Ever since the discovery of the porphyrin ring in "pigments of life", such as chlorophyll and hemoglobin, it has become a prime synthetic target for optoelectronic properties and in the design of metal complexes. During one such early expedition on the synthesis of porphyrin, Woodward proposed that condensing pyrrole with an aldehyde under acidic conditions yields the "precursor" porphyrinogen macrocycle. Its four-electron oxidation leads to the "transitory" 20π isophlorin, which undergoes subsequent two-electron oxidation to form the 18π "porphyrin". Due to its fleeting lifetime, it has been a synthetic challenge to stabilize the tetrapyrrolic isophlorin. This macrocycle symbolizes the antiaromatic character of a porphyrin-like macrocycle. In addition, the pyrrole NH also plays a key role in the proton-coupled, two-electron oxidation of isophlorin to the aromatic porphyrin. However, a major aspect of its unstable nature was attributed to its antiaromatic character, which is understood to destabilize the macrocycle upon conjugation. Antiaromaticity in general has not gained significant attention mainly due to the lack of stable 4nπ systems. In this regard, a stable isophlorin and its derivatives provide a glimmering hope to peek into the world of antiaromatic systems. This review will focus on the attempted synthesis of antiaromatic isophlorin ever since its conception. Based on recent synthetic advances, the chemistry of isophlorins can be expected to blossom into expanded derivatives of this antiaromatic macrocycle. Along with the synthetic details, the structural, electronic, and redox properties of isophlorin and its expanded derivatives will be elaborated.

Core-modified hexaphyrin and octaphyrin display four- and six-electron reversible redox reactions respectively, to alternate between aromatic and anti-aromatic states of a given macrocycle.

Steric hindrance induced by thiophene molecules in predesigned precursors favors the exclusive formation of three dimensional (3D) π-conjugated cage and quasi-cage like molecules instead of porphyrinoid macrocycle. Herein we report...

Multifunctional organic luminogens exhibiting simultaneous aggregation induced emission (AIE), room-temperature phosphorescence (RTP), and mechanochromism have recently attracted considerable attention owing to their potential applications in optoelectronics and bioimaging. However, a comprehensive correlation among these three distinguished properties is yet to be unveiled, which will help to decipher defined methodologies to design future generation multifunctional organic materials. Herein, we have demonstrated a route to obtain a multifunctional organic luminogen, starting from an ACQphore (TPANDI) by simple structural engineering. We have shown that a slight reduction in length of the planar acceptor moieties can effectively inhibit the undesirable π–π stacking interaction between molecules in the condensed state and thereby cause an ACQ to AIE type transformation from TPANDI to TPANMI and TPAPMI. Both TPANMI and TPAPMI exhibit RTP properties (even in ambient condition) because of the presence of a reasonably low singlet–triplet energy gap (ΔE ST). In our study, these two luminogens were found to be mechano-inactive. Interestingly, an insertion of cyano-ethylene group and benzene linker in between the triphenylamine and phthalimide moieties introduced another luminogen TPACNPMI, which can simultaneously exhibit AIE, RTP, and mechanochromic properties.

The electronic properties of a hexaphyrin was finetuned via core-modification leading to the formation of a Hückel aromatic 30π hexaphyrin which incorporates two pyrrole and four furan rings in the π-conjugated pathway. This Hückel aromatic hexaphyrin modified its conformation upon two-electron ring oxidation either with triflic acid or Meerwein salt [Et 3 O] + [SbCl 6 ] À to yield 28π Möbius aromatic dication species. Reversible aromatic transition was established by spectroscopic techniques and further supported by quantum chemical calculations.