Change In Molecular Polarizability Research Articles

Enhanced electro-optical nonlinear responses of doped nematic liquid crystals: Towards optoelectronic devices

Liquid crystals with high electro-optical responses play significant roles in optics and photonics. Providing a simple method for increasing the electro-optical behaviors of liquid crystals can be considered one of the most important and challenging research areas. In this work, left-handed chiral dopants and temperature effects on the electro-optical properties of pure and doped nematic liquid crystals with different percentages of chiral dopants were investigated. Despite decreasing the Kerr constant by increasing the percentage of chiral dopants, and temperature, the maximum values were observed for 4% of chiral dopants near the transition temperature. In this case, the value of Kerr constant is approximately 20 times that of a pure liquid crystal sample. This interesting result can be related to the changes in molecular polarizability due to molecular interactions. Hence, doped liquid crystals with high and controllable responses can be a good candidate for designing optoelectronic devices.

Stark Absorption Study of Perylene Doped in a Poly (Methyl Methacrylate) Polymer Film at Different Concentrations

In this article, electric field modulated absorption (Stark absorption, SA) study of perylene doped in a poly (methyl methacrylate) (PMMA) polymer film at the concentrations of 1.0 mol%, 2.0 mol% and 3.0 mol% is presented. The field effects on the absorption spectra were found to originate mostly from the Stark shift induced by a change in molecular polarizability between the ground and excited states. The analyses of SA spectra with the aid of Liptay formalism yielded essentially the same magnitude of the average change in molecular polarizability (Δα), evaluated for the S0 -> S1 transition, for all the concentrations. This finding implies that Δα of perylene molecule doped in a PMMA film does not depend on concentration. Moreover, the yielded positive Δα for both the preparations implies that SA spectroscopy is an essential technique to probe spectroscopically that perylene possesses larger molecular polarizability in the excited states than in the ground state.
 Journal of Bangladesh Academy of Sciences, Vol. 43, No. 2, 133-139, 2019

A Microscopic Model of the Electrochemical Vibrational Stark Effect: Understanding VSF Spectroscopy of (bi)Sulfate on Pt(111)

Much work over the past several decades has demonstrated that the vibrational frequencies of adsorbates on metal surfaces in ultra-high vacuum (UHV) shift with changing surface coverage. Extensive theoretical efforts have shown that this shift can be quantitatively understood as the result of dipole-dipole interaction. A similar phenomenon is known from spectro-electrochemistry (the so-called electrochemical Vibrational Stark Effect (VSE)): the vibrational frequency of the adsorbate is observed to change as a function of applied external potential. Typically this potential-dependent change in adsorbate frequency is assumed to be linear (or piece-wise linear) with applied bias and thus changes in slope are assigned to changes in adsorbate structure or chemical reactions. Here we measure the change in spectral response of the (bi)sulfate anion adsorbed on a Pt(111) working electrode with changes in applied bias and quantitatively interpret the results using a microscopic model of the VSE that explicitly accounts for the effect of dipole-dipole interaction. Using this model we show that changes in external bias that cause a linear change in molecular polarizability can induce a nonlinear change in the center frequency and intensity of the spectral response of the adsorbed (bi)sulfate: the nonlinearity of the VSE depends both on the local field at potential of zero charge and also the relative change induced by external bias. These results imply that essentially all spectroelectrochemical studies that have attributed chemical significance to changes in slope of spectral parameters with applied bias should be revisited, as potential dependent dipole-dipole interaction can induce such changes independent of any structural or chemical change.

Temperature Transferability of Force Field Parameters for Dispersion Interactions.

The accuracy of force fields is a key to the successful prediction of the thermodynamic properties of materials. In simulations of organic molecules over large temperature ranges, atomistic force fields that are parametrized at, or near, ambient temperatures are found to systematically underestimate the intermolecular dispersion interactions at elevated temperatures. Analysis of the underestimates using diatomic molecules indicates that a minor part is due to the change in molecular polarizability, while the major part is due to the reduced dielectric constant of the bulk liquid as the density decreases with increasing temperature. By establishing the dispersion parameter as a linear function of temperature, we have successfully enhanced the temperature transferability of atomistic force fields. This approach is tested on 66 molecular liquids covering four functional groups - alkane, aromatic, ether, and ketone-aldehyde - over a broad range of temperatures by calculating liquid density, heat of vaporization, isobaric heat capacity, and shear viscosity.

Can Lattice Models Predict the Density of States of Amorphous Organic Semiconductors?

We extend existing lattice models of small-molecule amorphous semiconductors by accounting for changes in molecular polarizability upon charging or excitation. A compact expression of this contribution to the density of states is provided. Although the lattice model and the description based on a microscopic morphology both qualitatively predict an additional broadening, shift, and an exponential tail (traps) of the density of states, a quantitative agreement between the two cannot be achieved.

Interfacial Charge-Transfer Effects in Semiconductor–Molecule–Metal Structures: Influence of Contact Variation

The charge-transfer resonance of Raman measurements in nanosized semiconductor–molecule–metal interfaces as a function of the excitation energy with four models (Cu–ZnO–PATP–Ag, Cu–Ag–PATP–ZnO, Cu–ZnO–Ag–PATP, and Cu–Ag–ZnO–PATP assemblies) to describe this dependence provides a powerful tool to study the chemical mechanism of surface enhanced Raman scattering (SERS). We measured the SERS spectra of self-assembled p-aminothiophenol (PATP) molecule junctions at 488, 514, 633, and 785 nm excitation wavelengths. We followed changes at the molecule junctions during the conditioning and eventually effect of charge-transfer (CT) through molecule–ZnO interfaces. Our results demonstrate that the interaction between the semiconductor bands and molecular energy levels can lead to novel charge behavior. The typical ZnO-PATP interfacial electron–hole recombination causes an increase in the CT resonance enhancement of Raman scattering, which is mainly responsible for the drastic change in molecular polarizability. We ...

Stark shifts and exciton dissociation in CdSe nanoparticle grafted conjugated polymer

Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of polymer films of CdSe quantum dots (QDs) incorporated sulfide-substituted poly(1,4-phenylene vinylene) derivative CdSe-S3PPV were measured. The observed Stark shift both in E-A and E-PL responses is likely to be caused by a substantial contribution of change in molecular polarizability (Δα¯) and change in electric dipole moment (|Δμ|) following photoexcitation. Together with Stark shift, field-induced photoluminescence (PL) quenching and enhancement were observed depending on excitation energy. The quenching of PL of CdSe-S3PPV film is interpreted in terms of an exciton model-a breaking of electron-hole pairs in the presence of electric field.

Comparative study of electroabsorption spectra of polar and nonpolar organic molecules in solution and in a polymer film

Electroabsorption (EA) spectra of polar and nonpolar molecules of coumarin 153 (C153) and pyrene in solution and in a polymer film of polymethylmethacrylate (PMMA) have been measured in the UV-visible region at room temperature. The shape of the EA spectra of C153 in benzene, 1,4-dioxane, or monochlorobenzene remarkably depends on the angle between the polarization direction of the absorption light and the applied electric field, whereas the EA spectra of C153 doped in PMMA show only the Stark shift and the field-induced change in spectral shape is negligible. These results demonstrate that C153 is reoriented by application of electric fields in solution, but the molecules are immobilized in a PMMA film. Based on the EA spectra, electric dipole moments both in the ground state and in the excited state have been evaluated for C153 in different solvents. In the EA spectra of pyrene, only the Stark shift is observed both in solution and in PMMA, indicating that the field-induced molecular reorientation does not occur both in solution and in PMMA. The change in dipole moment of C153 as well as the change in molecular polarizability of pyrene following absorption is much larger in solution than that in PMMA.

Electroabsorption and Electrophotoluminescence Spectra of Porphyrin Supramolecules in a Polymer Film

Abstract Electroabsorption and electrophotoluminescence spectra of supramolecular porphyrin dimers in a solid film of poly(methyl methacrylate), PMMA, have been examined. The electroabsorption spectra in the Soret band region are fitted by the deconvolution of the corresponding absorption spectrum into four components, indicating that electroabsorption measurements are useful to identify bands that are hard to distinguish in absorption spectra. The values of the changes in molecular polarizability and dipole moment following photoexcitation are evaluated at each absorption band.

Thermal and electrothermal sensitivity of polyglutamic acid with incorporated carbocyanine dyes in Langmuir–Blodgett films

Light-and electrosensitive carbocyanine dyes were incorporated into polyglutamic acid via covalent and non-covalent bonding. The reversible colour change on heating the Langmuir-Blodgett films of polyaminoacid has been studied by absorption spectroscopy and electroconductivity techniques. Characteristic shifts of the absorption spectrum are explained by formation of aggregates and hydrogen bonds. It was shown that H-aggregates and dimers are formed in Langmuir-Blodgett films; each type of assemblies contributes to the absorption spectrum. At elevated temperatures the ratio between concentrations of monomers, dimers and H-aggregates varies due to a breakdown of a part of aggregates into monomers. The change in molecular polarizability was also noticed. Electroconductivity of Langmuir-Blodgett films is observed in longitudinal and transverse directions. Electrochromic effect is noticed under weak electric current by change from red to colourless with iodine doping.

External Electric Field Effects on Absorption, Fluorescence, and Phosphorescence Spectra of Diphenylpolyynes in a Polymer Film

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.

Electric Field Effects on Absorption and Fluorescence Spectra of Trimethylsilyl- and Trimethylsilylethynyl-Substituted Compounds of Pyrene in a PMMA Film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.

Refractive index changes in polysilane films due to thermochromism

A large change in refractive index of about 0.07 was found in polysilane (poly(di-n-hexylsilane)) after a thermal treatment. The change was coincident reversibly with the material's thermochromism, suggesting that a part of the origin is from a change in molecular polarizability, i.e., the σ-conjugation length, arising from a switching of the Si backbone conformation between trans-planar and random-helix. By incorporating a pigment film for optothermal energy conversion, a possible application in reversible information recording using near-infrared light irradiation has been demonstrated.

Electric Field Effects on Fluorescence of Methylene-Linked Compounds of Phenanthrene andN,N-Dimethylaniline in a Poly(methyl methacrylate) Polymer Film

Electric field effects on photoinduced electron-transfer processes have been examined in a polymer film for methylene-linked compounds of phenanthrene and N,N-dimethylaniline, based on the measurements of the field effects on fluorescence spectra. Both intra- and intermolecular electron-transfer processes are enhanced by an electric field (F), depending on the methylene chain length, and LE fluorescence emitted from the locally excited state of phenanthrene chromophore is quenched by F. Exciplex fluorescence is also influenced by F. The magnitude of the change in molecular polarizability following fluorescence process has been determined from the Stark shift both for LE fluorescence and for exciplex fluorescence.

Electric-Field-Induced Quenching of Fluorescence of Tetraphenylporphyrin in a PMMA Polymer Film

Abstract Fluorescence of tetraphenylporphyrin (TPP) doped in a polymer film is quenched by an electric field. Field-induced enhancement of the internal conversion from S1 to the ground state is considered to be accelerated by a molecular association. Electroabsorption spectra and electrofluorescence spectra also show that change in molecular polarizability following excitation increases by a molecular association of TPP.

Absorption and electroabsorption spectra of carotenoid cation radical and dication

Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl 3 in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D 0→D 2 transition in cation radical is at 835 nm. The electroabsorption spectrum for the D 0→D 2 transition exhibits a negative change of molecular polarizability, Δ α=−1.2·10 −38 C·m 2/V (−105 A 3), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D 1 and D 2 excited states. Absorption spectrum of astaxanthin dication is located at 715–717 nm, between those of D 0→D 2 in cation radical and S 0→S 2 in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δ α=2.89·10 −38 C·m 2/V (260 A 3), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.

Electric field effects on absorption and fluorescence spectra of pyrene doped in a PMMA polymer film

Electric field modulated absorption, fluorescence and fluorescence excitation spectra of pyrene doped in a PMMA polymer film were measured with excitation from 220 to 380 nm. The observed spectra of the field-induced change in absorption intensity or fluorescence intensity for each electronic transition agree well with the first derivatives of the corresponding absorption or fluorescence spectrum, indicating that the field effect results from the Stark shift induced by a change in molecular polarizability between the ground state and the excited state. The magnitude of the change in polarizability was evaluated for the first, S 1( 1L b), the second, S 2( 1B a), the third, S 3( 1B b), and the fourth, S 4( 1L a), optically spin-allowed absorption bands. The change in average polarizability following excitation is found to be in the order of S 1<S 4<S 3<S 2, and all the polarizabilities in the excited states are larger than in the ground state. The third nonlinear order susceptibility ( χ (3)) of a pyrene-doped PMMA film was also calculated on the basis of the electroabsorption and electrofluorescence excitation spectra.

Altered reaction dynamics: lowered barriers and bound states induced by intense infrared laser fields

This Letter reports a study on the effect of the Stark shift for exchange reactions in intense infrared laser fields. It is shown for the H + H 2 exchange reaction that such fields reduce the potential barrier, thus enhancing the reaction rate at low temperatures. Another interesting effect, caused by the change in molecular polarizability as reactants approach, is the creation of (laser-induced) potential minima along the reaction path.

On the determination of molecular polarizability changes upon electronic excitation from the solvent shifts of absorption band maxima

A linear relationship was established between the polarizability differences of the ground and excited state molecules (Δα) from electrochromism measurements and pMW, where p is the slope of the plot of absorption band maxima against the Lorenz-Lorentz function ϕ( n 2) = ( n 2 − 2)/( n 2 + 2), n is the refractive index of the solvent and MW is the relative molecular weight of the solute. The Δα values of porphyrins, aromatic hydrocarbons, polar and ionic π-electronic molecules were estimated by using this dependence as a calibration line. The average polarizability is always larger in the excited states (exception: the S 1 state of azulene with Δα=0±2 Å 3). For different transitions in a given compound Δα increases, as their energy and oscillator strength grow (exceptions: the S 3 band of azulene at 296 nm and the S 2 band of bacteriochlorophyll a at 575 nm). The magnitudes of Δα are relatively small in the case of S 1 bands in porphyrins. A linear correlation was found between Δα/MW and the average matrix-induced dipole moment change responsible for linear broadening of spectral holes in an external electric field.

The evaluation of α′-elements for the knowledge of forms of molecular vibrations

Abstract Elements of the tensors of molecular polarizability changes in the single vibrations (the α′-elements) may be obtained from single crystal Raman work if the “oriented gas hypothesis” is applicable. From work on phosphonitrilic halides, relations between some of the α′ in the normal vibrations of triatomic groups have been extracted. Certain relations derived for p-chloroaniline are in accord with calculated values for benzene, but they require new concepts about the forms of the normal vibrations connected with the substituents. The apparent absence of interpretations of the α′ in most single crystal work is regretted.