Bis-(Fluorene)-Embedded Hexaphyrins

Abstract

The polyaromatic hydrocarbon containing expanded porphyrins, bis-(fluorene)-embedded hexaphyrins, were synthesized by condensing fluorene-based tripyrrane with pentafluorobenzaldehyde in CH2Cl2 in the presence of 1 equiv of BF3·OEt2 under an inert atmosphere followed by oxidation with DDQ in open air at room temperature. The reaction worked only when 1 equiv of BF3·OEt2 was added to the reaction mixture under concentrated reaction conditions. The bis-(fluorene)-embedded macrocycles were characterized and studied by high-resolution mass spectrometry (HRMS), nuclear magnetic resonance (NMR), absorption, electrochemical, and density functional theory (DFT)/time-dependent (TD)-DFT techniques. In 1H NMR, the hexaphyrins showed a few broad unresolved resonances at room temperature, but the NMR spectra were well-resolved at lower temperatures, indicating that the hexaphyrins were very flexible. The DFT-optimized structures indicated that the two fluorene units at the crossing point of the figure-eight loop makes an angle of ∼79.73° with each other, the fluorene moieties maintained their own planarity, and one of the fluorene moieties was not involved in conjugation with the rest of the macrocycle. The absorption spectra of hexaphyrins showed one intense sharp band in the higher energy region and a broad band in the lower energy region. The electrochemical studies indicated that expanded hexaphyrins are relatively electron-rich and showed three easier oxidations and one reduction. The DFT/TD-DFT studies are in agreement with the experimental observations.

Phenanthrene‐Embedded Carbaporphyrinoids and Related Systems: From Ligands to Cages and Molecular Switches

“…The presence of PAH subunits incorporated into the expanded porphyrinoid frameworks allowed to create the original group of molecular conformational switches, in which the external stimuli influence the shape and stereodynamic properties of the system. [38,39,50,81,84] Eventually, the intriguing molecular capsules have been recently reported, where the PAH-embedded macrocycles served as structural facets to construct three-dimensional cage architectures. [54,85,86] The examples discussed herein were selected to demonstrate the potential of phenanthrene-and structurally related hydrocarbon-embedded carbaporphyrinoids in organic and coordination chemistry.…”

“…A series of fluorene‐embedded macrocycles, including fluorenophyrin 5 and their oxa‐ 6 and thia‐derivates 7 , was reported by Ravikanth and coworkers [46–50] . The same group has also obtained the fluorenone‐embedded macrocycle 8 [51] …”

“…A series of fluorene-embedded macrocycles, including fluorenophyrin 5 and their oxa-6 and thia-derivates 7, was reported by Ravikanth and coworkers. [46][47][48][49][50] The same group has also obtained the fluorenone-embedded macrocycle 8. [51] The Sessler group has introduced several systems incorporating dibenzo[g,p]chrysene moiety, e. g. 9, which can be perceived as a pair of twinned phenanthriporphyrins.…”

“…The fluorene-incorporating tripyrrane analog 52 has been further exploited in synthesizing expanded fluorenoporphyrinoids (Scheme 23). [48,50] The condensation of 52 with bithiophene dicarbinol 53 incorporating various meso-substituents allowed obtaining a series of doubly fused fluorene-embedded sapphyrins 54. [48] Modification of the amount of DDQ -the oxidant used on the second step of the synthesis, allowed to observe the intermediate product, namely the macrocycle incorporating two sp 3 meso-carbons and only a single thiophene-pyrrole bridge.…”

“…The reaction of 52 with pentafluorobenzaldehyde followed by DDQ oxidation yielded difluorenohexaphyrin 55 (Scheme 23). [50] Interestingly, a modification of catalyst concentration allowed to direct the reaction towards expanded carbaporphyrinoid 55 , instead of fluorenophyrin 5 . The macrocycle demonstrated nonaromatic features and, in the solution, existed in a figure‐eight conformation.…”