Novel receptor systems based on porphyrins and related macrocycles

Il lavoro di questa tesi di dottorato riguarda essenzialmente la preparazione di recettori molecolari basati su porfirinoidi e lo studio dei sistemi supramolecolari a cui tali composti possono dar luogo in soluzione. La versatilità di sintesi e di funzionalizzazione delle porfirine, infatti, ha da sempre suscitato un grande interesse che, negli ultimi decenni, è stato rivolto non soltanto allo studio della loro reattività e delle loro proprietà chimiche e fisiche, ma anche alla preparazione e caratterizzazione di composti analoghi. L’intensa attività di ricerca su tali composti ne ha permesso diverse applicazioni pratiche, che vanno dalla costruzione di modelli biomimetici, alla preparazione di catalizzatori e materiali sensibili per sensori. La mia attività di ricerca, svolta nel corso del triennio di dottorato, può essere schematicamente suddivisa in tre principali tematiche, la prima riguardante la preparazione di recettori ditopici costituiti da porfirine e cavitandi e lo studio di particolari sistemi supramolecolari a cui essi possono dare origine in soluzione, la seconda riguardante lo studio del fenomeno di aggregazione di porfirinoidi anfifili chirali ed infine l’ultima riguardante lo studio della reattività dei corroli e relativi complessi metallici nel caso della reazione di nitrazione. In particolare, lo studio dei sistemi supramolecolari ditopici è stato focalizzato sulla caratterizzazione di sistemi “2+2” derivanti dalla complessazione di particolari ligandi da parte dei recettori preparati. La scelta dei ligandi si è riflessa nella formazione di capsule non covalenti dotate di uno spazio interno caratterizzato o meno da elementi di chiralità, come è stato ben dimostrato da studi di dicroismo circolare. Inoltre è stato studiata la bontà di tali strutture ditopiche in ambito sensoristico, utilizzandole come materiale sensibile dei sensori nanogravimentrici, che costituiscono il naso elettronico sviluppato presso i nostri laboratori. Lo studio del fenomeno di aggregazione di porfirinoidi anfifili è stato condotto utilizzando le principali tecniche spettroscopiche, quali spettroscopia UV-vis, fluorescenza, dicroismo circolare e Resonance Light Scattering. Dopo aver preparato un derivato anfifilo chirale cationico di una tetrafenilporfirina, non solo è stato analizzato il fenomeno di aggregazione di questo macrociclo, ma anche l’eteroaggregazione dello stesso su un template costituito da aggregati supramolecolari chirali formati dall’analogo macrociclo anionico. Sono stati inoltre condotti degli studi preliminari sull’aggregazione di un corrolo anionico, fenomeno mai indagato in letteratura fino ad oggi. Infine per quanto riguarda i corroli, è stata studiata la reattività delle posizioni periferiche di tali macrocicli nella reazione di nitrazione e messo a punto una nuova metodologia sintetica per la preparazione di nitroderivati di questi composti e di alcuni complessi metallici: l’ottenimento di tali strutture è infatti di notevole interesse, rappresentando queste delle ottime piattaforme di base per la costruzione di strutture fuse più complesse. Parole chiave: Sistemi Host-Guest, Porfirine, Resorcinareni, Riconoscimento Molecolare, Chiralità, Sensori chimici (Naso Elettronico), Aggregati supramolecolari, Corroli, Nitrazione.This PhD thesis deals with the preparation of molecular receptors based on porphyrins and related macrocycles, as well as the investigation of supramolecular systems that they originate in solution. The synthetic versatility of porphyrin systems together with their unique and remarkable chemical and physical characteristics account the great attention towards these compounds and their synthetic analogues. In the last decade, the massive research activities carried out on these macrocycles led to the exploitation of such systems in a large number of practical applications, such as the construction of biomimetic systems, sensors analyses and catalytic processes. This thesis is focused on three different topics, the first one concerning the preparation of ditopic receptors constituted by porphyrin-cavitand conjugates and the investigation of supramolecular systems they originate in solution; the second topic deals with kinetic and spectroscopic studies on the self-aggregation of amphiphilic, chiral porphyrinoids; the latter regards the preparation of functionalized corroles and metallocorroles in order to study their reactivity in a specific reaction. Two Resorcinarene-Zn-porphyrin conjugates have been synthesized to obtain new organic material for chemical sensors; among the main spectroscopic characterizations, the formation in solution of supramolecular systems under metal coordination of ligands properly shaped have been investigated. More in sight, the use of chiral, bidentate ligands originated supramolecular “2+2” capsules, possessing elements of chirality. The goodness of these new structures in terms of sensitivity has been also tested, exploiting them as CIMs in nanogravimetric sensors which constitute the electronic nose, developed in our laboratories. The study of aggregation phenomena of amphiphilic porphyrinoids has been carried out by different spectroscopic techniques, such as UV-vis spectroscopy, Fluorescence, Circular Dichroism and Resonance Light Scattering. These studies dealt in particular with the heteroaggregation of a chiral, cationic porphyrin derivative onto preformed supramolecular aggregates of the anionic chiral analogue, affording remarkable results about the amplification of chirality for the resulting binary system. Moreover, preliminary aggregation experiments were performed on an anionic corrole, giving some indications about the unknown aggregation properties of such class of molecules. Finally, the reactivity of the peripheral beta positions of corroles has been studied in the nitration reaction; a new synthetic strategy has been tuned to obtain nitroderivatives of silver, copper and iron corroles. The achievement of such nitrocompounds can be of fundamental importance for the construction of more elaborated β-fused structures, displaying intriguing optoelectronic properties. Keywords: Host-Guest Systems, Porphyrins, Resorcinarenes, Molecular Recognition, Chirality, Chemical sensors (Electronic Nose), Supramolecular Aggregates, Corroles, Nitration

Porphyrin-Based Nanostructures for Sensing Applications

The construction of nanosized supramolecular hosts via self-assembly of molecular components is a fascinating field of research. Such intriguing class of architectures, beside their intrinsic intellectual stimuli, is of importance in many fields of chemistry and technology, such as material chemistry, catalysis, and sensor applications. Within this wide scenario, tailored solid films of porphyrin derivatives are structures of great potential for, among others, chemical sensor applications. The formation ofsupramoleculesrelays on noncovalent interactions (electrostatic, hydrogen bond, , or coordinative interactions) driven by the chemical information stored on the assembling molecules, such as shape and functional groups. This allows, for example, the formation of large well-defined porphyrin aggregates in solution that can be spontaneously transferred onto a solid surface, so achieving a solid system with tailored features. These films have been used, covering the bridge between nanostructures and microsystems, for the construction of solid-state sensors for volatiles and metal ion recognition and detection. Moreover, the variation of peripheral substituents of porphyrins, such as, for example, chiral appended functionalities, can result in the formation of porphyrin aggregates featuring high supramolecular chirality. This would allow the achievement of porphyrin layers characterised by different chiroptical and molecular recognition properties

Recent advances in chemical sensors using porphyrin-carbon nanostructure hybrid materials

Porphyrins and carbon nanomaterials are among the most widely investigated and applied compounds, both offering multiple options to modulate their optical, electronic and magnetic properties by easy and well-established synthetic manipulations. Individually, they play a leading role in the development of efficient and robust chemical sensors, where they detect a plethora of analytes of practical relevance. But even more interesting, the merging of the peculiar features of these single components into hybrid nanostructures results in novel materials with amplified sensing properties exploitable in different application fields, covering the areas of health, food, environment and so on. In this contribution, we focused on recent examples reported in literature illustrating the integration of different carbon materials (i.e., graphene, nanotubes and carbon dots) and (metallo)porphyrins in heterostructures exploited in chemical sensors operating in liquid as well as gaseous phase, with particular focus on research performed in the last four years

Synthesis and characterization of new-type soluble β-substituted zinc phthalocyanine derivative of clofoctol

In this work, we have described the synthesis and characterization of novel zinc (II) phthalocyanine bearing four 2-(2,4-dichloro-benzyl)-4-(1,1,3,3-tetramethyl-butyl)-phenoxy substituents on the peripheral positions. The compound was characterized by elemental analysis and different spectroscopic techniques, such as FT-IR, 1H NMR, MALDI-TOF, and UV-Vis. The Zn (II) phthalocyanine shows excellent solubility in organic solvents such as dichloromethane (DCM), n-hexane, chloroform, tetrahydrofuran (THF), and toluene. Photochemical and electrochemical characterizations of the complex were performed by UV-Vis, fluorescence spectroscopy, and cyclic voltammetry. Its good solubility allows a direct deposition of this compound as film, which has been tested as a solid-state sensing material in gravimetric chemical sensors for gas detection, and the obtained results indicate its potential for qualitative discrimination and quantitative assessment of various volatile organic compounds, among them methanol, n-hexane, triethylamine (TEA), toluene and DCM, in a wide concentration range

Light-activated porphyrinoid-capped nanoparticles for gas sensing

The coupling of semiconductors with organic molecules results in a class of sensors whose chemoresistive properties are dictated by the nature of dyes. Organic molecules generally reduce conductivity, but in the case of optically active dyes, such as porphyrinoids, the conductivity is restored by illumination with visible light. In this paper, we investigated the gas sensing properties of ZnO nanoparticles coated with porphyrins and corroles. Under light illumination, the resistance of these materials increases with the adsorption of volatile compounds but decreases when these are strong electron donors. The behavior of these sensors can be explicated on the basis of the structural difference between free-base porphyrin and corrole, the influence of coordinated metal, and the corresponding electronic structures. These sensors are promising electronic noses that combine the selectivity to strong electron donors with the broad selectivity toward the other classes of chemicals. An efficient representation of the data of this peculiar array can be obtained by replacing the Euclidean distance with the angular distance. To this end, a recently introduced spherical principal component analysis algorithm is applied for the first time to gas sensor array data. Results show that a minimal gas sensor array (four elements) can produce a sort of chemotopic map, which enables us to cluster a very large class of pure chemical vapors. Furthermore, this map provides information about the composition of complex odor matrices, such as the headspaces of beef meat and their evolution over the time

Chiral selectivity of porphyrin-ZnO nanoparticle conjugates

Recognition of enantiomers is one of the most arduous challenges in chemical sensor development. Although several chiral systems exist, their effective exploitation as the sensitive layer in chemical sensors is hampered by several practical implications that hinder stereoselective recognition in solid state. In this paper, we report a new methodology to efficiently prepare chiral solid films, by using a hybrid material approach where chiral porphyrin derivatives are grafted onto zinc oxide nanoparticles. Circular dichroism (CD) evidences that the solid-state film of the material retains supramolecular chirality due to porphyrin interactions, besides an additional CD feature in correspondence of the absorbance of ZnO (375 nm), suggesting the induction of chirality in the underlying zinc oxide nanoparticles. The capability of hybrid material to detect and recognize vapors of enantiomer pairs was evaluated by fabricating gas sensors based on quartz microbalances. Chiral films of porphyrin on its own were used for comparison. The sensor based on functionalized nanostructures presented a remarkable stereoselectivity in the recognition of limonene enantiomers, whose ability to intercalate in the porphyrin layers makes this terpene an optimal chiral probe. The chiroptical and stereoselective properties of the hybrid material confirm that the use of porphyrin-capped ZnO nanostructures is a viable route for the formation of chiral selective surfaces. © 2019 American Chemical Society

Porphyrins Through the Looking Glass: Spectroscopic and Mechanistic Insights in Supramolecular Chirogenesis of New Self-Assembled Porphyrin Derivatives

The solvent driven aggregation of porphyrin derivatives, covalently linked to a L- or D-prolinate enantiomer, results in the stereospecific formation of species featuring remarkable supramolecular chirality, as a consequence of reading and amplification of the stereochemical information stored in the proline-appended group. Spectroscopic, kinetic, and topographic SEM studies gave important information on the aggregation processes, and on the structures of the final chiral architectures. The results obtained may be the seeds for the construction of stereoselective sensors aiming at the detection, for example, of novel emergent pollutants from agrochemical, food, and pharmaceutical industry

Tunable supramolecular chirogenesis in the self-assembling of amphiphilic porphyrin triggered by chiral amines

Supramolecular chirality is one of the most important issues in different branches of science and technology, as stereoselective molecular recognition, catalysis, and sensors. In this paper, we report on the self-assembly of amphiphilic porphyrin derivatives possessing a chiral information on the periphery of the macrocycle (i.e., D- or L-proline moieties), in the presence of chiral amines as co-solute, such as chiral benzylamine derivatives. The aggregation process, steered by hydrophobic effect, has been studied in aqueous solvent mixtures by combined spectroscopic and topographic techniques. The results obtained pointed out a dramatic effect of these ligands on the morphology and on the supramolecular chirality of the final self-assembled structures. Scanning electron microscopy topography, as well as fluorescence microscopy studies revealed the formation of rod-like structures of micrometric size, different from the fractal structures formerly observed when the self-assembly process is carried out in the absence of chiral amine co-solutes. On the other hand, comparative experiments with an achiral porphyrin analogue strongly suggested that the presence of the prolinate moiety is mandatory for the achievement of the observed highly organized suprastructures. The results obtained would be of importance for unraveling the intimate mechanisms operating in the selection of the homochirality, and for the preparation of sensitive materials for the detection of chiral analytes, with tunable stereoselectivity and morphology

Seeding chiral ensembles of prolinated porphyrin derivatives on glass surface: simple and rapid access to chiral porphyrin films

An easy and fast method to achieve chiral porphyrin films on glass is herein reported. The on-surface formation of organized supramolecular architectures with distinctive and remarkable chiroptical features strictly depends on the macrocycles used, the solvent chosen for the casting deposition, and most importantly, on the roughness of the glass slide. Dynamic light scattering studies performed on 10−4–10−6 M porphyrin solutions revealed the presence of small porphyrin aggregates, whose size and number increase depending on the initial concentration. Once transferred on surface, these protoaggregates act as nucleation seeds for the following, self-assembling into larger structures upon solvent evaporation, with a process driven by a fine balance between intermolecular and molecule–substrate interactions. The described method represents a straightforward way to fabricate porphyrin-based chiral surfaces onto a transparent and economic substrate in few minutes. The results obtained can be particularly promising for the development of sensors based on stereoselective optical active films, targeting the detection of chiral analytes of practical relevance, such as the so-called emerging pollutants released in the environment from agrochemical, food, and pharmaceutical manufacturing

Chiral recognition by supramolecular porphyrin-hemicucurbit[8]uril-functionalized gravimetric sensors

Enantiorecognition of a chiral analyte usually requiresthe abilityto respond with high specificity to one of the two enantiomers ofa chiral compound. However, in most cases, chiral sensors have chemicalsensitivity toward both enantiomers, showing differences only in theintensity of responses. Furthermore, specific chiral receptors areobtained with high synthetic efforts and have limited structural versatility.These facts hinder the implementation of chiral sensors in many potentialapplications. Here, we utilize the presence of both enantiomers ofeach receptor to introduce a novel normalization that allows the enantio-recognitionof compounds even when single sensors are not specific for one enantiomerof a target analyte. For this purpose, a novel protocol that permitsthe fabrication of a large set of enantiomeric receptor pairs withlow synthetic efforts by combining metalloporphyrins with (R,R)- and (S,S)-cyclohexanohemicucurbit[8]uril is developed. The potentialitiesof this approach are investigated by an array of four pairs of enantiomericsensors fabricated using quartz microbalances since gravimetric sensorsare intrinsically non-selective toward the mechanism of interactionof analytes and receptors. Albeit the weak enantioselectivity of singlesensors toward limonene and 1-phenylethylamine, the normalizationallows the correct identification of these enantiomers in the vaporphase indifferent to their concentration. Remarkably, the achiralmetalloporphyrin choice influences the enantioselective properties,opening the way to easily obtain a large library of chiral receptorsthat can be implemented in actual sensor arrays. These enantioselectiveelectronic noses and tongues may have a potential striking impactin many medical, agrochemical, and environmental fields