Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Abstract

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and -NH protons in the (1)H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by N-H···O, N-H···F, and C-H···F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.

Reversible Redox Reaction Between Antiaromatic and Aromatic States of 32π‐Expanded Isophlorins

“…Addition of TFA changed its color to deep-blue and absorbs at l = 580 and 611 nm with two other weak transitions in the region between l = 800-1000 nm. But, addition of triethylamine to [5] 2+ changed the deep-blue solution to the original reddish-brown color with the same absorption features as of 5 (see the Supporting Information), thus confirming the twisting of a planar molecule into a figureeight conformation. The compound 3 ( Figure 4) and 4 also show similar strong red-shifted absorption upon the addition of TFA.…”

“…[4] In such porphyrinoids, the ability of the pyrrolic nitrogen atom to oscillate between the imine and amine forms is crucial to regulating the conjugated pathway for facilitating such an alteration. [3,5] In the absence of a redox process, the flexible nature of expanded porphyrinoids encourages topology-based (Hückel and Mobius) alteration between aromatic and antiaromatic states with the same number of protons and p electrons. [2c, 6] In contrast, the 18paromatic porphyrin seldom adopts the 20p-antiaromatic form.…”

Hydrophobic Functionalization of ZnO Nanosheets by In Situ Center‐Substituted Synthesis for Selective Photocatalysis under Visible Irradiation

“…Compared with pure ZnTPP, the signals of porphyrinic carbons in ZnTPP for ZnTPP/ZnO NAs were with large chemical shifts. The zinc atoms coordinated to the center of tetraphenylporphyrin for the composites are also combined with the oxygen atoms of ZnO NAs, forming an axial coordination and resulting in the decrease of electron cloud density of metalloporphyrins and leading to a chemical shift toward low field …”

“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

“…Recently,F uruta and co-workers reported an eo-confused octaphyrin that contained seven meso bridges. [29] In at arget-oriented approach,alinear hexapyrrole (38) was condensed with a b-pyrrole carbinol( 39)t oy ield the octapyrrole oligomer (40)w ith terminal N-confused pyrrole units (Scheme 14). Oligomer 40 was subjected to oxidation by DDQ to yield a3 4p neo-confused octaphyrin (41)i n3 1% yield.…”

“…[38] They synthesized a3 6p tetrathiaoctaphyrin by employing the tetrapyrrane derivative of af used-dithienothiophene( DTT; 56). Under acidic conditions, compound 56 was condensed with pentafluorobenzaldehyde, followed by oxidation, to yield 36 p octaphyrin 57 (Scheme 22).…”