Effect of the Substitution Position on the Electronic and Solvatochromic Properties of Isocyanoaminonaphthalene (ICAN) Fluorophores

Preparation and Optical Study of 1-Formamido-5-Isocyanonaphthalene, the Hydrolysis Product of the Potent Antifungal 1,5-Diisocyanonaphthalene

Aromatic isocyanides have gained a lot of attention lately as promising antifungal and anticancer drugs, as well as high-performance fluorescent analytical probes for the detection of toxic metals, such as mercury, even in vivo. Since this topic is relatively new and aromatic isocyanides possess unique photophysical properties, the understanding of structure-behavior relationships and the preparation of novel potentially biologically active derivatives are of paramount importance. Here, we report the photophysical characterization of 1,5-diisocyanonaphthalene (DIN) backed by quantum chemical calculations. It was discovered that DIN undergoes hydrolysis in certain solvents in the presence of oxonium ions. By the careful control of the reaction conditions for the first time, the nonsymmetric product 1-formamido-5-isocyanonaphthalene (ICNF) could be prepared. Contrary to expectations, the monoformamido derivative showed a significant solvatochromic behavior with a ~50 nm range from hexane to water. This behavior was explained by the enhanced H-bond-forming ability of the formamide group. The significance of the hydrolysis reaction is that the isocyano group is converted to formamide in living organisms. Therefore, ICNF could be a potential drug (for example, antifungal) and the reaction can be used as a model for the preparation of other nonsymmetric formamido-isocyanoarenes. In contrast to its relative 1-amino-5-iscyanonaphthalene (ICAN), ICNF is highly fluorescent in water, enabling the development of a fluorescent turnoff probe.

Synthesis and Photophysical Properties of α-(N-Biphenyl)-Substituted 2,2'-Bipyridine-Based Push-Pull Fluorophores

A series of new α-(N-biphenyl)-substituted 2,2'-bipyridines were obtained through the combination of the ipso-nucleophilic aromatic substitution of the C5-cyano group, aza-Diels-Alder and Suzuki cross-coupling reactions, starting from 5-cyano-1,2,4-triazines. For the obtained compounds, photophysical and fluorosolvatochromic properties were studied. Fluorophores 3l and 3b demonstrated unexpected AIEE activity, while 3a and 3h showed promising nitroexplosive detection abilities.

Potential Original Drug for Aspergillosis: In Vitro and In Vivo Effects of 1-N,N-Dimethylamino-5-Isocyanonaphthalene (DIMICAN) on Aspergillus fumigatus

As the recent outbreak of coronavirus disease 2019 (COVID-19) has shown, viral infections are prone to secondary complications like invasive aspergillosis with a high mortality rate, and therefore the development of novel, effective antifungals is of paramount importance. We have previously demonstrated that 1-amino-5-isocyanonaphthalene (ICAN) derivatives are promising original drug candidates against Candida strains (Patent pending), even against fluconazole resistant C. albicans. Consequently, in this study ICANs were tested on Aspergillus fumigatus, an opportunistic pathogen, which is the leading cause of invasive and systematic pulmonary aspergillosis in immunosuppressed, transplanted and cancer- or COVID-19 treated patients. We have tested several N-alkylated ICANs, a well as 1,5-naphthalene-diisocyanide (DIN) with the microdilution method against Aspergillus fumigatus strains. The results revealed that the diisocyanide (DIN) was the most effective with a minimum inhibitory concentration (MIC) value as low as 0.6 µg mL⁻¹ (3.4 µM); however, its practical applicability is limited by its poor water solubility, which needs to be overcome by proper formulation. The other alkylated derivatives also have in vitro and in vivo anti-Aspergillus fumigatus effects. For animal experiments the second most effective derivative 1-N, N-dimethylamino-5-isocyanonaphthalene (DIMICAN, MIC: 7–8 µg mL⁻¹, 36–41 µM) was selected, toxicity tests were made with mice, and then the antifungal effect of DIMICAN was tested in a neutropenic aspergillosis murine model. Compared to amphotericin B (AMB), a well-known antifungal, the antifungal effect of DIMICAN in vivo turned out to be much better (40% vs. 90% survival after eight days), indicating its potential as a clinical drug candidate.

Isocyanonaphthol Derivatives: Excited-State Proton Transfer and Solvatochromic Properties

Fluorescent probes that exhibit solvatochromic or excited-state proton-transfer (ESPT) properties are essential tools for the study of complex biological or chemical systems. Herein, the synthesis and characterization of a novel fluorophore that reveals both features, 5-isocyanonaphthalene-1-ol (ICOL), are reported. Various solvatochromic methods, such as Lippert−Mataga and Bilot−Kawski, together with time-dependent density functional theory (TD-DFT) and time-resolved emission spectroscopy (TRES), were applied to gain insights into its excited-state behavior. To make comparisons, the octyloxy derivative of ICOL, 5-isocyano-1-(octyloxy)naphthalene (ICON), was also prepared. We found that internal charge transfer (ICT) takes place between the isocyano and −OH groups of ICOL, and we determined the values of the dipole moments for the ground and excited states of both ICOL and ICON. Furthermore, in the emission spectra of ICOL, a second band at higher wavelengths (green emission) in solvents of higher polarities (dual emission), in addition to the band present at lower wavelengths (blue emission), were observed. The extent of this dual emission increases in the order of 2-propanol < methanol < N,N-dimethylformamide (DMF) < dimethyl sulfoxide (DMSO). The presence of the dual fluorescence of ICOL in these solvents can be ascribed to ESPT. For ICOL, we also determined ground- and excited-state pKa values of 8.4 ± 0.3 and 0.9 ± 0.7, respectively, which indicates a considerable increase in acidity upon excitation. The TRES experiments showed that the excited-state lifetimes of the ICOL and ICON spanned from 10.1 ns to 5.0 ns and from 5.7 ns to 3.8 ns, respectively. In addition, we demonstrated that ICOL can be used as an effective indicator of not only the critical micelle concentration (cmc) of ionic (sodium lauryl sulfate (SLS)) and nonionic surfactants (Tween 80), but also other micellar parameters, such as partition coefficients, as well as to map the microenvironments in the cavities of biomacromolecules (e.g., BSA). It is also pointed out that fluorescence quenching by pyridine can effectively be utilized for the determination of the fractions of ICOL molecules that reside at the water−micelle interface and in the interior spaces of micelles.

Optical Study of Solvatochromic Isocyanoaminoanthracene Dyes and 1,5-Diaminoanthracene

Isocyanoaminoarenes (ICAAr-s) are a novel and versatile group of solvatochromic fluorophores. Despite their versatile applicability, such as antifungals, cancer drugs and analytical probes, they still represent a mostly unchartered territory among intramolecular charge-transfer (ICT) dyes. The current paper describes the preparation and detailed optical study of novel 1-isocyano-5-aminoanthrace (ICAA) and its N-methylated derivatives along with the starting 1,5-diaminoanthracene. The conversion of one of the amino groups of the diamine into an isocyano group significantly increased the polar character of the dyes, which resulted in a significant 50-70 nm (2077-2609 cm-1) redshift of the emission maximum and a broadened solvatochromic range. The fluorescence quantum yield of ICAAs is strongly influenced by the polarity of the solvent. The starting anthracene-diamine is highly fluorescent in every solvent (√f = 12-53%), while the isocyano derivatives are practically nonfluorescent in solvents more polar than dioxane. This phenomenon implies the potential application of ICAAs to probe the polarity of the medium and is favorable in practical applications, such as cell-staining, resulting in a reduced background fluorescence. The ICT character of the emission states of ICAAs are in good agreement with the computational findings presented in TD-DFT calculations and molecular electrostatic potential (MESP) isosurfaces.

Antifungal Activity of an Original Amino-Isocyanonaphthalene (ICAN) Compound Family: Promising Broad Spectrum Antifungals

: Multiple drug resistant fungi pose a serious threat to human health, therefore the development of completely new antimycotics is of paramount importance. The in vitro antifungal activity of the original, 1-amino-5-isocyanonaphthalenes (ICANs) was evaluated against reference strains of clinically important Candida species. Structure-activity studies revealed that the naphthalene core and the isocyano- together with the amino moieties are all necessary to exert antifungal activity. 1,1-N-dimethylamino-5-isocyanonaphthalene (DIMICAN), the most promising candidate, was tested further in vitro against clinical isolates of Candida species, yielding a minimum inhibitory concentration (MIC) of 0.04-1.25 µg/mL. DIMICAN was found to be effective against intrinsically fluconazole resistant Candida krusei isolates, too. In vivo experiments were performed in a severly neutropenic murine model inoculated with a clinical strain of Candida albicans. Daily administration of 5 mg/kg DIMICAN intraperitoneally resulted in 80% survival even at day 13, whereas 100% of the control group died within six days. Based on these results, ICANs may become an effective clinical lead compound family against fungal pathogens.