Influence of the bias field on dielectric properties of the SmCA* in the vicinity of the SmC*–SmCA* phase transition

In a large class of smectic mixtures prepared at our University, the phase transition between chiral ferroelectric smectic C (SmC*) and chiral antiferroelectric smectic C (SmC_A*) phases can be observed on cooling. Under bias field the temperature of the phase transition SmC*?SmC_A* decreases (ca. 100°C in the investigated mixture). The transition is called: unwound SmC*?twisted SmC_A* phase transition. The Goldstone mode in SmC* phase is reduced by a direct current field while two modes (P_H and P_L) in the SmC_A* phase are amplified. The amplitude of the fast X mode observed in the SmC_A* phase is reduced. The aim of this paper is to show how parameters of the modes in SmC_A* phase (calculated from Cole–Cole model) change with bias voltage—when twisted structure in SmC_A* phase is gradually unwound. The character of the modes observed in SmC_A* is discussed. A new effect is shown: a high value of dielectric loss is detected in the unwound SmC* phase, which is very close to SmC_A*.     

Reactions of 3-aroylacrylates with α-aminoazoles*

Dihydro derivatives of pyrazolo[3,4-b]pyridine-, pyrazolo[1,5-a]pyrimidine-, and [1,2,4]triazolo-[1,5-a]pyrimidinecarboxylates have been prepared by cyclocondensation of β-aroylacrylates with 5-aminopyrazoles and 3-amino-1,2,4-triazole. Heating dihydro[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylates with hydrazine hydrate led to recyclization of the pyrimidine ring to form 6-arylpyridazin-3(2H)-ones.     

Dielectric characterisation of a ferroelectric liquid crystalline material having SmA*–SmC*–SmBh* phase sequence

Thermodynamical, optical and dielectric characterisation of a material possessing ferroelectric SmC* and hexatic SmB* phases has been carried out. Phase identification has been done by miscibility studies. From the dielectric studies, a relaxation mechanism is observed in the low MHz region of the SmA* phase, which is related to the tilt fluctuation (soft mode) of the directors. In the SmC* phase, another collective relaxation mechanism has been observed in the kHz region, which is related to the phase fluctuation (Goldstone mode) of the directors. In the SmB_h* phase, 2-weak relaxation modes are observed in the kHz and MHz frequency range, respectively, due to individual molecular rotations.     

Spectral shifts of the n → π* and π → π* transitions of uracil based on a modified form of solvent reorganization energy

According to our recent studies on the nonequilibrium solvation, the solvent reorganization energy is found to be the cost of maintaining the residual polarization P', which equilibrates with the extra electric field E(ex). On the basis of this solvent reorganization energy and the well-established equilibrium solvation energy, a novel and reasonable expression for the spectral shift of the electronic absorption spectra is proposed in this work. Furthermore, the two lowest transitions of uracil in aqueous solution are investigated as test cases with the TDDFT/6-311++G** method. The obtained spectral shift is 0.48 eV for n → π* transition and -0.14 eV for π → π* transition, agreeing well with available experimental results. The contributions to the shift are discussed and the electrostatic plus polarization components are found to be crucial for the electronic absorption spectra of uracil in aqueous solution.     

Dielectric investigation of the liquid crystal compound with the directSmA*–SmCA* phase transition

New compound showing a direct SmA*–SmC_A* phase transition was synthesised. As far as authors know there are a few pure compounds showing para- and antiferroelectric phases without SmC* between them. Direct current (DC) field applied into a planar-oriented cell induces ferroelectric SmC* phase in an investigated compound. Typical for SmC*, Goldstone mode starts to be detectable. DC field also shifts down the temperature of a SmC_A* phase creation. Moreover, modes in the appearing antiferroelectic phase are enhanced by DC field. This paper shows and discusses relations between modes detected in SmA*, SmC_A* and SmC* (SmC* phase – nucleated by DC field) phases. Parameters of observed modes are calculated using the Cole–Cole relaxation model and a calculation procedure useful especially for high frequency relaxations (higher than 200 kHz).     

Synthesis of heterocyclic compounds using basic zeolite Csβ*

The application of the basic zeolite Csβ as catalyst for the interaction of methyl vinyl ketone (MVK) with 5-methoxybenzimidazole-2-thiol leads to a Michael heteroreaction exclusively at the N-nucleophilic center with the formation of a fairly unusual product of di-addition of MVK to thiol. The reaction of 1,2,4-triazole-3-thiol with MVK in the presence of zeolite Csβ proceeds both at the S- and also at the N-nucleophilic center and leads to the formation of products of mono- and diaddition according to Michael, and also to the product of heterocyclization. On interacting crotonaldehyde with salicylaldehyde in the presence of Csβ 2-methyl-2H-chromene-3-carbaldehyde is formed.     

Direct Observation of Singlet ππ* and nπ* Equilibrium State in 2-Amino-1,3,5-triazine Solution

1,3,5-Triazine molecules represent a class of molecules that may have been prebiotic information carriers in a primordial soup in early Earth and their excited state dynamics has received attention in recent years. In our previous study, one component with lifetime longer than 100 ps was discovered in 2-amino-1,3,5-trainzine (2-AT), but its nature has not been revealed. In this study, excited state dynamics of 2-AT is studied in different solvents by using femtosecond time-resolved transient absorption and fluorescence up-conversion spectroscopy. Interestingly, an equilibrium state consisting of the bright \begin{document}$ \pi\pi.* $\end{document} and dark n\begin{document}$ \pi.* $\end{document} states in 2-AT is directly observed in aqueous solution and its dynamics is solvent sensitive. The whole picture of the excited state deactivation mechanism of 2-AT is proposed based on our spectroscopy results.     

Energies of π*- and π-states of the conduction band and valence band of chiral carbon nanotubes

Nanotubes (NTs) are mainly represented by (n,p) chiral NTs with chirality indices 0 < p < n delimited by (n,0) and (n,n) for achiral NTs. In (n,p) chiral NTs, the unit cell hexagons have a helical arrangement on the cylindrical surface of an NT and common angular and axial translations. An analytical formula was derived for calculation of the band structure of both chiral and achiral NTs with chirality indices 0 ≤ p ≤ n and band diagrams of some chiral NTs. Chiral NTs significantly extend the range of semiconducting NTs. An equation for the band gap width ΔЕ of semiconducting chiral and achiral NTs was derived: \(\frac{{\vartriangle E}}{{{\gamma _0}}} = \frac{{2\pi }}{{\sqrt {3{n^2} + 3np + 3{p^2}} }}\). Tables of the band structure parameters of metallic and semiconducting chiral NTs are presented.     

Spin-orbit coupling of ππ* states in molecules with annelated oxazole ring

Within the framework of the -electron approximation, matrix elements have been calculated for the operator of spin-orbit coupling (SOC) between singlet and triplet terms of oxazole derivatives with condensed naphthalene, quinoline, coumarin, and carbostyryl fragments (24 structures were examined). Values calculated for the SOC have been compared with experimentally measured quantum yields of fluorescence for 15 compounds. It has been established that the values of the SOC matrix elements amount to 0.02–0.03 cm^–1 on the average.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 403–406, July–August, 1990.     

Theoretical study of the valence π → π* excited states of polyacenes: anthracene and naphthacene

The valence π → π ^* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon spectra and T-T spectra of low-lying valence π → π ^* excited states of anthracene and naphthacene. The present theory predicts the valence π → π ^* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions, but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can be used as a model for understanding the excited states of larger polyacenes. Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998     

Observation of Exceptionally Low-Lying π-π* Excited States in Oxidized Forms of Quadruple-Decker Phthalocyanine Complexes

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (～1000 cm(-1)). With the aid of theoretical calculations, these bands can be assigned to the π-π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps.     

The role of the πσ* state in intramolecular charge transfer of 4-(dimethylamino)benzonitrile

The solvent-polarity dependence and temporal characteristics of the transient absorption of 4-(dimethylamino)benzonitrile, DMABN, and 4-(dimethylamino)benzethyne, DMABE, demonstrate the presence of the πσ*-state absorption at about 700 nm and the ππ* (LE)-state absorption at about 520 nm and 450 nm. The rise and decay times of the πσ*-state transient differ from those of the ππ*-state transients in both compounds. Moreover, the peak position of the πσ*-state absorption is blue-shifted and more intense in acetonitrile as compared to n-hexane, whereas the band positions of the ππ*-state absorptions are essentially the same in the two solvents. For DMABN in acetonitrile, the rise time (～4.3 ps) of the twisted intramolecular charge transfer (TICT)-state transient at 330 nm is identical to the decay time of the πσ*-state transient. The 4.8 ns decay time of the TICT-state absorption of DMABN is longer than the 2.9 ns decay time of the intramolecular charge-transfer (ICT) fluorescence, indicating that the fluorescent ICT state differs from the TICT state observed in transient absorption. These results are consistent with the presence of a low-lying πσ* state in DMABN (and DMABE), and the role the πσ* state plays in the formation of the TICT state of DMABN.     

On the solvatochromism of the n ↔ π* electronic transitions in ketones

The solvatochromism of the n ? π* electronic transitions of acetone, which is determined in this work by means of absorption and emission spectroscopy, has been studied using the pure solvent scales for polarizability, dipolarity and acidity of the medium. From these analyses, the necessary reduction of the dipole moment and the increase of the polarizability for acetone on electronic excitation are evaluated using Abe's approach (Bull. Chem. Soc. Jpn. 1966, 39, 936). The influence that the increase of the aromatic structure (cf. acetophenone) and the lengthening of the size of the alkyl substituents R- (cf. R-CO-R) cause on the solvatochromism of acetone is discussed. Also, we have shown that the photophysical model proposed by Pimentel (J. Am. Chem. Soc. 1957, 79, 3323), which is widely accepted for explaining the n → π* blue shift phenomenon for hydrogen-bonded complexes, is mistaken due to ignoring the acid-base changes undergone on electronic excitation; accordingly, a new photophysical model has been proposed.     

Enhance photocatalytic properties of TiO2 using π-π* conjugate system

Abstract

A novel π-π* conjugate system photo-catalyst was successfully constructed using aniline, pyrrole and TiO₂. The structures of photo-catalyst were measured by XRD, SEM, BET, TEM, FT-IR, XPS and TGA in detail. The photocatalytic properties were investigated in rhodamine B and methyl orange solution, respectively. And the enhance mechanism of π-π* conjugate system was discussed in depth. The weight percentage of Ti weight about 47% in the compound and the π-π* conjugate system did not change the crystal structure of TiO₂. The photocatalytic degradation properties of the π-π* conjugate system photo-catalyst could get 99% for rhodamine B and methyl orange after 10 and 15?min, respectively. The introduction of the π-π* conjugate system into TiO₂ was beneficial to improve light harvest, photoelectric response and separation of electron-holes.