The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO 4 − mediated oxidations of the corresponding meso-tetraaryl-2,3dihydroxychlorins ( 7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2oxachlorin ( 11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q x band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC−CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.
A promising solution to address the challenges in plastics sustainability is to replace current polymers with chemically recyclable ones that can depolymerise into their constituent monomers for circular use of materials. Despite the progress, few depolymerisable polymers exhibit the excellent thermal stability and strong mechanical properties of traditional polymers. Here we report a series of chemically recyclable polymers that show excellent thermal stability (decomposition temperature > 370 ºC) and tunable mechanical properties. The polymers are formed via ring-opening metathesis polymerisation of cyclooctene with a trans-cyclobutane installed at the 5,6-positions. The additional ring converts the nondepolymerisable polycyclooctene into a depolymerisable polymer by reducing the ring strain energy in the monomer (from 8.2 kcal/mol in unsubstituted cyclooctene to 4.9 kcal/mol in the fused ring). The fusedring monomer enables a broad scope of functionalities to be incorporated, providing access to chemically recyclable elastomers and plastics that show promise as next-generation sustainable materials. Main TextSynthetic polymers, including synthetic rubber and synthetic plastics, have been used in nearly every aspect of our daily lives. The dominance of synthetic polymers is largely driven by their excellent stability and processability as well as their versatile mechanical properties. However, due to their high durability, waste materials composed of these polymers have accumulated in the ocean and have caused serious concerns for marine ecosystems 1 . In addition, since 90% of these polymers are derived from nite fossil feedstocks, the production of these materials is unsustainable if they cannot be recycled and reused 2 .
The new fluorescent chromophore BOPHY can be readily synthesized in two steps from commercially available reagents via the coupling of pyrrole-2-carboxaldehyde with hydrazine followed by reaction with BF3. The resultant symmetric and dimeric tetracycle is composed of two BF2 units in six-membered chelate rings appended with pyrrole units on the periphery. The quantum yields of fluorescence for the unmodified compound and the tetramethyl variant are near unity, with values of 95 and 92%, respectively, in CH2Cl2. We have probed the electronic structure of this compound via cyclic voltammetry and density functional theory analysis.
We have synthesized and characterized via single-crystal X-ray diffraction methods iron(II), ruthenium(II), and osmium(II) carbonyl derivatives of (1-methylimidazole) (5,10,15,20-tetraphenylporphyrinate) [(5,10,15,20-tetraphenylporphyrinate ) TPP)], Fe(TPP)(CO)(1-MeIm)‚toluene, Ru(TPP)(CO)(1-MeIm)‚ chloroform, and Os(TPP)(CO)(1-MeIm)‚chloroform, together with the osmium(II) pyridine adduct Os(TPP)-(CO)(py)‚2benzene. The crystallographic results permit a detailed structural comparison between all of the six carbonyl metalloporphyrins which can be prepared from TPP, Fe, Ru, Os, and the two axial bases 1-methylimidazole and pyridine. The structures of all three (Fe, Ru, Os) 1-methylimidazole complexes display major saddle distortions, with the extent of the distortions being Fe > Ru ∼ Os. For the pyridine complexes, deviations from planarity of the porphyrin ring are about an order of magnitude smaller than those for the 1-methylimidazole species. The M-C-O bond angles in all complexes are in the range 176.8-179.3°. We also determined the 13 C and 17 O NMR isotropic chemical shifts, the 13 C NMR chemical shift tensor elements, and, for the three 1-MeIm adducts, the 17 O nuclear quadrupole coupling constants. We then used density functional theory (DFT) to relate the experimental spectroscopic results to the experimental structures. For the 13 C and 17 O isotropic shifts, there are excellent correlations between theory and experiment ( 13 C, R 2 value ) ∼0.99; 17 O, R 2 value ) ∼0.99), although the slopes ( 13 C, ∼-0.97; 17 O, ∼-1.27) deviate somewhat from the ideal values. For the 17 O nuclear quadrupole coupling constant, our results indicate an rms error between theory and experiment of 0.20 MHz, for experimental values ranging from (+)1.0 to (-)0.40 MHz, where the signs are deduced from the calculations. The ability to predict spectroscopic observables in metalloporphyrin systems having relatively well characterized structures by using density functional theory provides additional confidence in the application of these theoretical methods to systems where structures are much less certain, such as heme proteins.
Four free-base corroles with electron-donating or electron-withdrawing groups on the para or 2 through 6-positons of the meso phenyl rings were prepared via either Paolesse or Gross conditions and investigated for their absorption and emission properties. The triaryl corroles 5,10,15-triphenylcorrole, 5,10,15-tris(pentafluorophenyl)corrole, 5,10,15-tris(p-nitrophenyl)corrole, and 5,10,15-tris(p-methoxyphenyl)corrole were examined. Absorption, steady-state, and time-resolved fluorescence measurements were performed on all compounds in both nonpolar (dichloromethane) and polar (dimethylacetamide) solvents. The experimental evidence points to hydrogen bonding with an internal N-H group as the most likely factor in the solvent-dependent photophysical behavior of these corroles, that is also highly dependent upon substitution.
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