The Hyperporphyrin Concept: A Contemporary Perspective

Abstract

The Gouterman four-orbital model conceptualizes porphyrin UV–visible spectra as dominated by four frontier molecular orbitals—two nearly degenerate HOMOs and two exactly degenerate LUMOS under D 4h symmetry. These are well separated from all the other molecular orbitals, and normal spectra involve transitions among these MOs. Unusual spectra occur when additional orbitals appear in this energy range, typically as a consequence of the central coordinated atom. For example, metals with empty d orbitals in a suitable energy range may lead to charge transfer from porphyrin (ligand) to metal, that is, so-called LMCT transitions. Metals with filled p or d orbitals may lead to charge transfer from metal to porphyrin, MLCT transitions. These cases lead to additional peaks and/or significant redshifts in the spectra and were classified as hyperporphyrins by Gouterman. Cases in which spectra are blueshifted were classified as hypsoporphyrins; they are common for relatively electronegative late transition metal porphyrins. Many of the same principles apply to porphyrin analogues, especially corroles. In this Perspective, we focus on two newer classes of hyperporphyrins: one reflecting substituent effects in protonated or deprotonated free-base tetraphenyporphyrins and the other reflecting “noninnocent” interactions between central metal ions and corroles. Hyperporphyrin effects on spectra can be dramatic, yet they can be generated by relatively simple changes and subtle structural variations, such as acid–base reactions or the selection of a central metal ion. These concepts suggest strategies for engineering porphyrin or porphyrinoid dyes for specific applications, especially those requiring far-red or near-infrared absorption or emission.

Hypercorroles Formed via the Tail that Wagged the Dog: Charge Transfer Interactions from Innocent Corroles to Meso-Nitrophenyl Substituents

“…Moreover, a comparison of spectra for the various ligated forms of 4 and 5 in Figure , where the electronic structure of the corrole remains constant (i.e., Cor 3– Co III (X) 2 ), affords another piece of data consistent with the presence of corrole-to-aryl charge transfer interactions where the bis-CN – adducts display the most red-shifted and broad Q-bands, consistent with the increased electron-donor ability (i.e., charge transfer interactions) of the corrole bearing two anionic ligands. Moreover, the 141–166 nm red shift of the Q-bands as compared to the 21–26 nm red shift in the Soret bands for the three ligated forms of 4 and 5 indicate that these LLCT interactions more significantly perturb the corrole orbital(s) responsible for this transition, a reoccurring phenomenon in reported hyperporphyrin spectra. ,, The change in orbital energy responsible for this perturbation of these UV-visible spectra can be ascribed to a destabilization of the corrole-based LUMO such that the lowest unoccupied corrole-centered orbitals are higher in energy than the LUMO of the redox-active NO 2 Ph groups on the macrocycle periphery resulting in a delocalization, which includes the π-systems of the meso -nitrophenyl groups.…”

“…Moreover, the 141−166 nm red shift of the Q-bands as compared to the 21−26 nm red shift in the Soret bands for the three ligated forms of 4 and 5 indicate that these LLCT interactions more significantly perturb the corrole orbital(s) responsible for this transition, 54 a reoccurring phenomenon in reported hyperporphyrin spectra. 5,6,10 The change in orbital energy responsible for this perturbation of these UV-visible spectra can be ascribed to a destabilization of the corrole-based LUMO such that the lowest unoccupied corrole-centered orbitals are higher in energy than the LUMO of the redoxactive NO 2 Ph groups on the macrocycle periphery resulting in a delocalization, which includes the π-systems of the mesonitrophenyl groups. More concrete, experimental confirmation for the proposed electronic orbital interactions leading to the hypercorrole spectra of 4 and 5 can be obtained by monitoring the changes in reduction potentials upon moving from the noninnocent forms of mono-DMSO corroles to the innocent mono-CN − and bis-CN − derivatives.…”

“…Gouterman defined these systems as hyperporphyrins whose dramatically altered spectra were caused by charge transfer interactions associated with ligand-to-metal (LMCT), metal-to-ligand (MLCT), and/or ligand-to-ligand (LLCT) − transitions. These charge transfers, summarized in a recent perspective, underscore a multitude of molecular designs options, which can be employed to fine tune the porphyrin − and/or tetrapyrrole electronic properties, − which play an important role in the areas of catalysis, , materials, and medicinal sciences, − the latter of which often relies on complexes, such as hyperporphyrins, which exhibit NIR absorptions for therapeutic purposes …”

Phenol- and resorcinol-appended metallocorroles and their derivatization with fluorous tags

“…In contrast, modest differences in peak positions were observed between the 4-methoxy/hydroxy and 3,5-dimethoxy/dihydroxy derivatives. Interestingly, the resorcinol-appended complexes M[T(3,5-OH)PC] (M = ReO, Au) were found to exhibit moderate solubility and modest spectral redshifts in 0.05 M aqueous KOH, consistent with (partial) deprotonation of the phenolic OH groups 74 – 78 .…”

Synthesis, Spectral, Redox, and Sensing Studies of β-Dicyanovinyl-Appended Corroles and Their Metal Complexes

“…Apart from their sensitive electronic configuration, corroles exhibit very high structural diversity, unique coordination chemistry, and strong light absorption/emission properties in comparison to porphyrins. − A judiciously selected functionalization of the corrole ring’s periphery can change not only the molecule’s electronic properties, redox potentials, and coordination chemistry , but also lead to new possibilities in the areas of nonlinear optics, , near-infrared phosphorescence, − catalysis, − and fluorescence. , In addition to these areas of research, corrole macrocycles are good prospects for a range of sensing material applications due to their distinctive optical characteristics and synthetic adaptability. , In fact, the unusual behavior, reactivity, and high acidity of corrole imino protons have been utilized for the sensing of basic anions like CN – , F – , OAc – , and H 2 PO 4 – ions through an anion-induced deprotonation mechanism . Lodeiro and co-workers have investigated several gallium­(III) corroles for sensing toxic cyanide and fluoride anions, , while Gros and co-workers developed cobalt corrole-based porous organic polymers (POPs) for selective CO adsorption .…”