Substituent effects on the photophysical properties of amino-aurone-derivatives

ABSTRACT

Aurones are potential candidates to be employed as fluorescent probes or as pharmacophores for biological applications. This work describes a density functional theory (DFT) and time-dependent -DFT study at the PBE0/6-31?+?G(d) level of theory to analyse the structural, electronic and photophysical properties of a series of new proposed 4′-amine-aurone derivatives in its E and Z isomeric conformations. The maximum absorption wavelength of the proposed aurones appears in the range 390???514?nm, while the most allowed emission pathways were computed in the range 493–530?nm. The bathochromic shift of these compounds with respect to the non-substituted aurone is modulated by the acceptor strength of the added 4-substituents, in addition to the ability of the substituents to localise the frontier molecular orbitals over the acceptor benzofuranone moiety without losing the tricyclic planarity, which favours the push–pull nature of these molecules. The influence of the 4-substituent is also evidenced in the Stokes shifts for the whole series; as the electron-withdrawing character of the 4-substituents enhances, higher is the polarisation of the structure resulting in higher Stokes shifts. As a result, -CF₃ and -NO₂ substituents were responsible of larger Stokes shifts, then compounds containing these substituents are proposed as potential fluorescence probes for useful applications in biological systems.     

Substituent effects on the excited-state properties of platinummeso-tetraphenylporphyrins

A study has been undertaken to examine the effect of peripheral substitution on the photophysics of platinum tetraarylporphyrins. The aim of the study was to provide better dyes for oxygen sensing. Substitution of electron rich aryl groups results in red shifts for both the absorption and emission spectra. These observations are explained using simple molecular orbital theory. The lifetime of the excited singlet state for platinum tetraphenylporphyrin is calculated to be shorter than 0.7 psec. The radiative rate constants are on the order of 10³ sec^–1 while nonradiative rate constants are on the order of 10⁴ sec^–1. The model used is sufficiently flexible to be applied to all Pt and Pd porphyrins.     

Substituent effects on the conformational stability of allyl fluorides

By the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCH?CHCH₂F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for Y?H, Cl, Me, Vinyl, CF₃, CN, CHO, and NO₂, whereas the gauche is favored for Y?OMe, OH. A correlation of ΔH with the substituent constants σ⁺(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH₂?CHCH₂F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C₂–C₃ vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C₂–C₃ vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive π_C?C→σ*_{C? F} interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Polarization effects in the photoemission spectra of trigonal tellurium

Strong and unequivocal polarization effects are observed on the photoemission spectra of vacuum-cleaved, $\text{(10}\text{1}\text{0)}$ Tellurium. Calculations of the energy distribution curves of the photoelectrons have been performed based on pseudopotential band structure and wave functions. The results correlate with the observations and allow for the interpretation of the measured EDC in terms of bulk properties and surface effects.     

A comment on the geometry of some scalar-tensor theories

We show that the scalar field in scalar-tensor theories such as the Jordan-Brans-Dicke theory has an interpretation as a potential for the torsion in a Riemannian manifold. The relation is similar to that of the metric to the connection.     

Substituent effects in the Nicholas epimerization of glycosides

The Nicholas reaction is a proven and versatile tool in glycoside chemistry. Its application is less clear‐cut, if the intermediate Nicholas cation has two reactive sides that enable different reactions. Density functional theory (DFT) calculations are used to rationalize the observed difference in product yields for the Nicholas epimerization and substitution obtained from experiments with very similar glycosides. Substituents, which can increase the rigidity of the backbone, should be avoided in epimerization reactions as they can slow down reorientation of the intramolecular ion pair so that the substitution reaction dominates.     

Substituent effects in cooperativity of chalcogen bonds

In the present work, substituent effects on cooperativity of S···N chalcogen bonds are studied in XHS···NCHS···4-Z–Py (X = F, Cl; Z = H, F, OH, CH₃, NH₂, NO₂, and CN; and Py = pyridine) complexes using ab initio calculations. An increased attraction or a positive cooperativity is observed on introduction of a third molecule to the XHS···NCHS and NCHS···4-Z–Py binary systems. The shortening of each chalcogen bond distance in the ternary systems is dependent on the substituent Z and is increased in the order Z = NH₂ > OH > CH₃ > H > F > CN > NO₂. The electronic aspects of the complexes are analysed using molecular electrostatic potential, and the parameters derived from the atoms in molecules and natural bond orbital methodologies. According to interaction energy decomposition analysis, the electrostatic energies are important in the interaction energy of S···N bonds and may be regarded as being responsible for the stability of these complexes.     

Substituent effects on the triplet-triplet absorption spectra of coumarin and its derivatives

Preliminary work on the triplet-triplet (T-T) absorption of coumarin and two of its 4-hydroxy substituted derivatives is extended to four additional substituted coumarins in order to investigate substituent effects on T-T absorption in these molecules. A steady cross Illumination technique is used in EPA glass at 77° K. Measurements of the decay of T-T absorption are carried out with a flash technique and a comparison made with phosphorescence lifetimes to verify the triplet nature of the absorption. The observed T-T spectra show a marked dependence on the presence and position of hydroxy substitution. An explanation is developed based on the mixing of $\text{nπ}{}^1$ character into the lowest $\text{ππ}{}^1$ triplet state. The effect of methyl substitution is also investigated and, although it leads to relatively small changes in the T-T spectra, the results suggest that it plays a role in the susceptibility of the molecules to photochemical change.     

Spin densities in some odd alternant radicals including linear polyene radicals

An approximate unrestricted Hartree-Fock method is used to calculate spin densities in the odd alternant radicals cyclohexadienyl, benzyl, perinaphthenyl and triphenylmethyl and the linear polyene radicals from allyl to C₁₉H₂₁. The results agree quite well with the exact calculations and with empirical estimates as well as with experimental data on hyperfine splitting constants and E.S.R. second moments.     

Luminescence properties of some platinum (II) complexes. Counter-ion and molecular geometry effects

Reflectance and luminescence spectra, and emission lifetimes of 14 charged and neutral Pt (II) complexes in the solid crystalline state are reported. The lifetimes (in the range of some tens of microseconds) indicate that the emissions are due to a spin-forbidden process. On the basis of spectral correlations, the phosphorescence is tentatively identified as due to the lowest d-d ligand field transition when the bonding of the ligand is essentially σ in character, and to a π1 → d charge-transfer transition for those complexes in which the ligands themselves have π orbital systems. Both the radiative (k_r) and non-radiative (k_n) rate constants are sensitive to changes in molecular geometry (cis, trans isomers) and counter-ions. Assuming unitary efficiency for the intersystem crossing to the emitting state, the counter-ion appears to predominantly affect k_n through vibrational coupling of the chromophore with the lattice. For the cis forms, both k_r and k_n are affected in a complex manner, with metal-metal interactions playing an important role. For the trans forms, however, the constancy of the quantum yield with respect to temperature suggests that k_n is negligible in comparison to k_r, and therefore the trans chromophores behave as isolated systems within the crystalline lattice.     

Substituent effects on the gas-phase fragmentation reactions of protonated peptides containing benzylamine-derivatized lysyl residues

Motivated by the need for chemical strategies designed to tune peptide fragmentation to selective cleavage reactions, benzyl ring substituent influence on the relative formation of carbocation elimination (CCE) products from peptides with benzylamine-derivatized lysyl residues has been examined using collision-induced dissociation (CID) tandem mass spectrometry. Unsubstituted benzylamine-derivatized peptides yield a mixture of products derived from amide backbone cleavage and CCE. The latter involves side-chain cleavage of the derivatized lysyl residue to form a benzylic carbocation [C(7)H(7)](+) and an intact peptide product ion [(MH(n))(n+) - (C(7)H(7))(+)]((n-1)+). The CCE pathway is contingent upon protonation of the secondary ε-amino group (N(ε)) of the derivatized lysyl residue. Using the Hammett methodology to evaluate the electronic contributions of benzyl ring substituents on chemical reactivity, a direct correlation was observed between changes in the CCE product ion intensity ratios (relative to backbone fragmentation) and the Hammett substituent constants, σ, of the corresponding substituents. There was no correlation between the substituent-influenced gas-phase proton affinity of N(ε) and the relative ratios of CCE product ions. However, a strong correlation was observed between the π orbital interaction energies (ΔE(int)) of the eliminated benzylic carbocation and the logarithm of the relative ratios, indicating the predominant factor in the CCE pathway is the substituent effect on the level of hyperconjugation and resonance stability of the eliminated benzylic carbocation. This work effectively demonstrates the applicability of σ (and ΔE(int)) as substituent selection parameters for the design of benzyl-based peptide-reactive reagents which tune CCE product formation as desired for specific applications.     

Scale and geometry effects on heat-recirculating combustors

A simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices. A three-dimensional (3D) numerical model of spiral counterflow ‘Swiss roll’ combustors was then used to confirm and extend the applicability of the identified parameters. It was found that without property adjustment to maintain constant values of these parameters, at low Reynolds number (Re) smaller-scale combustors actually showed better performance (in terms of having lower lean extinction limits at the same Re) due to lower heat loss and internal wall-to-wall radiation effects, whereas at high Re, larger-scale combustors showed better performance due to longer residence time relative to chemical reaction time. By adjustment of property values, it was confirmed that four dimensionless parameters were sufficient to characterise combustor performance at all scales: Re, a heat loss coefficient (α), a Damköhler number (Da) and a radiative transfer number (R). The effect of diffusive transport effect (i.e. Lewis number) was found to be significant only at low Re. Substantial differences were found between the performance of linear and spiral combustors; these were explained in terms of the effects of the area exposed to heat loss to ambient and the sometimes detrimental effect of increasing heat transfer to adjacent outlet turns of the spiral exchanger. These results provide insight into the optimal design of small-scale combustors and choice of operation conditions.     

Substituent effects on the pH‐dependent multiequilibria of flavylium salt analogs of anthocyanins

Substituent effects on the hydration, tautomerization, and isomerization equilibria of flavylium salts can be described by a series of linear free energy relationships (LSER) based on Hammett correlations. The positions on the flavylium rings were classified as either activated (para‐like) or nonactivated (meta‐like) to decide which σ value to employ (σ_R or σ_m, respectively), while the steric effects of substituents at C‐3 were included via the E_S parameter. Based on these relationships, we then show that it is possible to predict values of the “apparent pK_a” (pK_ap) of flavylium ions that were not included in the original data set, as well as those of several naturally occurring anthocyanins. The value of pK_ap provides a measure of the thermodynamic stability of the flavylium cation as a function of pH and is directly related to the pH range in which the color of the flavylium cation form of anthocyanins persists in aqueous solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Free-field representations and geometry of some Gepner models

The geometry of the k ^K Gepner model, where k + 2 = 2K, is investigated by a free-field representation known as the “bcβγ” system. Using this representation, we directly show that the internal sector of the model is given by Landau-Ginzburg ℂ ^K /ℤ_2K orbifold. Then we consider the deformation of the orbifold by a marginal antichiral-chiral operator. Analyzing the chiral de Rham complex structure in the holomorphic sector, we show that it coincides with chiral de Rham complex of some toric manifold, where toric data are given by certain fermionic screening currents. This allows relating the Gepner model deformed by the marginal operator to a σ-model on the CY manifold realized as a double cover of ℙ ^{K − 1} with ramification along a certain submanifold.     

A green's function calculation of the lowest ionization potential of some radicals

By using the so-called two particle-hole extended Tamm-Dancoff Approximation (2ph ex-TDA) to the one-particle Green's function (GF), we calculated the lowest ionization potentials (IP) for the radicals NO, NS, PO, PS, CF, NH₂, HCO and FCO. Though these molecules have an open-shell electronic structure, for which the usual (closed shell) GF development cannot be applied, we circumvented this difficulty by reversing in the actual calculation the physical order of the ionization process. The underlying ab initio SCF calculations were performed by using basis sets of double- to triple-zeta quality plus polarization functions. A comparison of the 2ph ex-TDA results with experiment shows the satisfactory usual accuracy (⋍0.2ev) of this method. A short comparison with other ΔSCF results is also done. A theoretical prediction for the ionization potential of the PS radical, where no experimental values are available, is finally proposed.