Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

Synthesis and photophysical and cation-binding properties of mono- and tetranaphthylcalix[4]arenes as highly sensitive and selective fluorescent sensors for sodium.

Abstract: The syntheses and properties of two calixarene-based fluorescent molecular sensors are reported. These comprise tert-butylcalix[4]arene either with one appended fluorophore and three ester groups (Calix-AMN1), or with four appended fluorophores (Calix-AMN4). The fluorophore is 6-acyl-2-methoxynaphthalene (AMN), which contains an electron-donating substituent (methoxy group) conjugated to an electron-withdrawing substituent (carbonyl group); this allows photoinduced charge transfer (PCT) to occur upon excitation. The investigated fluoroionophores thus belong to the family of PCT fluorescent molecular sensors. In addition to the expected red shifts of the absorption and emission spectra upon cation binding, a drastic enhancement of the fluorescence quantum yield—in an “off–on” fashion comparable to that seen in photoinduced electron transfer (PET) molecular sensors—was observed. For Calix-AMN1, it increases from 10−3 for the free ligand to 0.68 for the complex with Ca2+. This exceptional behaviour can be interpreted in terms of the relative locations of the nπ* and ππ* levels, which depend on the charge density of the bound cation. For Calix-AMN4, in addition to the photophysical effects observed for Calix-AMN1, interactions between the chromophores by complexation with some cations have been found in the ground state (hypochromic effect) and in the excited state (excimer formation). Steady-state fluorescence anisotropy measurements for the system Na+⊂Calix-AMN4, show a depolarization effect due to energy transfer (homotransfer) between the fluorophores. Regarding the complexing properties, a high selectivity for Na+ over K+, Li+, Ca2+ and Mg2+ was observed in ethanol and ethanol–water mixtures. The selectivity (Na+/other cations) expressed as the ratio of the stability constants was found to be more than 400.

Substituent effects on the physical properties and pKa of aniline

Abstract: The aniline molecule is nonplanar, with its NH2 group lying at an angle of approximately 42 to the plane of the benzene ring. Substituents on the phenyl ring alter this out-of-plane angle as well as other molecular properties such as the ring bond lengths and angles, the barrier to inversion Einv ,a nd the pKa of the amino group. Ab initio 6-311G quantum chemical calculations have been employed to examine these substituent influences and the extent to which they are interrelated. Electron-donating substituents increase the C—N bond length R(C—N),, Einv ,a nd the pKa ,w hereas electron-withdrawing substituents have the opposite effect. Among the molecular parameters that might serve as regression indicators for these changes, Hammett constants, which traditionally have been used to represent substituent electronic effects, yield fair to good correlations for R(C—N) (r 2 D 0.797), (r 2 D 0.804), Einv (r 2 D 0.829), and the amino group pKa (r 2 D 0.931) for aniline and 18 substituted anilines. Of several measures of atomic charge, the Mulliken and electrostatic charges on the amino nitrogen atom show essentially no correlation with these properties. In contrast, the natural charge Qn on the amino nitrogen is well correlated with the bond length R(C—N) (r 2 D 0.889), (r 2 D 0.932), Einv (r 2 D 0.839), and the amino group pKa (r 2 D 0.960). This latter result suggests that the natural charge, rather than either the Mulliken or electrostatic charges, may be the preferred charge descriptor for correlation purposes. Inclusion of electron correlation at the MP2 level increases the correlations of Einv with both (r 2 D 0.951) and Qn (r 2 D 0.892). c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 1107-1115, 2000

Synthesis and characterization of three-coordinate Ni(III)-imide complexes.

Abstract: A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.