Microwave-Assisted Synthesis and Spectral Properties of Pyrrolidine-Fused Chlorin Derivatives.

In this work we pursued research involving the microwave-assisted N-alkylation of a NH pyrrolidine-fused chlorin with methyl 4-(bromomethyl) benzoate and subsequent ester hydrolysis as a straightforward strategy to obtain carboxylic acid functionality in the pyrrolidine-fused chlorin, as a single reaction product. We studied the reaction's scope by extending the N-alkylation of the free-base chlorin and its corresponding Zn(II) complex to other alkyl halides, including 1,4-diiodobutane, N-(2-bromoethyl)phthalimide, and 2-bromoethanaminium bromide. In addition, two new chlorin-dansyl dyads were synthesized by reacting dansyl chloride with the 2-aminoethyl pyrrolidine-fused chlorin (dyad 6) and NH pyrrolidine-fused chlorin (dyad 7). According to spectral studies, the linker length between the two fluorophores influences the response of the dyads to the solvent polarity. Because of the simplicity of these approaches, we believe it will enable access to a vast library of custom-tailored N-functionalized chlorins while preserving their important absorption and emission spectra as photosensitizers in photodynamic therapy (PDT) of cancer and photodynamic inactivation (PDI) of microorganisms.

Selective Determination of Glutathione Using a Highly Emissive Fluorescent Probe Based on a Pyrrolidine-Fused Chlorin.

We report the use of a carboxylated pyrrolidine-fused chlorin (TCPC) as a fluorescent probe for the determination of glutathione (GSH) in 7.4 pH phosphate buffer. TCPC is a very stable, highly emissive molecule that has been easily obtained from meso-tetrakis(4-methoxycarbonylphenyl) porphyrin (TCPP) through a 1,3-dipolar cycloaddition approach. First, we describe the coordination of TCPC with Hg(II) ions and the corresponding spectral changes, mainly characterized by a strong quenching of the chlorin emission band. Then, the TCPC-Hg2+ complex exhibits a significant fluorescence turn-on in the presence of low concentrations of the target analyte GSH. The efficacy of the sensing molecule was tested by using different TCPC:Hg2+ concentration ratios (1:2, 1:5 and 1:10) that gave rise to sigmoidal response curves in all cases with modulating detection limits, being the lowest 40 nM. The experiments were carried out under physiological conditions and the selectivity of the system was demonstrated against a number of potential interferents, including cysteine. Furthermore, the TCPC macrocycle did not showed a significant fluorescent quenching in the presence of other metal ions.

Hybrid Zn-ß-Aminoporphyrin-Carbon Nanotubes: Pyrrolidine and Direct Covalent Linkage Recognition, and Multiple-Photo Response.

To unveil and shape the molecular connectivity in (metallo)porphyrin-carbon nanotube hybrids are of main relevance for the multiple medicinal, photoelectronic, catalytic, and photocatalytic applications of these materials. Multi-walled carbon nanotubes (MWCNTs) were modified through 1,3-dipolar cycloaddition reactions with azomethine ylides generated in situ and carrying pentafluorophenyl groups, followed by immobilization of the ß-amino-tetraphenylporphyrinate Zn(II). The functionalities were confirmed via XPS and FTIR, whereas Raman spectroscopy showed disruptions on the graphitic carbon nanotube surface upon both steps. The functionalization extension, measured via TGA mass loss and corroborated via XPS, was 0.2 mmol·g-1. Photophysical studies attest to the presence of the different porphyrin-carbon nanotube connectivity in the nanohybrid. Significantly different emission spectra and fluorescence anisotropy of 0.15-0.3 were observed upon variation of excitation wavelength. Vis-NIR absorption and flash photolysis experiments showed energy/charge transfer in the photoactivated nanohybrid. Moreover, evidence was found for direct reaction of amino groups with a carbon nanotube surface in the presence of molecular dipoles such as the zwitterionic sarcosine amino acid.

Biogenic Amine Sensing Based on Rosamine a N-Methylpyridinium Derivative Supported on Silica Materials from Rice Husk.

In this work new rosamine-silica composites were prepared and their sensing ability towards different amines was assessed. Rice husk wastes were used as a biogenic silica source. Silica was extracted by thermal treatment, before rice husk ash and after acid leaching with citric acid-treated rice husk ash. Mesoporous material (SBA-15) was also prepared using the extracted silica. The prepared materials were characterized by several techniques such as FTIR, XRD, SEM and N2 adsorption. The materials were then used as adsorbents of the chromophore N-methylpyridinium rosamine (Ros4PyMe). The obtained loaded composites were tested in solution for amines sensing (n-butylamine, aniline, putrescine and cadaverine). The detection studies were analyzed by fluorescence and revealed 40% and 48% quenching in fluorescence intensity for the composite Ros4PyMe@SBA in the presence of the biogenic amines cadaverine and putrescine, respectively. The composite was also sensitive in the powder form, changing the color from violet to pale pink in the presence of putrescine vapors with a fast response (around 2 min), the process being reversible by exposure to air.

Functionalization of Rhodamine Platforms with 3-Hydroxy-4-pyridinone Chelating Units and Its Fluorescence Behavior towards Fe(III).

Functionalization of xanthene fluorophores with specific receptor units is an important topic of research aiming for the development of new analytical tools for biological sciences, clinical diagnosis, food and environmental monitoring. Herein, we report a new dihydrorosamine containing two active amino groups, which was functionalized with 3-benzyloxy-1-(3'-carboxypropyl)-2-methyl-4-pyridinone through an amide coupling strategy. Benzylated mono- and di-functionalized dihydrorosamine derivatives (H in position 9 of the xanthene) were obtained, but with modest reaction yields, requiring long and laborious purification procedures. Looking for a more efficient approach, rhodamine 110 was selected to react with the carboxypropyl pyridinone, enabling the isolation of the corresponding mono- and di-functionalized derivatives in amounts that depend on the excess of pyridinone added to the reaction. The structure of all compounds was established by 1H and 13C NMR, MS (ESI) and their absorption and emission properties were evaluated in dichloromethane. The fluorescence behavior of the debenzylated mono-rhodamine 110 derivative in the presence of Fe(III) was studied, making it an interesting fluorogenic dye for future optical sensing applications.

Synthesis of Novel Diketopyrrolopyrrole-Rhodamine Conjugates and Their Ability for Sensing Cu2+ and Li.

The search for accurate and sensitive methods to detect chemical substances, namely cations and anions, is urgent and widely sought due to the enormous impact that some of these chemical species have on human health and on the environment. Here, we present a new platform for the efficient sensing of Cu2+ and Li+ cations. For this purpose, two novel photoactive diketopyrrolopyrrole-rhodamine conjugates were synthesized through the condensation of a diketopyrrolopyrrole dicarbaldehyde with rhodamine B hydrazide. The resulting chemosensors 1 and 2, bearing one or two rhodamine hydrazide moieties, respectively, were characterized by 1H and 13C NMR and high-resolution mass spectrometry, and their photophysical and ion-responsive behaviours were investigated via absorption and fluorescence measurements. Chemosensors 1 and 2 displayed a rapid colorimetric response upon the addition of Cu2+, with a remarkable increase in the absorbance and fluorescence intensities. The addition of other metal ions caused no significant effects. Moreover, the resulting chemosensor-Cu2+ complexes revealed to be good probes for the sensing of Li+ with reversibility and low detection limits. The recognition ability of the new chemosensors was investigated by absorption and fluorescence titrations and competitive studies.

Synthesis, characterization, and cellular investigations of porphyrin- and chlorin-indomethacin conjugates for photodynamic therapy of cancer.

Indomethacin is a potent non-steroidal anti-inflammatory drug (NSAID) with a strong selective inhibitor activity towards cyclooxygenase-2 (COX-2), an enzyme that is highly overexpressed in various tumour cells, being involved in tumourigenesis. Concomitantly, porphyrins have gained much attention as promising photosensitizers (PSs) for the non-invasive photodynamic therapy (PDT) of cancer. Herein, we report the design, and determine the singlet oxygen generation capacity and in vitro cellular toxicity of porphyrin- and chlorin-indomethacin conjugates (P2-Ind and C2-Ind). Both the conjugates were obtained in high yields and were characterized by 1H, 19F and 13C NMR as well as by high resolution mass spectrometry. The singlet oxygen generation properties were assessed by the 1,3-diphenylisobenzofuran singlet oxygen trap method, which showed that C2 and C2-Ind are the best singlet oxygen photosensitizers. In addition, it was found that the presence of indomethacin did not influence the singlet oxygen generation of porphyrin or chlorin. Cytotoxicity studies of the conjugate in human HEp2 cells revealed that the porphyrin- and chlorin-indomethacin conjugates have similar dark cytotoxicities, while chlorin C2 was shown to be the most phototoxic. Despite having lower cellular uptake than C2-Ind after 24 hours, chlorin C2 had a broad localization in HEp2 cells while the chlorin-indomethacin conjugate C2-Ind could be detected in the form of small aggregates. DFT calculations were performed to shed light on the reaction energy involved in the formation of the indomethacin conjugates and to compare the relative stability of selected isomers in solution. Moreover, the calculated energy of their first excited triplet state structures confirmed their use as suitable photosensitizers to generate singlet oxygen for PDT.

Synthesis of Catechol Derived Rosamine Dyes and Their Reactivity toward Biogenic Amines.

Functional organic dyes play a key role in many fields, namely in biotechnology and medical diagnosis. Herein, we report two novel 2,3- and 3,4-dihydroxyphenyl substituted rosamines (3 and 4, respectively) that were successfully synthesized through a microwave-assisted protocol. The best reaction yields were obtained for rosamine 4, which also showed the most interesting photophysical properties, specially toward biogenic amines (BAs). Several amines including n- and t-butylamine, cadaverine, and putrescine cause spectral changes of 4, in UV-Vis and fluorescence spectra, which are indicative of their potential application as an effective tool to detect amines in acetonitrile solutions. In the gas phase, the probe response is more expressive for spermine and putrescine. Additionally, we found that methanolic solutions of rosamine 4 and n-butylamine undergo a pink to yellow color change over time, which has been attributed to the formation of a new compound. The latter was isolated and identified as 5 (9-aminopyronin), whose solutions exhibit a remarkable increase in fluorescence intensity together with a shift toward more energetic wavelengths. Other 9-aminopyronins 6a, 6b, 7a, and 7b were obtained from methanolic solutions of 4 with putrescine and cadaverine, demonstrating the potential of this new xanthene entity to react with primary amines.

From Discrete Complexes to Metal-Organic Layered Materials: Remarkable Hydrogen Bonding Frameworks.

A series of metal-organic coordination complexes based on alkaline-earth metal centers [Mg(II), Ca(II), and Ba(II)] and the ligand 5-aminoisophthalate (aip2-) revealed notable structural diversity, both in the materials' dimensionality and in their hydrogen bonding networks: [Mg(H2O)6]∙[Mg2(Haip)(H2O)10]∙(Haip)∙3(aip)∙10(H2O) (1) and [Mg(aip)(phen)(H2O)2]∙(H2O) (2) were isolated as discrete complexes (0D); [Ca(aip)(H2O)2]∙(H2O) (3), [Ca(aip)(phen)(H2O)2]∙(phen)∙(H2O) (4), and [Ba2(aip)2(phen)2(H2O)7]∙2(phen)∙2(H2O) (5) revealed metal-organic chain (1D) structures, while the [Ba(aip)(H2O)] (6) showed a metal-organic layered (2D) arrangement. Furthermore, most of these metal-organic coordination materials revealed interesting thermal stability properties, being stable at temperatures up to 450 °C.

Subppm Amine Detection via Absorption and Luminescence Turn-On Caused by Ligand Exchange in Metal Organic Frameworks.

Luminescent metal-organic frameworks (LMOFs) are promising materials for lighting and sensing applications. Herein, exposure of the highly luminescent Zn2(bpdc)2(bpee) MOF (H2bpdc = 4,4'-biphenyldicarboxylic acid and bpee = 1,2-bipyridylethene) to subppm amine contents turns on a new absorption band unambiguously ascribed to free bpee molecules concomitant with the gradual appearance of a new photoluminescence band at shorter wavelengths. These findings combined with Fourier-transform infrared spectra, powder X-ray diffraction and thermogravimetric analysis of exposed LMOF powders confirm that bpee ligands are exchanged by amines and released inside the LMOF, triggering absorption and luminescence features which can be exploited for highly sensitive amine recognition. This principle was demonstrated in mixed matrix membranes (MMMs) prepared by a simple solvent-free method consisting of mixing Zn2(bpdc)2(bpee) with dimethylvinyl-terminated dimethylsiloxane and dimethylhydrogen siloxane. This method enabled the production of free-standing, permeable, and highly transparent MMMs which showed enormous potential and sensitivity to the detection of amines in gas phase and aqueous medium.

Synthesis of Pyridyl and N-Methylpyridinium Analogues of Rosamines: Relevance of Solvent and Charge on Their Photophysical Properties.

A series of pyridyl analogues of rosamines was prepared by employing two methodologies: (i) the conventional-heating condensation of a pyridinecarboxaldehyde with 3-(diethylamino)phenol in propionic acid, and (ii) the novel ohmic-heating assisted condensation under "on water" conditions, followed by oxidation. The 4-pyridyl substituted rosamine was further converted into the N-methylpyridinium derivative through N-alkylation using methyl iodide. The influence of the position and cationization of the nitrogen atom of the pyridyl ring in the physicochemical properties of fluorophores was investigated by 1 H, 13 C, 15 N NMR spectral analysis, UV/Vis and fluorescence spectroscopy, single-crystal X-ray diffraction (4-pyridyl and N-methylpyridinium derivatives) and thermal-behavior analysis. Curiously, for ethanolic solutions of 4-pyridyl and N-methylpyridinium derivatives an extinction of color and fluorescence over time was observed. This phenomenon was further studied and the data revealed that it is the result of nucleophilic addition of ethoxide ion to the central 9-position of the xanthene. The kinetics of the process is slower for the 4-pyridyl rosamine, which emphasizes the importance of the charge in the N-methylpyridinium analogue in the reactivity of the molecule towards a nucleophile agent. This phenomenon is reversible, meaning that the compounds can be rapidly recovered by decreasing the pH, opening new avenues in the sensing applications of this class of rosamines.

New fluorescent rosamine chelator showing promising antibacterial activity against Gram-positive bacteria.

The restricted number of antibiotics to treat infections caused by common multidrug resistant bacterial pathogens in the clinical setting demands a continuous search for new molecules with antibacterial properties. Bacterial iron deprivation represents a promising alternative, being iron chelators an attractive class for drug design in which particular compounds seem to have antibacterial effect. In this work, we report the synthesis and characterization of a new fluorescent 3-hydroxy-4-pyridinone (3,4-HPO) iron chelator functionalized with a carboxyrosamine fluorophore (MRB20). The antibacterial activity of MRB20 was assessed against representative strains from clinically relevant Gram-positive and Gram-negative bacterial species and further compared with the inhibitory effect of a set of structurally related iron chelators including Deferiprone (1,2-dimethyl-3-hydroxy-4-pyridinone). Compounds exhibiting a promising minimal inhibitory concentration (MIC < 10 mg/L) were further tested against a wider range of bacterial genera and species (Staphylococcus spp. Enterococcus spp. Listeria monocytogenes, Bacillus spp.), including multidrug resistant bacteria. With the exception of the novel compound (MRB20), all chelators inhibited the strains assayed at very high concentrations [minimum inhibitory concentrations (MIC) ranging from 70 mg/L to >180 mg/L]. MRB20 revealed a good antibacterial activity (6.7-13.2 mg/L) against Gram-positive strains from different genera and species, including clinically relevant species (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecium, Enterococcus faecalis), which might be eventually compatible with a therapeutic application or as adjuvant.

Design of a Water Soluble Fluorescent 3-Hydroxy-4-Pyridinone Ligand Active at Physiological pH Values.

In the present work we report the structure and the spectroscopic characterization of a new fluorescent 3-hydroxy-4-pyridinone ligand D-3,4-HPO. The synthesis of the compound was performed in two steps, which involve the reaction of the commercially available fluorophore dansyl chloride with a 3-hydroxy-4-pyridinone chelating unit and further deprotection. The new fluorescent chelator was characterized in the solid state by single-crystal X-ray diffraction and in solution by NMR, MS, absorption and fluorescence spectroscopies. The analysis of the variation of the absorption spectrum with pH allowed the determination of four pK a values (pK a1  = 3.50, pK a2  = 4.50, pK a3  = 9.60, pK a4  = 10.20) and establishment of the corresponding distribution diagram. The study of the fluorescence properties of the ligand show that in the pH range between 4 and 9 the fluorescence intensity is constant and has its maximum value thus allowing its further use at physiological pH values. The interaction of the ligand with copper(II) was accessed by fluorescence spectroscopy in MOPS buffer and the results show that the presence of copper(II) quenches the fluorescence of the ligand in ca 94 % at a ligand: metal ratio of 2:1. The latter result is consistent with the formation of a copper(II) complex with the bidentate ligand, as confirmed by the EPR spectroscopy. Graphical Abstract New water soluble fluorescent ligand active at physiological pH values.

Ohmic Heating-Assisted Synthesis of 3-Arylquinolin-4(1H)-ones by a Reusable and Ligand-Free Suzuki-Miyaura Reaction in Water.

Potential bioactive 3-arylquinolin-4(1H)-ones were synthesized under ohmic heating using an efficient, reusable, and ligand-free protocol developed for the Suzuki-Miyaura coupling of 1-substituted-3-iodoquinolin-4(1H)-ones with several boronic acids in water using Pd(OAc)2 as a catalyst and tetrabutylammonium bromide (TBAB) as the phase transfer catalyst. Good substrate generality, ease of execution, short reaction time, and practicability make this method exploitable for the generation of libraries of B ring-substituted 3-arylquinolin-4(1H)-ones. After a simple workup, the Pd/catalyst-H2O-TBAB system could be reused for at least seven cycles without significant loss of activity.

Isoxazolidine-fused meso-tetraarylchlorins as key tools for the synthesis of mono- and bis-annulated chlorins.

The microwave-assisted catalytic hydrogenation of the isoxazolidine-fused meso-tetrakis(pentafluorophenyl)chlorin afforded directly a mono-annulated chlorin with a singular 1-methyl-2,3-dihydro-1H-benzo[b]azepine ring that resulted from the cleavage of the isoxazolidine N-O bond followed by an intramolecular nucleophilic aromatic substitution of an o-F atom. The subsequent treatment of the mono-annulated chlorin with NaH induced a second intramolecular nucleophilic aromatic substitution, generating a bis-annulated chlorin having an additional 2H-pyran ring.

Synthesis of (E)-2-Styrylchromones and Flavones by Base-Catalyzed Cyclodehydration of the Appropriate ß-Diketones Using Water as Solvent.

A low cost, safe, clean and environmentally benign base-catalyzed cyclodehydration of appropriate ß-diketones affording (E)-2-styrylchromones and flavones in good yields is disclosed. Water was used as solvent and the reactions were heated using classical and microwave heating methods, under open and closed vessel conditions. ß-Diketones having electron-donating and withdrawing substituents were used to evaluate the reaction scope. The reaction products were isolated in high purity by simple filtration and recrystallization from ethanol, when using 800 mg of the starting diketone under classical reflux heating conditions.

Modelling Hyperglycaemia in an Epithelial Membrane Model: Biophysical Characterisation.

Biomimetic models are valuable platforms to improve our knowledge on the molecular mechanisms governing membrane-driven processes in (patho)physiological conditions, including membrane permeability, transport, and fusion. However, current membrane models are over simplistic and do not include the membrane's lipid remodelling in response to extracellular stimuli. Our study describes the synthesis of glycated dimyristoyl-phosphatidylethanolamine (DMPE-glyc), which was structurally characterised by mass spectrometry (ESI-MS) and quantified by NMR spectroscopy to be further incorporated in a complex phospholipid (PL) membrane model enriched in cholesterol (Chol) and (glyco)sphingolipids (GSL) designed to mimic epithelial membranes (PL/Chol/GSL) under hyperglycaemia conditions. Characterisation of synthesised DMPE-glyc adducts by tandem mass spectrometry (ESI-MS/MS) show that synthetic DMPE-glyc adducts correspond to Amadori products and quantification by 1H NMR spectroscopy show that the yield of glycation reaction was 8%. The biophysical characterisation of the epithelial membrane model shows that excess glucose alters the thermotropic behaviour and fluidity of epithelial membrane models likely to impact permeability of solutes. The epithelial membrane models developed to mimic normo- and hyperglycaemic scenarios are the basis to investigate (poly)phenol-lipid and drug-membrane interactions crucial in nutrition, pharmaceutics, structural biochemistry, and medicinal chemistry.

Ohmic heating-assisted synthesis and characterization of Zn(II), Cu(II) and Pd(II) complexes of heterocyclic-fused chlorins.

Chlorins are highly interesting compounds due to their spectroscopic properties in both UV-Vis and NIR regions. Upon coordination to a metal ion, the corresponding metallochlorins exhibit more valuable physicochemical properties that enable a broader range of applications, such as in photodynamic therapy (PDT), water splitting catalysis, optical sensor devices and dye-sensitized solar cells. Synthetic chemistry has been in a continuous quest to fulfil most green chemistry requirements through the development of efficient reactions. Being a heating process that does not depend on heat transfer to the reaction medium, ohmic heating accomplishes the mentioned requirements and allows a fast and uniform heating regime thanks to the ionic conductivity of the reaction medium. Herein, we report the metallation of pyrrolidine- and isoxazolidine-fused chlorins with Zn(II), Cu(II) and Pd(II) salts by ohmic heating, using non-toxic aqueous solutions, and their corresponding physico-chemical characterization. All pyrrolidine-fused chlorins showed higher yields, when compared with isoxazolidine ones. From the thermogravimetric analysis performed it is possible to infer that the metal enhances the steadiness of the macrocycle, making it easier to cause the thermal decomposition of the pyrrolidine- and isoxazolidine-fused chlorins. The Zn(II) complexes showed high absorption in the NIR spectral region, a low fluorescence quantum yield and a short excited singlet state, which indicate the high efficiency of intersystem crossing to the triplet state, making them very promising candidates as photosensitizers for PDT.

Multidimensional Ln-Aminophthalate Photoluminescent Coordination Polymers.

The development of straightforward reproducible methods for the preparation of new photoluminescent coordination polymers (CPs) is an important goal in luminescence and chemical sensing fields. Isophthalic acid derivatives have been reported for a wide range of applications, and in addition to their relatively low cost, have encouraged its use in the preparation of novel lanthanide-based coordination polymers (LnCPs). Considering that the photoluminescent properties of these CPs are highly dependent on the existence of water molecules in the crystal structure, our research efforts are now focused on the preparation of CP with the lowest water content possible, while considering a green chemistry approach. One- and two-dimensional (1D and 2D) LnCPs were prepared from 5-aminoisophthalic acid and Sm3+/Tb3+ using hydrothermal and/or microwave-assisted synthesis. The unprecedented LnCPs were characterized by single-crystal X-ray diffraction (SCRXD), powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM), and their photoluminescence (PL) properties were studied in the solid state, at room temperature, using the CPs as powders and encapsulated in poly(methyl methacrylate (PMMA) films, envisaging the potential preparation of devices for sensing. The materials revealed interesting PL properties that depend on the dimensionality, metal ion, co-ligand used and water content.

Antibacterial activity of naphthyl derived bis-(3-hydroxy-4-pyridinonate) copper(II) complexes against multidrug-resistant bacteria.

In this work, we report the synthesis and characterization of three novel copper(II) complexes of naphthyl derived 3-hydroxy-4-pyridinone chelators. Their antibacterial activity against several Gram-positive and Gram-negative reference strains and multidrug-resistant clinical isolates was assessed through determination of the minimum inhibitory concentration (MIC). The complex Cu(naph1pp)2 shows the highest antibacterial activity, including against multidrug-resistant isolates, nonetheless, being more active against Gram-positive than Gram-negative bacteria. Cu(naph1pp)2 was further explored in combinatorial tests with ciprofloxacin against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VRE). The combination of Cu(naph1pp)2 and ciprofloxacin is considered additive, i.e., the effect of the two compounds combined is stronger than that of the individual compounds in the equivalent concentration.