Photophysics of Reduced Silicon Tetraphenylporphyrin

Conformationally rigid, π-extended annulated porphyrinoids derived from the naphthobipyrrole motif

π-Extension in porphyrinoids can be achieved by fusing additional aromatic rings onto the macrocycle's periphery and such porphyrinoids are referred to as annulated porphyrinoids. Annulated porphyrinoids display contrasting properties in comparison with their non-annulated congeners. While an annulation strategy can create π-extended systems, the simultaneous incorporation of conformational rigidity in such porphyrinoids can ensure that they adopt a planar structure, and the advantages associated with the extended π-network can be leveraged. Hence, while synthesizing such porphyrinoids, judicial selection of the precursor becomes important. The ease of synthesis and the presence of a β-β'-linked o-phenylene bridge qualify 3,8-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole, commonly known as naphthobipyrrole, to be one such precursor suitable for the synthesis of conformationally rigid annulated porphyrinoids. This field of study has started to bloom only in the last decade and the examples reported so far are confined to the naphtho-versions of porphycenes (isomeric porphyrin), a few members of the aromatic/antiaromatic expanded porphyrinoids, and calix[n]bipyrroles. In view of this, the current review article aims to summarize the up-to-date developments in this area and discusses the synthesis, structure, and properties of the reported naphthobipyrrole-derived annulated porphyrinoids.

Changes in porphyrin’s conjugation based on synthetic and post-synthetic modifications

Porphyrins or more broadly defined porphyrinoids are the structures where the extended π-cloud can be significantly modified by several factors. The broad range of introduced structural motifs has shown a possibility of modification of conjugation by a controlled synthetic approach, leading to expected optical or magnetic behaviour, and also by post-synthetic modifications (i.e. redox or protonation/deprotonation), Both approaches lead to noticeab changes in observed properties but also open a potential for further utilization. Thus, this already constituted big family of macrocyclic structures with specific highly extended π-delocalization shows a significant contribution in several fields from fundamental studies, leading to understanding behaviour of skeletons like that with a substantial influence on biological studies and material science. The presented material focuses on the most significant examples of modifications of porphyrinoids skeleton leading to drastic changes in optical response and magnetic properties. Through the presentation, the focus will be placed on the changes leading to the most red-shifted transition as the parameter indicating extending the π-delocalization. Significantly different magnetic character will be also discussed based on the switching between aromatic/antiaromatic character assigned to macrocyclic structures that will be included.

Facile Redox-Induced Aromatic-Antiaromatic Interconversion of a β-Tetracyano-21,23-Dithiaporphyrin under Ambient Conditions

Facile redox-induced aromatic-antiaromatic interconversions were accomplished by using β-tetracyano-21,23-dithiaporphyrin (CN₄ S₂ Por). Introduced cyano groups not only increased the reduction potential of the porphyrin core but also stabilized the antiaromatic isophlorin (CN₄ S₂ Iph) by π conjugation. The reduction of CN₄ S₂ Por with hydrazine in polar solvents quantitatively affords CN₄ S₂ Iph, even under ambient conditions. CN₄ S₂ Iph retains a nearly planar conformation and exhibits considerable antiaromaticity. Aerobic oxidation of CN₄ S₂ Iph to CN₄ S₂ Por occurs in nonpolar solvents. This study was conducted to contribute to the understanding of the structure-antiaromaticity relationship.

On-Surface Synthesis of Nonmetal Porphyrins

We report the on-surface synthesis of a nonmetal porphyrin, namely, silicon tetraphenylporphyrin (Si-TPP), by the deposition of atomic silicon onto a free-base TPP layer on a Ag(100) surface under ultrahigh vacuum (UHV) conditions. Scanning tunneling microscopy provides insights into the self-assembly of the TPP molecules before and after Si insertion. Silicon coordinates with all four nitrogen atoms of the TPP macrocycle and interacts with a silver atom of the substrate as confirmed by scanning tunneling spectroscopy, X-ray photoelectron spectroscopy, and complementary density functional theory calculations. The Si-TPP complex presents a saddle-shaped conformation that is stable under STM manipulation. Our study shows how protocols established for the on-surface metalation of tetrapyrroles can be adopted to achieve nonmetal porphyrins. Complementary experiments yielding Si-TPP and Ge-TPP on Ag(111) highlight the applicability to different main group elements and supports. The success of our nonmetal porphyrin synthesis procedure is further corroborated by a temperature-programmed desorption experiment, revealing the desorption of Ge-TPP. This extension of interfacial complex formation beyond metal elements opens promising prospects for new tetrapyrrole architectures with distinct properties and functionalities.

Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom

A careful density functional theory reexamination of the geometric and electronic structures of reduced main-group porphyrin complexes E(Por)L₂ (E = Si or Ge; L = pyridine or tetrahydrofuran), B₂(Por), and C₂(Por) has confirmed these as pure isophlorin derivatives with normal-valent coordinated central atoms. Only axially unligated Ge(Por) and the dications [B₂(Por)]²⁺ and [C₂(Por)]²⁺ feature aromatic porphyrin ligands. The calculations faithfully reproduce the strong bond-length alternation along the outer rim of the macrocycle in the reduced complexes, consistent with antiaromatic character, as well as much stronger ruffling in the reduced group 14 complexes relative to their nonreduced counterparts such as E(Por)X₂ (E = Si or Ge; X = F or Cl). The latter is thought to reflect the lower barrier to nonplanar deformation for the antiaromatic systems. In addition, unlike B₂(Por) and its dication, which are planar, C₂(Por) and its dication are predicted to be strongly ruffled, reflecting the smaller size of the central C₂ unit. The calculations also predict characteristically low ionization potentials and singlet-triplet gaps for the antiaromatic complexes. A brief exploratory study of the as-yet-unknown group 15 complexes E(TPP)(Ph)(py), where E = P and As, also indicated an antiaromatic isophlorin macrocycle coordinated to a pentavalent group 15 center.

Formation and Isolation of a Four-Electron-Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin

The two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na₂ S₂ O₄ gave a corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph. Further reduction of Iph proceeded to form an unprecedented four-electron-reduced porphyrin (IphH₂ ), which was fully characterized by spectroscopic and X-ray crystallographic analysis. IphH₂ , with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton-coupled four-electron reversible redox system.

Structure and spectroscopic properties of porphyrinato group 14 derivatives: Part I – Phenylacetylido ligands

A series of bis(phenylacetylido)(porphyrinato) E(IV) (E=Si, Ge, Sn) derivatives were obtained by reacting lithium phenylacetylide with the corresponding bis(chlorido)(porphyrinato) E(IV) precursors. Crystal structure determinations demonstrated that the trans-coordinated acetylide ligands deviate from their expected ideal geometry. Density functional theory calculations and comparison of the packing of the molecules among the homologous series of Si(IV), Ge(IV) and Sn(IV) revealed causes for the deviation from the simple valence shell electron pair repulsion model. Fluorescence and phosphorescence of these organometallic group 14 derivatives were studied and the results are compared to the literature-known properties of the related bis(chlorido)(porphyrinato) E(IV) compounds (E=Si, Ge, Sn). The first crystal structure of a porphyrinato Ge(IV) compound with σ-bonded acetylide ligands is reported.

Chemo- and Regioselective Reduction of 5,15-Diazaporphyrins Providing Antiaromatic Azaporphyrinoids

Reagent-controlled chemo- and regioselective reduction of 5,15-diazaporphyrins has been developed. The selective reduction of carbon-carbon double bonds of diazaporphyrins provides 18 π aromatic isobacteriochlorin-type products, whereas the reduction of carbon-nitrogen double bonds leads to selective formation of 20 π N,N'-dihydrodiazaporphyrins in excellent yields. The distinct antiaromatic character of N,N'-dihydrodiazaporphyrins has been revealed. The free-base N,N'-dihydrodiazaporphyrin exhibits slower inner NH tautomerism than that in the corresponding 18 π porphyrins.

Synthesis and reactivity studies of a tin(II) corrole complex

A series of tris(pentafluorophenyl)corrole (TPFC) tin(IV) and tin(II) complexes were prepared and studied by various characterization techniques including (1)H, (19)F, and (119)Sn NMR and UV-vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The unusual 4-coordinate, monomeric, divalent tin(II) complex [(TPFC)Sn(II)](-) (2a) showed highly efficient reactivity toward alkenes and alkyl halides via a nucleophilic addition pathway leading to the quantitative formation of alkyl stannyl corrole compounds. DFT calculations confirmed the divalent nature of the tin center in 2a, and an NBO analysis showed about 99.99% Sn lone pair character, of which 83.6% was Sn 5s and 16.35% was Sn 5p character.