Conformationally Rigid Ethynylene–Cumulene Conjugated Aromatic [30] Heteroannulenes with NIR Absorption: Synthesis, Spectroscopic and Theoretical Characterization

Abstract

Two hitherto unknown conformationally rigid Hückel aromatic ethynylene–cumulene conjugated [30] heteroannulenes have been synthesized and characterized. A thorough solution-state spectroscopic characterization, combined with in-depth theoretical calculations, has been performed to arrive at the proposed geometry of the macrocycles. The most stable optimized structures for the free base form of both the macrocycles showed absolute planar geometries without any ring inversion with mean plane deviation (MPD) values of 0.00 and 0.00 Å, respectively, in accordance with the NMR spectroscopic observations. The induced correspondence of rigid ethynylene–cumulene moieties leading to near-infrared (NIR) absorption in neutral and protonated forms of macrocycles is the important highlight of this article. This noteworthy finding has been supported by DFT-level theoretical calculations. There is an increasing pursuit in designing such NIR-absorbing/-emitting systems due to their immense applications in medicine and biology for recognizing and transportation of various substrates. The geometry of the novel 30π aromatic heteroannulenes shows promise for evolution of such novel systems in near future.

2,3‐Diarylbenzo[b]arsole: Structural Modification and Polymerization for Tuning of Photophysical Properties

“…Zirconocene dichloride (ZrCp 2 Cl 2 ), diphenylacetylene, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh 3 ) 4 ), phenyl boronic acid, 1‐naphthaleneboronic acid, 9,9‐dimethylfluoren‐2‐boronic acid, and ethylvinyl ether were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). SiliaMetS(R) Thiol (40–63 μmol 60 Å, functionalized Silica) was purchased from SiliCycle, Inc. Phenyllithium (PhLi, 1.12 m in cyclohexane‐diethyl ether) was purchased from Kanto Chemical Co., Inc. Hexaphenylcyclohexaarsine (As 6 Ph 6 ), [13] bis( p ‐bromophenyl)acetylene, [14] bis( p ‐methoxyphenyl)acetylene, [15] bis( p ‐trifluoromethylphenyl)acetylene, [16] bis(3‐anisyl)acetylene, [17] bis(2‐pyridyl)acetylene, [14] bis(2‐thienyl)acetylene, [18] 2,2′‐(9,9‐dioctyl‐9 H ‐fluorene‐2,7‐diyl)bis[4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane, [19] tribromoarsine, [20] and 5‐(4‐bromophenyl)‐norbornene [21] were prepared according to literature procedures.…”

Tailor-made aromatic porphyrinoids with NIR absorption

“…In the increasing pursuit for designing NIR absorbing chromophores by incorporating functionalized bithiophene as the building block, Rath et al have successfully synthesized and thoroughly characterized first ever ethynylene–cumulene conjugated expanded aromatic [30] heteroannulenes 77 / 78 45 from easy to make precursors P75 / P76 (Scheme 16) via acid catalysed oxidative condensation with pyrrole.…”

NIR Absorbing Aromatic E-Ethylene Bridged Hexaphyrins (2.1.1.2.1.1): Synthesis, Characterization, and Protonation Studies

“…It is worth mentioning that the conformational nonrigidity of expanded porphyrinoids is more vulnerable for embedding functionalized bipyrrole/ bithiophene/bifuran-based building blocks to modulate the spatial conformation and hence aromaticity (antiaromaticity); 3 thus, in the increasing pursuit for designing NIR absorbing aromatic expanded porphyrinoids, we have very recently reported first ever ethynylene-cumulene conjugated expanded aromatic [30]π porphyrinoids with NIR absorption by incorporating ethynylene bithiophene as a building block. 5 The McMurry coupling method has been widely used for coupling the dialdehydes of thiophene oligomers into macrocycles with ethylene bridges. 6 In this context, Anand and coworkers have reported series of stable antiaromatic 32π expanded isophlorinoids using E-ethylene bridged bis-(thiophene)/furan building blocks (Chart S1).…”

“…Scheme 1 depicts the acid catalyzed Lindsey type condensation of E-ethylene linked bithiophene diol and pyrrole using BF 3 •Et dichloro-5,6-dicyano-1,4-benzoquinone) for synthesizing 8 and 9. 5 Column chromatographic separation over basic alumina followed by repeated silica gel (200−400 mesh) chromatographic separation yielded 10% of macrocycle 8 as an air-stable dark violet solid and 7% of macrocycle 9 as an air-stable green solid. The best yield was achieved with 0.1 equiv of BF 3 •Et 2 O as the catalyst.…”

“…Because of their enormous potential biomedical applications such as tissue diagnostics, photodynamic therapy dyes, and microscopic imaging agents, designing such NIR absorbing and/or emitting systems is of great interest. It is worth mentioning that the conformational nonrigidity of expanded porphyrinoids is more vulnerable for embedding functionalized bipyrrole/bithiophene/bifuran-based building blocks to modulate the spatial conformation and hence aromaticity (antiaromaticity); thus, in the increasing pursuit for designing NIR absorbing aromatic expanded porphyrinoids, we have very recently reported first ever ethynylene-cumulene conjugated expanded aromatic [30]­π porphyrinoids with NIR absorption by incorporating ethynylene bithiophene as a building block . The McMurry coupling method has been widely used for coupling the dialdehydes of thiophene oligomers into macrocycles with ethylene bridges .…”