Non-conjugated alkyl chain engineering to tune condensed state photophysical and supramolecular assembly properties

Techniques utilizing strategic molecular engineering to develop well-defined supramolecular fluorescent nano-assemblies hold significant scientific interest. However, a significant drawback in their practical application is the undesirable aggregation-caused quenching (ACQ) phenomenon in traditional fluorescent compounds, which usually occurs in planar aromatic cores. Herein, a series of seven planar naphthalimide (NC) derivatives have been constructed by appending non-conjugated linear alkyl chains that effectively modulate their condensed state emission and supramolecular nano-assembly. Structural characterization through single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) patterns indicates that the supramolecular nano-assemblies and their condensed state emission properties are intricately governed by the alkyl chain-driven intermolecular packing orientation. The steric constraints exerted by the non-conjugated linear alkyl chains appended to the NC core efficiently alter the intermolecular π-π stacking interactions of the planar aromatic core. The investigated role of linear alkyl chains in modulating the condensed state behavior of NC derivatives presents a conceptual insight and a promising avenue for constructing fluorescent nano-assemblies.

Seed-Induced Living Two-Dimensional (2D) Supramolecular Polymerization in Water: Implications on Protein Adsorption and Enzyme Inhibition

In biological systems, programmable supramolecular frameworks characterized by coordinated directional non-covalent interactions are widespread. However, only a small number of reports involve pure water-based dynamic supramolecular assembly of artificial π-amphiphiles, primarily due to the formidable challenge of counteracting the strong hydrophobic dominance of the π-surface in water, leading to undesired kinetic traps. This study reveals the pathway complexity in hydrogen-bonding-mediated supramolecular polymerization of an amide-functionalized naphthalene monoimide (NMI) building block with a hydrophilic oligo-oxyethylene (OE) wedge. O-NMI-2 initially produced entropically driven, collapsed spherical particles in water (Agg-1); however, over a span of 72 h, these metastable Agg-1 gradually transformed into two-dimensional (2D) nanosheets (Agg-2), favoured by both entropy and enthalpy contributions. The intricate self-assembly pathways in O-NMI-2 enable us to explore seed-induced living supramolecular polymerization (LSP) in water for controlled synthesis of monolayered 2D assemblies. Furthermore, we demonstrated the nonspecific surface adsorption of a model enzyme, serine protease α-Chymotrypsin (α-ChT), and consequently the enzyme activity, which could be regulated by controlling the morphological transformation of O-NMI-2 from Agg-1 to Agg-2. We delve into the thermodynamic aspects of such shape-dependent protein-surface interactions and unravel the impact of seed-induced LSP on temporally controlling the catalytic activity of α-ChT.

Self-Assembly Driven Formation of Functional Ultralong "Artificial Fibers" to Mitigate the Neuronal Damage Associated with Alzheimer's Disease

Fibrillation of amyloid beta (Aβ) is the key event in the amyloid neurotoxicity process that induces a chain of toxic events including oxidative stress, caspase activation, poly(ADP-ribose) polymerase cleavage, and mitochondrial dysfunction resulting in neuronal loss and memory decline manifesting as clinical dementia in humans. Herein, we report the development of a novel, biologically active supramolecular probe, INHQ, and achieve functional nanoarchitectures via a self-assembly process such that ultralong fibers are achieved spontaneously. With specifically decorated functional groups on INHQ such as imidazole, hydroxyquinoline, hydrophobic chain, and hydroxyquinoline molecules, these ultralong fibers coassembled efficiently with toxic Aβ oligomers and mitigated the amyloid-induced neurotoxicity by blocking the aforementioned biochemical events leading to neuronal damage in mice. These functional ultralong "Artificial Fibers" morphologically resemble the amyloid fibers and provide a higher surface area of interaction that improves its clearance ability against the Aβ aggregates. The efficacy of this novel INHQ molecule was ascertained by its high ability to interact with Aβ. Moreover, this injectable, ultralong INHQ functional "artificial fiber" translocates through the blood-brain barrier and successfully attenuates the amyloid-triggered neuronal damage and pyknosis in the cerebral cortex of wild-type mouse. Utilizing various spectroscopic techniques, morphology analysis, and in vitro, in silico, and in vivo studies, these ultralong INHQ fibers are proven to hold great promise for treating neurological disorders at all stages with a potential to replace the existing medications, reduce complications in the brain, and eradicate the amyloid-triggered neurotoxicity implicated in numerous disorders in human through a rare synergistic mechanism.

Pathway Complexity in Supramolecular Copolymerization and Blocky Star Copolymers by a Hetero-Seeding Effect

This study unravels the intricate kinetic and thermodynamic pathways involved in the supramolecular copolymerization of the two chiral dipolar naphthalene monoimide (NMI) building blocks (O-NMI and S-NMI), differing merely by a single heteroatom (oxygen vs sulfur). O-NMI exhibits distinct supramolecular polymerization features as compared to S-NMI in terms of its pathway complexity, hierarchical organization, and chiroptical properties. Two distinct self-assembly pathways in O-NMI occur due to the interplay between the competing dipolar interactions among the NMI chromophores and amide-amide hydrogen (H)-bonding that engenders distinct nanotapes and helical fibers, from its antiparallel and parallel stacking modes, respectively. In contrast, the propensity of S-NMI to form only a stable spherical assembly is ascribed to its much stronger amide-amide H-bonding, which outperforms other competing interactions. Under the thermodynamic route, an equimolar mixture of the two monomers generates a temporally controlled chiral statistical supramolecular copolymer that autocatalytically evolves from an initially formed metastable spherical heterostructure. In contrast, the sequence-controlled addition of the two monomers leads to the kinetically driven hetero-seeded block copolymerization. The ability to trap O-NMI in a metastable state allows its secondary nucleation from the surface of the thermodynamically stable S-NMI spherical "seed", which leads to the core-multiarmed "star" copolymer with reversibly and temporally controllable length of the growing O-NMI "arms" from the S-NMI "core". Unlike the one-dimensional self-assembly of O-NMI and its random co-assembly with S-NMI, which are both chiral, unprecedentedly, the preferred helical bias of the nucleating O-NMI fibers is completely inhibited by the absence of stereoregularity of the S-NMI "seed" in the "star" topology.

Bipolar 1,8-naphthalimides showing high electron mobility and red AIE-active TADF for OLED applications

Aiming to design bipolar organic semiconductors with high electron mobility and efficient red thermally activated delayed fluorescence (TADF), three donor-acceptor compounds were designed and synthesized selecting 1,8-naphthalimide as an acceptor and phenoxazine, 3,7-di-tert-butylphenothiazine or 2,7-di-tert-butyldimethyl-9,10-dihydroacridine as donor moieties. Aggregation induced emission enhancement was detected for the compounds causing efficient TADF in the solid-state. Photoluminescence quantum yields up to 77% were observed for the films of the compounds doped in a host. The compounds exhibited small singlet-triplet splitting (0.03-0.05 eV), and high reverse intersystem crossing rates of 2.08 × 10⁵-1.13 × 10⁶ s^-1. The compounds were characterized by satisfactory hole and electron-injecting properties with ionization potentials of 5.72-5.83 eV and electron affinities of 2.79-2.91 eV. Bipolar charge transport was revealed by time of flight measurements. Electron transport with low dispersity and mobilities exceeding 2 × 10^-3 cm² V^-1 s^-1 was observed at an electric field of 4.6 × 10⁵ V cm^-1. The compounds were used as emitters in red electroluminescent devices, which showed maximum external quantum efficiencies up to 8.2%. Utilization of host-guest systems as light-emitting materials with hosts preferably transporting holes and TADF guests which preferably transport electrons allowed maximum efficiencies to be achieved at a practical brightness of 700-2200 cd m^-2. DFT calculations of the geometry, electronic structure, absorption and photoluminescence spectra of all compounds were carried out to prove the conclusions drawn from the experiment. The results of the calculations clearly show that the first excited state for all compounds is the intramolecular charge transfer state. Quantitative analysis of the separation degree of electronic density during excitation allows the observed dependence of the blue shift value in the absorption and emission spectra on the increasing polarity of the solvent to be explained.

Target-triggered and controlled release plasmon-enhanced fluorescent AIE probe for conformational monitoring of insulin fibrillation

In this work, we constructed a target-triggered and controlled-release plasmon-enhanced fluorescent AIE probe to realize the purpose of conformational monitoring of insulin fibrillation. We synthesized a novel water-soluble anthracene derivative, 4,4',4'',4'''-(anthracene-9,10-diylbis(ethene-2,1,1-triyl))tetrakis(N,N,N-trimethylbenzenaminium) iodide (BDVAI), with AIE properties, high biocompatibility and good self-assembly effect. Gold nanocages (AuNCs) were selected as the substrate for PEF, and the inner space of hollow AuNCs was filled with BDVAI. Thiol-modified DNA chains were bonded to the surface of AuNCs by Au-S bonds, and an insulin aptamer was combined with the sulfhydryl chain to seal the AuNCs. This PEF-AIE sensor produces different fluorescence signals when interacting with native insulin and fibrillar insulin; thus, monitoring conformational changes in insulin can be realized by detecting fluorescence intensity changes during insulin fibrillation. Based on this design, this system realized sensitive detection of fibrillar insulin with a detection limit of 23.6 pM. This AIE molecular-based PEF fluorescence enhancement system improves the optical properties of fluorescent substances, which is of great significance in improving the detection sensitivity of amyloid fibrils conformational changes and providing a reliable basis for further understanding the pathogenesis of amyloidosis.

Turn-on fluorescent naphthalimide-benzothiazole probe for cyanide detection and its two-mode aggregation-induced emission behavior

Naphthalimide-benzothiazole conjugate (NBTZ) linked by cinnamonitrile was designed, synthesized, and fully characterized by NMR (¹H, ¹³C, DEPT, HSQC) and high-resolution mass spectrometry. NBTZ exhibited unique turn-on fluorescence in the presence of CN^- with relatively high selectivity compared to other anions such as SCN^-, HSO₄^-, ClO₄^-, NO₃^-, Cl^-, Br^-, I^-, and PO₄^-3 in tetrahydrofuran (THF). The detection limit for CN^- was found to be 3.35 × 10^-8 M in THF. The sensing mechanism was analyzed through ¹H, ¹³C, DEPT, and mass spectroscopy. NBTZ also showed two-mode aggregation-induced emission (AIE) in THF-H₂O mixtures. In a 30:70 THF-H₂O (v/v) mixture, the maximum AIE was observed at 430 nm (blue) because of the rotation of the CC bond between the naphthalimide ring and the phenyl ring was restricted. In 10:90 THF-H₂O (v/v), a new red-shifted AIE appeared at 490 nm (cyan), due to the extended π-conjugation induced by restriction of rotation of the CC bond between the benzothiazole and naphthalimide rings.

A conformational tweak for enhanced cellular internalization, photobleaching resistance and prolonged imaging efficacy

A strategy of conformational tweaking regulates the condensed state behavior of naphthalimide skeletal isomers (NSIs) and enhances their photophysical properties, cellular uptake and prolonged imaging capability. This salient approach results in a large Stokes shift (>120 nm), rapid cellular internalization, photobleaching resistance, and efficient bioimaging of the ribbon-like nano-assembly superior to that of its electronically similar micro-flower isomer.

Additive- and column-free synthesis of rigid bis-coumarins as fluorescent dyes for G-quadruplex sensing via disaggregation-induced emission

A novel and efficient method to synthesize rigid bis-coumarins based on the dimerization of coumarinyl aldehydes was developed. This procedure is additive- and column-free, providing a facile and environment-friendly way to prepare fluorophores. The prepared novel fluorescent bis-coumarins exhibit favorable photophysical properties with good sensitivity and selectivity towards G-quadruplexes (G4s).

ICT and AIE Characteristics Two Cyano-Functionalized Probes and Their Photophysical Properties, DFT Calculations, Cytotoxicity, and Cell Imaging Applications

Two probes, AIE-1 and AIE-2, were synthesized to investigate the effect of substitutional functional group on aggregation (aggregation-caused quenching (ACQ) or aggregation-induced emission (AIE)) and intramolecular charge transfer (ICT) behavior as well as on the cell imaging aspect. The yellow-color non-substituted probe AIE-1 showed weak charge-transfer absorption and an emission band at 377 nm and 432 nm, whereas the yellowish-orange color substituted probe AIE-2 showed a strong charge-transfer absorption and an emission band at 424 nm and 477 nm in THF solvent. The UV-Vis studies of AIE-1 and AIE-2 in THF and THF with different water fractions showed huge absorption changes in AIE-2 with high water fractions due to its strong aggregation behavior, but no such noticeable absorption changes were observed for AIE-1. Interestingly, the fluorescence intensity of AIE-1 at 432 nm gradually decreased with increasing water fractions and became almost non-emissive at 90% water. However, the monomer-type emission of AIE-2 at 477 nm was shifted to 584 nm with a 6-fold increase in fluorescence intensity in THF-H2O (1:9, v/v) solvent mixtures due to the restriction of intramolecular rotation on aggregation in high water fractions. This result indicates that the probe AIE-1 shows ACQ and probe AIE-2 shows AIE behaviors in THF-H2O solvent mixtures. Furthermore, the emission spectra of AIE-1 and AIE-2 were carried out in different solvent and with different concentrations to see the solvent- or concentration-dependent aggregation behavior. Scanning electron microscope (SEM) and dynamic light scattering (DLS) experiments were also conducted to assess the morphology and particle size of two probes before and after aggregation. Both of the probes, AIE-1 and AIE-2, showed less toxicity on HeLa cells and were suitable for cell imaging studies. Density functional theory (DFT) calculation was also carried out to confirm the ICT process from an electron-rich indole moiety to an electron-deficient cyano-phenyl ring of AIE-1 or AIE-2.

Stepwise elucidation of fluorescence based sensing mechanisms considering picric acid as a model analyte

The precise study of fluorescence-based sensing mechanisms and a step-by-step design experiment for the elucidation of the mechanism of sensing for newly designed sensing systems can be ascertained using the presented tutorial review.

ESIPT-active multi-color aggregation-induced emission features of triphenylamine–salicylaldehyde-based unsymmetrical azine family

A new family of ESIPT and AIE-active triphenylamine-appended unsymmetrical azine derivatives is reported.

An Unprecedented Blueshifted Naphthalimide AIEEgen for Ultrasensitive Detection of 4-Nitroaniline in Water via "Receptor-Free" IFE Mechanism

The development of a new naphthalene appended naphthalimide derivative (NMI) with aggregation-induced enhanced emission (AIEE) property for the sensitive detection of 4-nitroaniline (4-NA) in aqueous media is presented here. The newly designed naphthalimide AIEEgen has an exceptional blue-shifted condensed state emission that is devoid of any receptor site, accomplished ultrasensitive detection of 4-NA, which is one of the broad-spectrum pesticides that belong to the class III toxic chemical, at parts per billion level (LOD/36 ppb, K_sv =4.1×10⁴  m^-1 ) in water with excellent selectivity even in the presence of potentially competing aliphatic and aromatic amines. The reported probe is the first of its kind, demonstrating major advantages of receptor-free inner filter effect (IFE) mechanism for the sensitive detection of 4-NA using an AIEEgenic probe. Excellent sensitivity for 4-NA is also achieved on paper-based test-strip for low-cost on-site detection.

Functional group engineering in naphthalimides: a conceptual insight to fine-tune the supramolecular self-assembly and condensed state luminescence

Engineering well-defined supramolecular fluorescent nano-architectures based on organic conjugated small molecules has been an essential scientific challenge. Herein, a library of sixteen naphthalimide congeners (1-15 and HNI) has been strategically designed that unveils a conceptual insight into the functional group controlled condensed state emission and aggregation-induced enhanced emission (AIEE) in conventional strong aggregation-caused quenching (ACQ) active fluorophores. Along with the regulation of ACQ-to-AIEE transformation and tailoring of the condensed state emission, a simple yet potential design strategy of functional group engineering has been established for the first time to spontaneously generate and systematically tailor the supramolecular self-assembly of organic small molecules into highly defined nano-architectures. Single-crystal XRD analysis of six congeners revealed that, unlike the well-established electronic contribution of the functional groups in the molecularly dispersed state, the condensed state photophysical and morphological properties are dictated by the distinct intermolecular π-π stacking interaction of the planar aromatic core. This work demonstrates an unconventional influence of the functional motif in the condensed state that could emerge as a promising route to build a fluorescent supramolecular nanoassembly from non-fluorescent conjugated molecules for a variety of future applications.

Aggregation induced emission enhancement (AIEE) of tripodal pyrazole derivatives for sensing of nitroaromatics and vapor phase detection of picric acid

Organosoluble tris-pyrazole compounds aggregate in organic/aqueous solvent mixtures, showing aggregation-induced emission enhancement (AIEE), the latter being quenched by picric acid.

Unravelling substitution effects on charge transfer characteristics in cocrystals of pyrene based donors and 3,5-dinitrobenzoic acid

Ambipolar to p-type semiconductivity switching with the change of the ⋯DADADA⋯ to ⋯ADDADD⋯ packing arrangement in charge transfer cocrystals of pyrene based donors is discussed.

Insights into the aggregation-induced emission of 1,8-naphthalimide-based supramolecular hydrogels

The study of aggregation-induced emission (AIE) of a series of 1,8-naphthalimide derivatives in aqueous media is reported herein. Some of these molecules constitute the first examples of 1,8-naphthalimide-containing amino acid derivatives that form hydrogels with excellent photophysical and mechanical properties. The present study provides further insights for the rational design of water-compatible stimuli-responsive photonic materials presenting AIE. AIE was quantitatively evaluated by measuring the fluorescence quantum yields of the molecules. Gelators 1 and 2 exhibit self-assembled fibrillar morphologies and present the best performance regarding the AIE effect, showing a remarkable enhancement in fluorescence intensity of 4700% and reaching a notable fluorescence quantum yield (Φf) of 30%. Non-gelator molecules 6 and 7 form nanoparticles, which also present AIE, but with emissions corresponding to their excimers. Therefore, the AIE intensity and wavelength are regulated by the type of aggregate morphology: fibers, nanoparticles or soluble species.

Aggregation-Induced Emission-Active Ruthenium(II) Complex of 4,7-Dichloro Phenanthroline for Selective Luminescent Detection and Ribosomal RNA Imaging

The development of red emissive aggregation-induced emission (AIE) active probes for organelle-specific imaging is of great importance. Construction of metal complex-based AIE-active materials with metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) emission together with the ligand-centered and intraligand (LC/ILCT) emission is a challenging task. We developed a red emissive ruthenium(II) complex, 1[PF₆]₂, and its perchlorate analogues of the 4,7-dichloro phenanthroline ligand. 1[PF₆]₂ has been characterized by spectroscopic and single-crystal X-ray diffraction. Complex 1 showed AIE enhancement in water, highly dense polyethylene glycol media, and also in the solid state. The possible reason behind the AIE property may be the weak supramolecular π···π, C-H···π, and C-Cl···H interactions between neighboring phen ligands as well as C-Cl···O halogen bonding (XB). The crystal structures of the two perchlorate analogues revealed C-Cl···O distances shorter than the sum of the van der Waals radii, which confirmed the XB interaction. The AIE property was supported by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, and atomic force microscopy studies. Most importantly, the probe was found to be low cytotoxicity and to efficiently permeate the cell membrane. The cell-imaging experiments revealed rapid staining of the nucleolus in HeLa cells via the interaction with nucleolar ribosomal ribonucleic acid (rRNA). It is expected that the supramolecular interactions as well as C-Cl···O XB interaction with rRNA is the origin of aggregation and possible photoluminescence enhancement. To the best of our knowledge, this is the first report of red emissive ruthenium(II) complex-based probes with AIE characteristics for selective rRNA detection and nucleolar imaging.

Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects

This comprehensive review surveys the up-to-date development of aggregation-induced emission/aggregation-induced emission enhancement (AIE/AIEE) active naphthalimide (NI)-based smart materials with potential for wide and real-world applications and that serves as a highly versatile building block with tunable absorption and emission in the complete visible region. The review article commences with a precise description of the importance of NI moiety and its several restricted area of applications owing to its aggregation caused quenching (ACQ) properties, followed by the discovery and importance of AIE/AIEE-active NIs. The introduction section tracked an overview of the state of the art in NI luminogens in multiple applications. It also includes a few mechanistic studies on the structure-property correlation of NIs and provides more insights into the condensed-state photophysical properties of small aggregation-prone systems. The review aims to ultimately accomplish current and forthcoming views comprising the use of the NIs for the detection of biologically active molecules, such as amino acids and proteins, recognition of toxic analytes, fabrication of light emitting diodes, and their potential in therapeutics and diagnostics.

Aldehyde group driven aggregation-induced enhanced emission in naphthalimides and its application for ultradetection of hydrazine on multiple platforms

A new concept of formyl group induced ACQ to AIEE transformation is established in naphthalimide congeners. Also, ultradetection of hydrazine via Schiff base complexation over multiple platforms is presented.

Restriction of intramolecular motion (RIM) of rigid aromatic cores is the most universal mechanism so far that can successfully explain almost all AIE/AIEEgenic systems. By designing two novel naphthalimide derivatives (NIB and NIBD), we experimentally demonstrated the effect of a single formyl group that can efficiently transform an aggregation caused quenching (ACQ) system (NIB) into an AIEEgenic system (NIBD) by strengthening the RIM process. Besides, the newly designed naphthalimide AIEEgen (NIBD) accomplished ultrasensitive detection of hydrazine at the parts per trillion level (LOD/81 ppt) in aqueous media with high selectivity and enormous improvement over the existing state of the art. An exceptional sensitivity is also achieved in the vapor phase (LOD/0.003%) using a Whatman paper strip based portable device for simple and cost-effective on-site detection. The detection mechanism involved a reaction-based spontaneous formation of a non-fluorescent hydrazone Schiff base derivative (NIBDH). The in vitro potentiality of the AIEEgenic probe was also demonstrated in two mammalian cell lines i.e. HeLa (human cervical cancer cell line) and HEK293T (Human embryonic kidney cell line that expresses a mutant version of the SV40 large T antigen). Owing to the highly selective formation of the hydrazone Schiff base complex with hydrazine, NIBD responds to the existence of hydrazine in both these cell lines without any interference from other biologically rich metal ions and amino acids. These outcomes could initiate a much wider use of formyl group induced condensed state emission and a key hypothesis that could generate newer avenues for ACQ to AIEE transformations for several practical applications including hydrazone Schiff base complexation for probing and manipulating hydrazine biology associated with several metabolic activities.

Pendant chain engineering to fine-tune the nanomorphologies and solid state luminescence of naphthalimide AIEEgens: application to phenolic nitro-explosive detection in water

Strategically, a series of five angular "V" shaped naphthalimide AIEEgens with varying pendant chains (butyl, hexyl, octyl, cyclohexyl and methylcyclohexyl) have been synthesized to fine-tune their nanomorphological and photophysical properties. With similar aromatic cores and electronic states, unexpected tuning of the condensed state emission colors and nanomorphologies (reproducible on any kind of surface) of naphthalimides has been achieved for the first time simply by varying their side chains. Conclusive analysis by various spectroscopic techniques (SC-XRD, powder-XRD, DLS, FESEM) and DFT computational studies confirmed the full control of the pendant chain (in terms of bulkiness around the naphthalimide core, which restricts the ease of intermolecular π-π interactions) over the nanoaggregate morphology and solid state emissive properties of the AIEEgens; this can be rationalized to all aggregation-prone systems. These comprehensive studies establish a conceptually unique yet simple and effective method to precisely tune the nanomorphologies and the emission colors of aggregation-prone small organic molecules by judicious choice of the non-conjugated pendant chain. Thus, considering the prime role of the active layer nanomorphology in all organic optoelectronic devices, this methodology may emerge as a promising tool to improve device performance. Among all the congeners, the hexyl chain-containing congener (HNQ) forms well-defined nanoribbons with smaller diameters (as confirmed from DLS: 166 nm and FESEM: 150 nm) and provides a larger surface area. Consequently, the HNQ-nanoribbons were employed as a fluorescent sensor for the discriminative detection of trinitrophenol (TNP) in pure aqueous media. FE-SEM images revealed that, upon gradual addition of TNP (10 nM to 100 μM), these nanoribbons undergo an aggregation/disaggregation process, forming non-fluorescent co-aggregates with TNP, and provide highly enhanced sensitivity compared to existing state-of-the-art on aggregation-prone systems. Fluorescence titration studies confirmed that HNQ can detect the presence of TNP as low as 16.8 ppb and can serve as a cost-effective portable device incorporated with UV-light for on-site visual detection of TNP, even in the presence of potentially competing nitroaromatic compounds.