Aromaticity Switching in Porphyrinoids

Abstract

The aromaticity of porphyrinoids can be substantially altered by reversible modification of their original electronic structures. Well-defined modulators can be used as a means to initiate these modifications, including redox processes, acid-base chemistry, and conformational phenomena. This Focus Review emphasizes the situation for which a single macrocyclic frame alternatively adopts diatropic and paratropic features and both situations are readily and mutually exchangeable. Eventually, such a porphyrinoid transformation can be explored as a suitable element to construct switchable optoelectronic materials.

A Rhodium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m‐Benziporphyrin

“…To account for determined properties of 5,the DFT-optimized structure of the molecule was calculated ( Figure 2). The 1 HNMR spectrum of transient 5 reveals an increased multiplicity of carbaporphyrin resonance signals as compared to the C s -symmetric 3-CO substrate and reveals,a ccording to the magnetic criteria, [18] the basic features of aromatic metallocarbaporphyrinoids ( Figure 1B). [9] TheD FT-optimized model of 5 ( Figure 2) shows the structural constraints determined by NMR, including NOE connectivities.S uch an architecture imposes aR h III ···h 2 -C(22)C(26) arrangement, as reflected by the RhÀC(26) (2.031 ) and RhÀC(22) (2.308 ) bond lengths.…”

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

“…The complexes can also be obtained via a consecutive reduction and insertion process . The character of 5 a ( 5 b ) differs from that of 2 a ( 2 b ), since the shape of the absorption spectra of 5 a ( 5 b ) (Figure B) is characteristic for antiaromatic porphyrinoids with 4n π electrons . 5 a ( 5 b ) have intense Soret‐like band at 370 nm (365 nm) accompanied by a very broad red‐shifted transition with small oscillator strengths.…”

“…Extending the π‐electron system in conjugated molecules is an important area of research, because the conjugation pathway exhibits notable influence on the optical and magnetic properties . Linear extensions of the acene π cloud by annelation of molecular rings lead to different aromatic behavior of five‐ and six‐membered rings .…”

“…The aromatic character is analyzed based on the measured 1 H NMR spectra. The cations appear to be aromatic (Figure A, B, the assignment follows numbering scheme presented in Scheme ), since all resonances assigned to the perimeter of the macrocycle are significantly downfield shifted ( 2 a δ =9.31 ppm (4), δ =8.78 ppm (5), δ =8.95 ppm (9), δ =8.98 ppm (10), δ =8.61 ppm (14), δ =9.04 ppm (15) and for 2 b δ =9.08 ppm (4), δ =8.54 ppm (5), δ =8.84 ppm (9,10), δ =8.43 ppm (14), δ =8.73 ppm (15)) suggesting that the macrocycle sustains a global diatropic ring current, which is also expected to shift the 1 H NMR signal of the inner entrapped hydrogen upfield . However, the inner hydrogen of 2 a ( 2 b ) has a significant downfield shift of δ =9.98 ppm ( δ =11.35 ppm) in the 1 H NMR spectrum as shown in Figure .…”

“…The 1 H NMR spectrum for 4 a ( 4 b ) shows upfield relocation of the perimeter lines to the region of δ =5.2–6.2 ppm (Figures S18 and S22) with the two NH resonances shifted to δ =19.1 ppm (17.3 ppm) and δ =17.1 ppm (14.5 ppm) for 4 a ( 4 b ) suggesting a change of the aromatic character from aromatic to antiaromatic . Significantly more upfield‐shifted signals were recorded for the boron(III) complexes ( 5 a δ =5.29 ppm (4), δ =4.98 ppm (5), δ =4.45 ppm (9), δ =4.39 ppm (10), δ =5.26 ppm (14), δ =4.91 ppm (15) and for 5 b δ =5.69 ppm (4), δ =5.33 ppm (5), δ =4.78 ppm (9), δ =4.71 ppm (10), δ =5.15 ppm (14), δ =5.47 ppm (15)), which is typical for antiaromatic porphyrinoids . The chemical shifts of the axial aryl group attached to the central boron inside the macrocycle are sensitive probes for assessing the aromatic character of the macrocycle .…”