TICT formation in para- and meta-derivatives of N-phenylpyrrole

The photophysical properties of m- and p-cyano N-phenylpyrrole (m- and p-PBN) are compared. Both compounds show highly red-shifted and strongly forbidden emission in polar solvents, assigned to a charge transfer state. The forbidden nature is indicative of very weak coupling between the two pi-systems, and a twisted emissive structure is suggested (TICT state). Comparison to quantum chemical calculations indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group, i.e., the central bonds in the ring are lengthened instead of shortened. m-PBN is the first meta compound which shows a CT emission assignable to a TICT state. It differs from p-PBN by a less exergonic formation of the CT state from the LE/ICT quinoid state. Consequently, it shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence in this solvent (LE/ICT and TICT).     

Spectral characteristics and nonlinear studies of crystal violet dye

Solid-state dye-doped polymer is an attractive alternative to the conventional liquid dye solution. In this paper, the spectral characteristics and the nonlinear optical properties of the dye crystal violet are studied. The spectral characteristics of crystal violet dye doped poly(methylmethacrylate) modified with additive n-butyl acetate (nBA) are studied by recording its absorption and fluorescence spectra and the results are compared with the corresponding liquid mixture. The nonlinear refractive index of the dye in nBA and dye doped polymer film were measured using z-scan technique, by exciting with He-Ne laser. The results obtained are intercompared. Both the samples of dye crystal violet show a negative nonlinear refractive index. The origin of optical nonlinearity in the dye may be attributed due to laser-heating induced nonlinear effect.     

Helical aromatic oligoamides: reliable, readily predictable folding from the combination of rigidified structural motifs

Factors responsible for the folding of aromatic oligoamides with backbones rigidified by local three-center H-bonds were investigated. The stability of the three-center H-bonds was quantified by the half-lives of amide proton-deuterium exchange reactions, which show that the three-center H-bonds were largely intact at room temperature in the oligomer examined. This result is consistent with our current and previous 2D NMR studies. The overall helical conformation of nonamer 1 was found by variable-temperature NOESY studies to be dynamic. As temperature rose, the end-to-end NOEs rapidly disappeared, while the amide side chain NOEs were still readily detectable, corresponding to the "breath" and stretching of the helix by slightly twisting the local H-bonded rings. Based on the simple repetition of the same structural motif and local conformational preference, undecamer 2 was found to fold into well-defined helical conformation. The predictability of the folding of these backbone-rigidified aromatic oligoamides was demonstrated by a simple modeling method using structural parameters from oligomers with known crystal structures. The reliability and generality of the modeling methods were shown by the excellent agreement between the modeled structures corresponding to 1 and 2 and data from NOESY studies.     

Significance of cross correlations in the stress relaxation of polymer melts

According to linear response theory, all relaxation functions in the linear regime can be obtained using time correlation functions calculated under equilibrium. In this paper, we demonstrate that the cross correlations make a significant contribution to the partial stress relaxation functions in polymer melts. We present two illustrations in the context of polymer rheology using (1) Brownian dynamics simulations of a single chain model for entangled polymers, the slip-spring model, and (2) molecular dynamics simulations of a multichain model. Using the single chain model, we analyze the contribution of the confining potential to the stress relaxation and the plateau modulus. Although the idea is illustrated with a particular model, it applies to any single chain model that uses a potential to confine the motion of the chains. This leads us to question some of the assumptions behind the tube theory, especially the meaning of the entanglement molecular weight obtained from the plateau modulus. To shed some light on this issue, we study the contribution of the nonbonded excluded-volume interactions to the stress relaxation using the multichain model. The proportionality of the bonded/nonbonded contributions to the total stress relaxation (after a density dependent "colloidal" relaxation time) provides some insight into the success of the tube theory in spite of using questionable assumptions. The proportionality indicates that the shape of the relaxation spectrum can indeed be reproduced using the tube theory and the problem is reduced to that of finding the correct prefactor.     

Efficient metal-free organic room temperature phosphors

An innovative transformation of organic luminescent materials in recent years has realised the exciting research area of ultralong room-temperature phosphorescence. Here the credit for the advancements goes to the rational design of new organic phosphors. The continuous effort in the area has yielded wide varieties of metal-free organic systems capable of extending the lifetime to several seconds under ambient conditions with high quantum yield and attractive afterglow properties. The various strategies adopted in the past decade to manipulate the fate of triplet excitons suggest a bright future for this class of materials. To analyze the underlying processes in detail, we have chosen high performing organic triplet emitters that utilized the best possible ways to achieve a lifetime above one second along with impressive quantum yield and afterglow properties. Such a case study describing different classes of metal-free organic phosphors and strategies adopted for the efficient management of triplet excitons will stimulate the development of better candidates for futuristic applications. This Perspective discusses the phosphorescence features of single- and multi-component crystalline assemblies, host–guest assemblies, polymers, and polymer-based systems under various classes of molecules. The various applications of the organic phosphors, along with future perspectives, are also highlighted.

A summary of the extremely efficient organic phosphors that utilized the best possible ways to manipulate the fate of triplet excitons for achieving a long lifetime along with impressive quantum yield and afterglow properties is provided.     

Optimization of Enzymatic Clarification of Sapodilla Juice: A Statistical Perspective

Response surface methodology (RSM) was employed to establish optimum conditions for enzymatic clarification of sapodilla juice. Polygalacturonase obtained from Streptomyces lydicus had been purified to homogeneity and was used for the treatment. The independent variables were temperature (30–45 °C), enzyme concentration (0.5–1.5 U), and treatment time (30–90 min), whose effects on viscosity and clarity of the juice were evaluated using a Box–Behnken design. Significant regression models describing the changes of viscosity and clarity with respect to the independent variables were obtained, with the coefficient of determination, R ² , greater than 0.8. Based on response surfaces and contour plots, the optimum conditions for clarifying sapodilla juice were enzyme concentration 1.15 U, incubation temperature 34 °C, and incubation time 70 min.     

Magnetism,exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS₃ is a layered antiferromagnet (T _N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate⁵ T _2g ground state of the Fe²⁺ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σ_i [δ(L _iz ² −2)+|λ|L _i .S _i ]−Σ _ij J _ij S _i .S _j . The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,J _ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;J _nn k=27.7 K; andJ _nnn k=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT _N=122 K for FePS₃, which is remarkably close to the observed value of theT _N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS₃, for which the high temperature series expansion susceptibility is available.     

Thermal and Photochemical Transformations of Hetero-1,3,5-hexatrienes into Five-Membered Rings—Possible Pericyclic Reactions

Heterosubstituted 1,3,5-hexatrienes undergo thermal and photochemical cyclizations, most of which can be viewed as pericyclic reactions. Numerous cyclizations of this type have been reported in the literature. This article presents a systematic account of such reactions, classified according to the number and nature of the heteroatoms.