Rotation Of Aryl Ring Research Articles

5,10,15-Triarylcorrole atropisomerism

A series of 5,10,15-triarylcorroles has been prepared, with the meso-aryl rings functionalized with different substituents to investigate their influence on the aryl ring rotation with respect to the corrole plane. The study has been carried out by different NMR techniques, allowing the complete assignment of the 1H NMR spectra and giving insights on the kinetic and thermodynamic factors driving the atropisomerism in triarylcorrole derivatives.

Solid-State Emission from Mono- and Bichromophoric Boron Dipyrromethene (BODIPY) Derivatives and Comparison with Fluid Solution.

The syntheses and crystal structures of sterically crowded mono- and bichromophoric BODIPY-based dyes are reported. The "monomeric" compound is weakly fluorescent in the liquid phase due to fast internal conversion associated with rotation of aryl rings at the boron atom. The side-by-side "dimer" exhibits weak excitonic coupling between the dipyrrin units and is much more emissive in fluid solution. Solid samples of both molecular entities are strongly fluorescent under near-UV illumination. Thus, the mono-chromophore exhibits dual fluorescence from what appears to be a mixture of crystalline and possibly amorphous (or interfacial regions) distributions. The bi-chromophore packs in the crystal as pairs of chromophores with each unit being provided by a different molecule. This leads to excitonic splitting and the formation of a strong H-band in the absorption spectrum. Fluorescence occurs from the corresponding J-species and also from what appears to be an aggregated state associated with interfacial areas. Both bulk and interface-bound states show relatively long-lived fluorescence while the crystal structures indicate the likelihood for fast electronic energy migration between molecules.

Click Reaction Synthesis and Photophysical Studies of Dendritic Metalloporphyrins

AbstractSeveral dendritic zinc(II)‐porphyrins bearing carbazole units at the terminals have been prepared through click reaction of azide‐substituted Zn‐porphyrin precursors and carbazole‐based alkynes under [Cu(NCCH3)4][PF6] catalysis. This family of new dendritic metalloporphyrins shows dual luminescence from both the upper S2 and the lowest S1 singlet states. The observed trends in the spectroscopic data and the photophysical properties of the dendrimers have been rationalized in terms of the type of meso‐spacer between the macrocycle and dendritic shell. The key feature of the meso‐spacer is proposed to be the degree of hindrance towards aryl ring rotation relative to the mean porphyrin plane. Based on the observed differences in the S1→S0 fluorescence quantum yield on going from four‐ to five‐coordinate dendrimers, it was shown that dendrimer architectures with all four meso‐aryl spacers sterically hindered, are most appropriate to monitor processes related to axial ligation of the dendrimer core, since they provide larger luminescence response differences between the four‐coordinate and five‐coordinate forms. The blue S2→S0 fluorescence quantum yield has been measured and the observed trend has been rationalized in terms of the S2→S1 energy gap law. No significant differences were observed between the compounds either with different rotational degree of freedom of the meso‐spacers or with different ligation states of the dendrimer core.

Fluorescence enhancement of N2O2-type dipyrrin ligand in two step responding to zinc(II) ion

Fluorescence imaging is well-suited for the live imaging of biological Zn(II), which has no facile spectroscopic or magnetic signature. The successful application of this methodology requires the development of robust Zn(II) imaging agents that display high sensitivity, selectivity and temporal fidelity. In this study, a N2O2-type dipyrrin based bimolecular zinc(II) complex was produced and shown to have sharper, blue-shifted and more enhanced fluorescence emission. An approximate three-fold fluorescence enhancement was achieved within the micromolar concentration range, which is an important parameter for Zn(II) detection in vivo. The increase in emission intensity was due to the dominant role of aryl-ring rotation in governing the excited state dynamics and fluorescence properties of the dipyrrin dye. Fluorescence titration showed that the ligand complex exhibited very strong zinc(II) binding affinity when compared to that in the binuclear chloro complex. The fluorescence emission changes in the dipyrrin dye to zinc(II) ion could be observed not only using instruments but also by the naked-eye (violet→sky blue).

A green protocol for the synthesis of conformationally rigid sulfur linked bisquinolines by double Friedlander reaction in water

A series of bis(2-aryl-4-arylquinolin-3-yl)sulfanes and 1,2-bis(2-aryl-4-arylquinolin-3-yl)disulfanes were synthesised in good to excellent yields by double Friedlander reaction between 2-[(2-oxo-2-arylethyl)sulfanyl]-1-aryl-1-ethanones/2-[(2-oxo-2-arylethyl)disulfanyl]-1-aryl-1-ethanones and 2-aminobenzophenone with p-toluenesulfonic acid in water medium. An interesting restricted rotation of aryl rings has been noticed in bis(2-aryl-4-arylquinolin-3-yl)sulfanes as revealed by NMR and crystal data.

Structure and Dynamics of α-Aryl Amide and Ketone Enolates: THF, PMDTA, TMTAN, HMPA, and Crypt-Solvated Lithium Enolates, and Comparison with Phosphazenium Analogues

A variety of multinuclear NMR techniques, in combination with X-ray diffraction methods, were used to probe the solution structure of α-aryl lithium enolates of bis(4-fluorobenzyl) ketone (1-H), phenyl 4-fluorobenzyl ketone (2-H), and N,N-dimethyl 4-fluorophenylacetamide (3-H) in ethereal solvents and in the presence of cosolvent additives PMDTA, TMTAN, HMPA, and cryptand [2.1.1]. All three enolates were dimers in THF solution, and were converted to monomers by the triamine additives, PMDTA and TMTAN. The exchange of the triamine-solvated monomers with their ethereal-solvated dimer counterparts was probed by using dynamic NMR (DNMR). The cosolvent HMPA formed monomers along with minor amounts of lithiate species, (RO)(2)Li(-) and (RO)(3)Li(2-), which were also observed when cryptand [2.1.1] was used as a cosolvent, or when mixed lithium-phosphazenium enolate solutions were prepared. Dynamic exchange of lithiate species was investigated by DNMR spectroscopy. The barrier to rotation of the conjugated 4-fluorophenyl ring of these diverse enolate structures was measured and found to be consistent with a resonance picture where lower aggregation states lead to increased delocalization of negative charge. The lithium enolate aggregates identified were compared to the "naked" α-4-fluorophenyl enolates generated with the phosphazene base P4. The barrier to aryl ring rotation was 2.7 kcal/mol higher for the phosphazenium enolate 3-Li·P4H compared to the dimer (3-Li)(2). Structural characterization of a phosphazenium enolate through X-ray crystallography was obtained for the first time. Additional aspects of the Schwesinger base P4 were investigated which included characterization of the solution exchange behavior of the protonated and unprotonated forms as well as determination of the solid state structure by X-ray diffraction.

Synthesis, Structure, and Photophysical Characterization of Blue-Green Luminescent Zinc Complexes Containing 2-Iminophenanthropyrrolyl Ligands

New 2-iminophenanthro[9,10-c]pyrrole ligand precursors containing phenyl or 2,6-diisopropylphenyl groups at the imine nitrogen substituent, 2-arylformiminophenanthro[9,10-c]pyrroles (aryl = phenyl IIa, 2,6-diisopropylphenyl IIb) were synthesized and deprotonated in situ with NaH, originating solutions of the corresponding sodium salts (IVa, IVb). The reaction of these salts with zinc chloride gave the homoleptic bis-ligand Zn(II) complexes [Zn(kappa(2)N,N'-2-arylformiminophenanthro[9,10-c]pyrrolyl)(2)] (aryl = phenyl 2a, 2,6-diisopropylphenyl 2b). The new ligand precursors and complexes were characterized by NMR, elemental analysis, UV/vis spectroscopy, and X-ray crystallography, when possible. The photophysical characterization was carried out using steady-state and picosecond time-resolved luminescence techniques in solution. The influence of the pi-extended conjugation of the condensed phenanthro group on the deprotonated iminopyrrolyl ligands coordinated to Zn(2+) greatly enhances fluorescence quantum yields of the complexes (2a, 2b) in relation to those of their ligand precursors (IIa, IIb). Complex 2a shows emission in the green spectral region (lambda(max) = 494 nm), presenting the highest fluorescence quantum yield (phi(f) = 8.8%). In the case of the complex 2b (phi(f) = 3.9%), the bulkiness of the 2,6-diisopropyl substituents of the arylimino group highly restricts the aryl ring rotation toward coplanarity with the ligand framework, inducing a shift in the emission to the blue region (lambda(max) = 459 nm). The values of the radiative (k(f)) and radiationless rate constants (k(nr)) show that the fluorescence quantum yield enhancement in the complexes results from a 50-fold increase in k(f) values, indicating much more allowed pi-pi* transitions in complexes 2a and 2b than those occurring in the ligand precursors IIa and IIb, with an essentially n-pi* character. These assignments were confirmed by density-functional theory (DFT) and time-dependent DFT (TD-DFT) molecular orbital calculations. Simple 2-aryliminopyrrole ligand precursors (Ia, Ib) and their Zn(II) complexes (1a, 1b) were also prepared to compare their photophysical properties with those of the corresponding 2-aryliminophenanthro[9,10-c]pyrrolyl compounds.

Study of the Origin of the Hindered Rotation of an Aryl Ring in Chromium Aminocarbene Complexes Bearing an Aromatic Ring Attached to the Carbene Carbon Atom

The substituted chromium aminocarbene complexes cis-tetracarbonyl[(η2-N-allyl-N-allylamino)(4-X-phenyl)carbene]chromium(0) (X = MeO, Me, H, Cl, CO2Me, CF3) were prepared by the reaction of the corresponding N,N-diallylamides with Na2Cr(CO)5 and Me3SiCl. Using dynamic NMR, the rotational barrier of the aryl ring in these complexes was established. The obtained values are higher for the complexes bearing electron acceptors compared to those bearing donor substituents and correspond with the Hammett equation. This indicates that the restricted rotation of aryl ring in the chromium aminocarbene complexes is a balance between steric and conjugation effects; while the former increases the barrier, the latter decreases it by stabilizing the transition state of the rotation. This finding is compatible with the fact that in the related cis-tetracarbonyl[(N-alkyl-η2-N-allylamino)(phenyl)carbene]chromium(0) complexes (alkyl = Me, Et, i-Pr) the rotational barrier grows steadily with the growing steric demand of the s...

Structural control of the photodynamics of boron-dipyrrin complexes.

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.

Theoretical study of aryl-ring rotation in arylporphyrins

We carried out quantum chemical calculations for the aryl-ring rotations in phenylporphyrin and ortho-methoxyphenylporphyrin to determine the reaction path and the potential-energy barrier so that one can provide an insight into how such rotations can occur. It was found that along the reaction path, the structure changes from a planar porphyrin-ring at the stable state to a considerably distorted one at the transition state, and that the small potential-energy barrier, which is consistent with experiments, largely stems from the deformation of the porphyrin ring. An increase in the potential-energy barrier resulting from the substitution at the ortho position of the aryl group was also found to originate mainly from an additional deformation of the porphyrin ring at the transition state. These findings are in contrast with the calculated result for the aryl-ring rotation in the biphenyl, in which the direct steric hindrance between the aryl rings rather than the deformation makes large contributions to the overall potential energy barrier. We concluded that the deformation of the porphyrin ring is essential for the occurrence of the aryl-ring rotation in the arylporphyrins.

Conformational dynamics of 4-aryl-1,4-dihydropyridine calcium channel antagonists. 1. Quantitation of C4-C1′ bond rotational barriers

The solution state dynamics of six substituted 4-aryl-1,4-dihydropyridine calcium channel antagonist analogs were studied using saturation transfer, difference NOE and variable temperature NMR spectroscopy. Thermodynamic activation parameters for aryl ring rotation were determined for all six compounds from linear Eyring plots which span four orders of magnitude. The barrier to rotation about the ring juncture was also calculated at the AM1 level and these calculations correctly predict the general magnitude, though not the specific barriers, to rotation. The asymmetrically substituted compound, 1,4-dihydro-2,6-dimethyl-4-(2′-chloro-6-methylphenyl)-3,5-pyridine-dicarboxylic acid dimethyl ester ( 2a), showed a rotameric preference of 1.7:1 by NMR for having the aryl methyl group syn to the C4H of the dihydropyridine ring; AM1 correctly predicts this preference but overestimates its magnitude. In the crystal state, the rotameric ratio appears to be nearer 1:1.

The reactions of nitrosoarenes with cationic cyclohexadienyl complexes of iron tricarbonyl: an ESR study

The reactions of nitrosoarenes with the (cyclohexadienyl)Fe(CO)3 cation have been investigated by using electron spin resonance spectroscopy. The radicals produced are nitroxides of the type (OC)3Fe(C6H7)(Ar)N—O• but, in some cases, disproportionation and loss of the metal carbonyl fragment leads to the corresponding C6H5(Ar)N—O• radical. With bulky nitrosoarenes, such as C6Me5NO, isomers are observed in which the aryl ring rotation is slow on the ESR time scale. The analogous reactions with the cyclohexadienyl cation derived from the B ring of (ergosteryl acetate)Fe(CO)3 lead to initial attack not at one of the termini of the delocalized system but rather at the central carbon, i.e., at C-7. Subsequent hydrogen migration leads to the (5,7-diene)Fe(CO)3 complex bearing the arylnitroxide at the 7-position. The mechanisms of these reactions are discussed. Keywords: nitrosoarenes, iron cations, ESR.

ChemInform Abstract: Steric Effects on Atropisomerism in Tetraarylporphyrins.

AbstractThe rate of aryl ring rotation in the series of porphyrins (I) is studied by NMR spectroscopy and chromatography.

Restricted rotation of aryl rings in cis-1,2-diarylcyclopentanes and diarylmethylcyclobutanes

Restricted rotation of aryl rings in cis-1,2-diarylcyclopentanes and diarylmethylcyclobutanes