The Triplet State in Pyrene

Both emission and excitation spectra of phosphorescence and delayed fluorescence of pyrene single crystals have been investigated between 2 K and 300 K using cw dye laser excitation. The experiments prove that the lowest triplet state is excitonic in nature in the high and low temperature crystalline phases. No evidence for triplet excimer emission could be found. The so-called regular broad band triplet excimer emission was spectrally resolved at low temperature and identified as trap emission. On the basis of polarized high resolution excitation spectra of undoped normal and perdeuterated pyrene (supplemented by Zeeman spectra at helium temperature) and the concentration dependent phosphorescence excitation spectra of isotopically mixed crystals the triplet state symmetry and the excitonic Davydov splitting of the low temperature crystalline phase was determined. The resonance pair interaction between the dimer molecules was found to be an order of magnitude smaller than predicted from calculations reported so far.     

Statistical Study of Molecular Ordering in a Nematogenic Compound – A Computational Analysis

A computational Analysis has been carried out to determine the configurational preference of a pair of a nematogen, 4,4′‐azodiphenetole (C₂H₅‐O‐C₆H₄‐N=N‐C₆H₄‐O‐C₂H₅) [AZO] molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to compute the atomic charge and atomic dipole at each atomic centre. Configurational energy has been computed using modified Rayleigh‐Schrodinger perturbation method. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at phase transition temperature using Maxwell‐Boltzmann formula. Further, the flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. Molecular parameters like total energy, binding energy and total dipole moment have been given. Results have been discussed in the light of experimental as well as other theoretical observations.     

Hysteresis at the Lα–Pαβ Phase Transition in Hen Egg Lecithin

Infrared spectroscopy in the OH and CH stretching region of very thin samples of hydrated egg yolk lecithin reveals an hysteresis at the transition between the two liquid crystalline phases L_α (higher temperature phase) and P_αβ . Both of these phases havea lamellar structure with alternating hydrophilic and hydrophobic layers, but they differ as to the state of the interiors of the layers. The hysteresis can be explained in terms of a coupling between the structures and the dynamics of adjacent layers.     

The Electro-Optical Kerr Effect of Optically Active Liquids

The electro-optical Kerr effect of optically active liquids has been studied theoretically, applying the lamellar model of Jones, and experimentally, by measurement of the solution Kerr constant B₁₂ of a series of solutions of d-and dl-camphor in carbon tetrachloride. Equations are presented which allow the intrinsic phase retardation, δ, associated with the Kerr effect to be determined using the Sénarmont method in conjunction with either dc or pulsed electric fields. The method of determining δ, using pulsed electric fields is particularly useful for solutions having an appreciable electrical conductivity. The time-dependent behaviour of a relaxing system, measured using quadratic detection of the optical transient resulting from the application of a pulsed rectangular electric field, is predicted to be essentially unaffected by optical activity over a wide range of δ and optical rotation.     

Polycatenar oligophenylene liquid crystals

We report the synthesis of oligophenylene polycatenar liquid crystals incorporating 1,4‐disubstituted phenyl rings joined by a direct carbon carbon bond and some pyrimidine analogues. The nature of the linkages does appear to affect the mesomorphism significantly. The ratio of the aromatic core to the aliphatic chains is varied systematically by changing the number of 1,4‐disubstituted phenyl rings and the length of the aliphatic chains. This strongly influences the transition temperatures of the mesophases. Some of the compounds are columnar over an extended temperature range of more than 200°C with melting points below room temperature. We suggest that a combination of the poor overlap of the conjugated electron system of the molecular cores making up the columnar structure and the high concentration of aliphatic chains leads to a low charge‐carrier mobility.     

Conformational Behavior and Influence of Organic Solvents on a Strong Polar Group Nematogen at Room Temperature—A Computational Model

The conformational behavior and influence of organic solvents on a nematogen, 4’-n-alkyl-4-cyanobiphenyl, with strong polar group propyl (3CB) that is of commercial and application interest has been studied with respect to the translational and orientational motions. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrodinger Perturbation theory with the multicentered-multipole expansion method has been employed to evaluate the long-range interactions, and a “6-exp.” potential function has been assumed for the short-range interactions. The minimum energy configurations obtained during the different modes of interactions have been taken as input to calculate the configurational probability using the Maxwell–Boltzmann formula in nonpolar organic solvents, i.e., carbon tetrachloride (CCl₄), and chloroform (CHCl₃) at room temperature 300 K. It has been observed that the molecules show the interesting property in the organic solvents. The interaction energies of dimer complexes have been taken into consideration in order to investigate the most energetically stable configuration. An attempt has been made to develop an interesting computational model for nematogen at molecular level.     

Molecular ordering in a bipolar nematogenic cyanobiphenyl – a computer simulation approach

The molecular ordering of 4-(6-hydroxypropyloxy)-4′-cyanobiphenyl (H3CBP), a bipolar nematogen, has been carried out at room temperature (300 K) with respect to translatory and orientational motions. The complete neglect differential overlap (CNDO/2) method has been employed to compute the net atomic charge and atomic dipole moment at each atomic center. The modified Rayleigh–Schrodinger perturbation theory with multicentered-multipole expansion method has been employed to evaluate long-range intermolecular interactions, while a 6-exp potential function has been assumed for short-range interactions. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at room temperature using the Maxwell–Boltzmann formula. On the basis of stacking, in-plane, and terminal interactions energy calculations, all possible geometrical arrangements of the molecular pair have been considered. Further, an attempt has been made to develop a model for liquid crystallinity based on probability of dimer complexes.     

Phase stability and ordering of nematogen at molecular level: A computer-aided modeling

The present article deals with the phase stability and ordering of nematogen, viz., 6-octyloxy-2-naphthylyl-4-octoxybenzoate (ONOB) at molecular level. A comparative picture has been given between molecular charge distribution, and phase stability based on AM1, PM3, CNDO, and MNDO methods. The modified Rayleigh–Schrodinger perturbation method along with multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a “6-exp” potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature, nematic-isotropic transition temperature, and above transition temperature using the Maxwell–Boltzmann formula. Further, the entropy of each configuration has been computed during the different modes of interactions. An effort has been made to develop a computational model at molecular level based on configurational entropy, and translational rigidity parameters introduced in this article.     

Theoretical study of closo-decaborate novel nematogen: Thermodynamic behavior and phase stability

The present article deals with the thermodynamic behavior and phase stability of closo-decaborate nematogen viz. dinitrogen-10-(4-pentyl-1-thiacyclohexyl)-closo-decaborate (DPTD) at a molecular level. The atomic net charge and dipole moment at each atomic center have been evaluated using the complete neglect differential overlap (CNDO) method. The modified Rayleigh–Schrodinger perturbation method along with multicentered–multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature (300 K), nematic–isotropic transition temperature (435 K), and above transition temperature (500 K) using the Maxwell–Boltazman formula. Further, the entropy of each configuration has been computed during the different modes of interactions. The adopted framework provides valuable information on thermodynamic behavior, and phase stability of novel nematogen based on parameters, i.e., molecular and thermodynamic, introduced in this article.     

Excitons in α-Oxygen Crystals

The investigation results of exciton, exciton-magnon and biexciton absorption by α-oxygen at T = 1.5 K are reported.

It is shown that the doublet structure of exciton-magnon bands is due to anisotropy of the exciton and rnagnon bands; the doublet structure of the bands in the two-particle absorption results from the bound two-exciton states, the biexcitons, present in α-O₂.