Manuela Stefanelli

Porphyrinoids for Chemical Sensor Applications

Porphyrins and related macrocycles have been intensively exploited as sensing materials in chemical sensors, since in these devices they mimic most of their biological functions, such as reversible binding, catalytic activation, and optical changes. Such a magnificent bouquet of properties allows applying porphyrin derivatives to different transducers, ranging from nanogravimetric to optical devices, also enabling the realization of multifunctional chemical sensors, in which multiple transduction mechanisms are applied to the same sensing layer. Potential applications are further expanded through sensor arrays, where cross-selective sensing layers can be applied for the analysis of complex chemical matrices. The possibility of finely tuning the macrocycle properties by synthetic modification of the different components of the porphyrin ring, such as peripheral substituents, molecular skeleton, coordinated metal, allows creating a vast library of porphyrinoid-based sensing layers. From among these, one can select optimal arrays for a particular application. This feature is particularly suitable for sensor array applications, where cross-selective receptors are required. This Review briefly describes chemical sensor principles. The main part of the Review is divided into two sections, describing the porphyrin-based devices devoted to the detection of gaseous or liquid samples, according to the corresponding transduction mechanism. Although most devices are based on porphyrin derivatives, seminal examples of the application of corroles or other porphyrin analogues are evidenced in dedicated sections.

β-Nitro-5,10,15-tritolylcorroles.

Functionalization of the β-pyrrolic positions of the corrole macrocycle with -NO(2) groups is limited at present to metallocorrolates due to the instability exhibited by corrole free bases under oxidizing conditions. A careful choice of the oxidant can limit the transformation of corroles into decomposition products or isocorrole species, preserving the corrole aromaticity, and thus allowing the insertion of nitro groups onto the corrole framework. Here we report results obtained by reacting 5,10,15-tritolylcorrole (TTCorrH(3)) with the AgNO(2)/NaNO(2) system, to give mono- and dinitrocorrole derivatives when stoichiometry is carefully controlled. Reactions were found to be regioselective, affording the 3-NO(2)TTCorrH(3) and 3,17-(NO(2))(2)TTCorrH(3) isomers as the main products in the case of mono- and disubstitution, in 53 and 20% yields, respectively. In both cases, traces of other mono- and disubstituted isomers were detected, which were structurally characterized by X-ray crystallography. The influence of the β-nitro substituents on the corrole properties is studied in detail by UV-visible, electrochemical, and spectroelectrochemical characterization of these functionalized corroles. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations of the ground and excited state properties of these β-nitrocorrole derivatives also afforded significant information, closely matching the experimental observations. It is found that the β-NO(2) substituents conjugate with the π-aromatic system of the macrocycle, which initiates significant changes in both the spectroscopic and redox properties of the so functionalized corroles. This effect is more pronounced when the nitro group is introduced at the 2-position, because in this case the conjugation is, for steric reasons, more efficient than in the 3-nitro isomer.

Amination reaction on copper and germanium β-nitrocorrolates.

Copper and germanium complexes of β-substituted nitrocorroles were reacted with 4-amino-4H-1,2,4-triazole to give the corresponding β-amino-β-nitro derivatives, in moderate to good yields. This is the first successful example of a vicarious nucleophilic substitution performed on corrole derivatives, because the same reaction carried out on silver complexes afforded the corresponding 6-azahemiporphycenes by way of corrole ring expansion. The first step of this work is related to the modification of a synthetic protocol for preparation of the β-substituted nitro corroles. The nitration reaction was carried out on a copper corrole using NaNO(2) as the primary source of NO(2)(-) coupled with AgNO(2) used as oxidant. By variation of the molar ratio of the reagents it was possible to direct the product distribution toward mono- and dinitro derivatives. The reaction between mono- and dinitro derivatives of (TtBuCorrCu) with 4-amino-4H-1,2,4-triazole gave good results, leading to the isolation of 2-(NH(2))-3-(NO(2))-TtBuCorrCu and 2,18-(NH(2))(2)-3,17-(NO(2))(2)-TtBuCorrCu in moderate yields. To elucidate factors that influence the reaction, and to highlight the different behavior observed for different metal complex substrates, the electrochemistry of three copper complexes, TtBuPCorrCu, (NO(2))TtBuPCorrCu, and (NO(2))(2)TtBuPCorrCu, was studied by cyclic voltammetry and thin-layer UV-visible spectroelectrochemistry. The nitro groups on (NO(2))(x)TtBuPCorrCu are highly electron-withdrawing, which leads not only to a substantial positive shift of all redox potentials but also to a unique redox behavior and UV-vis spectrum of the singly reduced product as compared to the parent compound, TtBuPCorrCu. Finally, the amination reaction was carried out on a Ge(IV) nitrocorrolate, giving in good yield the 2-amino-3-nitroderivative, which was structurally characterized by single crystal X-ray crystallography.

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 of supramolecules relays on noncovalent interactions (electrostatic, hydrogen bond, 𝜋-𝜋, 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.

Demetalation of Silver(III) Corrolates

Several procedures for the demetalation of silver(III) corrolates have been tested. Acidic conditions induce removal of the silver ion but they can also promote concomitant oxidation of the corrole nucleus to an isocorrole species, the degree of which will depend upon the specific acidic media. This oxidation cannot be completely avoided by addition of hydrazine, particularly in the case of 3-NO(2) substituted complexes which are quantitatively converted into the corresponding 3-NO(2), 5-hydroxy isocorroles upon silver ion removal. Several beta-nitro isocorrole products were isolated, and one was structurally characterized. Electrochemical and chemical reductive methods for silver(III) corrolates demetalation were then tested with the aim to avoid the formation of isocorroles. While reaction with sodium borohydride was shown to be quite effective to demetalate unsubstituted silver corrolates this was not the case for the beta-nitro derivatives where the peripheral nitro group is reduced by borohydride giving the corresponding 3-amino free base corrole species. For the beta-nitro corrole silver complexes, a successful approach was obtained using DBU/THF solutions which afforded the 3-NO(2) corrole free-base compound as a single reaction product in good yield. These conditions were also effective for unsubstituted corroles although longer reaction times were necessary in this case. To study in greater detail the corrole demetalation behavior, selected Ag(III) derivatives were characterized by cyclic voltammetry in pyridine, and the demetalation products spectrally characterized after controlled potential reduction in a thin-layer spectroelectrochemical cell.

Nitration of iron corrolates: further evidence for non-innocence of the corrole ligand.

Mono- and di-substituted β-nitro derivatives have been obtained from the reaction of ttcorrFeCl with sodium nitrite in refluxing DMF. This result is unprecedented for iron corrolates and further evidences the non-innocent character of the corrole ligand.

Beta-nitro derivatives of germanium(IV) corrolates.

The reaction between germanium(IV) meso-triphenylcorrolates and nitrate salts affords the corresponding beta-nitro substituted corroles in good yield. Chromatographic separation of the crude reaction mixtures enables isolation of a mu-oxo dimer along with the corresponding monomers bearing a hydroxy or methoxy group at an axial position of the germanium central metal ion. Depending on the reaction conditions, mono- or dinitro substituted complexes can be obtained. The substitution is highly regioselective in each case, giving only the 3-nitro or 3,17-dinitro derivative among the different possible isomers. Five of the synthesized complexes were examined by cyclic voltammetry and UV-visible spectroelectrochemistry in dichloromethane, and the dinitro mu-oxo dimer is structurally characterized.

β-Nitro Derivatives of Iron Corrolates

Two different methods for the regioselective nitration of different meso-triarylcorroles leading to the corresponding β-substituted nitrocorrole iron complexes have been developed. A two-step procedure affords three Fe(III) nitrosyl products-the unsubstituted corrole, the 3-nitrocorrole, and the 3,17-dinitrocorrole. In contrast, a one-pot synthetic approach drives the reaction almost exclusively to formation of the iron nitrosyl 3,17-dinitrocorrole. Electron-releasing substituents on the meso-aryl groups of the triarylcorroles induce higher yields and longer reaction times than what is observed for the synthesis of similar triarylcorroles with electron-withdrawing functionalities, and these results can be confidently attributed to the facile formation and stabilization of an intermediate iron corrole π-cation radical. Electron-withdrawing substituents on the meso-aryl groups of triarylcorrole also seem to labilize the axial nitrosyl group which, in the case of the pentafluorophenylcorrole derivative, results in the direct formation of a disubstituted iron μ-oxo dimer complex. The influence of meso-aryl substituents on the progress and products of the nitration reaction was investigated. In addition, to elucidate the most important factors which influence the redox reactivity of these different iron nitrosyl complexes, selected compounds were examined by cyclic voltammetry and thin-layer UV-visible or FTIR spectroelectrochemistry in CH(2)Cl(2).

6-Azahemiporphycene: a new member of the porphyrinoid family.

The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-triazole provides another example of corrole ring expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of the corrole ring is a required step for the ring expansion and for this reason the reaction fails in the case of corroles bearing meso-phenyl groups carrying electron-withdrawing substituents. Steric requirements also limited the scope of the reaction, which is not successful in the case of 2,6-disubstituted meso-aryl corroles. The occurrence of an initial oxidation is further supported by formation of the 6-azahemiporphycene derivative when the reaction is carried out under the same conditions, using a 5- or a 10-isocorrole as starting material. (1)H NMR spectra and X-ray crystal characterization of 6-azahemiporphycene evidenced the presence of an intramolecular N-H...N hydrogen bond in the inner core of the macrocycle, while photophysical characterization confirmed the aromatic character of the novel macrocycle, showing an intense Soret-like band around 410 nm in the absorption spectrum. The fluorescence emission is very modest, and 6-azahemiporphycene showed higher photostability than the corresponding corrole species. Different metal complexes of 6-azahemiporphycene were prepared following synthetic protocols usually exploited for the preparation of metalloporphyrins, demonstrating good coordination properties for the macrocycle. Both the free-base and metal derivatives were characterized by cyclic voltammetry and spectroelectrochemistry in dichloromethane and benzonitrile. To further detail the behavior of this novel macrocycle, density functional theory (DFT) calculations were carried out on the basic structure of 6-azahemiporphycene with the aim of assessing aromaticity and tautomerism, as well as calculating its stability with respect to the 5-aza isomer.

Supramolecular chirality in solvent-promoted aggregation of amphiphilic porphyrin derivatives: kinetic studies and comparison between solution behavior and solid-state morphology by AFM topography.

The solvent-promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L-proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (nitrogen-quaternized L-proline, 3H(2)) or anionic (carboxylate residue, 6H(2)) group. Formation of anionic 6H(2) aggregates shows a diffusion-limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H(2) aggregates are formed by a cooperative, fractal-type decay, and appear as long-fibrous, looser structures. In the templated aggregation of 3H(2) over preformed 6H(2) aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter interchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid-state aggregate structures, corroborating the sergeant-soldier effect observed in the templated aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereoselective sensors and devices.