metal interactions. We address some of the limitations associated with current design strategies, such as restriction to a specific molecular interaction or the difficulty in controlling the assembly due to several complicated intermolecular interactions. Hydrophobic-effect-induced J-type aggregation and sodium-interaction-induced H-type aggregation of tryptophan-appended NDIs have been illustrated. (1)H NMR spectra further reveal sodium cation-π interactions in tryptophan-appended NDIs, while NMR and IR spectroscopic studies confirm the structural variations associated with the molecular assembly. In summary, the molecular organization has been successfully transformed from nanospheres to particles, nanobelts, fibers and fractals. Such drastic changes in the morphology are clear and striking evidence of the importance of non-trivial weak non-covalent forces.