Barrier-Lowering Effects of Baird Antiaromaticity in Photoinduced Proton-Coupled Electron Transfer (PCET) Reactions

<p>Baird antiaromaticity plays a central role in the photochemistry of proton-coupled electron transfer (PCET) reactions. We recognize that many popular organic chromophores that catalyze photoinduced PCET reactions are Hückel aromatic in the ground state, but gain significant Baird antiaromatic character in the lowest ππ* state, having important barrier-lowering effects for electron transfer. Two examples, 1) the photolytic O–H bond dissociation of phenol and 2) solar water splitting in the pyridine-water complex, are discussed. Contrary to an assumed homolytic O–H bond dissociation, both reactions proceed through loss (and gain) of an electron in the π-system (i.e., antiaromaticity relief), followed by heterolytic cleavage of the polar O–H bond near barrierlessly. Nucleus-independent chemical shifts (NICS), ionization energies (IE), electron affinities (EA), and excited-state PCET energy profiles of selected [4<i>n</i>] and [4<i>n</i>+2] π-systems are presented.<br></p>

Barrier-Lowering Effects of Baird Antiaromaticity in Photoinduced Proton-Coupled Electron Transfer (PCET) Reactions

<p>Baird antiaromaticity plays a central role in the photochemistry of proton-coupled electron transfer (PCET) reactions. We recognize that many popular organic chromophores that catalyze photoinduced PCET reactions are Hückel aromatic in the ground state, but gain significant Baird antiaromatic character in the lowest ππ* state, having important barrier-lowering effects for electron transfer. Two examples, 1) the photolytic O–H bond dissociation of phenol and 2) solar water splitting in the pyridine-water complex, are discussed. Contrary to an assumed homolytic O–H bond dissociation, both reactions proceed through loss (and gain) of an electron in the π-system (i.e., antiaromaticity relief), followed by heterolytic cleavage of the polar O–H bond near barrierlessly. Nucleus-independent chemical shifts (NICS), ionization energies (IE), electron affinities (EA), and excited-state PCET energy profiles of selected [4<i>n</i>] and [4<i>n</i>+2] π-systems are presented.<br></p>

Post Synthetic Modification of Planar Antiaromatic Hexaphyrin (1.0.1.0.1.0) by Regio-Selective, Sequential SNAr

In spite of unique structural, spectroscopic and redox properties, the synthetic variants of the planar, antiaromatic hexaphyrin (1.0.1.0.1.0) derivatives 2, has been limited due to the low yields and difficulty in access to the starting material. A chemical modification of the meso-substituents could be good alternative overcoming the synthetic barrier. Herein, we report a regio-selective nucleophilic aromatic substitution (SNAr) of meso-pentafluorophenyl group in rosarrin 2 with catechol. The reaction afforded benzodioxane fused rosarrin 3 as single product with high yield. The intrinsic antiaromatic character of the starting rosarrin 2 retained throughout the reactions. Clean, two electron reduction was achieved by treatment of 3 with SnCl2•2H2O affording 26π-electron aromatic rosarrin 4. The synthesized compounds exhibited noticeable changes in photophysical and redox properties compared with starting rosarrin 2.

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.

Post Synthetic Modification of Planar Antiaromatic Hexaphyrin(1.0.1.0.1.0) By Regio-selective, Sequential SNAr

In spite of unique structural, spectroscopic and redox properties, the synthetic variants of the planar, antiaromatic hexaphyrin (1.0.1.0.1.0) derivatives 2, has been limited due to the low yields and difficulty in access to the starting material. A chemical modification of the meso-substituents could be good alternative overcoming the synthetic barrier. Herein, we report a regio-selective nucleophilic aromatic substitution (SNAr) of meso-pentafluorophenyl group in rosarrin 2 with catechol. The reaction afforded benzodioxane fused rosarrin 3 as single product with high yield. The intrinsic antiaromatic character of the starting rosarrin 2 retained throughout the reactions. Clean, two electron reduction was achieved by treatment of 3 with SnCl2&bull;2H2O affording 26pi-electron aromatic rosarrin 4. The synthesized compounds exhibited noticeable changes in photophysical and redox properties compared with starting rosarrin 2. .

Tuning antiaromatic character and charge transport of pentalene-based antiaromatic compounds by substitution

Understanding structure−property relationships in antiaromatic molecules is crucial for controlling their electronic properties and designing new organic optoelectronic materials. Here we report the design, synthesis, and characterization of three new...

Carbenaporphyrins: A Missing Ligand in N-Heterocyclic Carbene Chemistry

The successful synthesis of a carbenaporphyrin ligand based on carbazole and triazolylidene (CTP) is reported. Instead of a macrocyclic aromatic or antiaromatic character, the aromaticity of each heterocyclic moiety is preserved, which results in optical properties different from porphyrins, e.g. fluorescence. In Li⁺ and Sc³⁺ complexes the ligand reveals striking geometric similarity to porphyrins, but is stronger electron donating.

Carbenaporphyrins: A Missing Ligand in N-Heterocyclic Carbene Chemistry

The successful synthesis of a carbenaporphyrin ligand based on carbazole and triazolylidene (CTP) is reported. Instead of a macrocyclic aromatic or antiaromatic character, the aromaticity of each heterocyclic moiety is preserved, which results in optical properties different from porphyrins, e.g. fluorescence. In Li⁺ and Sc³⁺ complexes the ligand reveals striking geometric similarity to porphyrins, but is stronger electron donating.

Carbenaporphyrins: A Missing Ligand in N-Heterocyclic Carbene Chemistry

The successful synthesis of a carbenaporphyrin ligand based on carbazole and triazolylidene (CTP) is reported. Instead of a macrocyclic aromatic or antiaromatic character, the aromaticity of each heterocyclic moiety is preserved, which results in optical properties different from porphyrins, e.g. fluorescence. In Li⁺ and Sc³⁺ complexes the ligand reveals striking geometric similarity to porphyrins, but is stronger electron donating.

Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle Between Pentalene and Benzocyclobutadiene?

According to the currently accepted structure-property relationships, aceno-pentalenes with angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with linear shape (fused to the 2,3-bond of the acene). <a>To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8π antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene are combined into, respectively, an angular and a linear topology via an unsaturated 6-membered ring. </a>The antiaromatic character of the molecules are supported experimentally by ¹H NMR, UV-Vis and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting 6-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4<i>n</i>]π subunits. We found that in the latter case pentalene drew the shorter straw. The BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with enhanced antiaromaticity. Thus, a reversed structure-antiaromaticity relationship when compared to aceno-pentalenes was achieved.